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Ethernet Specification

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1. 5 Q in the configuration 7 2 2 3 Attenuation The signal attenuation of any pair shall not exceed 3 dB measured at 10 MHz for the total length between the transceiver and the station 7 2 2 4 Velocity of Propagation The minimum velocity of propagation of the transceiver cable shall be 0 65 c 7 2 2 5 Pulse Distortion Pulse distortion shall not exceed 1 nS at the end of 50 meters of cable when driven with random 10 Mbit data encoded in accordance with 7 5 1 7 2 2 6 Resistance The resistance of the conductors used for the power pair shall not exceed 40 milliohms per meter 7 2 2 7 Transfer Impedance The common mode transfer impedance of the transceiver cable shall not exceed the values shown in Figure 7 2 as a function of frequency The differential mode transfer impedance of the cable with respect to any pair shall be 20 dB lower than the specified common mode transfer impedance 100 10 mQ meter Hn oe d 33 E H E HH I1 a BEBIH sii Sati SH 10KHz 100K 10M 100M Frequency Figure 7 2 Transceiver Cable Transfer Impedance 56 ETHERNET SPECIFICATION Physical Layer 7 2 3 Transceiver Cable Connectors The connectors used at the ends of the transceiver cable shall be 15 conductor D Subminiature types Cinch type DASM 15 or equivalent The end of the cable that mates with the transceiver must use a female connector with a slide lock assembly Cin
2. 1 Channel Goals and Non goals This section states the objectives underlying the design of the channel 7 1 1 1 Goals The following are the goals of the channel 1 Provide a means for communication between Ethernet Data Link Entities 2 Define physical interfaces which can be implemented corapatibiy among different manufacturers of hardware 3 Provide all clocks synchronization and timing required for both itself and the Ethernet Data Link 4 Provide high bandwidth and low bit error rates 5 Provide for ease of installability and serviceability 6 Provide for high network availability 7 Support the Ethernet Data Link to Physical Link interface 8 Low cost 7 1 1 2 Non Goals The following are not goals of the baseband coaxial channel design 1 Operation at data rates other than 10 megabits per second 2 Operation with media other than the specified coaxial cable 3 Simultaneous use of the channel by transmitters using signals not specified in this document 4 Protection against a malicious user or a malfunctioning Data Link Entity is not provided by the channel as specified However higher layers above the Data Link and or physical security means may be employed to acheive this 46 712 ETHERNET SPECIFICATION Physical Layer Characteristics of the Channel The channel provides and the data link assumes the following characteristics 1 7 1 3 The ability to send and receive information non simultane
3. Encoder 7 5 1 2 Decoder 7 5 1 3 Preamble Generation 7 5 2 Collision Detect Signal 7 5 3 Carrier Sense Signal 7 5 4 Channel Framing 7 5 4 1 Beginning of Frame Sequence 7 5 4 2 End of Frame Sequence 7 6 Channel Configuration Requirements 7 6 1 Cable Sectioning 7 6 2 Transceiver Placement 7 6 3 System Grounding 7 6 4 Repeaters 7 6 4 1 Carrier Detect and Transmit Repeat 7 6 4 2 Collision Detect and Collision Repeat 7 6 4 3 Repeater Signal Generation 7 6 4 3 1 Signal Amplification 7 6 4 3 2 Signal Timing 7 7 Environment Specifications 7 1 1 Electromagnetic Environment 7 7 2 Temperature and Humidity ETHERNET SPECIFICATION Contents Appendices APPENDIX A GLOSSARY APPENDIX B ASSIGNMENT OF ADDRESS AND TYPE VALUES APPENDIX C CRC IMPLEMENTATION APPENDIX D IMPLEMENTATION OF TRANSCEIVER CABLE DRIVER AND RECEIVER APPENDIX E INTERFRAME RECOVERY Figures and Tables Figure 4 1 Ethernet Architecture and Typical Implementation Figure 4 2 Architectural Layering Figure 4 3 Data Link Layer Functions Figure 4 4 Physical Layer Functions Figure 6 1 Data Link Layer Frame Format Figure 6 2 Structure of the Data Link Procedural Model Figure 6 3 Control Flow Summary Client Layer Processes Figure 6 4 Control Flow Summary Data Link Layer Processes Figure 7 1 Physical Channel Configurations Table 7 1 Physical Channel Propagation Delay Budget Figure 7 2 Maximum Transceiver Cable Transfer Impedance Figure 7 3 Typical
4. and attach terminators Three types of connectors may be necessary male plugs female jacks and female to female barrels Plugs are used exclusively at the ends of all cable sections Jacks are used to house cable terminators Barrels are used to join cable sections All connectors are N series 50 constant impedance types Since the frequencies present in the transmitted data are well below UHF range being band limited to approximately 20 MHz military versions of the connectors are not required but are acceptable Means must be provided to ensure that the connector shell which connects to the cable sheath does not make contact with any building metal or other unintended conductor A sleeve or boot to be slid over connector at installation time is suitable 7 3 1 3 Coaxial Cable Terminators Coaxial cable terminators are used to provide a termination impedance for the cable equal in value to its characteristic impedance thereby eliminating any reflection from the ends of the cables Terminators shall be packaged within an inline female jack connector The termination impedance shall be 50 2 1 measured from 0 50 MHz with the magnitude of the phase angle of the impedance not to exceed 5 degrees The terminator power rating shall be 1 watt or greater ETHERNET SPECIFICATION Physical Layer 61 7 3 1 4 Transceiver to Coaxial Cable Connections means must be provided to allow for attaching a transceiver to the coaxial cable
5. been presented the Data Link Layer sets transmitting to false to indicate the end of the frame The presence of a collision in the physical channel is signaled to the Data Link Layer via the variable collisionDetect var collisionDetect Boolean The collisionDetect signal remains true during the duration of the collision Note Since an entire collision may occur during the first invocation of TransmitBit e g during preamble removal the Data Link Layer must handle this possibility by monitoring collisionDetect concurrently with its transmission of outgoing bits See 6 5 for details The collisionDetect signal is generated only during transmission and is never true at any other time in particular it cannot be used during frame reception to detect collisions between overlapping transmissions from two or more other stations During reception the contents of an incoming frame are retrieved from the Physical Layer by the Data Link Layer via repeated use of the ReceiveBit operation function ReceiveBit Bit Each invocation of ReceiveBit retrieves one new bit of the incoming frame i e not including any preamble bits from the Physical Layer The ReceiveBit operation is synchronous in the sense that its duration is the entire reception of a single bit As with TransmitBit the first invocation of ReceiveBit make take significantly longer e g due to preamble removal Upon receiving a bit the Data Link Layer must immediate
6. between any two transceivers on the channel 7 6 4 1 Carrier Detect and Transmit Repeat Repeaters must implement the carrier sense function as specified in 7 5 3 for both segments between which it is connected Upon detection of carrier from one segment the repeater must retransmit all received signals from that segment onto the other segment Signals shall be retimed and amplified as specified in 7 6 4 3 The maximum steady state propagation delay through the repeater for the repeated signal not including startup delays carrier sense delay or retiming delays shell not exceed 800 nS 7 6 4 2 Collision Detect and Collision Repeat Repeaters must implement the collision detect function as specified in 7 5 2 for both segments between which it is connected If while repeating signals as specified in 7 6 4 1 collision is detected on either side the repeater must ensure that all stations involved in the collision recognize the event as a collision regardless of which side of the repeater the station is on The maximum time between the recongnition of the collision and the repeating of the collision indication not including carrier sense of retiming delays shall not exceed 200 nS 7 6 4 3 Repeater Signal Regeneration 7 6 4 3 1 Signal Amplification The repeater with its associated transceivers shall ensure that any signals repeated between segments shall have the same amplitude characteristics at the transceiver output of the repea
7. code type TransmitStatus transmitOK excessiveCollisionError 16 ETHERNET SPECIFICATION Inter Layer Interfaces Successful transmission is indicated by the status code transmitOK the code excessiveCollisionError indicates that the transmission attempt was aborted due to excessive collisions because of heavy traffic or a network failure Implementations may define additional implementation dependent status codes if necessary The Client Layer accepts incoming frames by invoking ReceiveFrame function ReceiveFrame var destinationParam AddressValue var sourceParam AddressValue var typeParam TypeValue var dataParam DataValue ReceiveStatus The ReceiveFrame operation is synchronous in the sense that the operation does not complete until a frame has been received The fields of the frame are delivered via the output parameters along with a status code type ReceiveStatus receiveOK frameCheckError alignmentError Successful reception is indicated by the status code receiveOK The code frameCheckError indicates that the frame received was damaged by a transmission error in the physical channel The code a ignmentError indicates that the frame received was damaged and that in addition its length was not an integral number of octets Implementations may define additional implementation dependent status codes if necessary 5 2 Data Link Layer to Physical Layer The interface through which the Data Link Layer uses th
8. crcSize see 6 5 1 2 note 3 slotTime 512 unit of time for collision handling type Bit 0 1 AddressValue array 1 addressSize of Bit array 1 typeSize of Bit DataValue array 1 dataSize of Bit CRCValue array 1 crcSize of Bit ViewPoint fields bits Two ways to view the contents of a frame Frame record Format of data link frame case view ViewPoint of fields destinationField AddressValue sourceField AddressValue typeField TypeValue dataField DataValue fcsField CRCValue bits contents array 1 frameSize of Bit end Frame ETHERNET SPECIFICATION Data Link Layer 35 6 5 2 1 2 Transmit State Variables The following items are specific to frame transmission See also 6 5 2 1 4 on interfaces const interFrameSpacing 9 6 minimum time between frames in microseconds attemptLimit 16 Max number of times attempt transmission backOffLimit 10 Limit on number of times to back off jamSize 32 jam may be 32 to 48 bits long var outgoingFrame Frame The frame to be transmitted currentTransmitBit lastTransmitBit 1 frameSize Positions of current and last outgoing bits in outgoingFrame deferring Boolean True implies any pending transmission must wait for the channel to clear frameWaiting Boolean Indicates that ou goingFrame is deferring attempts O attemptLimit Number of transmission attempts on outgoingFrame
9. individual station product requirements Hardware which does not meet he temperature and humidity requirements specified must state so in its published product specification ETHERNET SPECIFICATION Appendix 73 APPENDIX A GLOSSARY This section defines some of the essential terminology associated with the Ethernet baseband coaxial system A system whereby information is directly encoded and impressed on the coaxial transmission medium One information signal at a time can be present on the medium without disruption see collision binary exponential backoff The algorithm used to schedule retransmissions after a collision So called because the interval from which the retransmission time is selected is expanded exponentially with repeated collisions broadcast Describes the class of media for which the Ethernet is designed in which all stations are capable of receiving a signal transmitted by any other station Also describes the mode of usage of such a medium by the Data Link Layer in which all stations are instructed to receive a given frame carrier sense A signal provided by the Physical Layer to the Data Link Layer to indicate that one or more stations are currently transmitting on the channel channel logic The logical functions provided between the transceiver cable and the Data Link which support the defined interface between the data link and the physical layers Client Layer Collective term used to describe any la
10. nearest octet boundary If frame check sequence validation see 6 4 1 3 detects an error in such a frame the status code alignmentError is reported 6 4 1 2 Address Recognition The Ethernet data link controller is capable of recognizing physical and multicast addresses as defined in 6 2 1 6 4 1 2 1 Physical Addresses The data link controller recognizes and accepts any frame whose destination field contains the physical address of the station The physical address of each station is set by network management to a unique value associated with the station and distinct from the address of any other station on any Ethernet The setting of the station s physical address by network management allows multiple data link controllers connected to single station all to respond to the same physical address The procedures for allocating unique addresses are discussed in Appendix B 6 4 1 2 2 Multicast Addresses The data link controller recognizes and accepts any frame whose destination field contains the broadcast address The data link controller is capable of activating some number of multicast group addresses as specified by higher layers The data link controller recognizes and accepts any frame whose destination field contains an active multicast group address An active multicas group address may be deactivated 6 4 1 3 Frame Check Sequence Validation FCS validation is essentially identical to FCS generation If the bits of the incomi
11. on the acquisition time of the network Itis an upper bound on the length of a frame fragment generated by a collision See 6 4 2 1 It is the scheduling quantum for retransmission See 6 3 2 3 2 In order to fulfill all three functions the slot time must be larger than the sum of the Physical Layer round trip propagation time 450 bit times see 7 1 2 and the Data Link Layer maximum jam time 48 bit times see 6 3 2 3 1 The slot time is defined to be 512 bit times 6 3 2 3 1 Collision Detection and Enforcement Collisions are detected by monitoring the collisionDetect signal provided by the Physical Layer When a collision is detected during a frame transmission the transmission is not terminated immediately Instead the transmission continues until at least 32 but not more than 48 additional bits have been transmitted counting from the time collisionDetect went on This collision enforcement or jam guarantees that the duration of the collision is sufficient to insure its detection by all transmitting stations on the network The content of the jam is unspecified it may be any fixed or variable pattern convenient to the data link controller implementation but should not be the 32 bit CRC value corresponding to the partial frame transmitted prior to the jam ETHERNET SPECIFICATION Data Link Layer 25 6 3 2 3 2 Collision Backoff and Retransmission When a transmission attempt has terminated due to a collision it is r
12. recognition frame check sequence validation and frame disassembly to pass the fields of the received frame to the Client Layer Receive Link Management s main function is the filtering of collision fragments from complete incoming frames The performance of these functions by a receiving data link controller interacts with corresponding actions by other data link controllers to jointly implement the Ethernet data link protocol 6 4 1 Receive Data Decapsulation 6 4 1 1 Framing The data link controller recognizes the boundaries of an incoming frame by monitoring the carrierSense signal provided by the Physical Layer There are two possible length errors that can occur which indicate ill framed data the frame may be too long or its length may not be an integral number of octets 26 ETHERNET SPECIFICATION Data Link Layer 6 4 1 1 1 Maximum Frame Size The receiving data link controller is not required to enforce the frame size limit specified in 6 2 5 but it is allowed to truncate frames longer than 1518 octets and report this event as an implementation dependent error 6 4 1 1 2 Integral Number of Octets in Frame Since the format of a valid frame specifies an integral number of octets only a collision or an error can produce a frame with a length that is not an integral multiple of 8 Complete frames i e not rejected as collision fragments see 6 4 2 1 that do not contain an integral number of octets are truncated to the
13. recognizes the presence of activity on the medium through the carrier sense signal This is the first indication that the frame reception process should begin However dependent upon the physical configuration of the system there are some number of preamble bits to be received by the channel before the start of the data link frame as indicated by the double 1 at the end of preamble In addition the first signals received from the decoder may be invalid due to the first bit allowance of the transceiver see 7 4 2 2 The channel must wait no less than 8 bit times 800 nS before monitoring the output of the decoder for the double 1 indicating end of preamble and beginning of data link frame Upon reception of the double l the channel shall begin passing successive bits to the data link through the defined receive bit stream interface If after waiting the required 8 bit times a double 0 is encountered the physical channel shall not pass any bits of ETHERNET SPECIFICATION Physical Layer 69 the current frame to the data link Normal operation of the data link and channel shall resume on the subsequent frame 7 5 4 2 End of Frame Sequence As specified in 7 5 3 the carrier sense signal must be deasserted no later than 1 6 bit times 160 ns after the cessation of activity on the coaxial cable as seen by the channel logic The channel ensures that no extraneous bits will appear at the end of a frame following the last valid bit 7 6 Chan
14. that time Reception of multiple closely spaced incoming frames is a very desirable capability and is crucial for stations which tend to communicate with several other stations concurrently There is one important case in which a data link controller implementation cannot reasonably be expected to receive closely spaced incoming frames if the station hardware e g I O bus is intrinsically unable to accept the bits of a frame at the rate at which they arrive over the network each incoming frame must be bvffered to allow the station to accept it at some lower rate Assuming limited buffering resources e g a one frame buffer reception of subsequent frames cannot occur until sufficient buffer space is available This mode of operation is allowed for low performance stations Reception of an incoming frame immediately after transmission of an outgoing frame is a very important capability even for stations which do not tend to communicate with several other stations concurrently All stations low performance to high performance should allow reception of an incoming frame immediately after transmission of an outgoing frame
15. to indicate the end of the preamble and the beginning of the data link encapsulation portion of the frame The next bit transmitted is the bit originally submitted by transmission by the data link Preamble removal on reception is discussed in 7 5 4 1 1 TE jo 1 0 31 0 1 0 1 high level low level Figure 7 7 Preamble Encoding ETHERNET SPECIFICATION Physical Layer 67 7 5 2 Collision Detect Signal The channel must indicate to the data link when the signals on the coaxial cable imply simultaneous transmission attempts by more than one station This is normally indicated through the collision presence pair in the transceiver cable described in 7 2 1 3 The channel logic must assert the collision detect signal within 2 bit times 200 ns following the onset of collision presence This collision detect signal shall be asserted only when the data link is transmitting A functional logic description of the collision detect signal is shown in Figure 7 8 Following the loss of collision presence information the channel must deassert the collision detect signal within 1 6 bit cell times 160 ns Transceiver Cable Collision Pair Transition Detect collisionDetect signal to Data Link Layer transmitting Transition Detect output enabled if Data Link Layer an input transition has been detected within the previous 1 6 bit times 160 nS Figure 7 8 Functional Logic of collisionDetect Signal 7 5 3 C
16. to vary ETHERNET SPECIFICATION Data Link Layer 29 6 5 1 2 Use of Pascal in the Procedural Model Pascal was chosen for the procedural model because of its relative simplicity and clarity and its general acceptance Several observations need to be made about the way in which Pascal is used for the model including a Some limitations of the language have been circumvented in order to simplify the specification 1 The elements of the program variables procedures etc are presented in logical groupings in top down order Certain Pascal ordering restrictions have thus been circumvented to improve readability 2 The process and cycle constructs of the Pascal derivative Concurrent Pascal 7 have been introduced to indicate the sites of autonomous concurrent activity As used here a process is simply a parameterless procedure that begins execution at the beginning of time rather than being invoked by a procedure call A cycle statement represents the main body of a process and is executed repeatedly forever 3 The lack of variable array bounds in the language has been circumvented by treating frames as if they are always of a single fixed size which is never actually specified In fact of course the size of a frame depends on the size of its data field hence the value of the pseudo constant frameSize should be thought of as varying in the long term even though it is fixed for any given frame 4 The use of a varian
17. value indicates a detected error The extra logic to test for this value is not shown in Figure C 1 3 ETHERNET SPECIFICATION Appendix Sheol Dies Sali EERE ap gt J M O NOT ux 0 gt Output Giese Input Output Figure C 1 CRC Implmentation ETHERNET SPECIFICATION Appendix 79 One potential problem which is avoided in this implementation is insensitivity of the shift register to incoming zero bits when it is in the all zero state Following standard practice this state is avoided at the beginning and end of the frame by preloading the shift register with all 1 bits and by inverting each bit of the final CRC Logically these correspond respectively to the complementing of the first 32 bits of the frame and to the final complementing of the remainder as specified in the mathematical definition in 6 2 4 See also 9 for further discussion 80 ETHERNET SPECIFICATION Appendix D APPENDIX D IMPLEMENTATION OF TRANSCEIVER CABLE DRIVER AND RECEIVER This appendix presents circuit digrams for typical implementations of the transceiver cable drivers and receivers The use of these exact circuits is not necessary for conformance to the specification equivalent circuits may be used as long as the relevant specifications are met Figure D 1 depicts an implementation of the transceiver cable driver specified in 7 2 4
18. 1977 4 ETHERNET SPECIFICATION Goals and Non Goals 3 GOALS AND NON GOALS This section states the assumptions underlying the design of the Ethernet 3 1 Goals The goals of the Ethernet design are Simplicity Features which would complicate the design without substantially contributing to the meeting of the other goals have been excluded Low cost Since technological improvements will continue to reduce the overall cost of stations wishing to connect to the Ethernet the cost of the connection itself should be minimized Compatibility All implementations of the Ethernet should be capable of exchanging data at the data link level For this reason the specification avoids optional features to eliminate the possibility of incompatible variants of the Ethernet Addressing flexibility The addressing mechanisms should provide the capability to target frames to a single node a group of nodes or to all nodes on the network Fairness All nodes should have equal access to the network when averaged over time Progress No single node operating in accordance with the protocol should be able to prevent the progress of other nodes High speed The network should operate efficiently at a data rate of 10 Megabits per second Low delay At any given level of offered traffic the network should introduce as little delay as possible in the transfer of a frame Stability The network should be stable under all load conditions in t
19. IFICATION Physical Layer 7 5 1 3 Preamble Generation Because most of the channel circuitry is allowed to provide valid output some number of bit times after being presented valid input it is necessary for a preamble to be sent before the start of data link information to allow the channel circuitry to reach its steady state with valid outputs throughout the system Upon request by the data link to transmit the first bit of a new frame the channel shall first transmit the preamble a predetermined bit sequence used for channel stabilization and synchronization If while transmitting the preamble the channel logic asserts the collision detect signal as specified in 7 5 2 any remaining preamble bits shall not be sent The channel should immediately proceed with the transmission of the bit submitted by the data link The preamble is a 64 bit pattern to be presented to the channel encoder in the same manner as data link information The pattern is 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011 The bits are transmitted in order from left to right The nature of the pettern is such that when encoded it appears as a periodic waveform on the cable with a 5 MHz frequency Excepting the final two bits the only transitions present in the waveform are in the center of the bit cells This is depicted in Figure 7 7 The last two bits of the preamble contain transitions at both the bit cell centers and the edges and are used
20. It is suitable for use at either end of the transceiver cable as necessary i e it would be located at the station end to drive the transmit pair and at the transceiver end to drive the receive and collision presence pairs In addition it is capable of driving suitable isolation circuits required to be located within the transceiver Transceiver Cable Twisted Pair Vee Figure D 1 Typical Transceiver Cable Driver ETHERNET SPECIFICATION Appendix D 81 Figure D 2 depicts an implementation of the transceiver cable receiver specified in 7 2 5 It is suitable for use at either end of the transceiver cable as necessary i e it would be located at the station end to receive from the receive and collision presence pairs and at the transceiver to receive from the transmit pair It is capable of operating through suitable isolation circuits required to be located within the transceiver Data E Cable Twisted Pair Vee Data Valid Figure D 2 Typical Transceiver Cable Receiver 9 c ETHERNET SPECIFICATION Appendix APPENDIX E INTERFRAME RECOVERY It is important that data link controller implementations be able to receive a frame that arrives immediately after another frame has been transmitted or received Here immediately means 9 6 sec based on the minimum interframe spacing provided as recovery time for the data link See 6 3 22 It is important that the data link controller be able to resume reception within
21. K2 These conditions must be met in both the power off and the power on not transmitting states 7 4 1 2 Bias Current The transceiver must draw between 2 and 50 uA in the power off and the power on not transmitting states ETHERNET SPECIFICATION Physical Layer 63 7 4 1 3 Transmit Output Levels Signals received from the transceiver cable transmit pair must be transmitted onto the coaxial cable with the characteristics specified in 7 3 2 Note that 7 3 2 specifies the current level on the coaxial cable Since the coaxial cable proceeds in two directions away from the transceiver the current into the transceiver is actually twice the current measured on the coaxial cable Transmitted output asymmetry shall not exceed 2 ns for a 50 50 duty cycle input on the transceiver cable transmit pair 7 4 2 Transceiver to Transceiver Cable Interface 7 4 2 1 Transmit Pair The transceiver must present the transceiver cable receive characteristics specified in 7 2 5 to the transmit pair At the start of a frame transmission no more than 2 bits two 100 ns bit cells of information may be received from the transmit pair and not transmitted onto the coaxial cable The steady state propagation delay between the transmit pair input and the coaxial cable output shall not exceed 50 ns There are no signal inversion between the transceiver cable transmit pair and the coaxial cable 7 4 2 2 Receive Pair The transceiver must present the transcei
22. Sense Boolean indicates incoming bits transmitting Boolean indicates outgoing bits collisionDetect Boolean indicates channel contention procedure TransmitBit bitParam Bit Transmits one bit function ReceiveBit Bit Receives one bit procedure Wait bitTimes integer Waits for indicated number of bit times ETHERNET SPECIFICATION Data Link Layer 37 6 5 2 1 5 State Variable Initialization The procedure nitialize must be run when the Data Link Layer begins operation before any of the processes begin execution nitialize sets certain crucial shared state variables to their initial values other global variables are appropriately reinitialized before each use nitialize then waits for the channel to be idle and starts operation of the various processes procedure Initialize begin frameWaiting false deferring false newCollision false transmitting false In interface to Physical Layer see below receiving false while carrierSense do nothing Start execution of all processes end Initialize 38 ETHERNET SPECIFICATION Data Link Layer 6 5 2 2 Frame Transmission The algorithms in this section define data link frame transmission The function TransmitFrame implements the frame transmission operation provided to the Client Layer function TransmitFrame destinationParam AddressValue sourceParam AddressValue typeParam typeValue dataParam DataValue TransmitStatus p
23. The Ethernet A Local Area Network Data Link Layer and Physical Layer Specifications intg XEROX Digital Equipment Corporation Intel Corporation Xerox Co rporation Maynard MA Santa Clara CA Stamford CT Version 1 0 September 30 1980 IMPORTANT INFORMATION AND DISCLAIMERS 1 This specification includes subject matter relating to a patent s of Xerox Corporation No license under such patent s is granted by implication estoppel or otherwise as a result of publication of this specification Applicable licenses may be obtained from Xerox Corporation 2 This specification is furnished for informational purposes only Digital Intel and Xerox do not warrant or represent that this specification or any products made in conformance with it will work in the intended manner or be compatible with other products in a network system Nor do they assume responsibility for any errors that the specification may contain or have any liabilities or obligations for damages including but not limited to special indirect or consequential damages arising out of or in connection with the use of this specification in any way Digital Intel and Xerox products may follow or deviate from the specification without notice at any time 3 No representations or warranties are made that this specification or anything made from it is or will be free from infringements or patents of third persons ETHERNET SPECIFICATION Preface i Preface T
24. The connection must disturb the transmission line characteristics of the cable as little as possible it must present a predictably low shunt capacitance and therefore a negligibly short stub length For this reason the transceiver must be located as close to its cable connection as possible they are normally considered to be one assembly Long greater than 3 cm connections between the coaxial cable and the _ input of the transceiver are not acceptable The transceiver to coaxial cable connection shall present less than 2 picofarads shunt capacitance to the coaxial cable not including any transceiver electronics If the design of the connection is such that the coaxial cable must be severed to install the transceiver the coaxial cable segment must still meet the sectioning requirements of 7 6 1 Any coaxial connectors used on a severed cable must be type N as specified in 7 3 12 73 2 Coaxial Cable Signaling The AC component of the signal on the coaxial cable due to a single transceiver as measured on the coaxial cable immediately adjacent to the transceiver connection shall be 16 mA nominal 14 mA min 19 mA max The DC component shall be one half the AC component plus 4 5 mA 4 mA min 5 mA max The actual current measured at a given point on the cable is a function of the transmitted current and the cable loss to the point of measurement Positive current is defined as current out of the center conductor of the cable into the trans
25. Transceiver Cable Waveform Figure 7 4 Maximum Coaxial Cable Transfer Impedance Figure 7 5 Typical Coaxial Cable Waveform Figure 7 6 Manchester Encoding Figure 7 7 Preamble Encoding Figure 7 8 Functional Logic of collisionDetect Signal Figure 7 9 Functional Logic of carrierSense Signal Figure C 1 CRC Implementation Figure D 1 Typical Transceiver Cable Driver Figure D 2 Typical Transceiver Cable Receiver 73 76 77 80 82 10 ll 20 31 32 33 50 52 55 57 60 62 65 66 67 68 78 80 81 Vii ETHERNET SPECIFICATION Contents ETHERNET SPECIFICATION Introduction 1 1 INTRODUCTION The Ethernet local area network provides a communication facility for high speed data exchange among computers and other digital devices located within a moderate sized geographic area Its primary characteristics include Physical Layer Data rate 10 Million bits sec Maximum station separation 2 5 Kilometers Maximum number of stations 1024 Medium Shielded coaxial cable base band signalling Topology Branching non rooted tree Data Link Layer Link control procedure Fully distributed peer protocol with statistical contention resolution CSMA CD Message protocol Variable size frames best effort delivery The Ethernet like other local area networks falls in a middle ground between long distance low speed networks which carry data for hundreds or thousands of kilometers and specialized very high
26. able at the transceiver exceeds either that which could be produced by two transceiver outputs in the worst case if the transceiver in question is not transmitting or that which could be produced by that transceiver alone in the worst case if that transceiver is transmitting 7 4 2 4 Power Pair The transceiver cable provides power which may be used for operation of the transceiver electronics The power available shall be as described in 7 2 1 4 The distribution impedance of the transceiver cable is 4 Q maximum for a 50 meter cable with the resistance specified in 7 2 2 6 In order for the transceiver to derive its operating power from the power pair circuitry must be employed to provide the required electrical isolation specified in 7 4 3 7 4 3 Electrical Isolation The transceiver must provide electrical isolation between the transceiver cable and the coaxial cable The isolation impedance shall be greater than 250 measured between any conductor including shield of the transceiver cable and either the center conductor or shield of the coaxial cable at 60 Hz The breakdown voltage of the isolation means provided shall be at least 250 VAC rms 7 4 4 Reliability No single nor double component failure within the transceiver electronics shall impede communication among other transceivers on the coaxial cable Connectors and other passive components comprising the means of connecting the transceiver to the coaxial cable shall b
27. ally either the transmission succeeds or the attempt is abandoned on the assumption that the channel has failed or has become overloaded At the receiving end the bits resulting from a collision are received and decoded by the Physical Layer just as are the bits of a valid frame In particular collisions do not turn on the receiving station s collision detect signal which is generated only during transmission Instead the fragmentary frames received during collisions are distinguished from valid frames by the Data Link s Receive Link Management component by noting that a collision fragment is always smaller than the shortest valid frame Such fragments are discarded by Receive Link Management ETHERNET SPECIFICATION Inter Layer Interfaces 15 5 INTER LAYER INTERFACES The purpose of this section is to provide precise definitions of the interfaces between the architectural layers defined in Section 4 In order to provide such a definition some precise notation must be adopted The notation used here is the Pascal language in keeping with the procedural nature of the formal Data Link Layer specification see 6 5 Each interface is thus described as a set of procedures and or shared variables which collectively provide the only valid interactions between layers The accompanying text describes the meaning of each procedure or variable and points out any implicit interactions among them Note that the description of the interfaces in Pa
28. and suitability to a particular implementation technology or computer architecture In this context several important properties of the procedural model must be considered 6 5 1 1 Ground Rules for the Procedural Model a First it must be emphasized that the description of the Data Link Layer in a programming language is in no way intended to imply that a data link controller must be implemented as a program executed by a computer The implementation may consist of any appropriate technology including hardware firmware software or any combination b Similarly it must be emphasized that it is the behavior of Data Link Layer implementations that must match the specification not their internal structure The internal details of the procedural model are useful only to the extent that they help specify that behavior clearly and precisely 28 ETHERNET SPECIFICATION Data Link Layer c The handling of incoming and outgoing frames is rather stylized in the procedural model in the sense that frames are handled as single entities by most of the Data Link Layer and are only serialized for presentation to the Physical Layer In reality many data link controller implementations will instead handle frames serially on a bit octet or word basis A serial implementation would typically perform the required functions address recognition frame check sequence generation validation etc in an overlapped pipelined fashion This approach has not been r
29. arrier Sense Signal The channel must indicate to the data link the presence of carrier a signal transmission attempt on the coaxial cable by a station This is normally indicated through both the receive and collision presence pairs in the transceiver cable described in 7 2 1 The carrier sense signal shall be asserted when one or more station is attempting transmission on the cable regardless of whether the station sensing carrier is transmitting at that time The channel logic must assert the carrier sense signal within 2 bit times 200 ns following the onset of carrier presence information A 68 ETHERNET SPECIFICATION Physical Layer functional logic description of these signals is shown in Figure 7 9 Following the loss of carrier presence information receive transitions and collision presence information the channel must deassert the carrier sense signal within 1 6 bit cell times 160 ns Transceiver Cable Collision Pair C Transition Detect carrierSense signal Transceiver Cable Receive Pair to Data Link Layer C x C Transition Detect Transition Detect output enabled if an input transition has been detected within the previous 1 6 bit times 160 nS Figure 7 9 Functional Logic of carrierSense Signal 7 5 4 Channel Framing During reception the channel must provide the data link with signals to indicate beginning and end of frame 7 5 4 1 Beginning of Frame Sequence The channel logic
30. aterial The approach taken in the specification of the Data Link Layer in Section 6 is a procedural one in addition to describing the necessary algorithms in English and control flow charts the specification presents these algorithms in the language Pascal This approach makes clear the required behavior of Data Link Layer while leaving individual implementations free to exploit any appropriate technology Because the procedural approach is not suitable for specifying the details of the Physical Layer Section 7 uses carefully worded English prose and numerous figures and tables to specify the necessary parameters of this layer Some aspects of the Ethernet are necessarily discussed in more than one place in this specification Whenever any doubt arises concerning the official definition in such a case the reader should utilize the Pascal procedural specification of the Data Link Layer in Section 6 5 and the detailed prose specification of the Physical Layer in Sections 7 2 through 7 9 ii ETHERNET SPECIFICATION Preface One aspect of an overall network architecture which is not addressed by this specification is network management The network management facility performs operation maintenence and planning functions for the network Operation functions include parameter setting such as address selection Maintenance functions provide for fault detection isolation and repair Planning functions include collection of statisi
31. attempts 1 end loop if transmitSucceeding then TransmitLinkMgmt transmitOK else TransmitLinkMgmt excessiveCollisionError end TransmitLinkMgmt Each time a frame transmission attempt is initiated StartTransmit is called to alert the BitTransmitter process that bit transmission should begin procedure StartTransmit begin currentTransmitBit 1 lastTransmitBit frameSize transmitSucceeding true transmitting true end startTransmit Once frame transmission has been initiated TransmitLinkMgmt monitors the channel for contention by repeatedly calling WatchForCollision procedure WatchForCollision begin if transmitSucceeding and collisionDetect then begin newCollision true transmitSucceeding false end end WatchForCollision WatchForCollision upon detecting a collision updates newCollision to insure proper jamming by the BitTransmitter process 40 ETHERNET SPECIFICATION Data Link Layer After transmission of the jam has completed if TransmitLinkMgmt determines that another attempt should be made BackOff is called to schedule the next attempt to retransmit the frame var maxBackOff 2 1024 Working variable of BackOff procedure BackOff begin if attempts 1 then maxBackOff 2 else if attempts lt backOffLimit then maxBackOff maxBackOff 2 Wait slotTime Random 0 maxBackOff end BackOff function Random low high integer integer begin Random unif
32. be addressed largely in terms of the implementation interfaces compatibility Two important compatibility interfaces are defined within what is architecturally the Physical Layer Coaxial cable interface To communicate via the Ethernet all stations must adhere rigidly to the exact specification of coaxial cable signals defined in this document and to the procedures which define correct behavior of a station The medium independent aspects of the Data Link Layer should not be taken as detracting from this point communication via the Ethernet requires complete compatibility at the coaxial cable interface ETHERNET SPECIFICATION Functional Model of the Ethernet Architecture 7 Client Layer Data Link Layer Data Link Controller Physical Channel aa to Station Data Encapsulation ARCHITECTURE FUNCTIONS Transmit amp station Interface Receive Coax Ethernet Controller Board Transceiver Transceiver Cable Cable TYPICAL IMPLEMENTATION to I O bus etc Compatibility Interfaces Figure 4 1 Ethernet Architecture and Typical Implementation 8 ETHERNET SPECIFICATION Functional Model of the Ethernet Architecture Transceiver cable interface It is anticipated that most stations will be located some distance away from their connection to the coaxial cable While it is necessary to place a small amount of circuitry the transceiver directly adjacent to the coaxial cable the majority of the el
33. cal and usage information necessary for planned network growth While network management itself is properly performed outside the Ethernet Data Link and Physical Layers it requires appropriate additional interfaces to those layers which will be defined in a subsequent version of this specification ETHERNET SPECIFICATION Contents Table of Contents Preface 1 INTRODUCTION 2 REFERENCES 3 GOALS AND NON GOALS 3 1 Goals 3 2 Non Goals 4 FUNCTIONAL MODEL OF THE ETHERNET ARCHITECTURE 4 1 Layering 4 2 Data Link Layer 4 3 Physical Layer 4 4 Ethernet Operation and the Functional Model 4 4 1 Transmission Without Contention 4 4 2 Reception Without Contention 4 4 3 Collisions Handling of Contention 5 INTER LAYER INTERFACES 5 1 Client Layer to Data Link Layer 5 2 Data Link Layer to Physical Layer 6 ETHERNET DATA LINK LAYER SPECIFICATION 6 1 Data link Layer Overview and Model 6 2 Frame format 6 2 1 Address Fields 6 2 1 1 Destination Address Field 6 2 1 2 Source Address Field 6 2 2 Type Field 6 2 3 Data Field 6 2 4 Frame Check Sequence Field 6 2 5 Frame Size Limitations 6 3 Frame Transmission 6 3 1 Transmit Data Pepakon 6 3 1 1 Frame Assembly 6 3 1 2 Frame Check Sequence Generation 6 3 2 Transmit Link Management 5 nn tA iii iv ETHERNET SPECIFICATION Contents 6 3 2 1 Carrier Deference 6 3 2 2 Interframe Spacing 6 3 2 3 Collision Handling 6 3 2 3 1 Collision D
34. ceiver Cable loss is specified in 7 3 1 1 2 The 1095 9096 rise and fall times shall be 25 5 nsec Figure 7 5 shows typical waveforms present on the cable Harmonic content generated from a 10 MHz fundamental periodic input shall meet the following requirements Second and Third Harmonics 20 dB min Fourth and Fifth Harmonics 30 dB min Sixth and Seventh Harmonics 40 dB min All Higher Harmonics 50 dB min The signals as generated from the encoder described in 7 5 1 1 shall appear on the coaxial cable without any inversions 62 ETHERNET SPECIFICATION Physical Layer 7 0V 0 225V 1 825V 1 Voltages given are nominal worst case is given in text 2 Rise time is 25 nS nominal 3 Voltages are measured on coaxial cable adjacent to transceiver Figure 7 5 Typical Coaxial Cable Waveform 7 4 Transceiver Specifications The following sections specify the requirements for a transceiver 7 4 1 Transceiver to Coaxial Cable Interface The following sections describe the interface between the transceiver and the coaxial cable Positive current is defined as current into the transceiver out of the center conductor of the cable 7 4 1 1 Input Impedance The shunt capacitance presented to the coaxial cable by the transceiver circuitry not including the means of attachment to the coaxial cable shall not exceed 2 picofarads The shunt resistance presented to the coaxial cable shall be greater than 50
35. ch type DA 51220 1 or equivalent The transceiver must provide a mating male connector with locking posts The other end of the transceiver cable which mates with a female connector at the station must use a male connector with locking posts Cinch type D 53018 or equivalent The station must provide a female connector with the slide lock assembly Because of the end to end matching of the connectors transceiver cables may be extended by concatenating transceiver cable sections The transceiver cable sections function as extension cords cable with multiple sections must still meet the cable loss characteristics of 7 3 1 12 The pin assignment is given in the following table Transceiver Cable Connector Pin Assignment l Shield See note 2 Collision Presence 9 Collision Presence 3 Transmit 10 Transmit 4 Reserved Reserved 5 Receive 12 Receive 6 Powerreturn 13 Power 7 Reserved 14 Reserved 8 Reserved 15 Reserved Note Shield must be terminated to connector shell as well as pin 1 Metal metallized plastic or otherwise shielded connector backshells must be used to ensure shield integrity 7 2 4 Transceiver Cable Drive This section describes the requirements for driving any of the signal pairs in the transceiver cable transmit receive and collision presence The AC signal levels presented to the transceiver cable shall be 700 mV nominal balanced differential drive into 78 5 9 The co
36. dent of the value of carrierSense When transmission has completed or immediately if there was nothing to transmit the data link controller resumes its original monitoring of carrierSense When a frame is submitted by the Client Layer for transmission the transmission is initiated as soon as possible but in conformance with the rules of deference stated above 24 ETHERNET SPECIFICATION Data Link Layer 6 3 2 2 Interframe Spacing As defined in 6 3 2 1 the rules for deferring to passing frames insure a minimum _interframe spacing of 9 6 usec This is intended to provide interframe recovery time for other data link controllers and for the physical channel Note that 9 6 usec is the minimum value of the interframe spacing If necessary for implementation reasons a transmitting controller may use a larger value with a resulting decrease in its throughput The value should not exceed 10 6 psec 6 3 2 3 Collision Handling Once a data link controller has finished deferring and has started transmission it is still possible for it to experience contention for the channel As discussed in 4 4 3 collisions can occur until acquisition of the network has been accomplished through the deference of all other stations data link controllers The dynamics of collision handling are largely determined by single parameter called the slot time This single parameter describes three important aspects of collision handling It is an upper bound
37. e designed to minimize the probability of total network failure 7 5 Channel Logic The following sections describe the functions that must be performed to properly interface between the data link and the transceiver cable They are normally implemented as logic typically within the same device implementing the data link layer 7 5 1 Channel Encoding The channel shall use Manchester phase encoding with a data rate of 10 Mbps 019e measured at the encoder clock Thus each bit cell is 100 ns long The following section describes the requirements for encoding and decoding signals to be transmitted on or received from the coaxial or transceiver cables ETHERNET SPECIFICATION Physical Layer 65 7 5 1 1 Encoder The encoder is used to translate physically separate signals of clock synchronization and data into a single self synchronizable serial bit stream suitable for transmission on the coaxial cable by the transceiver During the first half of the bit cell time the serial signal transmitted is the logical complement of the bit value being encoded during that cell During the second half Of the bit cell time the uncomplemented value of the bit being encoded is transmitted Therefore there is always a signal transition either positive going or negative going depending on the bit being encoded in the center of each bit cell A timing diagram for a typical bit stream is given in Figure 7 6 The encoder output drives the t
38. e facilities of the Physical Layer consists of a function a pair of procedures and three Boolean variables Function Variables ReceiveBit collisionDetect Procedures carrierSense TransmitBit transmitting Wait During transmission the contents of an outgoing frame are passed from the Data Link Layer to the Physical Layer via repeated use of the TransmitBit operation procedure TransmitBit bitParam Bit Each invocation of TransmitBit passes one new bit of the outgoing frame to the Physical Layer The TransmitBit operation is synchronous in the sense that the duration of the operation is the entire transmission of the bit so that when the operation completes the Physical Layer is ready to accept the next bit immediately Note this does not imply that all invocations of TransmitBit are of exactly equal duration for example if the Physical Layer must perform some initial processing e g preamble generation before transmitting the first bit of a frame the first ETHERNET SPECIFICATION Inter Layer Interfaces 17 invocation of TransmitBit may take significantly longer The overall event of data being transmitted is signaled to the Physical Layer via the variable transmitting var transmitting Boolean Before sending the first bit of a frame the Data Link Layer sets transmitting to true to inform the Physical Link that a stream of bits will be presented via the TransmitBit operation After the last bit of the frame has
39. e user There is no attempt to protect the network from a malicious user at the data link level 6 ETHERNET SPECIFICATION Functional Model of the Ethernet Architecture 4 FUNCTIONAL MODEL OF THE ETHERNET ARCHITECTURE There are two important ways to view the Ethernet design corresponding to Architecture emphasizing the logical divisions of the system and how they fit together Implementation emphasizing the actual components and their packaging and interconnection Figure 4 1 illustrates these two views as they apply to a typical implementation showing how each view groups the various functions This document is organized along architectural lines emphasizing the large scale separation of the Ethernet system into two parts the Data Link Layer and the Physical Layer These layers are intended to correspond closely to the lowest layers of the ISO Model for Open Systems Interconnection 4 5 Architectural organization of the specification has two main advantages Clarity A clean overall division of the design along architectural lines raakes the specification clearer Flexibility Segregation of medium dependent aspects in the Physical Layer allows the Data Link Layer to apply to transmission media other than the specified coaxial cable As is evident in Figure 4 1 the architectural model is based on a set of interfaces different from those emphasized in the implementations One crucial aspect of the design however must
40. eceiver var b Bit begin cycle outer loop while receiving do begin inner loop b ReceiveBit Get next bit from physical link if carrierSense then begin append bit to packet incomingFrame currentReceiveBit b currentReceiveBit currentReceiveBit 1 end append bit to packet receiving carrierSense end inner loop end outer loop end BitReceiver 44 ETHERNET SPECIFICATION Data Link Layer 6 5 2 4 Common procedures The function CRC32 is used by both the transmit and receive algorithms to generate 32 bit CRC value function 2 f Frame CRCValue begin CRC32 The 32 bit CRC as defined in 6 2 4 end CRC32 Purely to enhance readability the following procedure is also defined procedure nothing begin end The idle state of a process i e while waiting for some event is cast as repeated calls on this procedure ETHERNET SPECIFICATION Physical Layer 45 7 ETHERNET PHYSICAL LAYER SPECIFICATION Baseband Coaxial System 7 1 Physical Channel Overview and Model The Ethernet physical channel henceforth referred to as the channel provides the lowest layer in the Ethernet architecture It performs all the functions needed to transmit and receive data at the physical level while supporting the Data Link to Physical Layer Interface described in 5 2 This section describes the requirements for interface and compatibility with a baseband coaxial implementation of the channel 7
41. ectronics the controller can and should be placed with the station Since it is desirable for the same transceiver to be usable with a wide variety of stations a second compatibility interface the transceiver cable interface is defined While conformance with this interface is not strictly necessary to insure communication it is highly recommended since it allows maximum flexibility in intermixing transceivers and stations 4 1 Layering The major division in the Ethernet Architecture is between the Physical Layer and the Data Link Layer corresponding to the lowest two levels in the ISO model The higher levels of the overall network architecture which use the Data Link Layer will be collectively referred to in this document as the Client Layer since strictly speaking the identity and function of higher level facilities are outside the scope of this specification The intent however is that the Ethernet Physical and Data Link Layers support the higher layers of the ISO model Network Layer Transport Layer etc The overall structure of the layered architecture is shown in Figure 4 2 Client Layer Interface Data Link Layer Interface Physical Layer Figure 4 2 Architectural Layering ETHERNET SPECIFICATION Functional Model of the Ethernet Architecture In the architectural model used here the layers interact via well defined interfaces The interface between the Client Layer and the Data Link Laye
42. eflected in the procedural model since this would only complicate the description of the functions without changing them in any way d The model consists of algorithms designed to be executed by a number of concurrent processes these algorithms collectively implement the Ethernet data link control procedure The timing dependencies introduced by the need for concurrent activity are resolved in two ways Processes vs External events It is assumed that the algorithms are executed very fast relative to external events in the sense that a process never falls behind in its work and fails to respond to an external event in a timely manner For example when a frame is to be received it is assumed that the data link procedure ReceiveFrame is always called well before the frame in question has started to arrive Processes vs Processes Among processes no assumptions are made about relative speeds of execution This means that each interaction between two processes must be structured to work correctly independent of their respective speeds Note however that the timing of interactions among processes is often in part an indirect reflection of the timing of external events in which case appropriate timing assumptions may still be made It is intended that the concurrency in the model reflect the parallelism intrinsic to the task of implementing the Ethernet data link although the actual parallel structure of the implementations is likely
43. el functions that increase the power of the communication facility and or tailor it to specific applications In particular it should be noted that all error recovery functions have been relegated to higher level protocols in keeping with the low error rates that characterize local networks One of the main objectives of this specification is compatibility As stated in Section 3 it is intended that every implementation of the Ethernet be able to exchange data with every other implementation It should be noted that higher level protocols raise their own issues of compatibility over and above those addressed by the Ethernet and other link level facilities This does not eliminate the importance of link level compatibility however While the compatibility provided by the Ethernet does not guarantee solutions to higher level compatibility problems it does provide a context within which such protlems can be addressed by avoiding low level incompatibilities that would make direct communication impossible ETHERNET SPECIFICATION References 3 2 REFERENCES The following three papers describe the Experimental Ethernet and are reprinted in The Ethernet Local Network Three Reports Xerox Palo Alto Research Center Technical Report CSL 80 2 February 1980 gl 3 Metcalfe M and Boggs D R Ethernet Distributed Packet Switching for Local Computer Networks Communications of the ACM 19 7 July 1976 Crane R C and Taf
44. erface and the transceiver cable interface the two points at which hardware compatibility is defined to allow connection of independently designed and manufactured components to the Ethernet contention Interference between colliding transmissions see collision Resolution of occasional contention is a normal part of the Ethernet s distributed link management procedure see CSMA CD controller The implementation unit which connects a station to the Ethernet typically comprising part of the Physical Layer much or all of the Data Link Layer and appropriate electronics for interfacing to the station CSM A CD Carrier Sense Multiple Access wita Collision Detection the generic term for the class of link management procedure used by the Ethernet So called because it a allows multiple stations to access the broadcast channel at will b avoids contention via carrier sense and deference and c resolves contention via collision detection and retransmission Data Link Layer The higher of the two layers in the Ethernet design which implements a medium independent link level communication facility on top of the physical channel provided by the Physical Layer deference A process by which a data link controller delays its transmission when the channel is busy to avoid contention with ongoing transmissions frame check sequence An encoded value appended to each frame by the Data Link Layer to allow detection of transmission errors in the
45. eserved for use by higher levels in particular to identify the Client Layer protocol associated with the frame The type field is uninterpreted at the Data Link Layer It is specified at this level because a uniform convention for the placement and value assignment of this field is crucial if multiple higher level protocols are to be able to share the same Ethernet network without conflict Appendix B discusses the assignment of type field values 22 ETHERNET SPECIFICATION Data Link Layer 6 2 3 Data Field The data field contains a sequence of n octets where 46 lt n lt 1500 Within this range full data transparency is provided in the sense that any arbitrary sequence of octet values may appear in the data field 6 2 4 Frame Check Sequence Field The frame check sequence FCS field contains a 4 octet 32 bit cyclic redundancy check CRC value This value is computed as a function of the contents of the source destination type and data fields ie all fields except the frame check sequence field itself The encoding is defined by the generating polynomial G x xX x36 x23 x x6 xR 4 xl x8 x 4 xt e 2 tx 1 This polynomial is also used in the Autodin II network its properties are investigated in 8 Mathematically the CRC value corresponding to a given frame is following procedure 1 The first 32 bits of the frame are complemented 2 The n bits of the frame are then conside
46. et the applicable requirements at either the transceiver cable or the coaxial cable interface as appropriate in addition to the Data Link compatibility required for all stations connected to the network All Ethernets must be compatible at the coaxial cable If a transceiver cable is used it should be the one specified in this document This allows device manufacturers to build hardware compatible with the Ethernet at the transceiver cable level without concerning themselves with the details of transceiver implementation Devices implementing transceiver cable compatibility should be capable of using transceivers designed and built by another manufacturer on the specified coaxial cable Equipment designed for connection to the specified coaxial cable either without a physically separate transceiver or with a non standard transceiver cable interface will be capable of communication However a sacrifice may have been made with respect to interchangeability with other stations This scheme of multiple compatibility interfaces allows individual designers some flexibility in making system tradeoffs yet allows cable manufacturers transceiver manufacturers and systems manufacturers to use standard commodity parts to produce a compatible communications system ETHERNET SPECIFICATION Physical Layer 49 7 1 5 Channel Configuration Model Certain physical limits have been placed on the physical channel These revolve mostly around maximum cab
47. etection and Enforcement 6 3 2 3 2 Collision Backoff and Retransmission 6 4 Frame Reception 6 4 1 Receive Data Decapsulation 6 4 1 1 Framing 6 4 1 1 1 Maximum Frame Size 6 4 1 1 2 Integral Number of Octets in Frame 6 4 1 2 Address Recognition 6 4 1 2 1 Physical Addresses 6 4 1 2 2 Multicast Addresses 6 4 1 3 Frame Check Sequence Validation 6 4 1 4 Frame Disassembly 6 4 2 Receive Link Management 6 4 2 1 Collision Filtering 6 5 The Data Link Layer Procedural Model 6 5 1 Overview of the Procedural Model 6 5 1 1 Ground Rules for the Procedural Model 6 5 1 2 Use of Pascal in the Procedural Model 6 5 2 The Procedural Model 6 5 2 1 Global Declarations 6 5 2 1 1 Common Constants and Types 5 2 1 2 Transmit State Variables 5 2 1 3 Receive State Variables 5 2 1 4 Summary of Interlayer Interfaces 5 2 1 5 State Variable Initialization 6 5 2 2 Frame Transmission 6 5 2 3 Frame Reception 6 5 2 4 Common Procedures 7 ETHERNET PHYSICAL LAYER SPECIFICATION 7 1 Physical Channel Overview and Model 7 1 1 Channel Goals and Non Goals 7 1 1 1 Goals 7 1 1 2 Non Goals 7 1 2 Characteristics of the Channel 7 1 3 Functions Provided by the Channel 7 1 4 Implementation of the Channel 7 1 4 1 General Overview of Channel Hardware 7 1 4 2 Compatibility Interfaces 7 1 5 Channel Configuration Model 7 1 6 Channel Interfaces 6 6 6 6 ETHERNET SPECIFICATION Contents 7 2 Transceiver Cable Compatibility Interface Specifications 7 2 1 T
48. etried by the transmitting data link controller until either it is successful or 16 attempts the original attempt plus 15 retries have been made and all have terminated due to collisions Note that all attempts to transmit a given frame are completed before any subsequent outgoing frames are transmitted The scheduling of the retransmissions is determined by a controlled randomization process called truncated binary exponential backoff At the end of enforcing a collision jamming the data link controller delays before attempting to retransmit the frame The delay is an integral multiple of the slot time See 6 3 2 3 The number of slot times to delay before the n retransmission attempt is chosen as a uniformly distributed random integer r in the range 0 lt lt 2K where min n 10 If all 16 attempts fail this event is reported as an error Note that the values given above define the most aggressive behavior that a station may exhibit in attempting to retransmit after a collision In the course of implementing the retransmission scheduling procedure a station may introduce extra delays which will degrade its own throughput but in no case may a station s retransmission scheduling result in a lower average delay between retransmission attempts than the procedure defined above 6 4 Frame Reception Frame reception includes both data decapsulation and link management aspects Receive Data Decapsulation comprises framing address
49. g the actual bits of the frame sends an encoded preamble to allow the receivers and repeaters along the channel to synchronize their clocks and other circuitry It then begins translating the bits of the frame into encoded form and passes them to the Channel Access component for actual transmission over the medium The Channel Access component performs the task of actually generating the electrical signals on the medium which represent the bits of the frame Simultaneously it monitors the medium and generates the collison detect signal which in the contention free case under discussion remains off for the duration of the frame When transmission has completed without contention the Data Link Layer so informs the Client Layer and awaits the next request for frame transmission 4 4 2 Reception Without Contention At the receiving station the arrival of a frame is first detected by the Receive Channel Access component of the Physical Layer which responds by synchronizing with the incoming preamble and by turning on the carrier sense signal As the encoded bits arrive from the medium they are passed to the Receive Data Decoding component ETHERNET SPECIFICATION Functional Model of the Ethernet Architecture 13 Receive Data Decoding translates the encoded signal back into binary data and discards the leading bits up to and including the end of the preamble It then passes subsequent bits up to the Data Link Layer Meanwhile the Rece
50. gure 6 2 and reflects the fact that the communication of entire frames is initiated by the Client Layer while the timing of collision backoff and of individual bit transfers is based on interactions between the Data Link Layer and the Physical Layer dependent bit time Figure 6 2 depicts the static structure of the procedural model showing how the various processes and procedures interact by invoking each other Figures 6 3 and 6 4 summarize the dynamic behavior of the model during transmission and reception focusing on the steps that must be performed rather than the procedural structure which performs them The usage of the shared state variables is not depicted in the figures but is described in the comments in 6 5 2 1 ETHERNET SPECIFICATION Data Link Layer 31 FrameTransmitter FrameReceiver T itDataEnca ReceiveDataDecai pore ENCAPSULATION CRC32 RecognizeA ddress DATA LINK LAYER TransmitLinkMgmt ReceiveLinkMgmt LINK Client Layer WatchForCollision BackOff StartReceive StartTransmit Random MANAGEMENT BitTransmitter StartJam RealTimeDelay Physical Layer TRANSMIT a RECEIVE Figure 6 2 Structure of the Data Link Procedural Model If 32 ETHERNET SPECIFICATION Data Link Layer no Start transmission ReceiveFrame Start receiving done receiving frame too small collision no yes collisionDetect no recognize address transmissi
51. he standard lengths described below 3 If uncontrolled cable sections must be used in building up a longer segment the lengths should be chosen such that reflections when they occur do not have a high probability of adding in phase This can be accomplished by using lengths which are odd integral multiples of a half wavelength in the cable at 5 MHz this corresponds to using lengths of 23 4 70 2 and 117 meters 0 5 meters for all sections These are considered to be the standard lengths for all cable sections Using these lengths exclusively any mix or match of cable sections may be used to build up a 500 meter segment without incurring excessive reflections 70 ETHERNET SPECIFICATION Physical Layer 4 As a last resort an arbitrary configuration of cable sections may be _ employed if it has been confirmed by analysis or measurement that the worst case signal reflection due to the impedance discontinuities at any point on the cable does not exceed 7 of the incident wave when driven by a transceiver meeting the specifications of 7 4 7 6 2 Transceiver Placement Transceivers and their associated connections to the cable cause signal reflections due to their non infinite bridging impedance While this impedance must be implemented as specified in 7 3 1 4 and 7 4 1 the placement of transceivers along the coaxial cable must also be controlled to insure that reflections from transceiver do not add in phase to a significant degree C
52. he notion of a data link between two network entities does not correspond directly to a distinct physical connection Nevertheless the two main functions generally associated with a data link control procedure are present Data encapsulation framing frame boundary delimitation addressing handling of source and destination addresses error detection detection of physical channel transmission errors Link management channel allocation collision avoidance contention resolution collision handling This split is reflected in the division of the Data Link Layer into the Data Encapsulation sub layer and the Link Management sub layer as shown in Figure 4 3 10 ETHERNET SPECIFICATION Functional Model of the Ethernet Architecture Client Layer Client to Data Link Interface Transmit Receive Data Encapsulation Data Decapsulation Data Link Layer Transmit Receive Link Management Link Management Data Link to Physical Interface Physical Layer Figure 4 3 Data Link Layer Functions In terms of the ISO model the Ethernet Data Link Layer provides a multi endpoint connection between higher layer entities wishing to communicate The connection provided is called a data link and is implemented between two or more Data Link Layer entities called data link controllers via a Physical Layer connection called the physical channel 4 3 Physical Layer The Physical Layer specified in this document provides a 10 MBit sec ph
53. he sense that the delivered traffic should be a monotonically non decreasing function of the total offered traffic Maintainability The Ethernet design should allow for network maintenance operation and planning Layered Architecture The Ethernet design should be specified in layered terms to separate the logical aspects of the data link protocol from the physical details of the communication medium ETHERNET SPECIFICATION Goals and Non Goals 5 3 2 Non Goals The following are not goals of the Ethernet design Full duplex At any given instant the Ethernet can transfer data from one source station to one or more destination stations Bi directional communication is provided by rapid exhange of frames rather than full duplex operation Error control Error handling at the data link level is limited to detection of bit errors in the physical channel and the detection and recovery from collisions Provision of a complete error control facility to handle detected errors is relegated to higher layers of the network architecture Security The data link protocol does not employ encryption or other mechanisms to provide security Higher layers of the network architecture may provide such facilities as appropriate Speed flexibility This specification defines a physical channel operating at a single fixed data rate of 10 Megabits per second Priority The data link protocol provides no support of priority station operation Hostil
54. he signal must pass through six 50 meter transceiver cables one at the transmitting station one at the receiving station and 2 at each repeater two repeaters possible The propagation velocity of the transceiver cable is assumed to be 65 c worst case The total round trip delay for all the transceiver cables is therefore 3 08 us worst case 5 A maximum of 1000 meters of point to point link anywhere in the system This will typically be used as a way of linking cable segments in different buildings Note that a repeater with this internal point to point link can be used to repeat signals between segments many hundreds of meters apart The worst case propagation velocity of the link cable is assumed to be 65 c the round trip propagation delay for 1000 meters is 10 26 us Table 7 1 summarizes the allocation of the round trip propagation delay to the individual components in the channel Figure 7 1 shows a minimum typical and large scale channel configuration 50 ETHERNET SPECIFICATION Physical Layer Coaxial Cable Segment 500 M max Transceiver Cable Coaxial Cable 50 M max Transceiver amp Connection to Coaxial Cable 100 max per segment Station Figure 7 1a Minimal Configuration Repeater Figure 7 1b A Typical Medium scale Configuration Segment 1 Segment 4 ETHERNET SPECIFICATION Physical Layer Station Repeater Segment 3 mum mum m Remote repeater Coaxial Cable Point to poin
55. his document contains the specification of the Ethernet a local area network developed jointly by Digital Equipment Corporation Intel Corporation and Xerox Corporation The Ethernet specification arises from an extensive collaborative effort of the three corporations and several years of work at Xerox on an earlier prototype Ethernet This specification is intended as a design reference document rather than an introduction or tutorial Readers seeking introductory material are directed to the reference list in Section 2 which cites several papers describing the intent theory and history of the Ethernet This document contains 7 sections falling into three main groups Sections 1 2 and 3 provide an overall description of the Ethernet including its goals and the scope of the specification Sections 4 and 5 describe the architectural structure of the Ethernet in terms of a functional model consisting of two layers the Data Link Layer and the Physical Layer Sections 6 and 7 specify the two layers in detail providing the primary technical specification of the Ethernet Readers wishing to obtain an initial grasp of the organization and content of the specification will be best served by reading Sections 1 3 and 4 Readers involved in actual implementation of the Ethernet will find Sections 5 6 and 7 to contain the central material of the specification Section 2 provides references and the appendices provide supplementary m
56. iated with one or more stations on a given Ethernet There are two kinds of multicast address Multicast group address An address associated by higher level convention with a group of logically related stations Broadcast address A distinguished predefined multicast address which always denotes the set of all stations on a given Ethernet The first bit of a data link address distinguishes physical from multicast addresses 0 physical address multicast address In either case the remainder of the first octet and all of the subsequent octets form a 47 bit pattern In the case of the broadcast address this pattern consists of 47 one bits There is no standard null address value The procedures for assigning suitably unique values for physical and multicast addresses are discussed in Appendix B 6 2 1 1 Destination Address Field The destination address field specifies the station s for which the frame is intended It may be a physical or multicast including broadcast address For details of address recognition by the receiving station s see 6 4 1 2 6 2 1 2 Source Address Field The source address field specifies the station sending the frame The source address field is not interpreted at the Data Link Layer It is specified at the data link level because a uniform convention for the placement of this field is crucial for most higher level protocols 6 2 2 Type Field The type field consists of a two octet value r
57. is the round trip propagation time of the physical channel This figure represents the maximum time required for a signal to propagate from one end of the network to the other and for a collision to propagate back The round trip propagation time is primarily but not entirely a function of the physical size of the network The round trip propagation time of the Physical Layer is defined to be at most 450 bit times see 7 1 2 ETHERNET SPECIFICATION Data Link Layer 19 6 ETHERNET DATA LINK LAYER SPECIFICATION 6 1 Data Link Layer Overview and Model As defined in Section 4 the Ethernet Architecture consists of the Data Link Layer and below it the Physical Layer Furthermore the Data Link Layer is divided into two sub layers see Figure 4 3 Data encapsulation framing addressing error detection Link management channel allocation contention resolution This model is used throughout this section to structure the detailed specification of the Data Link Layer An English description of the Data Link Layer is given in 6 2 6 3 and 6 4 A more precise algorithmic definition is given in 6 5 which provides a procedural model for the Data Link Layer in the form of a program in the language Pascal Note that whenever there is any apparent ambiguity concerning the definition of some aspect of the Data Link Layer it is the Pascal procedural specification in 6 5 which should be consulted for the definitive statement 6 2 Frame For
58. itting a square wave with a 10MHz fundamental frequency through the standard transceiver cable driver described in 7 2 4 An oscillator is used instead of a simple level shift to allow AC coupling at the transceiver Transceivers use the collision presence signal to indicate one of two conditions the transceiver is transmitting and there is an attempt by another station to transmit at the same time or there is a simultaneous transmission attempt by three or more stations regardless of whether the transceiver in question is transmitting 7 2 1 4 Power pair of wires is designated for providing power to the transceiver When the transceiver cable is implemented the station end of the cable must supply a voltage between 12 and 15 Vdc 5 with at least 0 5 Amperes available to the cable for remotely powering the transceiver The power source must meet applicable requirements for UL Class 2 wiring devices 7 2 2 Transceiver Cable Parameters 7 2 2 1 Mechanical Configuration The transceiver cable consists of four stranded twisted pair conductors plus an overall shield and insulating jacket The conductor and jacket insulating material may be polyethylene or other suitable material The flammability characteristics of the insulating material must be suitable for the installed environment ETHERNET SPECIFICATION Physical Layer 55 7 2 2 2 Characteristic Impedance The differential mode characteristic impedance of all pairs shall be 78 0
59. ive Link Management component of the Data Link Layer having seen carrier sense go on has been waiting for the incoming bits to be delivered Receive Link Management collects bits from the Physical Layer as long as the carrier serise signal remains on When the carrier sense signal goes off the frame is passed to Receive Data Decapsulation for processing Receive Data Decapsulation checks the frame s destination address field to decide whether the frame should be received by this station If so it passes the contents of the frame to the Client Layer along with an appropriate status code The status code is generated by inspecting the frame check sequence to detect any damage to the frame enroute and by checking for proper octet boundary alignment of the end of the frame 4 4 3 Collisions Handling of Contention If multiple stations attempt to transmit at the same time it is possible for their transmitting data link controllers to interfere with each others transmissions in spite of their attempts to avoid this by deferring When two stations transmissions overlap the resulting contention is called a collision A given station can experience a collision during the initial part of its transmission the collision window before its transmitted signal has had time to propagate to all parts of the Ethernet channel Once the collision window has passed the station is said to have acquired the channel subsequent collisions are avoided
60. l Std C17 E Periodic variations in impedance along a single piece of cable may be up to 3 Q sinusoidal centered around the average value with a period lt 2 meters Note that the proper operation of the network is dependent upon the cable characteristic impedance its value and tolerance are critical 7 3 1 12 Attenuation The attenuation of a cable segment shall not exceed 8 5 dB measured at 10 MHz nor 6 0 dB measured at 5 MHz 7 3 1 1 3 Velocity of Propagation The minimum acceptable velocity of propagation is 0 77 c ETHERNET SPECIFICATION Physical Layer 59 7 3 1 1 4 Mechanical Requirements The cable used should be suitable for routing in various environments including but not limited to dropped ceilings raised floors and cable troughs The jacket must provide insulation between the cable sheath and any building structural metal Also the cable must be capable of accepting coaxial cable connectors described in 7 3 1 2 The cable must in addition conform to the following requirements l The center conductor must be 0 0855 i 0005 diameter solid tinned copper The core dielectric material must be foamed The inside diameter of the innermost shield must be 242 minimum The outside diameter of the outermost shield must be 326 007 amp N The outermost shield must be greater than 90 coverage tinned copper braid 6 The jacket O D must be 0 405 nominal 7 The cable concentricity mus
61. le lengths or maximum propagation times as these affect the slot time as defined in the data link While the precise specification in later sections specify these maxima in terms of propagation times they were derived from the physical configuration model described here The maximum configuration is as follows 1 A coaxial cable terminated in its characteristic impedance at each end constitutes a cable segment A segment may contain a maximum of 500 meters of coaxial cable 2 A maximum of 2 repeaters in the path between any two stations Repeaters do not have to be located at the ends of segments nor is the user limited to one repeater per segment In fact repeaters can be used not only to extend the length of the channel but to extend the topology from one to three dimensional Repeaters occupy transceiver positions on each cable segment and count towards the maximum number of transceivers on a segment just as do the logically distinguishable stations 3 A maximum total coaxial cable length along the longest path between any two transceivers of 1500 meters The propagation velocity of the coaxial cable is assumed to be 0 77 c worst case c is the velocity of light in vacuo 300 000 kilometers per second The total round trip delay for all the coaxial cable in the system is therefore 13 us worst case 4 maximum of 50 meters of transceiver cable between any station and its associated transceiver Note that in the worst case t
62. lso fed directly back to Output When Control 0 the feedback paths are disabled and the shift register shifts the complement of its contents to Output Before CRC generation at the transmitting end initialization logic not shown in Figure C 1 preloads the shift register to all 1 s Control is then held at 1 while the address type and data fields of the outgoing frame are shifted into Input and the CRC is generated Meanwhile the same bits emerging at Output are transmitted over the network When the last bit of the data field has been processed Control is set to 0 and the complemented CRC is shifted out for transmission starting with the x l term see 6 2 4 CRC checking at the receiving end also begins with the shift register preloaded to all ls Control is then held at 1 while the incoming bits are shifted into Input to regenerate the CRC When the last bit of the data field has been processed the shift register should contain the CRC whose binary complement is about to arrive on the network Since this field boundary cannot be recognized by the receiver however Control remains at 1 and the bits of the CRC continue to feed into the the shift register until the end of the entire frame is reached If the two CRCs match the final contents of the shift register is the value 11000111 00000100 11011101 01111011 where the leftmost bit corresponds to the x term of the polynomial and the rightmost to the 0 term Any other final
63. ly request the next bit until all bits of the frame have have been received See 6 5 for details The overall event of data being received is signaled to the Data Link Layer via the variable carrierSense var carrierSense Boolean When the Physical Layer sets carrierSense to true the Data Link Layer must immediately begin retrieving the incoming bits via the ReceiveBit operation When carrierSense subsequently becomes fa se the Data Link Layer can begin processing the received bits as a completed frame Note that the true false 18 ETHERNET SPECIFICATION Inter Layer Interfaces transitions of carrierSense are not defined to be precisely synchronized with the beginning and end of the frame but may precede the beginning and lag the end respectively If an invocation of ReceiveBit is pending when carrierSense becomes false ReceiveBit returns an undefined value which should be discarded by the Data Link Layer See 6 5 for details The Data Link Layer must also monitor the value of carrierSense to defer its own transmissions when the channel is busy The Physical Layer also provides the procedure Wait procedure Wait bitTimes integer This procedure waits for the specified number of bit times This allows the Data Link Layer to measure time intervals in units of the physical channel dependent bit time Another important property of the Physical Layer which is an implicit part of the interface presented to the Data Link Layer
64. mat The data encapsulation function of the Data Link Layer comprises the construction and processing of frames The subfunctions of framing addressing and error detection are reflected in the frame format as follows Framing No explicit framing information is needed since the necessary framing cues carrierSense and transmitting are present in the interface to the Physical Layer Addressing Two address fields are provided to identify the source and destination stations for the frame Error detection A Frame Check Sequence field is provided for detection of transmission errors Figure 6 1 shows the five fields of a frame the addresses of the frame s source and destination a type field for use by higher layers see 6 2 2 a data field containing the transmitted data and the frame check sequence field containing a cyclic redundancy check value to detect transmission errors Of these five fields all are of fixed size except the data field which may contain any integral number of octets between the minimum and maximum values specified below see 6 2 5 20 ETHERNET SPECIFICATION Data Link Layer l octet Ee physical multicast bit 6 octeis Source Octets within 2oces frame transmitted Top to bottom Data 46 1500 octets Frame Check Sequence 4 octets Bits within octet transmitted gt left to right Figure 6 1 Data Link Layer Frame Format Relative to Figure 6 1 the octets of a frame are tran
65. mmon mode voltage presented to the transceiver cable shall not exceed that allowed at the receiver as specified in 7 2 5 2 Signal waveform shall be as shown in Figure 7 3 ETHERNET SPECIFICATION Physical Layer 57 Idle M e Preamble WT Data Idle rel o 0 7V gt P Voltages are measured differentially at output of transceiver cable driver 2 Rise and fall times meet 10 000 series ECL requirements Figure 7 3 Typical Transceiver Cable Waveform The transceiver cable driver must be capable of maintaining the specified minimum differential signal into the worst case low cable impedance 73 differential 18 5 Q common mode in the environment specified in section 7 7 The idle state of the output shall be high 700 mV nominal the first transition presented is negative going the last transition must be positive going Note that the presence of AC coupling may cause the voltage as specified at the output of the transceiver cable drive circuit not to appear on the transceiver cable in the idle state A typical transceiver cable drive circuit is given in Appendix D 7 2 5 Transceiver Cable Receive The following sections specify the requirements for receiving signals from any signal pair in the transceiver cable transmit receive and collision presence The circuit must be capable of receiving the signals from the transceiver cable driver specified in 7 2 4 through the cable specified in 7 2 2 in the worst ca
66. nel This signal is generated by the data encoder with the transceiver cable drive characteristics specified in 7 2 4 54 ETHERNET SPECIFICATION Physical Layer 7 2 1 2 Receive Signal The receive pair carries encoded data from the transceiver to the station It typically goes to the data decoder and the carrier sense circuitry In the steady state all transitions and lack of transitions on the coaxial cable become transitions and lack of transitions on the receive pair with the transceiver cable drive characteristics specified in 7 2 4 During startup the first few bits may be absorbed by the transceiver to attain steady state In the case of a station transmitting without collision interference the station s own transmit transitions on the coaxial cable will also appear on the receive pair after a delay due to propagation through the transceiver During collisions whether or not that transceiver is involved in the collision transitions on the receive lead are undefined they may or may not meet decoder phase requirements or they may not be present at all for extended periods Thus the receive signal on the transceiver cable cannot be used alone to deterministically generate the carrier sense signal This is described in more detail in 7 5 3 7 2 1 3 Collision Presence Signal The collision presence pair is used by the transceiver to indicate the presence of multiple transmission attempts on the coaxial cable This is done by transm
67. nel Configuration Requirements 7 6 1 Cable Sectioning The 500 meter maximum length coaxial cable segment need not be made from a single homogeneous length of cable The boundary between two cable sections joined by coaxial connectors two male plugs and a barrel represents a signal reflection point due to the impedance discontinuity caused by the batch to batch impedance tolerance of the cable Since the worst case variation from 50 Q is 2 Q see 7 3 1 1 1 a possible worst case reflection of 4 may result fiom the joining of two cable sections The configuration of long cable segments up to 500 meters from smaller sections must be made with care The following recommendations apply and are given in order of preference 1 If possible the total segment should be made from one homogeneous no breaks cable This is feasible for short segments and results in minimal reflections from cable impedance discontinuities 2 If cable segments must be built up from smaller sections it is highly desirable to ensure that all the sections are from the same manufacturer and lot This is equivalent to using a single cable since the cable discontinuities are due to extruder limitations and not extruder to extruder tolerances There are no restrictions in cable sectioning if this method is used However if a cable section in such a system is later replaced it must be replaced either with another cable from the same manufacturer and lot or with one of t
68. newCollision Boolean indicates that a collision has occurred but has not yet been jammed transmitSucceeding Boolean Running indicator of whether transmission is succeeding 6 5 2 1 3 Receive State Variables The following items are specific to frame reception See also 6 5 2 1 4 on interfaces var incomingFrame Frame The frame being received currentReceiveBit 1 frameSize Position of current bit in incomingFrame receiving Boolean indicates that a frame reception is in progress excessBits 0 7 Count of excess trailing bits beyond octet boundary receiveSucceeding Boolean Running indicator of whether reception is succeeding 36 ETHERNET SPECIFICATION Data Link Layer 6 5 2 1 4 Summary of Interlayer Interfaces The interface to the Client Layer defined in 5 1 is summarized below type TransmitStatus transmitOK excessiveCollisionError Result of TransmitFrame operation ReceiveStatus receiveOK frameCheckError alignmentError Result of ReceiveFrame operation function TransmitFrame destinationParam AddressValue sourceParam AddressValue typeParam typeValue dataParam DataValue TransmitStatus Transmits one frame function ReceiveFrame var destinationParam AddressValue var sourceParam AddressValue var typeParam TypeValue var dataParam DataValue ReceiveStatus Receives one frame The interface to the Physical Layer defined in 5 2 is summarized below var carrier
69. ng frame exclusive of the FCS field itself do not generate a CRC value identical to the one received an error has occurred and is reported as such Implementation issues are discussed in Appendix C ETHERNET SPECIFICATION Data Link Layer 27 6 4 1 4 Frame Disassembly The frame is disassembled and the fields are passed to the Client Layer via the output parameters of the ReceiveFrame operation see 5 1 6 4 2 Receive Link Management 6 4 2 1 Collision Filtering As specified in 62 5 the smallest valid frame must contain at least 64 octets Any frame containing less than 64 octets is presumed to be a fragment resulting from a collision and is discarded by the receiving data link controller Since occasional collisions are a normal part of the link management procedure the discarding of such a fragment is not reported as an error to the Client Layer 6 5 The Data Link Layer Procedural Model 6 5 1 Overview of the Procedural Model The functions of the Ethernet Data Link Layer are presented below modeled as a program written in the language Pascal 6 This procedural model is intended as the primary specification of the functions to be provided in any Ethernet Data Link Layer implementation It is important to distinguish however between the model and a real implementation The model is optimized for simplicity and clarity of presentation while any realistic implementation must place heavier emphasis on such constraints as efficiency
70. oaxial cables marked as specified in 7 3 1 1 6 have marks at regular 2 5 meters spacing a transceiver may be placed at any mark on the cable This guarantees both a minimum spacing between transceivers of 2 5 meters as well as controlling the relative spacing of transceivers to insure non alignment on fractional wavelength boundaries The total number of transceivers on a cable segment shall not exceed 100 7 6 3 System Grounding The sheath conductor of the coaxial cable shall not make electrical contact with any earth reference building structural metal ducting plumbing fixture or other unintentioned conductor Insuldtors may be used to cover any coaxial connectors used to join cable sections and terminators to insure that this requirement is met A sleeve or boot attached at installation time is acceptable The sheath conductor of the transceiver cable shall be connected to the earth reference or chassis of the device housing the station logic ETHERNET SPECIFICATION Physical Layer 71 7 6 4 Repeaters Repeaters are used to extend the channel length and topology beyond that which could be achieved by a single coaxial cable segment See the channel configuration model in 7 1 5 A repeater requires a transceiver on each of the segments between which it is repeating signals These transceivers must be as specified in 7 4 and must be counted towards the maximum specified in 7 6 2 A maximum of two repeaters may be in the signal path
71. on and enforcement and collision backoff and retransmission The performance of these functions by a transmitting data link controller interacts with corresponding actions by other data link controllers to jointly implement the Ethernet data link protocol 6 3 1 Transmit Data Encapsulation 6 3 1 1 Frame Assembly The fields of the data link frame are set to the values provided by the Client Layer as arguments to the TransmitFrame operation see 5 1 with the exception of the frame check sequence which is set to the CRC value generated by the data link controller 6 3 1 2 Frame Check Sequence Generation The CRC value defined in 6 2 4 is generated and inserted in the frame check sequence field following the fields supplied by the Client Layer Appendix C discusses CRC implementation 6 3 2 Transmit Link Management 6 3 2 1 Carrier Deference Even when it has nothing to transmit the data link controller monitors the physical channel for traffic by watching the carrierSense signal provided by the Physical Layer Whenever the channel is busy the data link controller defers to the passing frame by delaying any pending transmission of its own After the last bit of the passing frame i e when carrierSense changes from true to false the data link controller continues to defer for 9 6 usec to provide proper interframe spacing see 6 3 22 At the end of that time if it has a frame waiting to be transmitted transmission is initiated indepen
72. on done increment attempts yes disassemble frame yes too many attempts validate frame check sequence no compute backoff wait backoff time extra bits truncated from frame Done alignmentError Done receiveOK Done Done Done transmitOK excessiveCollisionError frameCheckError FrameTransmitter process FrameReceiver process Invoking Data Link TransmitFrame operation Invoking Data Link ReceiveFrame operation Figure 6 3 Control Flow Summary Client Layer Processes ETHERNET SPECIFICATION Data Link Layer 33 i channel busy yes yes wait interframe spacing deferring off frameWaiting Deference process transmission started transmit bit transmission done Bit Transmitter process receiving started be carrier sense on receiving done Bit Receiver process Figure 6 4 Control Flow Summary Data Link Layer Processes 34 ETHERNET SPECIFICATION Data Link Layer 6 5 2 1 Global Declarations 6 5 2 1 1 Common Constants and Types The following declarations of constants and types are used by the frame transmission and reception sections of each data link controller const addressSize 48 48 bit address 6 octets typeSize 16 16 bit protocol type 2 octets dataSize see 6 5 1 2 note 3 crcSize 32 32 bit CRC 4 octets frameSize 2 addressSize typeSize dataSize
73. ormly distributed random integer r such that low lt high end Random BackOff performs the truncated binary exponential backoff computation and then waits for the selected multiple of the slot time The Deference process runs asynchronously to continuously compute the proper value for the variable deferring process Deference begin cycle main loop while not carrierSense do nothing watch for carrier to appear deferring true delay start of new transmissions while carrierSense do nothing wait for carrier to disappear RealTimeDelay interFrameSpacing deferring false allow new transmissions to proceed while frameWaiting do nothing allow waiting transmission if any end main loop end Deference procedure RealTimeDelay usec real begin Wait for the specified number of microseconds end Rea TimeDelay ETHERNET SPECIFICATION Data Link Layer 41 The BitTransmitter process runs asynchronously transmitting bits at a rate determined by the Physical Layers TransmitBit operation process BitTransmitter begin cycle outer loop while transmitting do begin inner loop TransmitBit outgoingFrame currentTransmitBit send next bit to Physical Layer if newCollision then StartJam else NextBit end inner loop end outer loop end BitTransmitter procedure NextBit begin currentTransmitBit currentTransmitBit 1 transmitting currentTransmitBit lastTransmitBit end NexiBit p
74. orporation A block of addresses will be assigned to each licensee of Ethernet patents see below Others may obtain an address block or type field assignment by request A nominal fee to cover administrative costs will be charged Submit written requests to Xerox Corporation Ethernet Address Administration Office 3333 Coyote Hill Road Palo Alto CA 94304 Licensing Ethernet incorporates features that are protected by one or more patents assigned to Xerox Corporation Questions on the need for licensing particular uses of this specification should be directed to Director of Licensing Long Ridge Road Stamford CT 06904 ETHERNET SPECIFICATION Appendix T APPENDIX C CRC IMPLEMENTATION Every frame contains in its frame check sequence field a 32 bit cyclic redundancy check CRC code Because the formal mathematical definition of this code see 6 2 4 is not suggestive of an appropriate implementation this appendix outlines one possible implementation in terms of a feedback shift register This type of implementation is likely to be common in practice but is not a mandatory part of the specification j The feedback shift register see Figure C 1 is used to represent division of the pre scaled message by the generating polynomial The 32 bit register is accessed via the three signals Input Output and Control When Control 1 Input bits are shifted into the feedback shift register and a
75. ously between any two or more data link entities on the same network The ability to detect the presence of another station s transmission while not transmitting carrier sense The ability to detect the presence of another station s transmission while transmitting collision detect A total worst case round trip signal propagation delay including actual propagation time synchronization time for all intervening electronics and signal rise time degradation of 450 bit times equal to 45 ys for this 10 Mbit channel Functions Provided by the Channel The channel hardware provides the following functions in the performance of its role 1 7 14 Means for transmitting and receiving serial bit streams between the data link layer and the media Generation of clock for synchronization and timing Means for detecting carrier non idle channel Means for detecting collisions simultaneous transmission attempts by multiple stations Coding and decoding of the data link bit stream into a self synchronizable sequence of electrical signals suitable for transmission on the media provided by the channel Generation and removal of coding specific preamble information a synchronizing header sequence inserted before the first bit of the frame to ensure that all channel electronics are brought to a known steady state before the data link frame is transmitted Implementation of the Channel The phy
76. physical channel interframe spacing An enforced idle time between transmission of successive frames to allow receiving data link controllers and the physical channel to recover jam An encoded bit sequence used for collision enforcement Manchester encoding A means by which separate data and clock signals can be combined into a single self synchronizable data stream suitable for transmission on a Serial channel multicast An addressing mode in which a given frame is targeted to a group of logically related stations physical address The unique address value associated with a given station on the network An Ethernet physical address is defined to be distinct from all other physical addresses on all Ethernets ETHERNET SPECIFICATION Appendix 75 Physical Channel The implementation of the physical layer Physical Layer The lower of the two layers of the Ethernet design implemented by the physical channel using the specified coaxial cable medium The Physical Layer insulates the Data Link Layer from medium dependent physical characteristics preamble A sequence of 64 encoded bits which the Physical Layer transmits before each frame to allow synchronization of clocks and other Physical Layer circuitry at other sites on the channel repeater A device used to extend the length and topology of the physical channel beyond that imposed by a single segment up to the maximum allowable end to end channel length round trip p
77. r includes facilities for transmitting and receiving frames and provides per operation status information for use by higher level error recovery procedures The interface between the Data Link Layer and the Physical Layer includes signals for framing carrier sense transmit initiation and contention resolution collision detect facilities for passing a pair of serial bit streams transmit receive between the two layers and a wait function for timing These interfaces are described more precisely in Section 5 As mentioned in the preface additional interfaces are necessary to allow a higher level network management facility to interact with the Data Link Layer and Physical Layer to perform operation maintenance and planning functions 4 2 Data Link Layer The Data Link Layer defines a medium independent link level communication facility built on the medium dependent physical channel provided by the Physical Layer It is applicable to a general class of local area broadcast media suitable for use with the channel access discipline known as carrier sense multiple access with collision detection CSMA CD Compatibility with non contention media e g switched lines token passing rings etc while a worthwhile topic for further research is not addressed in this specification The Data Link Layer specified here is intended to be as similar as possible to that described in the ISO model In a broadcast network like the Ethernet t
78. ransceiver Cable Signals 7 2 1 1 Transmit Signal 7 2 1 2 Receive Signal 7 2 1 3 Collision Presence Signal 1 2 1 4 Power 7 2 2 Transceiver Cable Parameters 7 2 2 1 Mechanical Configuration 7 2 2 2 Characteristic Impedance 7 2 2 3 Attenuation 7 2 2 4 Velocity of Propagation 7 2 2 5 Pulse Distortion 7 2 2 6 Resistance 7 2 2 7 Transfer Impedance 7 2 3 Transceiver Cable Connectors 7 2 4 Transceiver Cable Drive 7 2 5 Transceiver Cable Receive 7 2 5 1 Load Impedance and Termination 7 2 5 2 Common Mode and CMRR 7 3 Coaxial Cable Compatibility Interface Specifications 7 3 1 Cable Component Specifications 13 1 1 Coaxial Cable Parameters 7 3 1 1 1 Characteristic Impedance 7 3 1 1 2 Attenuation 7 3 1 1 3 Velocity of Propagation 7 3 1 1 4 Mechanical Requirements 7 3 1 1 5 Pulse Distortion 7 3 1 1 6 Jacket Marking 7 3 1 1 7 Transfer Impedance 7 3 1 2 Coaxial Cable Connectors 7 3 1 3 Coaxial Cable Terminators 7 3 1 4 Transceiver to Coaxial Cable Connections 7 3 2 Coaxial Cable Signaling 7 4 Transceiver Specifications 7 4 1 Transceiver to Coaxial Cable Interface 7 4 1 1 Input Impedance 7 4 1 2 Bias Current 7 4 1 3 Transmit Output Levels 7 4 2 Transceiver to Transceiver Cable Interface 7 4 2 1 Transmit Pair 7 4 2 2 Receive Pair 7 4 2 3 Collision Presence Pair 7 4 2 4 Power Pair 7 4 3 Electrical Isolation vi ETHERNET SPECIFICATION Contents 7 4 4 Reliability 7 5 Channel Logic 7 5 1 Channel Encoding 7 5 1 1
79. ransmit pair of the transceiver cable and ultimately the coaxial cable through the transceiver The encoder output asymmetry must not exceed 0 5 ns The encoder shall provide the defined output for the first and all subsequent bits presented to its input All information submitted for encoding shall appear at the output of the encoder Typical data stream 11olo 11 0o 1 1 1 0 high level Encoded signal pattern low leve Figure 7 6 Manchester Encoding 7 5 1 2 Decoder i The decoder is used to separate the incoming phase encoded bit stream into a data stream and a clock signal The decoder must be able to provide data and clock signals usable by the data link under the asymmetry imposed by the worst case system configuration The decoder must provide usable output clock and data after no more than 8 bit cell times after reception of an encoded signal The first signals received from the transceiver at the beginning of frame reception may not constitute a valid properly encoded bit it is possible for the time from the first transition seen to the first true mid bit cell transition to assume any value from zero to 100 nS The decoder input is normally derived from the coaxial cable through the transceiver cable receive pair It is not necessary for the decoder to provide usable output when there is a collision on the coaxial cable regardless of whether the station using that decoder is involved in the collision 66 ETHERNET SPEC
80. red to be the coefficients of a polynomial M x of degree 1 The first bit of the destination address field corresponds to the 1 term and the last bit of the data field corresponds to the x term 3 M x is multiplied by 3 and divided by G x producing a remainder R x of degree 31 4 The coefficients of R x are considered to be a 32 bit sequence 5 The bit sequence is complemented and the result is the CRC The 32 bits of the CRC value are placed in the frame check sequence field so that the x3 term is the leftmost bit of the first octet and the 0 term is the rightmost bit of the last octet The bits of the CRC are thus transmitted in the order x2 x20 x1 0 Appendix discusses CRC implementation issues 6 2 5 Frame Size Limitations Given the limitations on the size of the data field specified in 6 2 3 and the 18 octet total size for the other four fields the smallest valid frame contains 64 octets and the largest valid frame contains 1518 octets ETHERNET SPECIFICATION Data Link Layer 23 6 3 Frame Transmission The Data Link frame transmission and reception are as follows Frame transmission includes data encapsulation and link management aspects Transmit Data Encapsulation includes the assembly of the outgoing frame from the values provided by the Client Layer and frame check sequence generation Transmit Link Management includes carrier deference interframe spacing collision detecti
81. rocedure StartJam begin currentTransmitBit 1 lastTransmitBit 2 jamSize newCollision false end StartJam BitTransmitter upon detecting a new collision immediately enforces it by calling StartJam to initiate the transmission of the jam The jam may contain 32 to 48 bits of arbitrary data StartJam uses the first 32 bits of the frame merely to simplify this program 42 ETHERNET SPECIFICATION Data Link Layer 6 5 2 3 Frame Reception The algorithms in this section define data link frame reception The procedure ReceiveFrame implements the frame reception operation provided to the Client Layer function ReceiveFrame var destinationParam AddressValue var sourceParam AddressValue var typeParam TypeValue var dataParam DataValue ReceiveStatus function ReceiveDataDecap ReceiveStatus nested function see body below begin repeat ReceiveLinkMgmt ReceiveFrame ReceiveDataDecap until receiveSucceeding end ReceiveFrame ReceiveFrame calls ReceiveLinkMgmt to receive the next valid frame and then calls the internal procedure ReceiveDataDecap to return the frame s fields to the Client Layer if the frame s address indicates that it should do so The returned ReceiveStatus indicates the presence or absence of detected transmission errors in the frame function ReceiveDataDecap ReceiveStatus begin receiveSucceeding RecognizeAddress incomingFrame destinationField if receiveSucceeding then
82. rocedure TransmitDataEncap nested procedure see body below begin TransmitDataEncap TransmitFrame TransmitLinkMgmt end TransmitFrame First TransmitFrame calls he internal procedure TransmitDataEncap to construct the frame It then calls TransmitLinkMgmt to perform the actual transmission The TransmitStatus returned indicates the success or failure of the transmission attempt TransmitDataEncap builds the frame and places the 32 bit CRC in the frame check sequence field procedure TransmitDataEncap begin i with outgoingFrame do begin assemble frame view fields destinationField destinationParam sourceField sourceParam typeField typeParam dataField dataParam fcsField CRCS2 outgoingFrame view bits end assemble frame end TransmitDataEncap ETHERNET SPECIFICATION Data Link Layer 39 TransmitLinkMgmt attempts to transmit the frame deferring first to any passing traffic If a collision occurs transmission is terminated properly and retransmission is scheduled following a suitable backoff interval function TransmitLinkMgmt TransmitStatus begin attempts 0 transmitSucceeding false while attempts lt attemptLimit and not transmitSucceeding do begin if attempts gt O then BackOff frameWaiting true while deferring do nothing defer to passing frame if any frameWaiting false StartTransmit while transmitting do WatchForCollision attempts
83. ropagation time In bit times the time required in the worst case for a transmitting station s collision detect signal to be asserted due to normal contention for the channel This delay is the primary component of the slot time slot time A multi purpose parameter which describes the contention behavior of the Data Link Layer It serves as a an upper bound on the collison vulnerability of a given transmission b an upper bound on the size of the frame fragment produced by a collision and c the scheduling quantum for collision retransmission station A single addressable site on the Ethernet generally implemented as a computer and appropriate peripherals and connected to the Ethernet via a controller and a transceiver transceiver The portion of the Physical Layer implementation that connects directly to the coaxial cable and provides both the electronics which send and receive the encoded signals on the cable and the required electrical isolation transceiver cable A four pair shielded cable used for the transceiver cable interface transceiver cable interface The electrical mechanical and logical interface which connects the transceiver to the controller The standard transceiver cable is a recommended compatibility interface 76 ETHERNET SPECIFICATION Appendix APPENDIX NOTES ON ADDRESS AND TYPE ASSIGNMENT AND LICENSING Address and Type Assignment The address and type fields will be administered by Xerox C
84. scal is a notational technique and in no way implies that they can or should be implemented in software This point is discussed more fully in 6 5 which provides complete Pascal declarations for the data types used in the remainder of this section Note also that the synchronous one frame at a time nature of the frame transmission and reception operations is a property of the architectural interface between the Client Layer and the Data Link Layer and need not be reflected in the implementation interface between a station and its controller 5 1 Client Layer to Data Link Layer The two primary services provided to the Client Layer by the Data Link aver are transmission and reception of frames The interface through which the Client Layer uses the facilities of the Data Link Layer therefore consists of a pair of functions Functions TransmitFrame ReceiveFrame Each of these functions has the components of a frame as its parameters input or Output and returns a status code as its result The Client Layer transmits a frame by invoking TransmitFrame function TransmitFrame destinationParam AddressValue sourceParam AddressValue typeParam TypeValue dataParam DataValue TransmitStatus The TransmitFrame operation is synchronous in the sense that its duration is the entire attempt to transmit the frame so that when the operation completes transmission has either succeeded or failed as indicated by the resulting status
85. se A typical receive circuit is given in Appendix D 7 2 5 1 Load Impedance and Termination The termination impedance shall be 78 Q 1 differential mode and 18 5 minimum common mode over the frequency range of 3 20 MHz 7 2 5 2 Common Mode and CMRR The common mode range and the common mode rejection ratio shall be sufficient to maintain a 5 1 signal to noise ratio in the environment specified in 7 7 measured at the input to the transceiver cable receiver The common mode DC voltage at the input of the receiver shall be in the range of zero to 5 Vde 58 ETHERNET SPECIFICATION Physical Layer 7 3 Coaxial Cable Compatibility Interface Specifications The coaxial cable is the common shared broadcast medium through which stations communicate It provides one of the compatibility interface points described in 7 1 42 7 3 1 Coaxial Cable Component Specifications The cable is of constant impedance coaxial construction It is terminated at each end by a terminator specified in 7 3 1 3 and connection provided for each transceiver Coaxial cable connectors are used to make the connection from the cable to the terminators and between cable sections if needed The cable has various electrical and mechanical requirements which must be met to ensure proper operation 7 3 1 1 Coaxial Cable Parameters 73 4111 Characteristic Impedance The average characteristic impedance of the cable shall be 50 2 Q measured according to Mi
86. sical channel specification is implementation dependent most of the channel hardware is fully specified and little leeway is given to the individual designer This is done in the interest of compatibility any system which allows different implementors to use different channel cables connectors clock speeds and the like will not be compatible across manufacturer boundaries Only the design of channel components which are not critical to system compatibility is left to the implementor ETHERNET SPECIFICATION Physical Layer 47 7 1 4 1 General Overview of Channel Hardware The channel minimally consists of the following functional blocks 1 The passive broadcast medium coaxial cable 2 The transceiver transmitter receiver for the coaxial cable G2 The means for connecting transceivers to a coaxial cable segment and for connecting coaxial cable segments together The channel clock The channel data encoder and decoder The preamble generator and remover aH tA A The carrier and collision detect circuits The coaxial medium is the only element common to the entire network A transceiver is required for each station connected to the medium The transceiver must be located adjacent to the coaxial cable The latter four components are generally located within and tightly coupled to the station hardware implementing the data link function It may be useful to be able to physically separate the transceiver from the re
87. since all other properly functioning stations can be assumed to have noticed the signal via carrier sense and to be deferring to it The time to acquire the channel is thus based on the round trip propagation time of the physical channel In the event of a collision the Transmit Channel Access component of a transmitting station s Physical Layer first notices the interference on the channel and turns on the collision detect signal This is noticed in turn by the Transmit Link Management component of the Data Link Layer and collision handling begins First Transmit Link Management enforces the collision by transmitting a bit sequence called the jam This insures that the duration of the collision is sufficient to be noticed by the other transmitting station s involved in the collision After the jam is sent Transmit Link Management terminates the transmission and schedules a retransmission attempt for a randomly selected time in the near future Retransmission is attempted repeatedly in the face of repeated collisions Since repeated collisions indicate a busy channel however Transmit Link Management attempts to adjust to the channel load by backing off voluntarily delaying its own retransmissions to reduce its load on the channel This is accomplished by 14 ETHERNET SPECIFICATION Functional Model of the Ethernet Architecture expanding the interval from which the random retransmission time is selected on each retransmission attempt Eventu
88. smitted from top to bottom and the bits of each octet are transmitted from left to right NOTE This document does not define an order of transmission for the octets of standard multi octet data types strings integers etc since no values of such data types appear in the data link frame format The order in which implementations of the Ethernet store the octets of a frame in computer memory and the manner in which higher level protocols interpret the contents of the data field as values of various multi octet data types are beyond the scope of this specification The Ethernet itself is also totally insensitive to the interpretation of bits within a octet as constituting the digits of an 8 digit binary numeric value Since some uniform convention is helpful however in avoiding needless incompatibility among different station types the interpretation is arbitrarily defined to be that the left most bit first transmitted is the low order 20 digit and the right most bit last transmitted is the high order 27 digit ETHERNET SPECIFICATION Data Link Layer 21 6 2 1 Address Fields Data link addresses are 6 octets 48 bits in length A data link address is of one of two types Physical address The unique address associated with a particular station on the Ethernet A station s physical address should be distinct from the physical address of any other station on any Ethernet Multicast address A multi destination address assoc
89. speed interconnections which are generally limited to tens of meters The Ethernet is intended primarily for use in such areas as office automation distributed data processing terminal access and other situations requiring economical connection to a local communication medium carrying bursty traffic at high peak data rates Use in situations demanding resistance to hostile environments real time response guarantees and so on while not specifically excluded do not constitute the primary environment for which the Ethernet is designed The precursor to the Ethernet specified in this document was the Experimental Ethernet designed and implemented by Xerox in 1975 and used continually since that time by thousands of stations The Ethernet defined here builds on that experience and on the larger base of the combined experience of Digital Intel and Xerox in many forms of networking and computer interconnection In specifying the Ethernet this document provides precise detailed definitions of the lowest two layers of an overall network architecture It thus defines what is generally 2 ETHERNET SPECIFICATION Introduction referred to as a link level facility It does not specify the higher level protocols needed to provide a complete network architecture Such higher level protocols would generally include such functions as internetwork communication error recovery flow control security measures e g encryption and other higher lev
90. st of the channel hardware This allows topological flexibility packaging advantages and improved system availability as well as allowing for independent manufacture of station hardware and transceivers To ensure that compatibility is maintained a physical interface known as the transceiver cable is identified and specified to connect the transceiver to the station Finally it may be necessary to add repeaters to the system to reach the maximum allowable distance between stations and to provide additional topological flexibility Repeaters are implemented using standard transceivers plus a simple non buffered finite state machine 48 ETHERNET SPECIFICATION Physical Layer 7 1 4 2 Compatibility Interfaces There are a number of possibilities for implementing systems or subsystems compatible in whole or in part with this specification It is important that all implementations be compatible at some point so that heterogenous systems from different manufacturers implementations can be interconnected on the same medium It is not necessary in every case to implement all of the components described herein e g it is possible to design an integrated station transceiver without requiring the transceiver cable The implementor must make the required trade offs between topological flexibility system availability configurability user needs and cost when designing the system For a device to be considered compatible it must me
91. t E A Practical Considerations in Ethernet Local Network Design Presented at Hawaii International Conference on System Sciences January 1980 Shoch J F and Hupp J A Measured Performance of an Ethernet Local Network Presented at Local Area Communications Network Symposium Boston May 1979 The following references describe the ISO Open Systems Model 4 5 Zimmermann H OSI Reference Model The ISO Model of Architecture for Open Systems Interconnection IEEE Transactions on Communication COM 28 4 April 1980 International Organization for Standardization ISO Reference Model of Open Systems Interconnection Document no ISO TC97 SC16 N227 June 1979 The following references describe the Pascal language used in the Data Link Layer 6 7 procedural model and its derivative Concurrent Pascal Jensen K and Wirth N Pasea User Manual and Report 2nd Edition Springer Verlag 1974 Brinch Hansen P Concurrent Pascal Report Technical Report CIT IS TR 17 California Institute of Technology 1975 The following references discuss the CRC code used for the frame check sequence 8 9 Hammond J L Brown J E and Liu S S Development of a Transmission Error Model and an Error Control Model Technical Report RADC TR 75 138 Rome Air Development Center 1975 Bittel R On Frame Check Sequence FCS Generation and Checking ANSI working paper X3 S34 77 43
92. t be 90 minimum The cable must also meet applicable flammability criteria and local codes for the installed environment Different e g polyethylene and Teflon dielectric types of cable sections may be interconnected while meeting the sectioning requirements of 7 6 1 7 3 1 1 5 Pulse Distortion Pulse distortion shall not exceed x 7 nS at the end of 500 meters of cable when driven with random 10 Mbit data encoded in accordance with 7 5 1 7 3 1 1 6 Jacket Marking The cable jacket must be marked with annular rings in a color contrasting with the background color of the jacket The rings must be spaced at 2 5 meter 5 cm regularly along the entire length of the cable It is permissible for the 2 5 meter spacing to be interrupted at discontinuities between cable sections joined by connectors See 7 6 2 for transceiver placement rules which mandate cable markings 7 3 1 1 7 Transfer Impedance The transfer impedance of the cable shall not exceed the values shown in Figure 7 4 as a function of frequency 60 ETHERNET SPECIFICATION Physical Layer 100 LER So ee rR ttt etn eae et T HHHS LL SENN EE ini i LLL 10 mQ meter Hn H He HHR 10KHz 100K 10M 00M 0 1 Frequency Figure 7 4 Maximum Coaxial Cable Transfer Impedance 7 3 1 2 Coaxial Cable Connectors Coaxial cable connectors are used to join cable sections
93. t link 1000M max Figure 7 1c A Typical Large scale Configuration 51 Segment 2 Segment 5 52 ETHERNET SPECIFICATION Physical Layer Table 7 1 Physical Channel Propagation Delay Budget Note 1 Element Unit Uni 4 Units Units Steady State Startup Forward Return Delay Delay Path Note 2 Path Encoder 0 1 uS 0 3 3 Transceiver Cable 5 13 nS M 0 300 M 300 M Transceiver 0 50 uS 0 2 pS 3 3 transmit path Transceiver 0 50 uS 0 5 pS 3 0 receive path Transceiver 0 0 5 uS 0 3 collision path Coaxial Cable 4 33 n M 0 1500M 1500M Point to Point 5 13 nS M 0 1000 M 1000 M Link Cable Repeater 0 8 pS 0 2 0 repeat path Repeater 0 2 uS 0 0 2 collision path Decoder 0 1 pS 0 8 uS 2 0 Carrier Sense 0 0 2 pS 3 0 Collision Detect 0 0 2 pS 0 3 Signal Rise Time 0 0 1 pS 3 0 to 70 in 500 M i Note 3 Signal Rise Time 0 2 7 uS 0 3 50 to 94 in 500 M Note 4 Total Worst Case Round Trip Delay a eee Note 1 All quantities given are worstcase both number of units and unit delays per unit Note 2 The propagation delay has been separated into forward path and return path delay This is because in one direction it is carrier sense which is being propagated through the channel and in the return direction it is collision detect which is being propagated The two signals have different propagation delays Note 3 In the worst case the propagated signal must reach 70 of i
94. t record to represent a frame both as fields and as bits follows the letter but not the spirit of the Pascal Report since it allows the underlying representation to be viewed as two different data types It also assumes that this representation is as shown in Figure 6 1 b The model makes no use of any explicit interprocess synchronization primitives Instead all interprocess interaction is done via carefully stylized manipulation of shared variables For example some variables are set by only one process and inspected by another process in such a manner that the net result is independent of their execution speeds While such techniques are not generally suitable for the construction of large concurrent programs they simplify the model and more nearly resemble the methods appropriate to the most likely implementation technologies e g microcode hardware state machines etc 30 ETHERNET SPECIFICATION Data Link Layer 6 5 2 Procedural Model The procedural model used here is based on five cooperating concurrent processes _Of these three are actually defined in the Data Link Layer The remaining two processes are provided by the Client Layer and utilize the interface operations provided by the Data Link Layer The five processes are thus Client Layer Frame Transmitter Process Frame Receiver Process Data Link Layer Bit Transmitter Process Bit Receiver Process Deference Process This organization of the model is illustrated in Fi
95. ted to segment as they did at the output of the transmitter on the repeated from segment allowing for transceiver output tolerances as specified in 7 4 13 Any loss of signal to noise ratio due to cable loss and noise pickup is thus regained at the output of the repeater 7 6 4 3 Signal Timing The repeater must ensure that the symmetry characteristics of the signals at the transceiver output of the repeated to segment are the same as those at the output of the transmitter on the repeated from segment allowing for transceiver and transceiver cable tolerances Any loss of symmetry due to transceivers and cable distortion is thus regained at the output of the repeater 72 ETHERNET SPECIFICATION Physical Layer 7 Environment Specifications The following sections specify the physical environment in which all channel components must operate to be considered compatible 7 1 1 Electromagnetic Environment The physical channel hardware shall meet its specifications when operating in the following ambient plane wave fields 2 Volts Meter from 10 KHz through 30 MHz 5 Volts Meter from 30 MHz through 1 GHz 7 1 3 Temperature and Humidity All physical channel hardware with the possible exception of the channel logic components shall operate over the ambient temperature range of 5 to 50 degrees Celsius and humidity range of 10 to 95 non condensing The channel logic components are normally part of the station hardware and are thus subject to
96. to Physical Interface Transmit Receive Data Encoding Data Decoding Physical Layer Transmit Receive Channel Access Channel Access Ethernet Coaxial Cable Figure 4 4 Physical Layer Functions 12 ETHERNET SPECIFICATION Functional Model of the Ethernet Architecture 4 4 Ethernet Operation and the Functional Model This section provides an overview of frame transmission and reception in terms of the functional model of the architecture This overview is descriptive rather than definitional the formal specifications of the operations described here are pen in Sections 6 and 7 4 4 1 Transmission Without Contention When the Client Layer requests the transmission of a frame the Transmit Data Encapsulation component of the Data Link Layer constructs the frame from the clientsupplied data and appends a frame check sequence to provide for error detection The frame is then handed to the Transmit Link Management component for transmission Transmit Link Management attempts to avoid contention with other traffic on the channel by monitoring the carrier sense signal and deferring to passing traffic When the channel is clear frame transmission is initiated after a brief interframe delay to provide recovery time for other data link controllers and for the physical channel The Data Link Layer then provides a serial stream of bits to the Physical Layer for transmission The Data Encoding component of the Physical Layer before sendin
97. ts final value to be detected as valid carrier at the end of 500 meters of coaxial cable This rise time must be included in the propagation delay budget Note 4 In the worst case the propagated collision on the return path must reach 94 of its final value to be detected as a collision at the end of 500 meters of coaxial cable ETHERNET SPECIFICATION Physical Layer 53 7 1 6 Channel Interfaces The channel specification hinges around three well defined entities the transceiver and coaxial cables shown as compatibility interfaces in Figure 4 1 and the logical interface between the physical channel and the data link controller shown in Figure 4 4 Note that the former two are physical interfaces specific to the channel and are specified in the interest of compatibility The latter is provided as _ a means by which the data link controller can interact with the physical channel The channel access component of the logical interface discussed in 4 4 1 comprises the collision and carrier detect functions described in 7 5 2 and 7 5 3 as well as the actual transmission of signals on the media The data encoding and decoding functions described in 4 4 1 comprise the generation and decomposition of encoded signals suitable for transmission described in 7 5 1 the generation and removal of code specific preamble described in 7 5 1 3 and 7 5 4 1 and the serial bit stream interface between the layers Section 5 describes the interface bet
98. ver cable transmit characteristics specified in 7 2 4 to the receive pair Asymmetry as seen on the receive pair shall not exceed 2 nsec for a 200 mV peak sinusoidal input from the coaxial cable The signal from the coaxial cable shall pass through AC coupling with an appropriate time constant before proceeding to the receive pair The time constant should compensate for the coaxial cable pulse distortion At the start of a frame reception from the coaxial cable no more than 5 bits five 100 ns bit cells of information may be received from the coaxial cable and not transmitted onto the receive pair In addition it is permissible for the first bit sent over the receive pair to contain encoding phase violations or invalid data however all successive bits of the frame shall be valid and meet encoding rules The steady state propagation delay between the coaxial cable and the receive pair output shall not exceed 50 ns There are no signal inversions between the coaxial cable and the transceiver cable receive pair 64 ETHERNET SPECIFICATION Physical Layer 7 4 2 3 Collision Presence Pair The transceiver must present the transmitter characteristics specified in 7 2 4 to the collision presence pair The signal presented to the collision presence pair shall be a periodic waveform with a 10 MHz 15 frequency This signal shall be presented to the collision presence pair no more than 5 bit times 500 nS after the average signal on the coaxial c
99. ween the data link and phissical layers as a series of Pascal procedures functions and shared variables The data link specification in section 6 shows how the data link uses this interface to communicate between client layers However this specification will not attempt to model the operation of the physical channel in Pascal The interface between layers is supported by the physical hardware which provides the ability to send and receive bit streams provide timing and signal carrier sense and collision detect to the data link The remainder of this section specifies the requirements for compatibility at both the transceiver cable and the coaxial cable In addition the specifications for the transceiver which interfaces the transceiver cable to the coaxial cable is given as well as the specification for the logic required between the transceiver cable and the interface to the data link 7 2 Transceiver Cable Compatibility Interface Specifications The transceiver cable is the means by which a physically separate transceiver is connected to a station It provides one of the compatibility interfaces described in 7 1 42 7 2 1 Transceiver Cable Signals The transceiver cable carries four signals Transmit Receive Collision Presence and Power Each signal is carried on a twisted pair of conductors in the cable 7 2 1 1 Transmit Signal The transmit pair carries encoded data for which the data link is requesting transmission on the chan
100. with incomingFrame do begin disassemble frame view fields destinationParam destinationField sourceParam sourceField typeParam typeField dataParam dataField if fcsField OROA2 incomingFrame t then ReceiveDataDecap receiveOK else if excessBits O then ReceiveDataDecap frameCheckError else ReceiveDataDecap alignmentError view bits end disassemble frame end ReceiveDataDecap ETHERNET SPECIFICATION Data Link Layer 43 function RecognizeAddress address AddressValue Boolean begin RecognizeAddress Returns true for the set of physical broadcast and multicast group addresses corresponding to this station end RecognizeAddress ReceiveLinkMgmt attempts repeatedly to receive the bits of a frame discarding any fragments from collisions by comparing them to the minimum valid frame size procedure ReceiveLinkMgmt begin repeat StartReceive while receiving do nothing wait for frame to finish arriving excessBits frameSize mod 8 frameSize frameSize excessBits truncate to octet boundary receiveSucceeding frameSize gt slotTime reject collision tragments until receiveSucceecing end ReceiveLinkMgmt procedure StartReceive begin currentReceiveBit 1 receiving true end StartReceive The BitReceiver process run asynchronously receiving bits from the channel at the rate determined by the Physical Layers ReceiveBit operation process BitR
101. yer of a network architecture which use the Ethernet Data Link and Client interface coaxial cable A two conductor concentric constant impedance transmission line coaxial cable interface The electrical mechanical and logical interface to the shared coaxial cable medium This is mandatory compatibility interface which must be correctly implemented by every Ethernet implementation coaxial cable section An unbroken piece of coaxial cable fitted with coaxial connectors at its ends used to build up coaxial cable segments coaxial cable segment A length of coaxial cable made up from one or more coaxial cable sections and coaxial connectors terminated at each end in its characteristic impedance A 500 meter segment is the longest configuration possible without repeaters collision The result of multiple transmissions overlapping in the physical channel resulting in garbled data and necessitating retransmission collision detect A signal provided by the Physical Layer to the Data Link Layer to indicate that one or more other stations are contending with the local station s 74 ETHERNET SPECIFICATION Appendix A transmission It can be true only during transmission collision enforcement Transmission of extra encoded jam bits after a collision is detected to insure that the duration of the collision is sufficient to guarantee its detection by all transmitting stations compatibility interfaces The coaxial cable int
102. ysical channel through a coaxial cable medium Because one purpose of the layered architecture is to insulate the Data Link Layer from the medium specific aspects of the channel the Physical Layer completely specifies the essential physical characteristics of the Ethernet such as data encoding timing voltage levels etc Implementation details are left unspecified to retain maximum flexibility for the implementor In all cases the criterion applied in distinguishing between essential characteristics and implementation details is guaranteed compatibility any two correct implementations of the Physical Layer specified here will be capable of exchanging data over the coaxial cable enabling communication between their ETHERNET SPECIFICATION Functional Model of the Ethernet Architecture 11 respective stations at the Data Link Layer The Physical Layer defined in this specification performs two main functions generally associated with physical channel control Data encoding preamble generation removal for synchronization bit encoding decoding between binary and phase encoded form Channel access bit transmission reception of encoded data carrier sense indicating traffic on the channel collision detection indicating contention on the channel This split is reflected in the division of the Physical Layer into the Data Encoding sub layer and the Channel Access sub layer as shown in Figure 4 4 Data Link Layer Data Link

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