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Implementation of the Link Access Procedure on the ISDN D

1. Awaiting Establishment DL ESTABLISH CONFIRM Multiple Frame Established Peer Le management Layer amp Layer 3 FIGURE 6 5 Test for data link layer setup Multiple Frome Estoblished DL_DATA INDICATION Peer Le management Layer 2 Layer 3 FIGURE 6 6 Test for data link layer information transport CHAPTER 6 TESTING 50 If an extensive conformance test is to be performed in the future I recommend the use of the tests supplied by the Conference of European Posts and Telecommunication Ad ministrations CEPT Their technical specifications are known as Normes Europ enne de T l communication NET and are recognized by most European countries In this specific case NET 3 would apply 10 6 2 Performance test It is essential for LAPD to function real time in order to be able to function conform its specifications As most of LAPD is implemented in software on a relatively slow PC the question arises if the implementation is fast enough Conformance testing can perhaps reveal correct response to specific stimuli but it does not say anything about the overall performance A first indication that the implementation might not be functioning 100 percent came when activating all three data links After a while one or more data links would release itself after a timeout situation Even without the time consuming functions associated with the protocol analyzer this problem returned In ord
2. number of defined processes union scratch unsigned char byte unsigned int word unsigned char pbyte unsigned int pword struct process byte PublStat Status Blocked or Running byte Priorid Priority struct state far State byte Substate Extra for LAP D processes union scratch Datal Parameters union scratch Data2 union scratch Data3 union scratch Data4 union scratch Data5 union scratch Data6 union scratch Data union scratch Data8 struct process Process MAXPROC FIGURE 3 4 Definition of a process descriptor Each descriptor consists of twelve elements First of all the PublStat field holds the status of the process It is either Blocked or Running A Blocked process cannot receive messages until it becomes Running Once a process is Blocked it can only be unBlocked by another unBlocked process If for example no packet data is supported layer management may block the packet data entity process The Priorid field holds the process s priority All messages for a certain process are placed in the queue with the corresponding priority value The State field provides information about the current state the process is in It points to the state table that should be used if a message is received Some processes need to register several important conditions Own Receiver Busy Peer Receiver Busy These are saved in the Substate field Datai Data8 are storage
3. read the status of the FIFO s int far readstat void read the interrupt register define rd_ireg inportb 0xb04 Initializations necessary to link the SNIC to the software Program SNIC to generate IRQ signal outportb 0xb00 inportb 0xb00 0x04 Note that selection with B2 of Master Control Register is as follows B2 O selects NDA function B2 1 selects IRQ function MITEL Databook p4 67 has been proven wrong on this with the use of a logic analyzer enable INT 15 generated by IRQ from SNIC outportb 0x21 inportb 0x21 amp Ox7F redirect INT 15 to user SNIC interrupt service routine APPENDIX A SNIC REGISTERS AND ACCESS FUNCTIONS oldvecOF getvect 15 setvect 15 Sisr set direction of D channel port to microprocessor port outportb 0xb02 0x58 enable SNIC interrupts outportb 0xb04 Ox7f 60 Appendix B LAP D formats TABLE B 1 Format of layer 2 frames Contents 1 Opening Flag 0 1 1 1 m 0 SAP ORo as TEI 6 mation Fid NB NA N Closing Flag 0 1 1 1 1 1 1 0 TABLE B 2 Command Response bit definition Command ue Response network gt user 0 E eee 61 APPENDIX B LAP D FORMATS TABLE B 3 SAPI definitions SAPI value Related layer 3 or management entity 0 Call control procedures 1 Packet mode communication conforming to Q 931 Packet mode communication conforming to X 25 layer 3 Layer 2 M
4. LAP D implementation As could be seen in the previous chapter LAP D is specified as a state machine Each data link layer entity is always in a defined state The entities communicate with adjacent layers by primitives We can regard the different data link layer entities as processes that communicate by messages This yields the schematic of figure 3 1 The lines between the entities or processes indicate communication messages 3 LM layer management me encoding 1 2 3 layers 1 2 3 Si signalling entity B channel S2 signalling entity B2 channel B channels Fra amp decoding FIGURE 3 1 The basic data link layer entities P packet data entity To accommodate the LAP D protocol on the ISDN terminal board an operating system supporting the above approach has been created by Oudelaar 8 It was developed keeping in mind the following considerations e it should provide support for a large number of processes entities This enables us to implement layer 3 in the same way this has been done by 7 e it should allow a priority mechanism for the processes Layer 2 processes have stricter timing constraints than layer 3 processes and thus sometimes need priority treatment e it should allow a small number of hardware interrupts The interrupt handlers of those interrupts must be fast in order not to stall any process 20 CHAPTER 3 LAP D IMPLEMENTATION 21 Process A Process B P
5. ISDN using ISDN basic access part 1 layers 1 and 2 aspects Technical report CEPT CEPT Liaison Office Seilerstrasse 22 CH 3008 Bern 1988 11 A S Tanenbaum Computer Networks Prentice Hall International Inc London second edition 1989 12 S van de Kuilen Software ontwikkeling rond de Mitel ISDN Express Card Graduation Report EB266 Eindhoven University of Technology Digital System Group Eindhoven August 1990 56 BIBLIOGRAPHY 57 13 D F van Egmond Software manual of the ISDN project december 1991 December 1991 14 P J G Weterman Design and Building of an ISDN Terminal Board Graduation Report EB200 Eindhoven University of Technology Digital System Group Eindhoven August 1989 Appendix A SNIC registers and access functions A 1 Address map TABLE A 1 Description of the SNIC registers Address Waite Red A Oxb00 Master Control Register veiy lt S 0xb01 ST BUS Control Register verily Y 0xb02_ HDLC Control Register 1____ verify SCS N 0xb03__ HDLC Control Register HDLC Status Register C 0xb04 HDLC Interrupt Mask Register HDLC Interrupt Status Register Oxb05_ HDLC Tx FIFO HDICRAFIFO 7 Oxb06 HDLC Address Byte 1 Register verify O 0xb07 HDLC Address Byte 2 Register very S 0xb08 C channel register DSTiCohamnl Y 0xb09__ DSTo C chann C channel Status Register N 0xb0a S Bu
6. 2194 tx XID 2889 putqueue room in queue 353 putqueue queue full 183 timecheck 0 timers running 1118 no timer expires during check timecheck x timers running 1118 x 61 each expiring timer adds 487 runtimer no timers running 145 runtimer x timers running 145 x 39 stoptimer timer at array x 140 x 98 timer still running stoptimer 2676 timer expired time out message removed dispatcher cycle no action 1600 tx_packet x bytes 536 x 156 rcv_packet x bytes 540 x 160 In the following text 0 denotes the message queue of the highest priority 1 denotes the queue of a lesser priority A Dispatcher cycle includes the getmsg call CHAPTER 6 TESTING Initiating side start T200 start T2002 start T2003 action time us tx SABME Dispatcher cycle 2237 tx_packet 3 bytes 1004 Dispatcher cycle 2237 dlestreq 4 2600 Dispatcher cycle 2237 tx_packet 3 bytes 1004 Dispatcher cycle 2237 dlestreq_4 2600 Dispatcher cycle 2237 tx_packet 3 bytes 1004 Responding side Assume all three SABME s are received in sequence This will affect the response times in a negatively they increase slightly Assume further that all message queues are empty receive 3 bytes interrupt receive 3 bytes interrupt receive 3 bytes interrupt Dispatcher cycle rev_packet 3 bytes Dispatcher cycle rev_packet 3 bytes Dispatcher cycle rev_packet 3 bytes Dispatcher cycle
7. Acknowledge pending flag As mentioned above the State field points to a state table The state tables for all processes involved were derived from the CCITT Recommendation Q 921 Annex B 9 Annex B provides the SDL representation of the point to point procedures of the data link layer entity The SDL diagrams do not describe all of the possible actions and conditions of the data link layer entity The shortcomings are minor and of little concern to the basic performance of the data link layer entities It should be noted though that the text of the recommendation prevails over any SDL inconsistency The translation of the SDL diagrams is illustrated in figure 3 2 Here I assume a data link layer entity to be in the TELASSIGNED state receiving a SABME command The relation between the diagram on the left and the code on the right speaks for itself I will not go into the detail of discussing precise implementation of VS VA NR etc All implemented functions are collected in state tables These state tables are made up of rows containing a possible input message and the corresponding function that must be executed on receipt of CHAPTER 3 LAP D IMPLEMENTATION 25 struct state word nucleo The incoming message void transi void The function to be executed transi y void dummy void Empty function Does nothing Each state defines a state table which is an array of the above structure state State
8. Block diagram of the ISDN Express Card Block diagram of the ISDN Terminal Board Functional block diagram of the Subscriber Network Interface Circuit SNIC Receive packet algorithms using polling and interrupts Transmit packet algorithm with the use of interrupts Outline of the interrupt handler Timer interrupt handler Test configuration for testing transmission and reception of packets Test configuration for monitoring the functional behaviour of the protocol Layout of the protocol analyzer Test for data link layer release Test for data link layer setup Test for data link layer information transport 10 11 12 13 15 16 17 18 List of Tables 2 1 5 1 5 2 6 1 6 2 A l B 1 B 2 B 3 B 4 B 5 B 6 B 7 B 8 C 1 Protocol mechanisms in LAP D 14 HDLC Transceiver frame format 35 Description of the SNIC registers 35 Duration of basic functions measured by ZTimer 51 Response times of three consecutive data link layer setup procedures estimation 53 Description of the SNIC registers 58 Format of layer 2 frames 61 Comman
9. CHAPTER 4 HARDWARE ASPECTS 31 T1 Trunk CEPT Trunk Digital Crosspoint Switch 1544Kbit s 23B D 2048Kbit s S interface SNIC 192Kbit s 192Kbit s 2B D 2B D HDLC Controller HDLC Controller Clock Generator and DPLL PC Interface FIGURE 4 1 Block diagram of the ISDN Express Card e microprocessor interface offering full control over the SNIC circuit Via the microprocessor interface the D channel can be rerouted to the PC Interface offering the PC full control over the SNIC This could also be achieved through the HDLC controllers be it with slightly more overhead Appendix A contains full information about the SNIC including a number of functions to make the D channel accessible by the PC The SNIC can then be used to send and receive frames on the D channel Interfacing the chip to the LAP D software will be discussed in the next chapter 4 1 2 ISDN Evaluation Software System The IES software that comes with the Mitel card is a menu driven application It allows the user to manipulate control registers and display status registers More important the D channel connection between two Mitel cards is made using this software Unfortunately the sources of the IES could not be adapted for integration into the ISDN software being discussed in this report Initialization of the Mitel card is therefore done with the IES The soft
10. CONFIRM DATA LINK CONNECTION RELEASED DL ESTABLISH REQUEST AWAITING DA DL ESTABLISH INDICATION DL ESTABLISH INDICATION AWAITING RELEASE DL ESTABLISH INDICATION DL ESTABLISH CONFIRN DL RELEASE INDICATION DL RELEASE INDICATION DATA LINK CONNECTION ESTABLISHED DL ESTABLISH CONFIRM DL RELEASE REQUEST DL ESTABLISH INDICATION DL DATA REQUEST INDICATION FIGURE 2 5 State transition diagram of the data link connection endpoints between layers 2 and 3 figure 2 5 e link connection released state stable state In this state the data link is idle It will be activated when layer 3 asks for a data link establishment DL_ESTABLISH_REQUEST or when layer 2 indicates an established data link DL_ESTABLISH_INDICATION e awaiting establishment state transition state A data link has signalled its peer to establish a data link As soon as a positive ac knowledgement is received DL_ESTABLISH CONFIRM a data link is considered es tablished e awaiting release state transition state A data link is waiting for its peer to acknowledge the request to release the data link e link connection established state stable state In this state information transfer is possible The illustration shows that primitives occur in all four basic states In a normal situation however a data link connection endpoint passes through the states counter clockwise Its peer only reacting to requests merely
11. assign it to the data link layer entity This is done using an MDL_ASSIGN REQUEST If the user equipment is of the automatic TEI assignment category an Identity request message is sent to the network Several parameters are included in the message one of which is a Reference number This randomly generated number helps the network to distinguish between different requests since no other identification of the data link is available no TEI assigned The network can respond with an Identity assigned message confirming the assignment of a certain TEI which is included in the message It is also possible for the network to deny a request e g when all available TEI information resources are exhausted In such a case the network will on its own start verifying each TEI it has assigned This might reveal multiple TEI assignments one TEI value assigned to more than one terminal After a TEI value has been assigned the data link layer entity will try to make a con nection It initiates a request for the multiple frame operation by transmitting the SABME command to its peer If its peer is able to comply it will respond with a UA response The connection is now established and acknowledged information transfer is possible Both data CHAPTER 2 LINK ACCESS PROCEDURE D 17 link layer entities are in the MULTIPLE_FRAME ESTABLISHED state In case the peer data link layer does not respond with a UA frame a data link cannot be properly set up and appro
12. for layer 2 management The TEI can take on any value in the range of 0 127 Value 127 the group TEI is assigned to the broadcast data link connection Values 0 63 are selected by the user CHAPTER 2 LINK ACCESS PROCEDURE D 10 Opening Flag AP ORTO Closing Flag 0 1 1 1 1 1 octet 2 octet 3 octet 4 5 octet 6 octets N 2 N 1 octet N 1 0 FIGURE 2 2 Format of layer 2 frames non automatic TEI assignment values 64 126 are selected by the network automatic TEI assignment The user sends command frames with the C R bit set to 0 and response frames with the C R bit set to 1 the network does the opposite Control field This field identifies the type of frame which will be either a command or a response frame Three types of control field formats are defined 1 numbered information transfer I format Used to perform layer 3 information transfers Send and receive sequence numbers are included to enable acknowledge ment 2 supervisory format S format Used for acknowledgements and requests for re transmission 3 unnumbered information transfer and control functions U format Used for data link control functions All three formats include a P F bit Its use is defined by the peer to peer procedures P stands for Poll F stands for Final indicating if a response is required Information field The contents of this field are determined by the control field In an information frame
13. it will contain layer 3 information and in e g a Frame Reject frame it will contain more specific information about the rejection Frame check sequence These 2 octets are added to detect possible transmission errors A full description of the frame format is located in appendix B The SAPI and the TEI together form the Data Link Connection Identifier DLCI The DLCI identifies a LAP D data link connection Figure 2 3 illustrates the identification of a LAP D connection As can be seen by this figure one terminal can have more than one TEI assigned to it see TE2 it is assigned TEI values 3 and 8 A terminal is not identified by one TEI the TEI merely identifies a specific connection endpoint within a service access point In other words within the signalling entity a service access point more than one data link connection can be processed CHAPTER 2 LINK ACCESS PROCEDURE D 11 User Network TEI Packet Signalling Signalling Packet entity entity d j entity entity AI 1 B LE D channel SAPI 0 SAPI 16 Point to point data link connection cea link layer service acces point rennes Broadcast data link connection DLCI SAPI TEI data link layer connection endpoint FIGURE 2 3 Multiplexing of data links using SAPI and TEI 2 4 Modes of operation The data link layer supports two modes of operation for layer 3 information transfer Both modes may exist on one D channel at the same
14. layer Physical media S T U FIGURE 2 1 The ISDN protocol reference model layers 1 to 3 basic access NT2 not shown would be positioned between TE and NT1 It much resembles the TE the D channel This can be between the terminals and NT2 between NT2 and ET net work node or exchange termination or between user terminals and the subscriber serving exchange NT1 see figure 2 1 LAP D supports multiple terminal installations at the user network interface and multiple layer 3 entities To achieve this LAP D performs the following functions 1 demarcation by means of flags alignment and transparency of the transported frames 2 the multiplexing of several data links on the same D channel 3 frame sequence maintenance in case of numbered frames 4 detection of errors 5 flow control LAP D is based on the High Level Data Link Control HDLC protocol This protocol is described in 4 together with the necessary changes 2 3 LAP D frame format All layer 2 peer to peer information is transmitted in frames conforming to the format in figure 2 2 The format defines several fields which are described below Opening amp Closing Flag Used to synchronise frames Address field Consists of a Service Access Point Identifier SAPI a Terminal Endpoint Identifier TEI and a Command Response bit The SAPI indicates the type of service which can be signalling SAPI 0 or packet data SAPI 16 whereas SAPI 63 is used
15. layer 3 frame Figure 3 3 shows what information is usually stored in the five parameters Messages are collected in a FIFO First In First Out queue This is done in the form of a pointer to a buffer which holds the actual message The message queue is entirely controlled by the dispatcher A process can only put messages in the queue not remove them there is one exception that will be explained in chapter 6 In order to favour the retrieval of a certain message over others several message queues are used Each queue resembles a priority Priorities can range from 0 highest priority to MAXPRIO lowest priority The value of MAXPRIO is for this time unrelevant but will be given in chapter 6 CHAPTER 3 LAP D IMPLEMENTATION 22 struct message word nucleo Holds the message type byte dest Destination process byte orig Originating process union scratch param Received N S union scratch param_2 Received N R union scratch param 3 Received P F union scratch param_4 Received C R union scratch param_5 Pointer to buffer of information field FIGURE 3 3 Declaration of the message structure The queues are accessed by three functions These are described in below void message word Mes byte Dest byte Orig byte Pari byte Par2 byte Par3 byte Par4 byte Par5 This function is called by the various processes to place a message in the message queue indicated by the priority of the D
16. registers and access functions Asl Address Map aora e Stas A Se AAA a A 2 Implemented access functions B LAP D formats C LAP D parameters 44 44 50 55 58 58 58 58 61 64 List of Figures 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 1 3 2 3 3 3 4 3 5 3 6 3 7 3 8 4 1 4 2 5 1 5 2 5 3 5 4 5 5 6 1 6 2 6 3 6 4 6 5 6 6 The ISDN protocol reference model layers 1 to 3 basic access Format of layer 2 frames wa oe ae Bh pere te a L na ges Multiplexing of data links using SAPI and TEIL Layer 2 primitives associated with the interaction between layer 3 and the MANagement entities a 5d Va Bae A We ee RRS State transition diagram of the data link connection endpoints between layers Control field parameters of an I frame Example of a procedure to release a data link connection The basic data link layer entities o Schematic view of the operating system Declaration of the message structure Definition of a process descriptor SDL representation of SABME response and corresponding C code Declaration of the state structure General structure of the dispatcher The basic data link layer entities shown as processes
17. space for variables unique to a process These can be send and receive state variables a retransmission counter etc As an example I have included the declaration of the signalling entity for the B channel Process LAPD S1 PublStat RUNNING Process LAPD S1 Priorid acl CHAPTER 3 LAP D IMPLEMENTATION 24 void far sabme_4 void sanas lt if ABLE_TO_ESTABLISH r r tx UA P 1 F P response message DL_ESTABLISH_INDICATION LAYER3 gt ProcAct 0 0 0 O 0 STABLIS vs 0 VA 0 VR 0 stoptmr T200 TIME_DUT ProcAct C an runtimer T203 TIME DUT ProcAct T203 SUBSTATE 0 clear exceptions NEWSTATE MULTIPLE_FRAME_ESTABLISHED DL else ESTABLISH NDICATION tx DM P 1 F P response STOP T200 START T203 7 MULTIPLE FRAME ESTABLISHED FIGURE 3 5 SDL representation of SABME response left and corresponding C code right Process LAPD S1 State TEI UNASSIGNED Process LAPD S1 Substate 0 Process LAPD S1 Datal byte 0 SAPI Process LAPD_S1 Data2 byte 200 TEI 200 not assigned Process LAPD S1 Data3 byte 0 VS Send state variable Process LAPD S1 Data4 byte 0 VA Acknowledge variable Process LAPD_S1 Data5 byte 0 VR Receive state variable Process LAPD S1 Data6 byte 0 RC Retransmission counter Process LAPD S1 Data7 byte 0 Able to establish flag Process LAPD_S1 Data8 byte 0
18. switches between the link connection released and established states The next section discusses the data link connection procedures in more detail CHAPTER 2 LINK ACCESS PROCEDURE D 14 TABLE 2 1 Protocol mechanisms in LAP D Function Protocol Mechanism connection establishment SABME and UA frames connection release DISC and UA frames addressing amp TEI and SAPI framing flow control error detection amp FCS test correction sliding window time out REJ frames P F checkpointing SABME and UA frames 2 6 LAP D protocol mechanisms This section will describe how data links are established how information transfer happens and what functions the management entity performs Table 2 6 shows how the most important protocol functions are realized in LAP D These functions are discussed below In the previous section a state diagram of the data link connection endpoints was shown figure 2 5 When looked at from the layer 2 side a more complex state diagram can be seen see figure 2 6 on the next page Nonetheless the four basic states can be seen the top four 1 4 form the link connection released state the lower two 7 8 form the link connection established state The awaiting establishment and awaiting release states have similar names 5 6 A global description of each state is included in figure 2 6 CHAPTER 2 LINK ACCESS PROCEDURE D 15 1 TE TEl removal unassigned TEI assignment E
19. to be disconnected 7 MULTIPLE FRAME ESTABLISHED In this state both kinds of information transfer num bered and unnumbered are possible A datalink is established 8 TIMER RECOVERY The datalink layer entity is recovering from a synchronization problem FIGURE 2 6 State transition diagram of a data link in layer 2 CHAPTER 2 LINK ACCESS PROCEDURE D 16 TEI Unassigned lemas MDL ASSIGN INDICATION ID REQUEST fo ID ASSIGNED Estoblish Awaiting TEI MDL ASSIGN REQUEST TE SABME CMD Awoiting Establishment UA RESP DL ESTABLISH CONFIRM Multiple Frame Established L2 management Layer 2 Layer 3 FIGURE 2 7 Example of the procedure to establish a data link connection 2 6 1 Connection establishment There are several states a data link must go through before it can transfer information As suming the data link is in the TELUNASSIGNED state it must first obtain a TEI value Without this TEI value the data link could not be identified properly Considering layer 2 receives a DL_ESTABLISH_REQUEST primitive from layer 3 the data link will ask man agement for a TEI value MDL_ASSIGN_INDICATION The data link will await a response from management in the ESTABLISH AWAITING TEI state This process is illustrated in figure 2 7 If the user equipment is of the non automatic TEI assignment category management can choose any unoccupied TEI value in the range of 0 63 and
20. Control Register 1 This is a write only read to verify register It can be used to enable disable the transmitter as well as the receiver It is also used to direct the D channel to the HDLC transceiver as the D channel is normally routed to the STBUS interface see figure 5 1 The HDLC Control Register 2 is used to tag bytes that are written into the transmit FIFO Bytes can be tagged as end of packet eop indicating it is the last byte of the packet or as frame abort fa indicating the packet should be aborted after the byte This register reads back the HDLC Status Register It reports the condition of the transmitter and receiver FIFO s full empty and the status of the packet being received top of receive FIFO is first byte of packet last byte of packet packet is good or bad The transmitter and receiver FIFO s are accessible through their corresponding registers HDLC Tx FIFO and HDLC Rx FIFO The HDLC Interrupt Mask Register is used to un mask interrupts generated by the transceiver Reading the HDLC Interrupt Status Register shows which interrupts occurred interrupts are reset after this action The above registers are the most important in relation to the transmission and reception CHAPTER 5 IMPLEMENTATION OF LOW LEVEL DRIVERS 36 of bytes on the D channel The Data Book includes algorithms to accomplish this I have used these algorithms as a basis from which I developed a more advanced method Before describing this met
21. E DISC UA I frame etc must be answered within T200 seconds a LAP D parameter If no response is received in time the command except the I frame is retransmitted Retransmission may occur up to N200 times If after N200 retransmissions still no response is received a connection error is assumed Depending on sev eral variables a data link may try to re establish itself or will return to the TELASSIGNED state 2 6 5 Errors There is a number of errors that may occur during data transportation Those errors that cannot be resolved by the data link layer entity itself are passed to the layer management for processing In Recommendation Q 921 only a handful of error responses is listed most other responses are left for the designer to design and implement Summary In this chapter the ISDN layer 2 protocol LAP D on the user network interface was de scribed The peer to peer frame format has been shown as well as the interaction between layers 1 and 3 and layer 2 management After discussing the four basic states of a data link connection endpoint a detailed view of the behaviour of a data link connection itself was given Three elements were discussed in particular connection establishment information transfer and connection release The next step towards implementing LAP D is creating the appropriate data structures to accommodate the state machine behaviour of LAP D The following chapter will elaborate on this Chapter 3
22. EB 348 CES Implementation of the Link Access Procedure on the ISDN D channel D F van Egmond Department of Electrical Engineering Digital Information Systems Group Eindhoven University of Technology Eindhoven December 6 1991 Supervisor Prof Ir M P J Stevens Coach Ir M J M van Weert The department of Electrical Engineering of the Eindhoven University of Technology does not accept any responsibility regarding the contents of student project and graduation reports Abstract The ISDN layer 2 protocol is called Link Access Procedure on the D channel LAP D It is specified by the CCITT as a state machine Using an oper ating system that can accommodate this state machine behaviour LAP D is implemented This implementation is device independent and will ultimately be used on an ISDN Terminal Board developed by the Digital Information Systems Group Device dependent hardware drivers link the LAP D software to the hardware realizing transmission and reception of data For testing pur poses LAP D is implemented on two PC s using Mitel ISDN Express Cards An interrupt handler serving the Mitel interrupts is designed Most errors of an earlier version of the LAP D software have been corrected Some hard to find errors however still prevent the software from being used Testing is done using a tool called the protocol analyzer This tool enables manipulating and observing specific items states messages of the software
23. Further development of this tool is recommended Testing has further revealed that the chosen implementation can meet all the timing restrictions imposed by the CCITT recommendations in spite the relatively slow system on which the software is installed Acronyms and abbreviations CCITT CEPT DLCI DSC EOP ET FA FCS FIFO HDLC IDPC IES ISDN LAPD LT NET NT OSI PC RxFIFO SAPI SNIC TEI TxFIFO Commit Consultatif International de T l graphique et T l phonique Conference of European Posts and Telecommunication Administrations Data link connection identifier Digital Subscriber Controller End of Packet Exchange Termination Frame Abort Frame Check Sequence First In First Out is a queue operating on that principle High level Data Link Control Integrated Data Protocol Controller ISDN Evaluation Software Integrated Services Digital Network Link Access Procedure on the D channel Line Termination Normes Europ enne de T l communication Network Termination Open Systems Interconnection Personal Computer Receiver FIFO Service Access Point Identifier Subscriber Network Interface Circuit Terminal End Point Identifier Transmitter FIFO Contents 1 Introduction 2 Link Access Procedure D 2 1 ISDN User Network Interface 2 2 LAP D functions td oe 8 Gn ee EN ge e the AE s Bs 2 3 LAP D frame format 2 4 Modes of oper
24. PH DATA INDICATION 1 UA Dispatcher cycle 2237 rev_packet 3 bytes 1600 0 PH DATAINDICATION 1 UA UAz Dispatcher cycle 2237 rcv_packet 3 bytes 1600 0 1 UA UAz UAs Dispatcher cycle 2237 Stoptimer T200 uas 2600 0 1 UA gt UAz Dispatcher cycle 2237 Stoptimer T2002 ua_B 2600 0 1 UAs Dispatcher cycle 2237 Stoptimer T2003 ua5 2600 0 1 It is now possible to calculate the response times i e the time between each tx SABME and receiving a UA The results are listed in table 6 2 Although these results are inaccurate an important conclusion may be drawn from them the implementation is able to handle bursts of short packets A burst consists of at most 3 packets since there are only three data link layer entities After transmitting a packet each entity will await a response TABLE 6 2 Response times of three consecutive data link layer setup procedures estimation entity response time 1 76 3 ms 2 73 0 ms 3 70 0 ms Another interesting analysis concerns information transfer In this case three data link layer entities transmit an information frame of maximum size Again the analysis is done by summing up all data link layer actions Transmitting a frame is considered to take 16 5 ms which is calculated from the D channel transmission speed I will not include the complete analysis but only the results The results show that all three transmitted Iframes are re sponded in one second The minim
25. PRI DinB PSC DCR Deck FIGURE 5 1 Functional block diagram of the Subscriber Network Interface Circuit SNIC 34 CHAPTER 5 IMPLEMENTATION OF LOW LEVEL DRIVERS 35 TABLE 5 1 HDLC Transceiver frame format DATA FIELD one n bytes two one byte n gt 2 bytes byte TABLE 5 2 Description of the SNIC registers _ Address Write Read A Obod Master Control Register verify o i S 0xb01 ST BUS Control Register verify Y 0xb02 HDLC Control Register 1 verify N 0b03 HDLC Control Register 3 HDLC Status Reger C bo HDLC Interrupt Mask Register HDLC Interrupt Status Register ES HDLC Tx FIFO DLC Rx FIFO 0x608 HDLC Address Byte p Register verify 0x507 HDLC Address Byte 2 Register verify S Oxb08 O channel register DST Cabana o Y 02508 DSTO C channel Channel Status Register 7 N H0xb02 S Bus Tx D channel DST D chann 7 ee 0x600 S bus Tx Bi channel DSi Bi channel 0x0 DST Bi chamnel S Bus Rx Bechan 0x60e S Bus Tx Bz channe DST Bachan 0601 DSTo B2 channe Sbus Rx B2 channel 7 illustrated in figure 2 2 A valid frame should have a data field of at least 16 bits The HDLC transceiver is controlled through several registers They are listed in table 5 2 For more information see 6 or appendix A Most of the asynchronous registers ASYNC are relevant to data communication on the D channel First there is the HDLC
26. Table J messagea functiona messageb function b message_c function c DEF dummy y FIGURE 3 6 Declaration of the state structure the message Figure 3 6 shows the necessary declarations The state table of a data link layer entity in the TELASSIGNED state would look as follows state TEI_ASSIGNED DL_ESTABLISH REQUEST dlestreq_4 E DL_RELEASE REQUEST dlrelreq 4 DL_UNIT_DATA_REQUEST dlunitdatareq4 UI_FRAME_IN_QUEUE uiframe 4 MDL_REMOVE REQUEST mdlerrresp_2 SABME sabme 4 DISC disc 4 On PRA AR A AAA DEF dummy y Every state table must terminate with the entry DEF dummy This entry will guard the process against messages that are not allowed in the current state by executing a dummy empty function should such a message occur see figure 3 6 Besides the three data link layer entities several other processes were created They are summed up below 1 LAPD S1 2 LAPD_S2 CHAPTER 3 LAP D IMPLEMENTATION 26 LAPD_P A MANAGEMENT TRANSMISSION an 6 MONITOR 7 layer 3 processes The LAPD processes are implemented using the SDL diagrams The MANAGEMENT pro cess is derived from the text of Q 921 It is constantly in one state Through this process TEI assignment and removal are realized as well as the other functions of the layer 2 management entity described in the previous chapter The TRANSMISSION process is the link to the hardware Besi
27. abled until the reception of the next opening flag It is now possible to design an algorithm to receive packets through the microprocessor interface The algorithm offered by the data book is shown in figure 5 2 left The upper dashed box shows the loop where the software waits for the beginning of a packet If a byte is received the Rx FIFO will not be empty it is checked if it is indeed the first byte of a packet If it is the next byte is read If it is the last byte the packet is tested on its FCS If it isn t the last byte more bytes will be read when the Rx FIFO is not empty Should during this receiving process the Rx FIFO run empty a waiting loop is entered lower dashed box Please note that first byte last byte and good FCS conditions apply to the byte that is in front in the FIFO This explains the extra read RxFIFO in figure 5 2 right after the good FCS test The dashed boxes show where the algorithm uses the polling technique to wait for a Rx FIFO not empty situation This is very time consuming I have chosen to use the generated interrupts to skip the waiting parts of the algorithm Four of the interrupts are used The RxFF and EOPD interrupts are a signal to start reading the receive FIFO In the RxFF case the receiver is still receiving bytes and an Rx FIFO empty situation may occur while reading the FIFO Reading the FIFO is suspended until the next RxFF or EOPD interrupt in the EOPD case a comp
28. al response time was shown to be greater than 0 6 s Theoretically the implementation of the data link layer should function properly When CHAPTER 6 TESTING 54 layer three functions are added and activated response times should not increase because layer 3 is assigned a lower priority than layer 2 including the TRANSMISSION process When the above two situations were put into practice the results were different Starting three data link layer connections was no problem After a few minutes however one by one the data link connections were released The reason for this has yet to be discovered The error might be found in buffer queue management although in some cases a SNIC interrupt remained unserviced thus suggesting the error to be found in the interrupt handler The reason why the error has not been found is the fact that every error situation showed a different probable cause Summary A tiny set of tests has been done on the software The results are encouraging but not perfect Transmission of large packets is still a problem an essential one when looked at from layer 3 since layer 3 makes use of this facility Two brief performance tests revealed that the implementation can very well fullfill the timing restrictions imposed by LAP D Unfortunately a practice run with the protocol analyzer revealed a serious error The software is not capable of maintaining a data link connection for a long period of time It is therefore far f
29. anagement procedures Reserved Others TABLE B 4 TEI definitions TEI value User type 0 63 Non automatic TEI assignment user equipment Automatic TEI assignment user equipment 127 Broadcast TEI TABLE B 5 Commands and responses ENCODING 2 3 4 56 7 8 N S N R 62 APPENDIX B LAP D FORMATS TABLE B 6 Use of P F bit P F function Command Poll P 1 solicits a response from the peer Response Final F 1 indicates answer to a poll TABLE B 7 Management message structure 8 7 6 5 4 3 2 1 N Ww ot TABLE B 8 Codes for messages concering TEI management procedures Message name Management Reference Message Action indicator Ai entity number Ri type identifier Identity request Ox0F 0 65535 0x01 Ai 127 any TEI value user to network acceptable Identity assigned 0x0F 0 65535 0x02 Ai 64 126 assigned TEI Ai 64 126 denied TEI value Identity 0x0F 0 65535 0x03 denied network to user Ai 127 no TEI value Identity check re 0x0F Not used 0x04 Ai 127 check all TEI val coded 0 quest network to ues user Ai 0 126 TEI value to be Identity check re Identity remove 0x0F network to user Identity verify user to network Not used 0x06 Ai 127 request for re coded 0 moval of all TEI values Ai 0 126 TEI value to be CIN 0x0F Not used 0x07 coded 0 Ox0F 0 65535 0
30. and send the closing flag after the last byte is sent As the transmit FIFO is being emptied the HDLC transceiver will generate a TxFL interrupt as soon as only 4 bytes remain in the FIFO If the packet is at most 4 bytes long this interrupt will not be generated It is only generated when there have been at least 5 bytes in the FIFO The TxFL interrupt is a signal to refill the FIFO if necessary This is why a Transmit Buffer Byte Count variable is used If non zero the packet is not yet completely transmitted If zero the packet has been transmitted and the TxFL interrupt can be ignored As soon as the FIFO runs empty and the last byte in the FIFO was marked as the last byte of a packet a TEOP interrupt is generated Should the FIFO run empty and expect more bytes a TxFun interrupt is generated and the transmitter should be reset In both cases the transmit variables are reset and the TRANSMISSION process s state returns to IDLE Another packet can now be transmitted CHAPTER 5 IMPLEMENTATION OF LOW LEVEL DRIVERS 40 gt N FIGURE 5 3 Transmit packet algorithm with the use of interrupts 5 5 Interrupt handler The interrupts are serviced in an interrupt handler On the following page Figure 5 4 sums up the two previous sections resulting in an outline of the interrupt handler CHAPTER 5 IMPLEMENTATION OF LOW LEVEL DRIVERS while HDLC Interrupt Status register not 0 if RxFov o
31. ation 2 5 Layer 2 interfaces 4 24 eue demie ee 8e een e 2 6 LAP D protocol mechanisms 2 6 1 Connection establishment 2 6 2 Acknowledged information transfer 2 6 3 Connection release 264 Timing restTictions se ue E Sie Dre de fre te te AS 2 6 5 CELOS de ho dise SB So A N E Se Se 3 LAP D implementation Sl Messages 2 475 AA A BAAS Ba Me Le dE 3 2 PTOCESSOS orar Ee aca EA RA A al as fes NA A 3 3 The dispateher s mu men dus dan Re a a BL 3 4 Interrupts 2 45 eee ca a elbow ie ee Soe ls NO ced ie at 4 Hardware aspects 4 1 ISDN Express Card Kit 4 1 1 Hardware overview and block diagram 4 1 2 ISDN Evaluation Software System 4 2 ISDN Terminal Board 5 Implementation of low level drivers Sil SNICG access ia A A a E HE a A E 5 2 HDLC transceiver interrupts 5 3 Receiving Packets s s yeu de a A LR ne Pie eas 5 4 Transmitting packets 5 5 Interrupt handler 4 Ei AD Gen ed re OA RS ie a a 5 6 Timer interrupts CONTENTS 6 Testing 6 1 Limited conformance test 6 2 Performance test 4 4 4 7 Conclusions Appendices A SNIC
32. cates that the Tx FIFO is empty without being given the end of packet indication The HDLC will transmit an abort sequence after encountering an underrun condition Receive FIFO Full Indicates that the HDLC controller has accumulated at least 15 bytes in the Rx FIFO Receive FIFO Overflow Indicates that the Rx FIFO has overflown i e an attempt to write to a full Rx FIFO The HDLC will always disable the receiver once the receive overflow has been detected The receiver will be re enabled upon detection of the next flag In order to make use of the interrupt mechanism of the SNIC chip several registers must be initiated correctly on the Mitel card e HLDC Interrupt Mask register interrupts should be unmasked e Master Control register enable RQ NDA pin for HDLC interrupts on the PC e Interrupt Mask register of 8259 programmable interrupt controller unmask hard ware interrupt number 7 IR7 e Redirect software interrupt number 15 which is generated upon IR7 to the SNIC interrupt service routine CHAPTER 5 IMPLEMENTATION OF LOW LEVEL DRIVERS 37 These actions are implemented in the function inithardw An interesting but annoying problem occurred trying to program the Master Control register bit B2 should be set to 0 according to the data book to enable the NDA IRQ pin for HDLC interrupts However tests on the PC revealed that no interrupt 15 would occur when an EOPD interrupt occurred on the Mitel card A lo
33. ces of the interrupts I will name a few e the receiver has detected a flag on the D channel indicating the start of a packet e the receiver has been receiving a packet and has now filled its relatively small buffer e the receiver has detected an end of packet flag e the transmit buffer is running empty e the transmitter is signalling it has transmitted a complete packet The interrupt handlers servicing one of the above requests generate if necessary messages for the TRANSMITTER process This process is described further in chapter 5 An entirely different type of low level software concerns timers Timers are necessary in the LAP D communication process as explained in section 2 6 4 In the current implementation each timer tick generates an interrupt after which the active timers are updated An expiring timer generates a TIME_OUT message for the process that initiated the timer CHAPTER 3 LAP D IMPLEMENTATION 29 Summary An overview of the LAP D implementation was given It is best illustrated by figure 3 8 An operating system was created by Oudelaar 8 to accommodate the LAP D state machine It regards data link layer entities as processes communicating by messages We have also seen that a dedicated dispatcher is in control The behaviour of the data link layer entities was translated from SDL diagrams into state tables The link with the hardware elements was shown to involve interrupts and interrupt handlers The behav
34. d Response bit definition 61 SAPDdennitians lt Meta sean PH o SAO Bes ele 4 nt 62 TEI definitions s une Be ee E ee eee gS dns 62 Commands and responses 62 Use ot P F bite oa a EA a ew ee Pa Pe Se ed ag 63 Management message structure 63 Codes for messages concering TEI management procedures 63 List of used LAP D parameters 64 Chapter 1 Introduction Until the mid 1970s telecommunication services were limited to voice and written commu nication During the last 15 years or so new demands on telecommunication services have arisen such as videotext fax communicating microcomputers etc To meet a variety of needs by a large number of specialized networks has certain disadvantages for both the cus tomer and the service providing companies These disadvantages involve mainly high costs and inefficiency 5 As aresult of advances in technology digital techniques are being intensively introduced in many countries The result has been a general consensus in the world of telecommunication to lay down through the International Telegraph and Telephone Consultative Committee CCITT the basis elements of a universal network the Integrated Services Digital Network ISDN The main feature of the ISDN concept is the support of a wide range of voice and nonvoice applications in the same network A key element of se
35. des frame assembly and disassembly the process initiates the transmitter on the S T interface and can receive frames from the receiver on the S T interface This process is discussed in detail in chapter 5 The MONITOR process can be used to monitor the status of the data links It was originally introduced by Oudelaar 8 to be used on the ISDN terminal board I have not used this process To monitor the data links I made use of the protocol analyzer written by Lemmens 7 The analyzer is discussed in a later chapter 3 3 The dispatcher In figure 3 2 where a schematic view of the operating system is shown the dispatcher can be seen to control the message queue The dispatcher is always active Its structure is shown in figure 3 7 It checks the message queues for messages starting with the highest priority queue Once the dispatcher finds a message in one of the queues the destination process and current state of that process will automatically have been determined by getmsg If the destination process is running not Blocked the corresponding state table is searched for the message s nucleo The function that is found is executed When no entry for the nucleo is found a dummy empty function is executed Next the message queue is updated and the priority is set to 0 to start the next scan for messages with the highest priority If the destination process is Blocked the message is discarded by updating the queue This procedure will th
36. e frame it expected N S V R The information field in the frame is discarded and no acknowledgement takes place If the frame is otherwise error free N R is used as acknowledgement as explained above A REJect frame is transmitted to initiate recovery of the correct frame sequence The side that receives the REJ frame will use the N R in the REJ frame as the next send sequence number V A is also set to N R which indicates the last frame that was received correct The requested frame N R is then retransmitted as soon as possible as well as any other frames with a sequence number greater than N R go back n principle N R sequence error This error occurs when N R is not in the range of V A V S If N R lt V A a frame is acknowledged for the second time If N R gt V S a frame is acknowledged that never has been sent The information field of the received frame if in sequence is passed on to layer 3 An N R sequence error causes the data link layer entity to initiate re establishment of the data link 2 6 3 Connection release A connection is released in much the same way as it is established see figure 2 9 Instead of a SABME command a DISConnect command is used The data link will wait in the AWAITING_RELEASE state until a UA frame is received After release the data link layer entity is in the TELASSIGNED state CHAPTER 2 LINK ACCESS PROCEDURE D 19 2 6 4 Timing restrictions Every transmitted command SABM
37. e window size is 3 Information from layer 3 in a DL_ DATA REQUEST primitive is transmitted in a layer 2 I frame An I frame can be transmitted only when VS lt V A k i e when less than k frames are outstanding On transmission of an I frame the control field parameters N S and N R are assigned the values of V S and V R respectively On reception of an I frame with N S equal to the current V R the information field of the received frame is passed to layer 3 using the DL DATA INDICATION primitive Furthermore V R is incremented by one The I frame is acknowledged with another I frame or Supervisory frame with N R set to V R Acknowledgement occurs upon reception of a valid I frame or Supervisory frame The acknowledging entity included N R in the control field of the response frame saying it next CHAPTER 2 LINK ACCESS PROCEDURE D 18 Multiple Frame Established DL RELEASE REQUEST DISC CMD Awaiting Release UA RESP DL RELEASE CONFIRM TEI Assigned Le management Layer 2 Layer 3 FIGURE 2 9 Example of a procedure to release a data link connection expects a frame with sequence number N R The peer entity treats N R as the acknowl edgement for all the I frames that have been transmitted with a send sequence number N S up to and including the received N R 1 The acknowledge state variable is set to N R Sequence errors N S sequence error In this case the receiver did not receive th
38. en repeat itself The dispatcher can be integrated into a larger structure as has been done in the case of the protocol analyzer Each message that is read by the dispatcher is displayed on the screen providing information on the status of the data links An important condition is that the dispatcher s body is executed sufficient times to guarantee continuation of the various processes controlled by the dispatcher CHAPTER 3 LAP D IMPLEMENTATION dispatcher byte Pri word AuxComp struct state far TablState Pri 0 High priority while 1 if getmsg Pri Present message present if destination process Blocked look_up nucleo_in_state_table TablState gt transi execute function updatequeue Pri discard message Pri 0 else destinationprocess blocked updatequeue Pri discard message else no message present Pri Pri MAXPRIO increment priority endless while loop FIGURE 3 7 General structure of the dispatcher 27 CHAPTER 3 LAP D IMPLEMENTATION 28 3 4 Interrupts For layer 2 to communicate with its peer it must transmit and receive frames over the S T interface The link with the transmitter and receiver hardware is laid through interrupt handlers that control the hardware These interrupt handlers are different for each hardware configuration the ISDN software is installed on What can be the sour
39. er to gain a better and more substantial insight in the performance of the implemen tation C code execution time has been measured For this reason special assembler code was used This code known as the ZTimer was found in 1 It enables accurate up to 1 us measurements of code performance It makes use of the built in timers of the IBM PC There are two restrictions the ZTimer imposed first interrupts should be disabled inside the code segment to be measured Second the code could not take longer than 54 ms to execute if not the measured time will read 1 There is a ZTimer version without this last restriction but all segments I needed to measure were shorter than 54 ms The general use of the ZTimer is as follows ZTimerOn Start timer code to be measured ZTimerOff Stop timer time ZTimerReport Calculate used time not counting overhead Most functions have separately been timed with the use of ZTimer in combination with Turbo Debug The results are listed in table 6 1 The first column lists the functions that were measured tx SABME for example takes 1620 ys to execute under the condition that a free buffer is found at once If this is not the case the execution times measured under this condition rise slightly Looking at the figures we see that as the information field in creases and thus the number of bytes to be sent execution time tends to rise sharply It is unfortunately impossible t
40. ere put into an initial state and on one side a message was generated The ISDN software is meant for the TE side of the configuration It is however possible to modify the code so the software can be used on the NT side as well layer 2 peer to peer only This involves the coding of the Command Response bit The use of this bit is as follows Command Response direction C R value Command network gt user 1 user gt network 0 Response network gt user 0 user gt network 1 CHAPTER 6 TESTING 48 entities can be manipulated Using two TE s users implementing the above convention the receiver not an NT but as TE will always receive a command when a response was sent and receive a response when a command was given The solution is to invert the received C R bit on both sides It should be noted that in case the TE is connected to a real NT this inversion is not necessary The following tests were successfully completed the first primitive or command was gen erated by the tester Multiple Frame Establishe y DL RELEASE REQUEST DISC CMD Awaiting Release UA RESP DL RELEASE CONFIRM TEI Assigned Peer Le management Layer 2 Layer 3 FIGURE 6 4 Test for data link layer release CHAPTER 6 TESTIN G 49 El Unassigned 01 ESTABLISA REQUES NDL ASSIGN INDICATION yD REQUEST Establish Awaiting TE 1D ASSIGNED NDL ASSIGN REQUEST TEI
41. estination process Mes message Dest destination process Orig originating process Prio priority of message Par information byte getmsg byte Prio This function checks the queue for messages and returns the message that is in the front of the queue Prio indicates which queue should be checked getmsg returns a byte pointing to a message buffer holding the actual message elements The message is not removed from the queue it is just read This is done in order to prevent the buffer to be replaced by a new message destroying the current contents void updatequeue byte Prio This function is called by the dispatcher after a message has been processed At that time the information in the message buffer is used and the buffer no longer needed updatequeue does the required removal by updating the queue pointers where Prio indicates which queue is to be updated 3 2 Processes The messages described in the previous section activate the various processes These pro cesses each representing a data link layer entity all have an associated process descriptor This descriptor holds vital information about the process and is saved each time a process is deactivated All process descriptors are held in an array where the indexes identify the processes This is illustrated in figure 3 4 CHAPTER 3 LAP D IMPLEMENTATION 23 define unsigned char byte define MAXPRIO maximum priorities of process 0 define MAXPROC
42. gic analyzer was therefore applied which showed a 125s clock signal This is what the data book describes as the New Data Available NDA signal which should only appear on the NDA J RQ pin when bit B2 is set to 1 My suspicions were then confirmed the programming of bit B2 should be exactly opposite a 1 for interrupt functioning a 0 for NDA functioning 5 3 Receiving packets When the HDLC receiver is enabled it can receive incoming packets An incoming packet is examined on a bit by bit basis the FCS is calculated and the data bytes are written into the 19 byte receive FIFO However the FCS and other control characters i e flag and abort are never stored in the receive FIFO All the bytes written to the receive FIFO are flagged with two status bits The status bits are found in the HDLC status register and indicate wether the byte to be read from the FIFO is the first byte of the packet the middle of the packet the last byte of the packet with good FCS or the last byte of the packet with bad FCS This status indication is valid for the byte that is to read from the receive FIFO The incoming data is always written to the FIFO in a byte wide manner Receive overflow occurs when the receive section attempts to load a byte to a full receive FIFO All attempts to write to the full FIFO will be ignored until the receive FIFO is read When overflow occurs the rest of the present packet is ignored as the receiver will be dis
43. hod I shall first introduce the interrupts generated by the HDLC transceiver I will then describe the method to receive packets and transmit packets respectively 5 2 HDLC transceiver interrupts The SNIC chip on the ISDN Express Card is able to generate eight maskable interrupts These occur upon a certain condition in the HDLC transceiver These interrupts are GA EOPD TEOP FA TxFL TxFun RxFF RxFov Indicates that a go ahead sequence has been detected on the received HDLC D channel I will not use it End of Packet Detected Indicates that an end of packet has been detected on the HDLC receiver The packet can either be a good packet an invalid packet or an aborted packet Transmit End of Packet According to the Data Book this indicates that the receiver has finished sending the closing flag of the last packet in the Tx FIFO Note If the transmitter is disabled immediately after the TEOP interrupt occurs the receiving party will receive an aborted packet Clearly the TEOP interrupt occurs sometime before the transmitter sends the closing flag Advise do not disable the transmitter after a TEOP Frame Abort Indicates that the receiver has detected a frame abort sequence on the received data stream Transmit FIFO Low Indicates that the device has only 4 bytes remaining in the Tx FIFO This bit has significance only when the Tx FIFO is being depleted and not when it is getting loaded Transmit FIFO Underrun Indi
44. inal board upon completion The link between the Mitel card and the LAP D software will be explained in the next chapter Chapter 5 Implementation of low level drivers This chapter discusses the implementation of the low level interrupt handlers in reference to the operating system in figure 3 2 These interrupt handlers together form the TRANSMIS SION process which is the link to the Mitel card hardware The TRANSMISSION process can access the SNIC circuit and so control the D channel access 5 1 SNIC access Figure 5 1 shows the functional block diagram of the SNIC The microprocessor interface allows complete control of the HDLC transceiver and access to all data control and status registers The HDLC transceiver handles the bit oriented protocol structure and formats the D channel as per level 2 of the X 25 packet switching protocol defined by CCITT It transmits and receives the packetized data information of control serially in a format shown in table 5 1 The data field refers to the Address SAPI TEI Control and Information fields D channel priority mechanism LTx S Bus link interface DSTi Vbias STBUS interface LRx DSTo F0od HDLC transceiver C4b Link activation FOb and controller VDD control STAR Rsto NT TE Microprocessor interface E HE NE E E E Rsti Cmode HALF ADO 7 R W WR DS RD AS ALE ts TRQ NDA AFT
45. ing is comparable to testing a sequential state machine in hardware in regard to the number of different input output combinations In 3 a total of 238 different transition identifiers is recognized During some random tests some persistent and unexplainable errors occurred The subse quent debugging did cost considerable time preventing me from actually performing thorough testing What I did do however was test if the basic functions were working This will be discussed in the next sections Additionally I will give an answer to the question raised in the introduction about the performance of the LAP D software using measurements of the code s execution time 6 1 Limited conformance test Frame level In this section the interrupt handlers for receiving and transmitting frames are tested The test configuration is shown in figure 6 1 The tests and their results are listed below Though the tests are meant to check the transmission process they implicitly include some functional aspects as to the response of the TE 44 CHAPTER 6 TESTING 45 tester NT TE S T Mitel IES ISDN software FIGURE 6 1 Test configuration for testing transmission and reception of packets On the left PC the Mitel IES software is used to transmit and receive packets to from the PC on the right where the ISDN software is installed The ISDN software is running under Turbo Debugger to ease examination of variables and monitoring program progress Tes
46. iour of LAP D is fully documented by the CCITT In order to discuss the behaviour and more important the implementation of the lower level handlers we must first explore the associated hardware This is done in the next chapter after which the implementation of the lower level handlers can be discussed e Dos C layer 3 processes network layer 000 Gate ticks em message queues T dispatcher TRANSMITTER low level coding decoding handlers l data link layer multiplexer physical layer FIGURE 3 8 The basic data link layer entities shown as processes the circles The service primitives are exchanged as messages via the message queues The dispatcher keeps track of the queues and delivers the messages to the processes D channel B channels layer manage ment Chapter 4 Hardware aspects The software described in this report will eventually be implemented on the ISDN Terminal Board developed in the Digital Systems Group The software is first tested on the Mitel ISDN Express Card This chapter will describe the two systems 4 1 ISDN Express Card Kit The ISDN Express Card Kit with ISDN Evaluation System IES software is an integrated hardware software package giving easy access to several ISDN reference points The hardware can handle low level protocol functions while the software can control all of the components on the card 4 1 1 Hardware overview and block diag
47. lete packet has been received and can be read from the FIFO The RxFov and FA interrupts are a signal to abort the current packet In the RxFov case one or more bytes have been lost due to a full FIFO In the FA case a frame abort sequence has been received The algorithm in figure 5 2 right shows the actions to be taken when an RxFF or EOPD CHAPTER 5 IMPLEMENTATION OF LOW LEVEL DRIVERS 38 enable receiver amp A AA read byte status reed RxFIFO Reset buf ter read RxFIFO clr Rev busy valid packet FIGURE 5 2 Receive packet algorithm using polling left and interrupts right interrupt occurs A status indicator Rcv_busy has been added to indicate that a packet is being read Thus when reading a byte that is not the beginning of a packet a test should be made to see if Rcv_busy is true In that case the byte and its successors are part of a packet of which the first part 2 or more bytes has already been read If Rcv_busy is false an error has occurred and it is best to reset the Rx FIFO The function readRxFIFO reads the receive FIFO and places the byte into a receive buffer Once a packet is received it is passed on to the TRANSMISSION process This is done by generating a PH DATA INDICATION message in which the packet s buffer pointer is passed along The message is put in the highest priority queue 0 Please note that the use of the PH DATA_INDICATION primitive i
48. maximum rate Would the operating system impose any restrictions as to the speed of the data transportation These questions will be answered in chapter 6 where the implementation performance is examined The next chapter will give the reader a brief introduction to the ISDN user network interface and the ISDN layer 2 protocol in particular Chapter 3 discusses the implementation of the layer 2 protocol independent of the hardware configuration Chapter 3 does not require the knowledge of the layer 2 protocol Chapters 4 and 5 go into detail about the hardware configuration that was used and the corresponding software In order to fully understand chapter 5 it is advisable to read chapter 4 first Chapter 6 will then look at the real time performance of the implementation The conclusions are summarized in chapter 7 along with a few words on future work Chapter 2 Link Access Procedure D This chapter describes in detail the ISDN protocol for the Data Link layer the Link Ac cess Procedure on the D channel LAP D Additional information can be found in CCITT recommendations Q 920 and Q 921 9 2 1 ISDN User Network Interface The ISDN user network interface is located at the S T reference point illustrated in fig ure 2 1 Two access scenario s are currently defined on this point Basic access An interface at a usable rate of 144 Kbit s It supports two B channels at the rate of 64 Kbit s and one D channel at the rate of 16 Kbi
49. o and from layer 1 concern layer 2 frames that are transmitted and received Layer 3 not only passes down its own information frames but also commands to layer 2 These commands control the data links Communication with management includes TEl assignment and error report Figure 2 4 shows the layer 2 inter faces and the possible primitives Layer 1 communicates lower left with layer 2 through the PH_DATA primitives This concerns layer 2 frames that have been received or are to be trans mitted Layers 2 and 3 exchange similar primitives now concerning layer 3 frames upper left The second group of primitives exchanged between layers 2 and 3 concern the initiation and termination of a data link upper right This leaves the primitives exchanged with layer 2 management Here there are three types of primitives those concerning management infor mation frames MDL_DATA those concerning TEI assignment or removal MDL_REMOVE MDL ASSIGN and those concerning error handling MDL_ERROR These primitives are conceptual and are not necessarily implemented They provide the procedural means to specify conceptually how a data link user can invoke a service Through the use of primitives a point to point data link layer connection endpoint can be triggered from one state to the other A data link layer connection endpoint can be in one of four basic states illustrated in CHAPTER 2 LINK ACCESS PROCEDURE D 13 DL RELEASE INDICATION DL RELEASE
50. o be removed 03 OF 00 00 04 FF Results Using step by step tracing after the breakpoint was reached correct re ception of the bytes was revealed The command was also correctly directed to the management process On the tester the expected sequence was received Test 4 Conditions NT and TE are connected and in the Active state concerning bus ac tivity The TE should be in the MULTIPLE FRAME ESTABLISHED state Layer 3 is assumed sending a maximum size information packet Tester NT TE I frame maximum size Results Sometimes a three byte frame was received on the tester instead of a 264 byte frame The error was that no provision had been made for back tracking along the transmission queue This has been corrected Sometimes a randomly positioned byte is received twice increasing the length of the frame by one The error could be either at the transmitter or the receiver side Debugging showed it must be an interrupt handler problem or hardware problem The error has not yet been corrected CHAPTER 6 TESTING 47 The tests show that information transfer is only guaranteed for short packets Not all packet sizes have been tested Fortunately most commands and supervisory frames are very short and are transmitted correctly Protocol level The results of the previous section enable us to continue with the functional aspect of the test as long as only small packets are involved For
51. o gain a more precise insight by experiment The unpredictable occurrence of interrupts which may not be disabled prevent using the ZTimer A theoretical approach where the timers are not considered and transmission speed is assumed equal to the speed on the D channel 16 Kbit s is used from here on One worst case situation will show if the implementation is able to deal with a burst of short packets Assume three data link layer entities start to set up a data link connection at the same time As soon as each data link layer entity executes tx SABME time becomes a crucial factor since a response must be received in one second 1 will sum up all actions by the ISDN software on both sides of the connection to obtain an indication of the duration of the entire procedure CHAPTER 6 TESTING 51 TABLE 6 1 Duration of basic functions measured by ZTimer Measured on an IBM PC running on 4 77 MHz Functions implemented with Turbo C 2 0 In some functions the interrupt enable disable commands were disabled Function Duration us Condition getmsg none found 16 getmsg msg found 637 updatequeue empty queue 14 updatequeue queue used 96 message room in queue 997 message queue full 117 tx SABME 1620 getbuff finds a free buffer at once tx RR 1711 tx DISC 1696 tx UA 1726 tx I_FRAME max inf field 40846 tx UI_FRAME max inf field 41508 tx DM 1578 tx RNR 1681 tx REJ 1636 tx FRMR
52. or future services can be implemented and tested A dedicated operating system has been developed by Oudelaar 8 It accommodates the ISDN lower layer protocols extremely well An important advantage of the operating system CHAPTER 1 INTRODUCTION 7 is that the protocols can be implemented hardware independently The only link to the hardware are the low level interrupt handlers that communicate with the operating system To assist with the implementation of the ISDN protocols the Digital Systems Group is in the possession of two Mitel ISDN Express Cards These make it possible to establish a 2B D connection S interface between two personal computers PC s Using this connection and the D channel layers 2 and 3 can be implemented and tested When the implementation is completed the result can be copied onto the ISDN terminal board with only minor modifi cations Layer 3 the Network layer has been implemented by Lemmens 7 Layer 2 the Data Link layer was only partially functioning In this report it is described how the layer 2 implementation is completed starting with the work of van de Kuilen 12 Besides various errors in the software the layer 2 software could not communicate properly with layer 1 Low level drivers had to be added to make data transportation possible A question that arose during the project involved the performance of the implementation What restrictions or problems would arise as LAP D would perform at its
53. priate action as specified by LAP D is taken 2 6 2 Acknowledged information transfer This section only applies to I frames because U frames require no special treatment I frames however are transmitted with several control field parameters Through these parameters flow control is realised LAP D makes use of the sliding window protocol go back n The parameters and the associated variables are listed below Figure 2 8 shows how the control field is made up Encoding of bits 1 23 4 5 6 7 8 fof OnO FIGURE 2 8 Control field parameters of an I frame N S Send sequence number send sequence number of transmitted I frames It is included in each transmitted I frame V S Send state variable the sequence number of the next I frame to be transmitted V A lt V S lt V A amp V A Acknowledge state variable identifies the last frame V A 1 that has been acknowl edged by its peer N R Receive sequence number V A lt N R lt V S expected send sequence number of next received I frame It is included in each transmitted I frame V R Receive state variable denotes the sequence number of the next in sequence I frame expected to be received k determines the size of the sliding window Sequence numbers are numbered modulo 128 For information about the sliding window mechanism see 11 LAP D uses a window size of 1 in the case the data link is used for signalling When used for packet data th
54. process The TRANSMISSION process can be in one of two states IDLE or BUSY These states only influence the transmit procedure not the receive procedure In the BUSY state the transmitter is currently transmitting a packet The PH_DATA_REQUEST message is then regenerated to be processed in another dispatcher cycle through the message queues This does not affect the sequence of the packets as the PH DATA REQUEST message only indicates that a packet is available for transmission The sequence of the packets is entirely determined by the order in which they appear in the transmission queue If the TRANSMISSION process is in the IDLE state packet transmission can be started First the packet in front of the transmission queue is fetched Then the function xmt_buff is called It initializes several transmit variables address of the first byte of the packet number of bytes in the packet disables the transmitter and fills the transmitter FIFO of the HDLC transceiver The FIFO can hold 19 bytes The transmitter is then enabled and the remaining bytes are sent by the interrupt handler Filling the transmitter FIFO is accomplished using the algorithm in figure 5 3 Bytes are written to the FIFO as long as it isn t full If it is full the transmitter is enabled and the algorithm is suspended until the next call Before writing the last byte to the transmit FIFO it should be tagged as end of packet This will tell the transmitter to calculate the FCS
55. r FA reset Rx FIFO clear Rev busy Js if RxFF or EOPD if first byte OR Rcv_busy set Rev_busy while Rx FIFO not empty read Rx FIFO if last byte if good FCS read last byte from Rx FIFO clear Rev busy pass information to LAP D else read last byte from Rx FIFO clear Rev_busy F exit else reset Rx FIFO TFL if Transmit Buffer Byte Count gt 0 fill FIFO with bytes TEOP reset transmit variables TRANSMISSION newstate IDLE TFUN reset Tx FIFO reset transmit variables TRANSMISSION newstate IDLE out 0x20 0x20 end of interrupt signal to interrupt controller FIGURE 5 4 Outline of the interrupt handler 41 CHAPTER 5 IMPLEMENTATION OF LOW LEVEL DRIVERS 42 void far timecheck void word timer for timer 0 timer lt MAXTIMER timer if Timer timer Used USED timedec timer void timedec word timer if Timer timer MainTimer 0 Send_Time out_message_to_ process Timer timer Used FREE FIGURE 5 5 Timer interrupt handler 5 6 Timer interrupts LAP D makes use of several timers to ensure correct data transfer These timers are listed in appendix C Implementation of these timers is accomplished through software interrupt 0x1C which happens once every 54 95 ms on an IBM XT The timer interrupt handler illustrated in figure 5 5 checks which timer
56. ram The hardware includes components for the basic access reference points S amp U primary rate reference points T1 and CEPT a digital telephone set component with speakerphone ca pability and devices for digital switching clock generation and synchronization and low level protocol functions Figure 4 1 shows a block diagram of the card The Digital Crosspoint switch in the middle can interconnect any of its ports allowing for several different config urations In this case a connection between the S interface and the Digital Phone will be used see the user manual 13 on how to make such a connection The connection is illus trated by a dashed line The clock generator and DPLL provide necessary timing signals to synchronise all the hardware and optionally the interfaces on the various reference points All four reference points on this card are connected to specialized circuits that perform layer 1 and part of layer 2 functions The most important device in relation to LAP D is the device connected the S interface namely the Subscriber Network Interface Circuit SNIC It is a device that implements the CCITT 1 430 Recommendation for the ISDN S and T reference points The SNIC may be used at either side of the subscriber line NT or TE Some of its features are e point to point point to multipoint and star configurations e full duplex 2B D e on chip HDLC D channel protocoller LAP D is a subset of HDLC see section 2 2 30
57. rmance is still out of the question The reason for this can be ascribed to difficulties in the transmitter process of layer 2 and to the lack of testing and correcting any errors of LAP D In order to find the errors I suggest improving an important tool the protocol analyzer More parts of the software status of relevant variables should be observable A facility through which test patterns preferably NET3 can be applied to LAP D would greatly aid conformance testing These improvements should always be evaluated as to the pressure they cause on the timing requirements and restrictions of the protocol When the layer 2 software can at last be released the time comes to test layer 3 Testing layer 3 in a similar configuration as used for layer 2 is not possible The layer 3 protocol specification Q 931 is not symmetrical for TE and ET Lemmens 7 suggests using CEPT Recommendation T S 46 30 as a solution to enable testing of layer 3 in an end to end situation with two terminals One should always carry in mind how the protocols are defined for TE or NT and the configuration in which one wants to apply them 55 Bibliography 1 Michael Abrash Zen of Assembly Language Volume I Knowledge Assembly Language Programming Series Scott Foresman and Company London 1990 2 J J P H Beijnsberger Hardware en software debugging van het ISDN terminal board Graduation Report EB276 Eindhoven University of Technology Digital System G
58. rocess C MSG Control Dispatcher Message queue FIFO Low level interrupt handlers INT Nar fe AP ste ath Hardware FIGURE 3 2 Schematic view of the operating system Figure 3 2 gives a schematic view of what is from now on called the Oudelaar operating system As it turned out a very interesting property of the operating system was that it is hardware independent It allows implementation and testing on different systems In my case this means I can use ISDN hardware that is operational known good to test the approach of this chapter When it is finished it will be transported to the ISDN terminal board 3 1 Messages All action is based on the exchange of messages They can be generated by a certain process to pass on information to another process In LAP D for example the information may be service primitives like DL_RELEASE_REQUEST Each message unit must at least contain in formation about the type of message and the destination of the message Figure 3 3 shows the data structure of the messages implementation is in C language The nucleo field holds the message type this can be DL ESTABLISH_REQUEST SABME etc The dest and orig fields hold the destination and originating processes identities respectively The five param eter fields constitute the information field of the message A DL ESTABLISH REQUEST primitive has no information fields whereas a DL UNIT_DATA INDICATION contains a
59. rom ready to be used by layer 3 let alone ready to enable development of ISDN services The only conclusion that can be made is that the software should be tested exhaustively 1One of the problems causing the data link connection has been found It concerns an action of layer 2 management after a collision of commands This action is to send an ID_VERIFY REQUEST to the NT The test configuration however does not provide any NT services The request remains unanswered causing layer 2 management to remove the TEI value of the data link in question I have disabled this managemement reaction as the data link layer entities are capable of resolving this collision by themselves Chapter 7 Conclusions The Keyword during the continuation of the ISDN project should be Testing A much over looked and underestimated factor Where previous students working on the project declared a piece of work tested and functional I have encountered the opposite Many errors not discussed in the previous chapters could have been detected if each function was tested indi vidually This can be done using a debugger and evaluating the function s results Much to my dismay testing is something I have not been able to pursue to the maximum extent Those parts I did test showed mixed results Although the various parts of the software seem to function it is not yet possible for layer 3 to access an error free transmission channel Likewise maximum rate perfo
60. roup Eindhoven October 1990 3 T Boyce T Grenier R L Probert and H Ural Formalization of ISDN LAP D for conformance testing In Proceedings of the Eigth Annual Joint Conference of the IEEE Computer and Communications Societies Technology Emerging or Converging vol ume 1 Ottawa Ont Canada 1989 4 O B P Rikkert de Koe OSI protocollen lagen 1 tm 4 een inleiding tot een beschrijving van de OSI standaarden Kluwer Technische Boeken Deventer 1990 5 Dicenet Design and Prospects for the ISDN Artech House Inc London 1987 6 Mitel Documentation Microelectronics Data Book issue 6 edition Canada 1988 7 R J M Lemmens Implementatie van het ISDN laag 3 signaleringsprotocol voor het D kanaal Graduation Report EB308 Eindhoven University of Technology Digital System Group Eindhoven June 1991 8 H Oudelaar Implementation of a CCITT Protocol on an ISDN Terminal Board Gradu ation Report EB222 Eindhoven University of Technology Digital System Group Eind hoven December 1989 9 CCITT Recommendations Q 920 Q 921 Digital Subscriber Signalling System No 1 DSS 1 Data Link Layer User Network Management In Documents of the 9th Plenary Assembly Melbourne 14 25 Nov 1988 Geneva 1989 Blue Book Fascicle VI 10 10 Technical Recommendations Application Committee European Telecommunication Standard 3 Approval requirements for terminal equipment to connect to integrated services digital network
61. rvice integration for an ISDN is the provision of a range of services using a limited set of con nection types and multi purpose user network interface arrangements Extract from Recommendation I 120 The ISDN is modelled according to the Open Systems Interconnection OSI concept A brief discussion of the ISDN protocol reference model is given in 7 The above citation speaks of a limited set of connection types and multi purpose user network interfaces The ideal situation would be one device that on the one hand connects to the network and on the other hand provides a network access to a telephone a fax a personal computer etc In practice however this will certainly not be the case due to the diversity and enormous amount of interfaces the device should handle The Digital Systems Group of the Faculty of Electrical Engineering Eindhoven University is conducting research involving the ISDN As a part of this an ISDN terminal board has been designed This board provides access to the ISDN and its services The board is equipped with a parallel interface for connection to e g a personal computer a connection for a digital phone and the so called S interface this is the point where the network is terminated roughly where the actual wire enters the customer premises An Intel 80186 microprocessor resides at the heart of the hardware The terminal will be used as an experimental ISDN terminal on which available and
62. s Tx D channel DSTiD channel C 0xb06 DSTo D chann S Bus Rx D channel Oxb0e_ S bus Tx Blechannel DSTi BI channel 0xb0d DSTo Bichannel S Bus Rx Bi chamnel 0xb e S Bus Tx B2 chann DSTiB2chamnel ObO DSTo B2 channe S bus Rx B2channel A 2 Implemented access functions Relevant implemented access functions and structures are struct Dscstruc address xmtnext Address of next transmit byte address rcvfree Address of next free location 58 APPENDIX A SNIC REGISTERS AND ACCESS FUNCTIONS 59 Dscstruc DscRam Ju read one byte from the RxFIFO define readRFIFO DscRam gt rcvfree inportb 0xb05 enable the receiver define enable_REN outportb 0xb02 inportb 0xb02 0x48 clear the receiver void resetRFIFO void mark next byte in TxFIFO as last byte in packet define tag_eop outportb 0xb03 1 abort packet after next byte define tag_fa outportb 0xb03 2 write a byte to the TxFIFO define writeTFIFO outportb 0xb05 DscRam gt xmtnext disable the transmitter define disable_TEN outportb 0xb02 inportb 0xb02 amp Ox7F enable the transmitter define enable_TEN outportb 0xb02 inportb 0xb02 0x80 clear the transmitter void resetTFIFO void check for activity on the S bus define lineact inportb 0xb09 amp 0x60
63. s are active and have to be updated Timers which expire result in a message for the process that started the timer The timers are accessed by the following functions void runtimer word msg byte dest word time Start a timer with length time void stoptmr word msg byte dest Stop a certain timer If the timer has already gen erated a Time out message the function remove_tmr_msg is called void restarttimer word msg byte dest word time Resets timer count to time byte timer_stat word msg Checks if a certain timer is running Status is returned as a byte 0 if the timer isn t not running 1 if it is remove tmr msg word msg byte dest Removes a timer message from the message queue when a timer has expired but must be stopped This is an exception of the rule that only the dispatcher can access the message queues Summary In this chapter the LAP D software was expanded with the TRANSMISSION process This process controls the transmission and reception of LAP D packets It uses the SNIC circuit on the Mitel card From a description of this SNIC circuit it was possible to derive transmit and receive algorithms on an interrupt basis The resulting interrupt handler code outline CHAPTER 5 IMPLEMENTATION OF LOW LEVEL DRIVERS 43 was shown To complete the low level functions the implementation of the LAP D timers was discussed The LAP D software has now been completed and can be tested There are two types of tests on
64. s within layer 2 and not between layer 1 and 2 as illustrated in figure 2 4 A new receive buffer is then allocated to receive the next packet The TRANSMISSION process will upon reception of the PH_DATA INDICATION mes sage decode the received packet When the destination process is determined a message is generated for that process and the information will then find its way further up the OSI layers or stay in layer 2 if it is a peer to peer message The TRANSMISSION process has the highest priority in the operating system The reason for this are the timing restrictions that apply to layer 2 peer to peer messages All CHAPTER 5 IMPLEMENTATION OF LOW LEVEL DRIVERS 39 transmitted packets must have a response within T200 seconds T200 is default 1 second CCITT parameter As soon as a packet is received it is to be decoded and given to the data link layer entity in question In chapter 6 I will verify if the implementation can function under the current one second restriction 5 4 Transmitting packets To transmit a packet the reverse of the actions in the previous section are taken Management or a data link layer entity will queue information in a special transmission queue which is ac cessed through the function putqueue Li QUEUE UIQUEUE information buffer pointer The transmitting entity will then signal the TRANSMISSION process that a packet is ready to be transmitted by means of a PH DATA REQUEST message to the TRANSMISSION
65. sabme 4 Dispatcher cycle tx_packet 3 bytes Dispatcher cycle sabme_4 Dispatcher cycle tx_packet 3 bytes Dispatcher cycle sabme_4 Dispatcher cycle tx_packet 3 bytes 1000 1000 1000 2237 1600 2237 1600 2237 1600 2237 3387 2237 1004 2237 3387 2237 1004 2237 3387 2237 1004 52 message queue after action 0 PH DATA REQUEST 1 2 x DL ESTABLISH REQUEST 0 1 2 x DL ESTABLISH REQUEST 0 PH DATA REQUEST 1 DL ESTABLISH REQUEST 0 1 DL ESTABLISH REQUEST 0 PH DATA_REQUEST 1 0 1 0 PH DATA INDICATION 1 0 2 x PH DATA INDICATION 1 0 3 x PH DATA INDICATION 1 0 2 x PH DATA INDICATION 1 SABME 0 PH DATA INDICATION 1 SABME SABME gt 0 1 SABME SABME2 SABMEs 0 PHDATA REQUEST 1 SABME gt SABMEs 0 1 SABME gt SABME 0 PHLDATA REQUEST 1 SABMEs 0 1 SABMEz 0 PH DATA REQUEST 1 0 1 CHAPTER 6 TESTING 53 action time us message queue after action Initiating side Assume initiating party starts receiving UA responses in sequence Again this assumption increases the response times Teceive 3 bytes interrupt 1000 0 PH DATA INDICATION 1 recelve 3 bytes interrupt 1000 0 2 x PH_DATAINDICATION 1 receive 3 bytes interrupt 1000 0 3 x PH DATA INDICATION 1 Dispatcher cycle 2237 rcv_packet 3 bytes 1600 0 2 x
66. sor is an Intel 80186 This processor is equipped with a clock generator two independent DMA channels a programmable interrupt controller three programmable 16 bit timers a local bus controller Unfortunately two DMA channels are insufficient as four are required the two B channels are bidirectional An 82258 DMA Coprocessor has been added to provide the needed capacity CHAPTER 4 HARDWARE ASPECTS 33 The terminal board is equipped with 512K Ram This fairly large amount was chosen with future expansion of the board s functions as the board will be used to develop new ISDN services For more detail on the board s hardware I recommend the reading of the graduation report of Beijnsberger 2 Comparing the Mitel card and the ISDN terminal board we can see that the last could be a part of the first It would roughly replace the SNIC circuit with the DSC and the two HDLC controllers with the two IDPC s which actually are two HDLC controllers The PC interface indicated in figure 4 1 would be located at the parallel and or serial interface of the terminal board Summary A brief description of the Mitel ISDN card and the ISDN terminal board were given On the Mitel card the SNIC chip was pointed out as the relevant circuit for LAP D Its equivalent on the terminal board is the DSC Both ISDN systems are accessible through a PC interface LAP D software is developed on the Mitel card and will be transported to the term
67. stablish request 3 Establish 2 Assign awaiting TE f awaiting TEI TEI TE assignment assignment 4 TE Establish assigned errs request complete peer SE ee initiated 6 Awaiting 5 Awaiting release establishment establishment release release establishment request complete 7 Multiple frame established Timing recovered T200 timer expiry 8 Timer recover 1 TELUNASSIGNED This is the initial state a data link layer entity is in Only acknowledged information transfer is possible 2 ASSIGN_AWAITING_TEI The data link layer entity is waiting for TEI assignment from man agement Layer 3 has indicated it wants to transport numbered information This is an indirect DATALINK ESTABLISH_REQUEST 3 ESTABLISH AWAITING TEI The datalink layer entity is waiting for TEI assignment from management Layer 3 has requested the establishment of a datalink connection 4 TEI_ASSIGNED The datalink layer entity is ready for action which will be the establishment of a datalink connection or the removal of the TEI value 5 AWAITING_ESTABLISHMENT A request for a datalink connection has gone out to the peer of the datalink layer entity This is a wait state waiting for response from the peer in order to enter multiple frame operation 6 AWAITING_RELEASE The datalink layer entity is waiting for response form its peer A datalink connection is about
68. t 1 Conditions NT and TE are connected and in the Active state concerning bus ac tivity All of the TE s data link layer entities are in the TELUNASSIGNED state Tester NT TE ID_ASSIGN byte sequence FE FF breakpoint at rcv_broadcast should 03 OF 00 01 02 03 be reached Results Using step by step tracing after the breakpoint was reached correct re ception of the bytes was revealed The ID_ASSIGN command was also directed correctly to the management process Test 2 Conditions NT and TE are connected and in the Active state concerning bus ac tivity All of the TE s data link layer entities are in the TELUNASSIGNED state TEI value 1 is assigned to a data link layer entity Tester NT TE ID CHECK byte sequence FE FF breakpoint at rcv broadcast should 03 OF 00 00 06 FF be reached Response should be byte sequence FE FF 03 0F 05 03 where indicates a random value Results Using step by step tracing after the breakpoint was reached correct re ception of the bytes was revealed Management did respond with the correct byte sequence CHAPTER 6 TESTING 46 Test 3 Conditions NT and TE are connected and in the Active state concerning bus ac tivity All of the TE s data link layer entities are in the TELUNASSIGNED state At least one TEI value is assigned to a data link layer entity Tester NT TE ID REMOVE byte sequence FE FF All TEI values are t
69. t s Primary rate access An interface at a usable rate of either 1984 Kbit s7 Europe or 1536 Kbit s7 USA amp Japan This interface supports 30 or 23 B channels 64 Kbit s and one D channel 64 Kbit s7 The B channels are used for 64 Kbit s data transport whereas the D channel is used to transport signalling information 16 Kbit s71 From here on basic access will be used 2B D As figure 2 1 shows the characteristics of the physical layer is specified by CCITT rec ommendation 1 430 This layer performs multiplexing of the D and B channels D channel contention resolution synchronization and activation deactivation The characteristics of the D channel of the data link layer are specified by CCITT recommendations Q 920 Q 921 These are called the Link Access Procedures on the D channel or LAP D The network layer s characteristics are specified by CCITT recommendations Q 930 Q 940 On these two layers no B channel restrictions are imposed I will therefore restrict myself to the D channel Since the network layer has already been implemented 1 will furthermore concentrate on the data link layer 2 2 LAP D functions The Link Access Procedures on the D channel provide an error free data link connection to convey information between layer 3 entities across the ISDN user network interface using CHAPTER 2 LINK ACCESS PROCEDURE D 9 TE NTI LT ET user network Network layer Data link layer Q 920 Q 921 Physical
70. this purpose a slightly different test configuration is used illustrated in figure 6 2 tester NT TE S T ISDN software ISDN software Protocol Analyzer active on both PC s FIGURE 6 2 Test configuration for monitoring the functional behaviour of the pro tocol On both PC s the ISDN with the protocol analyzer is used The analyzer displays both states and incoming messages of each data link layer entity A single step feature enables a close watch of the dispatcher s actions A protocol analyzer was developed by Lemmens 7 It is used to observe and manipulate the actual states of the data link layer entities The analyzer makes use of several windows in which the states incoming and outgoing messages are displayed of the various entities including layer 3 entities Figure 6 3 shows the initial screen of the analyzer To assist in the observation of the processes it is possible to slow down the dispatcher It can be run freely put in Single Step mode and deactivated In Single Step mode the dispatcher executes a single cycle This means that the dispatcher will check the message queue once and process the present message In any state of the dispatcher it is possible to generate an incoming message for an entity or alter its current state A complete description of the analyzer and its functions is given in the software manual see 13 The following tests were performed using the analyzer Both sides of the S T interface w
71. time Unacknowledged information transfer This mode can also be named the connectionless mode No error correction or flow control is used to transfer U frames I frames cannot be transferred in this mode The mode can be used for point to point communication between a user and the network or for multi point communication for the broadcast of frames to several terminals e g TE verify procedure Acknowledged information transfer This mode is only possible when a data link connection has been established A connection is made using the Set Asynchronous Balanced Mode Extended SABME command This is an inheritance of HDLC CHAPTER 2 LINK ACCESS PROCEDURE D 12 INFORMATION TRANSFER DATA LINK CONTROL DL RELEASE CONFIRM DL ESTABLISH CONFIRM DL RELEASE INDICATION DL ESTABLISH INDICATION DL DATA INDICATION DL UNIT DATA INDICATION DL ESTABLISH REQUEST DL DATA REQUEST DL RELEASE REQUEST DL UNIT DATA REQUEST MDL REMOVE REQUEST MDL ASSIGN REQUEST MDL UNIT DATA REQUEST NDL ERROR REQUEST PH DATA INDICATION MDL UNIT DATA INDICATION Layer 2 MDL ASSIGN INDICATION Management PH DATA REQUEST MDL ERROR INDICATION FIGURE 2 4 Layer 2 primitives associated with the interaction between layer 3 and the management entities 2 5 Layer 2 interfaces Layer 2 communicates with layers 1 and 3 as well as the management layer through messages called service primitives The messages t
72. ware manual 13 includes a full description of this procedure Information on how to use the IES is can also be found in the Designer s Manual 6 4 2 ISDN Terminal Board The ISDN Terminal Board see 14 has been designed so that it can operate as a stand alone ISDN Terminal It is equipped with multiple interfaces to allow various kinds of data com CHAPTER 4 HARDWARE ASPECTS 32 Parallel interface Oo PNC Processor 80186 82258 Serial interface IDPC IDPC FIGURE 4 2 Block diagram of the ISDN Terminal Board munication The block diagram in figure 4 2 of the Terminal Board is very similar to that of the ISDN Express Card The Digital Subscriber Controller DSC handles the layer 1 multi plexing It is comparable to the SNIC of the Mitel card as they perform the same functions It gives the two Integrated Data Protocol Controllers IDPC s access to the B channels An optional phone can be connected directly to the DSC offering voice communication The DSC can also perform some layer 2 functions such as Frame Check Sequence FCS computation All other protocol functions have to be implemented in software The IDPC s can be used to Each IDPC has a serial interface to which a data terminal can be connected handle bit oriented protocols for the B channels HDLC LAP D LAP B X 25 Each IDPC has a serial interface to which a data terminal can be connected The used microproces
73. which I have focussed briefly The first concerns the functionality of LAP D It shows whether the data link layer entities follow the correct procedures The second concerns the real time performance of the software This test can reveal if the software has the capacity to perform within the timing restrictions of LAP D In other words how efficient is the implemented code The next chapter will answer this question as the results of both tests are discussed Chapter 6 Testing Before claiming that the software works it should be subjected to tests As I found out testing takes a considerable amount of time There is no such thing as The Test after which conclusions can be drawn In fact I recognized several types of tests each giving answers to different questions e Coverage test is all code reachable Is there code that has no function e Decision test do conditional statements provide for all conditions that may occur e Protocol implementation test or conformance test does the code actually implement the protocol e Performance test does the code operate within the restrictions Does it allow in this case the terminal to be connected to an existing ISDN terminal Making a suitable test case that is comprehensive enough to facilitate this validation is a major problem The main reason for this is the fact that the number of inputs of the system is substantial Due to the state machine behaviour of the protocol its test
74. x05 Ai 0 126 TEI value in sponse user to use network Ai 0 126 TEI value to be checked 63 Appendix C LAP D parameters TABLE C 1 List of used LAP D parameters 5 6 T200 must be greater then the maximum time between trans mission of command frames and the reception of their corresponding response or acknowledgement frames T203 10s Maximum time allowed without frames being exchanged N200 Maximum number of retransmission of a frame system parameter N201 260 Maximum number of octets in an information field T200 Default value for timer T200 at the end of which transmission of a Minimum time between retransmission of TEI Identity check messages N202 Maximum number of transmissions of a T EI Identity request message Size of sliding window in case of basic access signalling services ls frame may be initiated according to the procedures described in Q 921 T201 T200 T202 2s Minimum time between retransmission of TEI Identity request messages Size of sliding window in case of basic access packet services declared in file DECLARE 64

Implementation of the Link Access Procedure on the ISDN D

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