Data Link Control in Data Communication
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1. it discovers the discrepancy and return NAK Lost Acknowledgement Lost ACK and NAK frames are treated by selective ARQ just as they are by go back n ARQ When the sending device reaches either capacity of its window or the end of its transmission it sets a timer If no acknowledgment arrives in the time allotted the sender retransmits all of the frames that remain unacknowledged In most cases the receiver will recognize any duplication and discard them Comparison between Go Back n and Selective Reject Although retransmitting only specific damaged or lost frames may seem more efficient than resending undamaged frames as well it is in fact less so Because of the complexity of the sorting and storage required by the receiver and the extra logic needed by the sender to select specific frames for retransmission selective reject ARQ is expensive and not often used In other words selective reject gives better performance but in practice it is usually discarded in favor of go back n for simplicity of implementation VI Conclusion From all the techniques of DLC in Data link control we can conclude frames received from the physical layer are corrected and make sure that frames is error free flow control and not corrupted during transferring of data from physical layer to network layer www 1losrjournals org 46 Page Data Link Control In Data Communication References 1 A Harish Babu P Ashesh Babu E Vamsi Krishna IETE Members and2. several popular mechanisms for continuous transmission error control two protocols are the most popular go back n ARQ and selective reject ARQ both based on sliding window flow control To extend sliding window to cover retransmission of lost or damaged frames three features are added to the basic flow control mechanism The sending device keeps copies of all transmitted frames until they have been acknowledged If frames 0 through 6 have been transmitted and the last acknowledgment was for frame 2 the sender keeps copies of frames 3 through 6 until it knows that they have been received undamaged In addition to ACK frames the receiver has the option of returning a NAK frame if the data have been received damaged The NAK frame tells the sender to retransmit a damaged frame Because sliding window is a continuous transmission mechanism both ACK and NAK frames must be numbered for identification Note that data frames that are received without errors do not have to be acknowledged individually Like stop and wait ARQ the sending device in sliding window ARQ is equipped with a timer to enable it to handle lost acknowledgment In sliding window ARQ n 1 frame the size of the window may be sent before an acknowledgment must be received By retransmitting the data frames two possibilities are covered lost data and lost NAK If the lost frame was an ACK frame the receiver can recognize the redundancy by the number on the frame and discard
3. transmitted the sending system finishes with an end of transmission EOT frame Poll Select The poll select method of line discipline works with topologies where one device in designated as a primary station and the other devices are secondary stations WORKING Whenever a multipoint link consists of a primary device and must be secondary devices using a single transmission line all exchanges must be such through the primary device even when the ultimate destination is a secondary device The primary device controls the link the secondary devices follow its instructions It is up to the primary to determine which devices allowed to use the channel at a given time The primary therefore always the initiator of a session If the primary wants to receive data it asks the secondary s if they have anything to send this function is called polling If the primary device to send data it tells the target secondary to get ready to receive Addresses For point to point configuration there is no need for addressing any transmission put onto the link by one device can be intended only for the other For the primary device in a multipoint topology to be able to identify and communicate with a specific secondary device however the must be an addressing convention For this reason every device on a link has address that can be used for identification Poll select protocol identify each frame as being either to or from a specific device on the link Ea
4. IOSR Journal of Electronics and Communication Engineering IOSR JECE ISSN 2278 2834 ISBN 2278 8735 Volume 4 Issue 1 Nov Dec 2012 PP 39 47 www tlosrjournals org Data Link Control in Data Communication Alluri Sri Amith Varma Changanti Veera Sankara Sarma Pathalapu Sai 3 3 Madan kumar Thumati Ravi 123 Final Year B Tech Dept of ECE KL University Vaddeswaram AP India Associate Professor B Tech Dept of ECE KL University Vaddeswaram AP India Abstract In this paper Data link control in data communication is discussed The control done by the Data link layer is Data link control It mainly does three controls that are its functions and their advantages I Introduction DLC is the service provided by the Data Link layer of function defined in the Open Systems Interconnection OSI model for network communication The Data Link layer is responsible for providing reliable data transfer across one physical link or telecommunications path within the network Technical communication systems are developed to transmit messages from a sender to a receiver beside the task of sending and receiving information over a channel there are many other tasks a communication system has to do One important task is to guarantee the correctness of the received information No existing channel is completely error free In order to receive correct information redundancy is often sent On the receiver side the redundancy is needed to corre
5. Research students in kl university 2 C Balanis Antenna Theory Analysis and Design 3rd edition New York Wiley 2005 2 3 4S Telecom P LTD BANGALORE User Manual Biography Alluri Sri Amith Varma was born in 1992 in West Godavari District He is pursuing his B tech from K L University He is interested in Communications and Wireless Networks Changanti Veera Sankara Sarma is born in Rajahmundry East Godavari District Andhra Pradesh India on 4 June1992 and currently pursuing B TECH 4 year in Electronics and Communication Engineering in K L University with specialization in Communications Areas m Aim of interests are Linear integrated circuit applications Digital Logic Design and Antennas P Sai Madan Kumar was born in 1992 at prakasam district He is currently pursuing B Tech from K L University He is interested in Wireless systems and Telecommunication a A i Thumati Ravi is working as Associate Proffesor in KL University He is interested in Image Processing www 1losrjournals org 47 Page
6. amount of data that can be sent before receiving acknowledgment It also provides the receiver s acknowledgment of frame received intact and so is linked to error control Error control means error detection and correction It allows the receiver to inform the sender of any frames lost or damaged in transmission and coordinates the retransmission of those frames by the sender 3 Error control III Line Discipline Whatever the system no device in it should be allowed to transmit until that device has evidence that the intended receiver is able to receive and is prepared to accept the transmission if the receiving device does not expect a transmission it is busy or is out of commission With no way to determine the status of the intended receiver the transmitting device may waste its time sending data to a nonfunctioning receiver or may interfere with signals already on the link The line discipline functions of the data link layer over see the establishment of links and the right of a particular device to transmit at a given time www iosrjournals org 39 Page Data Link Control In Data Communication Line discipline can be done in two ways enquiry acknowledgment ENQ ACK and poll select The first method is used in peer to peer communication the second method is used in_primary secondary communication ENQ ACK Enquiry acknowledgment ENQ ACK is used in systems where there is no question of the wrong receiver getting the transmissi
7. ch secondary device has an address that differentiates it from the others In any transmission that address will appear in a specified portion of each frame called an address field or header depending on the protocol If the www 1losrjournals org 40 Page Data Link Control In Data Communication transmission comes form the primary device the address indicates the recipient of the data If the transmission comes from a secondary device the address indicates the originator of the data Select The select mode is used whenever the primary device has something to send Remember that the primary controls the link If the primary is not either sending or receiving data it knows the link is available If it has something to send it sends it What it does not know however is whether the target device is prepared to receive usually prepared to receive means on So the primary must alert the secondary to the upcoming transmission and wait for an acknowledgment of the secondary s ready status Before sending data the primary creates and transmits select If the secondary is awake and running it returns an ACK frame to the primary The primary then sends one or more data frames each addressed to the intended secondary Poll The polling function is EAT by the primary device to solicit transmissions from the secondary devices the secondary s are not allowed to transmit data unless asked By keeping all control with the primary the mu
8. ct errors which have occurred while transmission Data link layer receives data from next higher layer adds some control bits 1 e frames are formed at this layer frame is handed over to the physical layer Data link layer removes the control bits and check for errors If there is no error it hands over the received data to the physical layer The specified set of rules and procedures for carrying out data link control functions is called as Data Link protocol Data Link layer ensures that an initial connection has been set up divides output data into data frames and handles the acknowledgments from a receiver that the data arrived successfully It also ensures that incoming data has been received successfully by analyzing bit patterns at special places in the frames IL Functions Of Data Link Control Data link layer due to possibility of transmission errors receiver may need to control the rate at which data arrive so Synchronization and interfacing techniques are not sufficient It is necessary to have a layer of control in each communicating device that provides certain functions The most important responsibilities of the data link layer are line discipline flow control and error control Collectively these functions are known as data link control Application Presentation discipline Data link Line discipline coordinates the link systems It determines which device can and when it can send Flow control coordinates the
9. e flow of data must not be allowed to overwhelm the receiver Any receiving devices has a limited speed at which it can process incoming data and a limited amount of memory in which to store incoming data The receiving device must be able to inform the sending device before those limits are reached and to request that the transmitting device send fewer frames or stop temporarily Incoming data must be checked and processes before they can be used www 1losrjournals org 41 Page Data Link Control In Data Communication Two methods have been developed to control flow of data across communications links stop and wait and sliding window Stop and wait In a stop and wait method of flow control the sender waits for an acknowledgement after every frame it sends Only when an acknowledgement has been received is the next frame sent This process of alternately sending and waiting repeats until the sender transmits an end of transmission EOT frame The advantage of stop and wait is simplicity each frame is checked and acknowledged before the next frame is sent The disadvantage 1s inefficiency stop and wait slow Each frame must travel all the way to the receiver and an acknowledgment must travel all the way back before the next frame can be sent Sliding window In the sliding window method of flow control the sender can transmit several frames before needing an acknowledgment Frames can be sent one right after another meaning that the li
10. er may now accept six frames before it 1s required to send an ACK If frames 0 through 3 have arrived but have not been acknowledged the window with contains three frame spaces Receiver window r wn wall moves to the nght This wall moves to the right when an ACK 15 sent when 2 frame is received As each ACK a is sent out the receiving window expands to include as many new placeholders as newly acknowledged frames The window expands to include a number of new frame spaces equal to the number of the most recently acknowledged frame minus the number of the previously acknowledged frame In a seven frame window if the prior ACK was for frame 2 and the current ACK is for frame 5 the window expands by three 5 2 If the prior ACK was for frame 3 and the current ACK is for frame 1 the window expands by six 1 8 3 vV Error Control In the data link layer the term error control refers primarily to methods of error detection and retransmission Automatic Repeat Request ARQ Error correction in the data link layer is implemented simply anytime an error detected in an exchange a negative acknowledgement NAK is returned and the specified frames are retransmitted This process is called automatic repeat request ARQ ARQ error control is implemented in the data link layer as an adjunct to flow control in fact stop and wait flow control is usually implemented as stop and wait ARQ and sliding window i
11. ltipoint system guarantees that only one transmission can occur at a time thereby ensuring against signal collisions without requiring elaborate precedence protocols When the primary is ready to receive data it must ask poll each device in turn 1f it has anything to send When the first secondary is approached it responds either with a NAK frame 1f it has nothing to send or with data in the form of a data frame it does If the response is negative a NAK frame the primary then polls the next secondary in the same way until it finds one with data to send When the response is positive a data frame the primary reads the frame and returns an acknowledgment ACK frame verifying its receipt The secondary may send several data frames one after the other or it may be required to wait for an ACK before sending each one depending on the protocol being used There are two possibilities for terminating the exchange either the secondary sends all its data finishing with an end of transmission EOT frame or the primary says Time s up Which of these occurs depends on the protocol and the length of the message Once a secondary has finished transmitting the primary can poll the remaining devices IV Flow Control The second aspect of data link control is flow control In most protocols flow control is a set of procedures that tells the sender how much data it can transmit before it must wait for an acknowledgment from the receiver Th
12. nk can carry several frames at once and its capacity can be used efficiently The receiver acknowledges only some of the frames using a single ACK to confirm the receipt of multiple data frames When the receiver sends an ACK it includes the number of the next frame it expects to receiver In other words to acknowledge the receipt of a string of frames ending in frame 4 the receiver sends an ACK containing the number 5 When the sender sees an ACK with number 5 it knows that all frames up through number 4 have been received The window can hold n 1 frames at either end therefore a maximum of n 1 frames may be sent before an acknowledgment is required Sender window At the beginning of a transmission the sender s window contains n frame As frames are sent out the left boundary of the window moves inward shrinking the size of the window Given a window of size w if three frames have been transmitted since the last acknowledgement then the number of frames left in the window is w 3 Once an ACK arrives the window expands to allow in a number of new frames equal to the number of frames acknowledged by that ACK Sender window oes ee TW wes CEE SIITODY CUES ES CG Ea gt Direction 7 Jo 1 2134 Js Jel Direction This wall moves to the right when a frame is sent This wall moves to the neht when an ACK is received Given a window of size 7 1f frames 0 through 4 have been sent and no ack
13. nowledgment has been received the sender s window contains two frames numbers 5 and 6 Now if an ACK numbered 4 is received four frames 0 through 3 are known to have arrived undamaged and the sender s window expands to include the next four frames in its buffer At this point the sender s window contains six frames numbers 5 6 7 0 1 2 If the received ACK had been numbered 2 the sender s window would have expanded by only two frames to contain a total of four www losrjournals org 42 Page Data Link Control In Data Communication Receiver Window At the beginning of transmission the receiver window contains not n 1 frames but n 1 spaces for frames As new frames come in the size of the receiver window shrinks The receiver window therefore represents not the number of frames received but the number of frames that may still be received before an ACK must be sent Given a window of size w if three frames are received without an acknowledgment being returned the number of spaces in the window is w 3 As soon as an acknowledgement is sent the window expands to include places for a number of frames equal to the number of frames acknowledged In the figure the window contains spaces for seven frames meaning that seven frames may be received before an ACK must be sent With the arrival of the first frame the receiving window shrinks moving the boundary from space 0 to 1 The window has shrunk by one so the receiv
14. nsert the retransmitted frame into its proper place in the sequence To make such selectivity possible a selective reject ARQ system differs from a go back n ARQ system in the following ways The receiving device must contain sorting logic to enable it to reorder frames received out of sequence The sending device must contain a searching mechanism that allows it to find and select only the requested frame for retransmission A buffer in the receiver must keep all previously received frames on hold until all retransmissions have been sorted and any duplicate frames have been identified and discarded To aid selectivity ACK numbers like NAK numbers must refer to the frame received or lost instead of the next frame expected This complexity requires a smaller window size than is needed by the go back n method if it is to work efficiently Damaged Frames A situation in which a damaged frame is received As you ca n see frames 0 and received but not acknowledged Data 2 arrives and is found to contain an error NAK is returned Like NAK frames in go back n error correction a NAK Here both acknowledges the intact receipt of any previously unacknowledged data frames and indicates an error in the current frame Lost Frames Although frames can be accepted out of sequence they cannot acknowledge out of sequence If a frame is lost the next framed will arrive out of sequence When the receiver tries to reorder the existing frames to include it
15. on that is when there is a dedicated link between two devices so that the only device capable of receiving the transmission is the intended one In both half duplex and full duplex transmission the initiating device establishes the session In half duplex the initiator then sends its data while the responder waits The responder may take over the link when the initiator 1s finished or has requested a response In full duplex both devices can transmit simultaneously once the session has been established WORKING The initiator first transmits a frame called an enquiry ENQ asking if the receiver is available to receive data The receiver must answer either with an acknowledgement ACK frame if it 1s ready to receive or with a negative acknowledgement NAK frame if it is not By requiring a response even if the answer is negative the initiator knows that its enquiry was in face received even if the receivers currently unable to accept a transmission If neither an ACK nor a NAK is received within a specified time limit the initiator assumes that the ENQ frame was lost in transit disconnects and sends a replacement An initiating system ordinarily makes three such attempts to establish a link before giving up If the response to the ENQ is negative for the three attempts the initiator disconnects and begins the process again at another time If the response is positive the initiator is free to send its data Once all of its data have been
16. s usually implemented as one of two variants of sliding window ARQ called go back n or selective reject Stop and Wait ARQ Stop and wait ARQ is and a form of stop and wait flow control extended to include retransmission of data in case of lost or damaged frames For retransmission to work four features are added to the basic flow control mechanism The sending device keeps a copy of the last frame transmitted until it receives an acknowledgment for that frame Keeping a copy allows the sender to retransmit lost or damaged frames until they are received correctly cA data O frame is acknowledged by an ACK 1 frame indicating that the receiver has gotten data 0 and is now expecting data 1 This numbering allows of identification of data frames in case of duplicate transmission If an error is discovered in a data frame a NAK frame is returned NAK frames which are not numbered tell the sender to retransmit the last frame sent Stop and wait ARQ requires that the sender wait until it receives an acknowledgment for the last frame transmitted before it transmits the next one The sending device is equipped with a timer If an expected acknowledgment is not received within an allotted time period the sender assumes that the last data frame was lost in transit and sends it again Damaged Frames When a frame is discovered by the receiver to contain an error it returns a NAK frame and the sender retransmits the last frame For example the sender
17. the NAK as well as any frames that it had transmitted after the lost one Data and data arrive intact but data 2 is lost The next frame to arrive at the receiver is data 3 The receiver is expecting data 2 and so considers data 3 to be an error discards it and returns a NAK 2 indicating the O and 1 have been accepted but 2 is in error in this case lost Lost Acknowledgement The sender is not expecting to receive an ACK frame for every data frame it sends It cannot use the absence of sequential ACK numbers to identify lost ACK or NAK frames Instead it uses a timer The sending device can send as many frames as the window allows before waiting for an acknowledgment Once that limit has been reached or the sender has no more frames to send it must wait If the ACK or especially if the NAK sent by the receiver has been lost the sender could wait forever To avoid tying up both device the sender is equipped with a timer that begins counting whenever the window capacity is reached If an acknowledgment has not been received within time limit the sender retransmits every frame transmitted www 1losrjournals org 45 Page Data Link Control In Data Communication Selective Reject ARQ In selective reject ARQ only the specific damaged or lost frame is retransmitted if frame is corrupted in transit a NAK is returned and the frame is resent out of sequence The receiving device must be able to sort the frames it has and i
18. the redundant data www losrjournals org 44 Page Data Link Control In Data Communication Go Back n ARQ In this sliding window go back n method If one frame is lost or damaged all frames sent since the last frame acknowledged are retransmitted Damaged Frame What if frames 0 1 2 and 3 have been transmitted but the first acknowledgment received is a NAK 3 Remember that a NAK means two things 1 a positive acknowledgment of all frames received prior to the damaged frame and 2 a negative acknowledgment of the frame indicated If the first acknowledgment is a NAK 3 it means that data frames O 1 and 2 were all received in good shape Only frame 3 must be resent What if frames 0 through 4 have been transmitted before a NAK is received for frame 2 As soon as the receiver discovers an error It stops accepting subsequent frames until the damaged frame has been replaced correctly Lost Data frame Sliding window protocols require that data frames be transmitted sequentially If one or more frames are so noise corrupted that they become lost in transit the next frame to arrive at the receiver will be out of sequence The receiver checks the identifying number on each frame discovers that one or more have been skipped and returns a NAK for the first missing frame A NAK frame does not indicate whether the frame has been lost or damaged just that it needs to be resent The sending device then retransmits the frame indicated by
19. transmits a data frame data 0 The receiver returns an ACK 1 indicating that data O arrived undamaged and it is now expecting data 1 The sender transmits its next frame data 1 It arrives undamaged and the receiver returns ACK 0 The sender transmits its next www 1losrjournals org 43 Page Data Link Control In Data Communication frame data 0 The receiver discovers an error in data O and returns a NAK The sender retransmits data 0 This time data 0 arrives intact and the receiver returns ACK 1 Lost Frame Any of the three frame types can be lost in transit Lost Data Frame the sender is equipped with a timer that starts every time a data frame is transmitted If the frame never makes it to the receiver the receiver can never acknowledge it positively or negatively The sending device waits for an ACK or NAK frame until its timer goes off at which point it tries again It retransmits the last data frame restarts its timer and waits for an acknowledgment Lost Acknowledgment In this case the data frame has made it to the receiver and has been found to be either acceptable or not acceptable But the ACK or NAK frame returned by the receiver is lost in transit The sending device waits until its timer goes off and then retransmits the data frame The receiver checks the number of the new data frame If the lost frame was a NAK the receiver accepts the new copy and returns the appropriate ACK Sliding Window ARQ Among the
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