Data Link Control Protocols

2 Introduction Data link layer –Concerned with the transfer of data over a serial data link Link –Point-to-point physical circuit –Radio-based channel –Physical or logical link through a switched network

3 Introduction (cont ’ d) User service (depending on application) –Best-try (connectionless) service –Reliable (connected- oriented) service

4 Introduction (cont ’ d) Connectionless service –Although error check bits are used to detect errors –Any frames that are found to contain transmission errors are simply discarded –Unacknowledged service

5 Introduction (cont ’ d) Connection-oriented service –employs the error and flow control procedures –A high probability that the data will be error free without duplicates and that messages will be delivered in the same sequence as they were submitted

6 Application environments End-to-end basis –The data link protocols are located in the two communications DTEs computers Logical significance –It operates over the local link connecting

7 Application environments (cont ’ d)

8 Point-to-point circuit –Operates on an end-to-end basis –A reliable connection-oriented service is normally used –Kermit and X-modem Multipoint topology –Such architectures are normally used in applications that involve a single master computer communicating with a distributed community of slave computers

9 Application environments (cont ’ d) –A connection-oriented data link protocol WANs (fig 5.2.c) –The link protocol has only local significance and operates between the DTE and the local DCE LANs (fig 5.2.d) –The user of relatively short, low bit error rate links that operate at high bit rates

10 Application environments (cont ’ d) –Errors are relatively infrequent and the end-to-end frame transfer time is very fast –It operates in a connectionless, best-try mode –All retransmission and flow control functions are left to a higher protocol layer in two DTEs

11 Character-oriented protocols Selected transmission control characters –To perform the various transmission control functions associated with link management, start-of-frame and end-of- frame delimiting, error control, and data transparency

12 Simplex protocols Kermit –It is used extensively for the transfer of the contents of a specified file or group of files from one computer to another over a point-to-point data link –Link A circuit set up through the switched telephone network using modems A pair of twisted-pair lines –Synchronous transmission is used –Idle RW protocol

13 Simplex protocols (cont ’ d) If modems are being used –Originate mode –Answer mode

14 Simplex protocols (cont ’ d) Kermit is not simply data link protocol –It performs a number of additional functions –Such as file reading/writing and file segmentation and reassembly

15 Simplex protocols (cont ’ d) Frame format –Fig 5.4 The contents of a text file are sent as a sequence of 80 character blocks The binary files are sent simply as a string of 8-bit bytes

16 Simplex protocols (cont ’ d)

17 Half-duplex protocols Most character-oriented protocols –Operate character-oriented protocols operate in the half-duplex, stop-and-wait mode BSC (Binary Synchronous Control) –Developed by IBM –Be used with a synchronous control scheme –Be used in multipoint applications

18 Half-duplex protocols (cont ’ d) –Topology Multipoint network Multidrop bus network

19 Half-duplex protocols (cont ’ d) Transmission control characters –Table 5.1

20 Half-duplex protocols (cont ’ d) Data transparency –The use of DLE character –When transmitting pure binary data rather than character strings –DLE/STX, DLE/ETX –Difference when operating in the transparent mode is error control Instead of a simple 8-bit longitudinal parity check per block A mode sophisticated polynomial code with each block terminated by a 16-bit CEC rather than an 8-bit BCC

21 Half-duplex protocols (cont ’ d) The master computer –Scheduling all transmissions on each shared data link –Poll control message Be used to request a specific slave computer to send any waiting data message it may have –Select control message Be used to ask the selected slave whether it is ready to receive a data message

22 Half-duplex protocols (cont ’ d) Retransmission –ACK, NACK 에 의해 결정 Timeout mechanism – 전송 data 가 완전히 파괴된 경우에 사용 Sequence number –Sending site Only increment –Receiving site Modulo-2

23 Half-duplex protocols (cont ’ d)

24 Half-duplex protocols (cont ’ d) User Interface –It is important to discriminate between the services provided by the link layer and the detailed operation of the link layer protocol entity

25 Half-duplex protocols (cont ’ d) Protocol Performance –An important performance parameter The average time taken to poll or select all the slave stations on a link –In practice Because of the low link utilization of idle RQ Message transmitting time is the dominant time in a poll or select sequence

26 Half-duplex protocols (cont ’ d) Example –Average message : 1000 bits –Control message : 30 bits –Transmission rate : 10kbps »Message transmit time : 0.1s »Control message transmit time : 0.003s

27 Duplex protocols Duplex protocol –Be used in the early ARPANET –It operated over the point-to-point duplex link –The transmission of information in both directions simultaneously –It utilizes a continuous RQ transmission control scheme for both directions

28 Duplex protocols (cont ’ d) –It operates with an effective send window of either 8 for terrestrial links or 16 for satellite links –To ensure a continuous flow of frames, 8(or 16 for satellite) separate stop-and-wait information flows can be in progress at any instant

29 Duplex protocols (cont ’ d) Each frame is treated as a separate entity –On receipt of a frame to be forwarded, the sending data link protocol simply scans the busy/idle bit associated with each logical channel Whether a channel is free

30 Duplex protocols (cont ’ d) –If so inserts the appropriate send sequence number and the logical channel number in the frame header Starts a timer for the frame and initiates its transmission –If channel is not free The frame is left in the input queue to wait for a free channel Bit-oriented protocols –Protocols defines bit patterns rather than transmission control characters to signal the start and end of a frame –Frame delimiting »Unique start-of-frame and end-of-frame bit patterns as flags ( ) »A unique start-of-frame bit pattern, known as the start delimiter( ), and a length(byte) count in the header at the start of the frame

31 Bit-oriented protocols (cont ’ d) –Unique start-of-frame and end-of-frame delimiters that include bit encoding violations

32 High-level data link control HDLC –An international standard by ISO –For use on both point-to-point and multipoint data links –It supports full-duplex, transparent-mode operations

33 HDLC (cont ’ d) 3 operational mode –NRM Normal response mode It is used in unbalanced configurations Slave stations can transmit only when specifically instructed by the master station

34 HDLC (cont ’ d) –ARM Asynchronous response mode It is used in unbalanced configurations It allows a secondary to initiate a transmission without receiving permission from the primary –ABM Asynchronous balanced mode It is used mainly on duplex point-to-point links Each station has an equal status and performs both primary and secondary functions

35 HDLC (cont ’ d) Frame formats –3 class Unnumbered frames –Be used for such functions as link setup and disconnections –They do not contain any acknowledgement information, which is contained in sequence numbers

36 HDLC (cont ’ d) Information frames –It carries the actual information or data and are normally referred to simply as I-frames –I-frames can be used to piggyback acknowledgement information relating to the flow of I-frames in the reverse direction when the link is being operated in ABM or ARM Supervisory frames –It is used for error and flow control and hence contain send and receive sequence numbers

37 HDLC (cont ’ d) Flag –Start-of-frame and end-of-frame delimiter FCS –16-bit CRC for complete contents enclosed between the two flag delimiters Address field –Depend on the mode of operation (group address, broadcast address)

38 HDLC (cont ’ d) Frame types –Unnumbered frames are used for link management SNRM and SABM-frames are used both to set up a logical link between the primary and a secondary station and to inform the secondary station of the mode of operation to be used –To clear a logical link, send a DISC frame

39 HDLC (cont ’ d) –UA frame is used as a acknowledgement –Supervisory frame RR, RNR –Be used in both NRM and ABM REJ and SREJ-frame –Be used only in ABM which permits simultaneous two-way communication across a point-to-point link SREJ-frame –Be used with a selective repeat transmission procedure REJ-frame –Be used with a go-back-N procedure

40 HDLC (cont ’ d) Protocol operation –Link management Before any information may be transmitted –A logical connection between the two communicating parties must be established –This is accomplished by the exchange of two unnumbered frames

41 Link management Connection establishment –SNRM(multidrop link) –SARM(point-to-point link) Clear link –DISC frame Acknowledgement –UA frame

42 HDLC (cont ’ d) –Data transfer The unnumbered poll(UP) frame with the P bit set to 1 if normally used by the primary to poll a secondary Error control –It uses a continuous RQ procedure with either a selective repeat or a go-back-N retransmission strategy Flow control –It is based on a window mechanism

43 Data transfer

44 Data transfer (cont ’ d)

45 HDLC (cont ’ d) –User interface

46 Link access procedure version B LAPB –A subset of HDLC –It is used to control the transfer of I-frames across a point-to-point duplex data link –An extended version of LAPA(link access procedure version A)

47 LAPB (cont ’ d) Applicability –In fig 5.2.c –LAPB is used to control the transfer of information frames across the local DTE-DCE interface –It has local significance Frames –RR- and REJ-frames are used for error control –RNR is used flow control

48 LAPB (cont ’ d)

49 Multilink procedure HDLC –Controls the transfer of information frames across a single duplex link –As a SLP(single link procedure) MLP –It simply treats the set of single link procedures as a pool of links available to transfer user information

50 Multilink procedure (cont ’ d) Additional control field –MLC field –Add to the head of each frame –It receives for transmission prior to passing the frame to an SLP –Two octets –Contains a 12-bit sequence number

51 Link access procedure for modems LAPM –The protocol used in error correction modems such as the V.32 modem Each modem comprises two functional units –User interface part (UIP) –Error correcting part (ECP)

52 LAPM (cont ’ d) XID (exchange identification) –The operational parameter values are negotiated when the two ECPs exchange two special unnumbered frames –One as a command and the other as a response

53 Link access procedure D-channel LAPD –The HDLC subset for use with the ISDN –It has been defined to control the flow of I- frames associated with the signaling channel(D-channel) –Two types of service Unacknowledged (best-try) service Acknowledged (connection-oriented) service

54 LAPD (cont ’ d)

55 LAPD (cont ’ d)

56 Logical link control LLC –The HDLC derivative used with LANs –With LANS The data link layer is comprised of two sublayers –MAC(medium access control) sublayer »It implements the distributed access control algorithm –LLC sublayer

57 LLC (cont ’ d) User services –Unacknowledged connectionless service It allows the user to initiate the transfer of service data units with a minimum of protocol overheads –Connection-oriented services It allow the user to establish a link-level logical connection before initiating the transfer of any service data units and, if required, to implement error recovery and sequencing of the flow of these units across an established connection

58 LLC (cont ’ d) –Acknowledged connectionless service Obtain reply service –It allows an item of data to be requested from a remote user without a connection first being established

59 LLC (cont ’ d) Protocol operations –Frame Address fields refer to the LLC service access point only No FCS field The complete LLC frame is passed to the MAC sublayer in the form of a primitive MAC sublayer handles the network addressing and error-detection functions

60 Protocol operations(cont ’ d) Two types of operation –Type 1 To support the unacknowledged connectionless service –Type 2 To support the connection-oriented service

61 LLC (cont ’ d) MAC services –User service primitives MA_UNITDATA.request MA_NITDATA.indication MA_UNITDATA.confirmation

62 MAC services (cont ’ d)