DATA LINK CONTROL PROTOCOLS

2 Introduction Data link control layer – often abbreviated simply to data link layer – is concerned with the transfer of data over a serial data link The transmission mode may be either asynchronous or synchronous and based on either a character-oriented or a bit-oriented transmission control protocol The data link layer is fundamental to the operation of all data communication applications

3 Introduction Depending on the application, the user service provided by the data link layer may be either a simple best-try (connectionless) service or a reliable (connection-oriented) service

4 Introduction The connectionless service means that although error check bits are used to detect errors, any frames that are found to contain transmission errors are simply discarded by the link layer protocol entity. It is also referred to as an unacknowledged service Retransmission becomes a function of a higher layer protocol

5 Introduction The connection-oriented service Data link protocol employs error and flow control procedures to provide a reliable service To achieve this, prior to sending any data (information frames), a logical connection between the two data link layer protocol entities is established (using L_Connect service) All data is transfer using a suitable retransmission and flow control protocol When all data has been exchanged, the logical connection is clear (using L_Disconnect service)

6 Introduction

7 Application Environments In some instances the data link protocols are located in the two communicating DTEs, and the protocol is said to operate on an end-to- end basis In others, data link protocols operates over the local link connecting, for example, the DTE to a network, the protocol is said to have only local significance

8 Application Environments The data link topology may be Point-to-point circuit Direct physical connection Operates on an end-to-end basis

9 Application Envirionments

10 Application Environments The data link topology may be Multipoint or multidrop topology Single transmission line – known as a bus or data highway – is used to connect all the computer together. Normally used in applications that involve a single master (supervisory) computer communicating with a distributed community of slave computers

11 Applications Environments

12 Application Environments

13 Character-oriented Protocols Character-oriented protocols are in use in both point-to-point and multipoint applications Are characterized by the selected transmission control characters used to perform the various transmission control function associated with link management, start of frame and end of frame – frame delimiting, error control and data transparency

14 Character-oriented Protocols Simplex protocols Half-duplex protocols Full-duplex protocols

15 Character-oriented Protocols – Simplex protocols Simplest since it allows a simplex – one direction only – transfer of data from one DTE to another Point-to-point data link topology The most widely used protocols for this function is Kermit

16 Character-oriented Protocols – Simplex protocols Kermit: Synchronous transmission (normally) Point-to-point data link Stop and Wait ARQ

17 Character-oriented Protocols – Simplex protocols

18 Character-oriented Protocols – Simplex protocols Kermit: Frame format

19 Character-oriented Protocols – Simplex protocols Kermit: Frame format

20 Character-oriented Protocols – Simplex protocols Kermit Protocol operation

21 Character-oriented Protocols – Half-duplex Protocols Most character-oriented protocols operate in the half-duplex, stop and wait mode The best known is BSC

22 Character-oriented Protocols – Half-duplex Protocols BSC: Synchronous transmission Connection-oriented protocol Multipoint network/ multidrop bus network

23 Character-oriented Protocols – Half-duplex Protocols BSC

24 Character-oriented Protocols – Half-duplex Protocols BSC: Frame format Data supervisory

25 Character-oriented Protocols – Half-duplex Protocols BSC

26 Character-oriented Protocols – Half-duplex Protocols BSC

27 Character-oriented Protocols – Half-duplex Protocols BSC

28 Character-oriented Protocols – Half-duplex Protocols BSC Protocol operation

29 Character-oriented Protocols – Half-duplex Protocols BSC

30 Character-oriented Protocols – Half-duplex Protocols BSC

31 Character-oriented Protocols – Half-duplex Protocols BSC Protocol performance

32 Character-oriented Protocols – Duplex Protocols A few character-oriented protocols operate in full-duplex mode Character-oriented, full duplex protocols ARPANET

33 Character-oriented Protocols – Duplex Protocols ARPANET

34 Bit-oriented Protocols All new data link protocols are bit- oriented protocols Such protocols use defined bit patterns rather than transmission control characters to signal the start and end of frame – known as frame delimiting

35 Bit-oriented Protocols – HDLC (High-level Data Link Control) Network configuration Point-to-point with single primary and secondary Multipoint with single primary and multiple secondary Point-to-point with two primaries and two secondaries

36 Bit-oriented Protocols – HDLC (High-level Data Link Control)

37 Bit-oriented Protocols – HDLC (High-level Data Link Control) Operational modes Normal response mode (NRM) used in balanced configurations Slave stations (or secondaries) can transmit only when specially instructed by the master (primary) station The link may be point-to-point or multipoint (in this case only one primary station is allowed)

38 Bit-oriented Protocols – HDLC (High-level Data Link Control) Operational modes Asynchronous response mode (ARM) Used in unbalanced configurations Allows a secondary to initiate a transmission without receiving permission from the primary. Normally used with point-to-point configurations and duplex links

39 Bit-oriented Protocols – HDLC (High-level Data Link Control) Operational modes Asynchronous balanced mode (ABM) Used mainly on duplex point-to-point links Each station has an equal status and performs both primary and secondary functions

40 Bit-oriented Protocols – HDLC (High-level Data Link Control) Frame format

41 Bit-oriented Protocols – HDLC (High-level Data Link Control) Frame format Flag field: a string of bits Receiver hunts for flag sequence to synchronize Bit stuffing used to avoid confusion with data containing inserted after every sequence of five 1s If receiver detects five 1s it checks next bit If 0, it is deleted If 1 and seventh bit is 0, accept as flag If sixth and seventh bits 1, sender is indicating abort

42 Bit-oriented Protocols – HDLC (High-level Data Link Control)

43 Bit-oriented Protocols – HDLC (High-level Data Link Control) Frame format Address field: Depend on the mode operation In NRM, on multidrop line, every secondary station is assigned a unique address. Whenever the primary station communicates with a secondary, the address field contains the address of the secondary Not used in this way in ABM because only direct point-to- point links are involved. Instead, it is used to indicate the direction of commands and their associated responses

44 Bit-oriented Protocols – HDLC (High-level Data Link Control) Frame format FCS field 16-bit CRC with G(x) = x 16 + x 12 + x 5 + 1

45 Bit-oriented Protocols – HDLC (High-level Data Link Control) Frame format Control field Unnumbered frames used for such functions as link setup and disconnect Information frames carry the actual information or data and are normally referred to simply as I-frame Supervisory frames used for error and flow control and hence contain send and receive sequence numbers

46 Bit-oriented Protocols – HDLC (High-level Data Link Control) Frame format Control field

47 Bit-oriented Protocols – HDLC (High-level Data Link Control) Frame format

48 Bit-oriented Protocols – HDLC (High-level Data Link Control) Frame format

49 Bit-oriented Protocols – HDLC (High-level Data Link Control) Protocol operation Link management Before any information (data) may be transmitted, a logical connection between the two communicating parties must be established

50 Bit-oriented Protocols – HDLC (High-level Data Link Control)

51 Bit-oriented Protocols – HDLC (High-level Data Link Control) Protocol operation Data transfer Error control uses a continuous RQ with either selective repeat or go-back-N retransmission strategy Flow control is based on a window mechanism

52 Bit-oriented Protocols – HDLC (High-level Data Link Control)

53 Bit-oriented Protocols – HDLC (High-level Data Link Control) Protocol operation Data transfer

54 Bit-oriented Protocols – HDLC (High-level Data Link Control) Protocol operation Data transfer

55 Bit-oriented Protocols – HDLC (High-level Data Link Control)

56 Bit-oriented Protocols – HDLC (High-level Data Link Control) User interface

57 Bit-oriented Protocols – Link Access Procedure version B (LAPB) LAPB Subset of HDLC that is used to control the transfer of I-frame across a point-to-point duplex data link that connects a computer to a public or private packet-switching network (X.25 networks) LAPB is an extended version of an earlier subset LAPA (Link Access Procedure version A)

58 Bit-oriented Protocols – Link Access Procedure version B (LAPB) The computer is the DTE and the packet switching exchange is the data circuit-terminating equipment (DCE) LAPB is used to control the transfer of information frames across the local DTE-DCE interface (local significance)

59 Bit-oriented Protocols – Link Access Procedure version B (LAPB) LAPB used asynchronous balanced mode with DTE and DCE and All I-frame are treated as command frames

60 Bit-oriented Protocols – Link Access Procedure version B (LAPB)

61 Bit-oriented Protocols – Link Access Procedure version B (LAPB)

62 Bit-oriented Protocols – Link Access Procedure version B (LAPB)

63 Bit-oriented Protocols – Link Access Procedure version B (LAPB)

64 Bit-oriented Protocols – Link Access Procedure – D channel (LAPD) ISDN (ITU-D) ABM Always 7-bit sequence numbers (no 3-bit) 16 bit address field contains two sub- addresses One for device and one for user (next layer up

65 Bit-oriented Protocols – Logical Link Control (LLC) IEEE 802 Different frame format Link control split between medium access layer (MAC) and LLC (on top of MAC) No primary and secondary - all stations are peers Two addresses needed Sender and receiver Error detection at MAC layer 32 bit CRC Destination and source access points (DSAP, SSAP)