1Page: 1© 2002 A.W. KringsCS420/520: Data Communications, Lecture 21Data Link Control ProtocolsuHDLC (bit-oriented)–High-level Data Link Control (HDLC) is an enhanced derivative ofSDLC from IBM.

–It’s the most important data link protocol. It is the default seriallink protocol for Cisco routers.

–International standard defined by ISO for point-to-point andmultipoint links.

»Supports full-duplex, transparent-mode operation–Terminology/Features

»Frames sent by primary to secondary are commands, from secondaryto primary are responses

»unbalanced configuration: e.g. point-to-point with single primary andsecondary, or multipoint with single primary and multiple secondaries

»balanced configuration: point-to-point with two primaries and twosecondaries. (Stations are called combined stations)

Page: 2© 2002 A.W. KringsCS420/520: Data Communications, Lecture 21Data Link Control Protocols–Three operational modes»Normal response mode (NRM)nused in unbalanced conf.nsecondaries talk only if asked

»Asynchronous Balanced Mode (ABM)

neach station has equal statusnperforms primary and secondary functions

nused in X.25

»Asynchronous Response mode (ARM)

nused in unbalanced conf.nallows secondaries to initiate transmission without explicitpermission from primary2

Page: 3© 2002 A.W. KringsCS420/520: Data Communications, Lecture 21Data Link Control ProtocolsuHDLC Frame Format:

FlagAddressControlFCSFlag88/168/1616/328Start-of-FrameDelimiterFrame HeaderInformation0 to NInformationFieldFrame CheckSequenceEnd-of-Frame

Delimiter

direction of transmissionnumberof bits

Control field uses three different frame formats:- unnumbered frames- information frames- supervisory frames

Page: 4© 2002 A.W. KringsCS420/520: Data Communications, Lecture 21Data Link Control Protocols–Frame Format: Unnumbered frames» for link setup and disconnection

Unnumbered Commands:M = Set Asynchronous Response Mode (SARM) Set Asynchronous Response Mode Extended (SARME) Set Normal Response Mode (SNRM) Set Normal Response Mode Extended (SNRME) Set Asynchronous Balanced Mode (SABM) Set Asynchronous Balanced Mode Extended (SABME) Reset (RSET) Frame Reject (FRMR) Disconnect (DISC)

1 2 3 4 5 6 7 81MMP/F1

Unnumbered Responses:M = Unnumbered Acknowledge (UA) Command Reject (CMDR) Frame Reject (FRMR) Disconnect Mode (DM)

Unnumbered frame control part:P/F = Poll/final bit3

Page: 5© 2002 A.W. KringsCS420/520: Data Communications, Lecture 21

Data Link Control Protocols–Frame Format: Information Frames»for transfer of user information, includes sequence numberingand piggy-backed acknowledgments

»often called I-frames

1 2 3 4 5 6 7 80N(S)N(R)P/F

Information Frame Control Part:

N(S) = Send Sequence #N(R) = Receive Sequence #P/F = Poll/final bit

Sta97 fig 6.10

Page: 6© 2002 A.W. KringsCS420/520: Data Communications, Lecture 21Data Link Control Protocols–Frame Format: Supervisory frames»for error and flow control and thus contain sequence numbers

S = Receiver Ready (RR) Receiver not Ready (RNR) Reject (REJ) Selective Reject (SREJ)

N(R) = sequence number

1 2 3 4 5 6 7 81SN(R)P/F0

Supervisory frame control part:

Sta97 fig 6.104

Page: 7© 2002 A.W. KringsCS420/520: Data Communications, Lecture 21

Data Link Control Protocols–Extended Frame Formats:

extended formats increase the maximum window to 127Sta97 fig 6.10

Page: 8© 2002 A.W. KringsCS420/520: Data Communications, Lecture 21Data Link Control Protocols–Frame Formats»Uses special flag (01111110) as start and end of framedelimiter wit zero-bit insertion.

»Uses 16-bit CRC-CCITT

nenhanced by shifting in 1’s and not 0’s and inverting remaindernso remainder is special pattern 0001 1101 0000 1111 and not 0.

»P/F bit in the control field is known as poll/final bit

nfor unbalanced link access controlnif sender sets this bit, receiver must acknowledge the frame

nthe acknowledge frame then has P/F bit set => know as the final bit

Gxxxx()=+++161251