Data 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 to primary are responsescommands, from secondary»unbalanced configurationsecondary, or multipoint with single primary and multiple secondaries: e.g. point-to-point with single primary and»balanced configuration:secondaries. (Stations are called point-to-point with two primaries and twocombined stations)© 2002 A.W. KringsPage: 1CS420/520: 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 functionsnused in X.25»Asynchronous Response mode (ARM)nused in unbalanced conf.nallows secondaries to initiate transmission without explicitpermission from primary© 2002 A.W. KringsPage: 2CS420/520: Data Communications, Lecture 211Data Link Control ProtocolsuHDLC Frame Format:Start-of-FrameFrame HeaderInformationFrame CheckEnd-of-FrameDelimiterFieldSequenceDelimiter88/168/160 to N16/328numberof bitsFlagAddressControlInformationFCSFlagdirection of transmissionControl field uses three different frame formats:- unnumbered frames- information frames- supervisory frames© 2002 A.W. KringsPage: 3CS420/520: Data Communications, Lecture 21Data Link Control Protocols–Frame Format: Unnumbered frames» for link setup and disconnectionUnnumbered frame control part:1 2 3 4 5 6 7 811MP/FMP/F = Poll/final bitUnnumbered Commands:Unnumbered Responses:M = Set Asynchronous Response Mode (SARM)M = Unnumbered Acknowledge (UA) Set Asynchronous Response Mode Extended (SARME) Command Reject (CMDR) Set Normal Response Mode (SNRM) Frame Reject (FRMR) Set Normal Response Mode Extended (SNRME) Disconnect Mode (DM) Set Asynchronous Balanced Mode (SABM) Set Asynchronous Balanced Mode Extended (SABME) Reset (RSET) Frame Reject (FRMR) Disconnect (DISC)© 2002 A.W. KringsPage: 4CS420/520: Data Communications, Lecture 212Data Link Control Protocols–Frame Format: Information Frames»for transfer of user information, includes sequence numberingand piggy-backed acknowledgments»often called I-framesInformation Frame Control Part:1 2 3 4 5 6 7 8N(S) = Send Sequence #0N(S)P/FN(R)N(R) = Receive Sequence #P/F = Poll/final bitSta97 fig 6.10© 2002 A.W. KringsPage: 5CS420/520: Data Communications, Lecture 21Data Link Control Protocols–Frame Format: Supervisory frames»for error and flow control and thus contain sequence numbersSupervisory frame control part:1 2 3 4 5 6 7 810SP/FN(R)S = Receiver Ready (RR) Receiver not Ready (RNR) Reject (REJ) Selective Reject (SREJ)N(R) = sequence number Sta97 fig 6.10© 2002 A.W. KringsPage: 6CS420/520: Data Communications, Lecture 213Data Link Control Protocols–Extended Frame Formats:extended formats increase the maximum window to 127Sta97 fig 6.10© 2002 A.W. KringsPage: 7CS420/520: 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-CCITTG(x)=x16+x12+x5+1nenhanced 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 bitnfor unbalanced link access controlnif sender sets this bit, receiver must acknowledge the framenthe acknowledge frame then has P/F bit set => know as the final bit© 2002 A.W. KringsPage: 8CS420/520: Data Communications, Lecture 214Data Link Control Protocols–HDLC Commands and responses»Sta00, table 7.1© 2002 A.W. KringsPage: 9CS420/520: Data Communications, Lecture 21© 2002 A.W. KringsPage: 10CS420/520: Data Communications, Lecture 215Data Link Control Protocolsexample: data transfer»NRM (Normal response mode) assumed,ni.e. transferred under control of primarynunnumbered poll (UP) with poll bit set is send to secondarynsecondary return RR-frame with F bit set => no data to send© 2002 A.W. KringsPage: 11CS420/520: Data Communications, Lecture 21Data Link Control Protocols© 2002 A.W. KringsSta00 fig 7.12Page: 12CS420/520: Data Communications, Lecture 216Data Link Control ProtocolsSta00 fig 7.12© 2002 A.W. KringsPage: 13CS420/520: Data Communications, Lecture 217