Page: 1© 2002 A.W. Krings CS420/520: Data Communications, Lecture 21
Data Link Control Protocols
u HDLC (bit-oriented)
–High-level Data Link Control (HDLC) is an enhanced derivative of SDLC from IBM.
–It’s the most important data link protocol. It is the default serial link protocol for Cisco routers.
–International standard defined by ISO for point-to-point and multipoint links.
»Supports full-duplex, transparent-mode operation
–Terminology/Features
»Frames sent by primary to secondary are commands , from secondary to primary are responses »unbalanced configuration : e.g. point-to-point with single primary and secondary, or multipoint with single primary and multiple secondaries »balanced configuration: point-to-point with two primaries and two secondaries. (Stations are called combined stations )
Page: 2© 2002 A.W. Krings CS420/520: Data Communications, Lecture 21
Data Link Control Protocols
–Three operational modes
»Normal response mode (NRM)
n used in unbalanced conf.
n secondaries talk only if asked
»Asynchronous Balanced Mode (ABM)
n each station has equal status
n performs primary and secondary functions
n used in X.25
»Asynchronous Response mode (ARM)
n used in unbalanced conf.
n allows secondaries to initiate transmission without explicit
permission from primary
Page: 3© 2002 A.W. Krings CS420/520: Data Communications, Lecture 21
Data Link Control Protocols
u HDLC Frame Format:
Flag Address Control FCS Flag
88/168/1616/328Start-of-Frame Delimiter Frame Header Information 0 to N Information Field Frame Check Sequence End-of-Frame Delimiter direction of transmission
number
of bits
Control field uses three different frame formats:
- unnumbered frames
- information frames
- supervisory frames
Page: 4© 2002 A.W. Krings CS420/520: Data Communications, Lecture 21
Data 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 8
1M M P/F 1Unnumbered Responses:M = Unnumbered Acknowledge (UA) Command Reject (CMDR) Frame Reject (FRMR) Disconnect Mode (DM)Unnumbered frame control part:
P/F = Poll/final bit