48 Communication Networks
Copyright © 2005 PragSoft
1 1
1 Send
1 2
Sequence Supervisory
Unnumbered 3
Number Code
Code 4
PF PF
PF 5
Receive Receive
6 Sequence
Sequence Unnumbered
7 Number
Number Code
Figure 3.34 summarizes some of the supervisory and unnumbered commands and responses. The supervisory frames use the receive sequence number for
acknowledgment and rejection, as discussed earlier. There are a number of HDLC-related protocols, often referred to as HDLC
subsets. These include:
•
The Link Access Procedure LAP is based on the SARM command of HDLC. Under LAP, the transmitting station sends a SARM to the receiving
station. The latter responds with a UA. At its discretion, the receiving station may interpret the receiving of a SARM command as a request for transmission in the
opposite direction, in which case the roles are reversed.
•
The Link Access Protocol Balanced LAP-B is an ABM subset of HDLC designed for use with X.25 see Chapter 4. It is used for establishing a link
between a DCE and a DTE. LAP-B does not support the SREJ command, and only supports a limited number of the unnumbered commands.
•
The Link Access Protocol, D channel LAP-D is an HDLC subset designed for use with ISDN. It will be described in Chapter 11.
•
The Logical Link Control LLC is an HDLC subset designed as a part of the IEEE 802 series of standards for use with LANs. It will be described in Chapter
10.
3.5. Further Reading
Martin and Leben 1988, Black 1989, Gitlin et al 1992, Halsall 1992, and Stallings 1994 provide comprehensive descriptions of the data link layer protocols
and standards. LAN data link layers are discussed in Chapter 9. Chapter 11 describes the ISDN data link layer.
www.pragsoft.com Chapter 3: The Data Link Layer
49
Figure 3.34 Sample HDLC commands and responses. Mode
Command Response Description
Information I
I Used for exchange of user data.
Supervisory RR
RR Receive Ready. Station is ready to receive frames
and acknowledged receipt of earlier frames. RNR
RNR Receive Not Ready. Station is unable to receive
frames, but acknowledged receipt of earlier frames.
REJ REJ
Reject. Rejects a frame according to the Go-Back- N scheme.
SREJ SREJ
Selective Reject. Selectively rejects a frame. Unnumbered
SNRM Set Normal Response Mode.
SARM Set Asynchronous Response Mode.
SABM Set Asynchronous Balanced Mode.
SIM Set Initialization Mode.
UI UI
Unnumbered Information. Used for exchange of data in unsequenced frames.
UP Unnumbered Poll. Unsequenced poll frame.
UA Unnumbered Acknowledgment. Unsequenced
acknowledgment frame. DISC
Disconnect. Forces a slave station into disconnect mode.
DM Disconnect Mode. Used by a slave to indicate it is
in disconnect mode. RSET
Reset. Enables stations to reset their sendreceive sequence numbers.
3.6. Summary
•
Data link protocols are divided into synchronous and asynchronous . A synchronous protocol is either character-oriented user data is a sequence of
delimited characters or bit-oriented user data is encapsulated by frames. Asynchronous protocols are all character-oriented.
•
A data link protocol may use the master-slave model based on the polling technique or the peer-to-peer model all stations have the same status.
•
Link layer protocols comprise a number of functions: acknowledgment to ensure that data is correctly passed on by stations, timers to impose time limits
on operations, error checking to facilitate error-free transmission, retransmission
to overcome errors, and flow control to manage the
differences in device speeds.
•
Parity checking
is a simple error checking method used with character-oriented
protocols. CRC is a more sophisticated error checking method used with bit- oriented protocols.
50 Communication Networks
Copyright © 2005 PragSoft
•
The sliding window protocol is a widely-used flow control protocol which relies on the transmitter keeping track of transmitted but unacknowledged frames, and
the receiver keeping track of frames that are expected to be received.
•
The BSC or BISYNC protocol is a widely-used synchronous, character- oriented protocol devised by IBM.
•
The HDLC is a widely-used bit-oriented protocol with a number of variations: LAP, LAP-B, LAP-D, and LLC. HDLC offers three modes of operation
NRM, ARM, and ABM. It transmits information in frames, of which there are three types: information frames, supervisory frames, and unnumbered
frames
.
3.7. Exercises
