The document discusses HDLC configurations and frame types. There are two common HDLC configurations: 1) NRM uses an unbalanced configuration with a primary station that can send commands and secondary stations that can respond, supporting both point-to-point and multipoint links. 2) ABM uses a balanced configuration where any station can be primary/secondary, supporting only point-to-point links. There are three types of HDLC frames: I-frames carry user data, S-frames carry control information for flow control, and U-frames are for link management. The document further describes the fields within HDLC frames and provides examples of frame exchanges.

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HDLC
Configurations and Transfer Modes
 Two Common Modes
 NRM

The station configured as unbalanced .

On primary and multiple secondary stations

A primary station can send commands and secondary
can respond

Used for both point-to-point and multipoint links
 ABM

The configuration is balanced

Link is point to point

Each station can function as primary/secondary

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Frames
 Three types of frames
 I-frames(Information frames)

To transport user data or control information for user
data like piggy backing
 S-frames(Supervisory frames)

To transport Control frames
 U-frames(Unnumbered frames)

For link management

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11.18 HDLC frame types

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Fields
 Flag Field
 It is an 8-bit sequence with a bit pattern of 01111110
 Identify start and end of the frame
 Serve for the synchronization Bi stuffing is used to avoid the
appearance of this number in that data
 Address field
 Address of the secondary station
 Whether the station is the originator or the destination
 If primary creates the frame it contains “to” otherwise “from”
 It can be 1 or several byte long
 Always the last bit ends with 1
 If more than one byte all byte’s last bit ,except the last byte ,will end
with 0.
 Ethernet does not use primary/secondary environment ,uses two
address fields

Sender address
 Receiver address

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Fields
 Control Field
 1 or 2 byte
 Used for error and flow control
 Interpretation is different for different frame types
 Information Field
 User’s data from the network layer or Network
management information layer
 Length can vary but fixed for one network
 FCS
 Frame check sequence
 It contain 2 or 4 byte ITU-T CRC

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FRAME TYPES
 I-frame

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11.20 S-frame control field in HDLC
Cod
e
Types of S-frames Meaning
00 RR Acknowledges a safe receipt of frame /frames
10 RNR Acknowledges the receipt of frames but tells that
receiver is not ready yet to receive further frames
01 REJ Used in Go-back-N to improve the efficiency and
tells about the lost frame to the sender before its
timer expires
11 SREJ Used in selective Repeat

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11.21 U-frame control field in HDLC

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Table 11.1 U-frame control command and responseTable 11.1 U-frame control command and response
Command/response Meaning
SNRMSNRM Set normal response mode
SNRMESNRME Set normal response mode (extended)
SABMSABM Set asynchronous balanced mode
SABMESABME Set asynchronous balanced mode (extended)
UPUP Unnumbered poll
UIUI Unnumbered information
UAUA Unnumbered acknowledgment
RDRD Request disconnect
DISCDISC Disconnect
DMDM Disconnect mode
RIMRIM Request information mode
SIMSIM Set initialization mode
RSETRSET Reset
XIDXID Exchange ID
FRMRFRMR Frame reject

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Example 3Example 3
Figure 11.22 shows an exchange using piggybacking where is
no error. Station A begins the exchange of information with an
I-frame numbered 0 followed by another I-frame numbered 1.
Station B piggybacks its acknowledgment of both frames onto
an I-frame of its own. Station B’s first I-frame is also
numbered 0 [N(S) field] and contains a 2 in its N(R) field,
acknowledging the receipt of A’s frames 1 and 0 and indicating
that it expects frame 2 to arrive next. Station B transmits its
second and third I-frames (numbered 1 and 2) before accepting
further frames from station A. Its N(R) information, therefore,
has not changed: B frames 1 and 2 indicate that station B is still
expecting A frame 2 to arrive next.

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11.22 Example 3

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Example 4Example 4
In Example 3, suppose frame 1 sent from station B to
station A has an error. Station A informs station B to
resend frames 1 and 2 (the system is using the Go-Back-
N mechanism). Station A sends a reject supervisory frame
to announce the error in frame 1. Figure 11.23 shows the
exchange.

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11.23 Example 4