HDLC Frame Types

I-frame S-frame U-frame

I-Frame: Information Frame

Flag Address Control Info FCS flag

Information in I-frame is the user information or payload coming from upper layers.Consider the control field of I-frame:

The first bit is ‘0’ informing the receiver that it is an I-frame N(S): Sequence number of frame sent P/F bit: Poll/Final bit: P=1 denotes that Primary is active; F=1 denotes that

secondary is active. (for details, refer to book) N(R): Sequence number of next frame expected

0 (1-bit)

N(S)

(3-bits)P/F

(1-bit)N(R) (3-bits)

S-Frame: Supervisory Frame

Flag Address Control FCS flag

10(2-bits)

Code(2-bits)

P/F(1-bit)

N(R)(3-bits)

The first 2 bits (10) show that it is an S-frame.

Code: Applicable for S and U-frames only. A 2 bit field which is interpreted as follows:

o 00: RR (Receive Ready)o 10: RNR (Receive Not Ready)o 01: REJ (Reject)o 11 SREJ (Selective Reject)

U-Frame: Unnumbered Frame

Flag Address Control Info FCS flag

1 1(2-bits)

Code(2-bits)

P/F(1-bit)

Code(3-bits)

The first 2 bits (11) show that it is a U-frame.

U-frame codes are divided into two sections:

2 bit prefix before P/F bit 3 bit suffix after P/F bit

The important combinations are as shown in the table below:

2 bit Code 3 bit code Functional Description

00 001 SNRM: Set Normal Response Mode

11 000 SARM: Set Asynchronous Response Mode

11 100 SABM: Set Asynchronous balance Mode

10 001 FRMR: Frame Reject

Information in U-frame is the Management Information which is required or managing the network.