HDLC is a bit-oriented protocol that organizes data into frames for transmission between devices over point-to-point or multipoint links. An HDLC frame consists of opening and closing flags bounding address, control, information, and frame check sequence fields. The control field contains sequence numbers for flow and error control. There are three classes of frames: information frames for data, unnumbered frames for link management, and supervisory frames for flow and error control using sequence numbers when piggybacking is not possible.

1. HDLC Frame
Structure
(HIGH LEVEL DATA LINK CONTROL)

2. INTRODUCTION:
 HDLC is a bit oriented code transparent Synchronous data
link layer protocol.
 It is developed by International Standard Organization
(ISO).
 Current standard for HDLC is ISO 13239.
 It provides both connection-oriented and connectionless
service .
 In HDLC, data is organized into a unit (called a frame) and
sent across a network to a destination that verifies its
successful arrival.

3. contd.
 The HDLC protocol embeds information in a data frame that allows devices
to control data flow and correct errors.
 HDLC supports both half-duplex and full-duplex communication over
point to point & multipoint link.

4. HDLC FRAME STRUCTURE
Flag Address Control Information FCS Flag
8 bits 8 or more bits 8 or 16 bits
Variable length, n
* 8 bits
16 or 32 bits 8 bits
Flag Address Control Information FCS Flag
8 bits 8 or more bits 8 or 16 bits
Variable length, n
* 8 bits
16 or 32 bits 8 bits
Flag Address Control Information FCS Flag
8 bits 8 or more bits 8 or 16 bits
Variable length, n
* 8 bits
16 or 32 bits 8 bits

5. HDLC FRAME FORMAT
 Flag : 01111110 -indicates start and ending of frames.
 FCS (Frame Check Sequence) : 16-bit CRC using generating polynomial
 G(x) =x 16 + x 12+ x 5 + 1
 Address Field:
➢ When a primary station is sending a frame, the field contains the
receiver identity.
➢ If a secondary station is sending the frame, the address field contains
the sender identity.
➢ In some cases, it contains a group or broadcast address.

6. Frame Types
● There are three different classes of frames used in HDLC
➢ Unnumbered frames, used in link setup and
disconnection, and hence do not contain ACK.
➢ Information frames, which carry actual information.
Such frames can piggyback ACK in case of ABM.
➢ Supervisory frames, which are used for error and flow
control purposes and hence contain send and receive
sequence numbers.

7. HDLC CONTROL FIELDS

8. CONTROL FIELDS
 In I-frames, N(s) is the sequence number of the frame being sent.
 N(r) is the sequence number of the frame being expected.
 The P/F bit, known as the poll/final bit, is used with
different meaning in different contexts.
➢ It is used to indicate polling, to indicate the final I- frame,
etc

9. 3 DIFFERENT CLASSES OF
FRAMES
 Information frames, or I-frames, transport user data from the network
layer. In addition they can also include flow and error control
information piggybacked on data.
 Unnumbered frames, or U-frames, are used for various miscellaneous
purposes, including link management. Some U-frames contain an
information field, depending on the type.
 Supervisory Frames, or S-frames, are used for flow and error control
whenever piggybacking is impossible or inappropriate, such as when
a station does not have data to send. S-frames do not have
information fields.

10. contd.
 There are four different supervisory frames
➢ SS=00, Receiver Ready (RR), N(R),ACKs all frames received up to and including the one
with sequence number N(R) - 1.
➢ SS=10, Receiver Not Ready (RNR), and N(R) has the same meaning as above
➢ SS=01, Reject; all frames with sequence number N(R) or higher are rejected, which
in turns ACKs frames with sequence number N(R) -1 or lower.
➢ SS=11, Selective Reject; the receive rejects the frame with sequence number N(R)

11. contd.
 The unnumbered frames can be grouped into the following categories:
➢ Mode-setting commands and responses
➢ Recovery commands and responses
➢ Miscellaneous commands and responses