The document discusses High-Level Data Link Control (HDLC), a bit-oriented synchronous data link layer protocol developed by ISO. It describes HDLC's operation modes including Normal Response Mode, Asynchronous Response Mode, and Asynchronous Balanced Mode. The document also covers HDLC frame format, frame classes including unnumbered, supervisory, and information frames, and HDLC protocol operation for link management and data transfer with error and flow control.

1. H D L C
Akhil Kaushik, A.P CSE Deptt 1

2. Introduction
• High-Level Data Link Control (HDLC) –It is a
bit-oriented synchronous data link layer protocol
developed by the ISO.
• HDLC provides both connection-oriented and
connectionless service.
• Many other layer 2 protocols are also dependent
on HDLC like SDLC, LAPB, LAPD, etc
Akhil Kaushik, A.P CSE Deptt 2

3. Introduction
• The HDLC protocol is defined by ISO for use
on both point-to-point and multipoint
(multidrop) data links.
• It supports full duplex transparent-mode
operation and is now extensively used in both
multipoint and computer networks.
Akhil Kaushik, A.P CSE Deptt 3

4. HDLC Operation Modes
1. Normal Response Mode (NRM ) -
-It is used in unbalanced configurations.
-In this mode, slave stations (or secondary) can
only transmit when specially instructed by the
master (primary station).
-The link may be point-to-point or multipoint. In
the latter case only one primary station is
allowed
Akhil Kaushik, A.P CSE Deptt 4

5. HDLC Operation Modes
2. Asynchronous Response Mode (ARM ) -
-This mode is used in unbalanced configurations.
- It allows a secondary station to initiate a transmission
without receiving permission from the primary station.
-This mode is normally used with point-to-point
configurations and full duplex links and allows the
secondary station to send frames asynchronously with
respect to the primary station)
Akhil Kaushik, A.P CSE Deptt 5

6. HDLC Operation Modes
3. Asynchronous Balanced Mode (ABM) –
- It is used mainly on full duplex point-to-point links for
computer to computer communications and for
connections between a computer and a packed
switched data network
- In this case each station has an equal status and
performs the role of both primary and secondary
functions.
- This mode is used in the protocol set known as X.25.)
Akhil Kaushik, A.P CSE Deptt 6

7. HDLC Frame Format
• HDLC frames can be transmitted over
synchronous or asynchronous links.
Akhil Kaushik, A.P CSE Deptt 7
Opening
Flag
Address Control Information FCS Flag
8 bits 8 or more bits 8 or 16 bits
Variable
length, 0 or
more bits
16 or 32 bits 8 bits

8. HDLC Frame Format
• Those links have no mechanism to mark the
beginning or end of a frame, so the beginning
and end of each frame has to be identified.
• This is done by using a frame delimiter, or
flag, & this sequence is '01111110' or 7E(hex).
• Each frame begins and ends with a frame
delimiter.
Akhil Kaushik, A.P CSE Deptt 8

9. Frame Classes
1. Unnumbered frames –
- Unnumbered frames are used for link
management
- Used to set up the logical link between the
primary station and a secondary station
- To inform the secondary station about the
mode of operation which is used.
2. Supervisory frames - are used for error and
flow control.
Akhil Kaushik, A.P CSE Deptt 9

10. Frame Classes
3. Information frames –
- Information frames are those who carry the
actual data.
-The Information frames can be used to
piggyback acknowledgment information
relating to the flow of Information frames in the
reverse direction when the link is being
operated in ABM or ARM.
Akhil Kaushik, A.P CSE Deptt 10

11. Protocol Operation
The two basic functions in the protocol
are : -
• Link management
• Data transfer (which includes error and
flow control).
Akhil Kaushik, A.P CSE Deptt 11

12. Protocol Operation
1. Link Management:-
• Before any transmission a logical connection is
accomplished by the exchange of two unnumbered
frames.
• The procedure for setting a logical connection (In a
multidrop link) is given below:-
The SNRM is first sent by the primary station with
the poll bit set to 1 and the address of the appropriate
secondary in the address field. The secondary
responds with a UA frame with the final bit set and its
own address in the address field.
Akhil Kaushik, A.P CSE Deptt 12

13. Protocol Operation
• After all the data has
been transferred, the
link is cleared by the
primary sending a DISC
frame and the
secondary responding
with a UA between the
two communication
parties must be
established.
Akhil Kaushik, A.P CSE Deptt 13

14. Protocol Operation
2. Data transfer:-
• In NRM all data (information frames) is transferred
under the control of the primary station. The
unnumbered poll frame with the P bit set to 1 is
normally used by the primary to poll a secondary.
• If the secondary has no data to transmit, it returns an
RNR frame with the F bit set. If data is waiting, it
transmits the data, typically as a sequence of
information frames.
Akhil Kaushik, A.P CSE Deptt 14

15. Protocol Operation
• The two most important aspects associated with the
data transfer phase are error control and flow control.
• Essentially, error control uses a continues RQ
procedure with either a selective repeat or a go back N
transmission strategy.
• Flow control is based on a window mechanism.
Akhil Kaushik, A.P CSE Deptt 15

16. T H A N K S