hdlc & basic protocols

H D L C

Akhil Kaushik, A.P CSE Deptt 1

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


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.


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

allowedAkhil Kaushik, A.P CSE Deptt 4


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)



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.)


HDLC Frame Format

• HDLC frames can be transmitted over

synchronous or asynchronous links.


Opening

FlagAddress 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

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.


http://en.wikipedia.org/wiki/File:NrziEncodedFlags.png

http://en.wikipedia.org/wiki/File:NrziEncodedFlags.png

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

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.


Protocol Operation

The two basic functions in the protocol

are : -

• Link management

• Data transfer (which includes error and

flow control).


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

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.


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.


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.


T H A N K S

hdlc & basic protocols

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