high-level data link control(hdlc)

7/28/2019 High-Level Data Link Control(HDLC)

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H igh-level Data L ink Control

(HDLC)

HDLC is abit-orientedprotocol for

communication over point-to-point andmultipoint l inks.

I t implements the ARQ mechanisms


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most important

data link

control protocol

specified as ISO3009, ISO 4335

basis for other

data link

control

protocols

Station types:

Primary -controls

operation oflink

Secondary -under control

of primary

station Combined -

issuescommands and

responses

Linkconfigurations

Unbalanced - 1

primary,1 ormultiple secondary

Balanced - 2combined stations

HDLC Terminologies


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HDLC transfer modes

Normal response mode(NRM):

configuration is unbalanced. We have one primarystation and multiple secondary stations.

A primary station can send commands; a secondary

station can only respond. The NRM is used for bothpoint-to-point and mul tiple-point l inks.

Asynchronous balanced mode(ABM). In asynchronous balanced mode (ABM), the

configuration is balanced. The link is point-to-point, andeach station can function as a primary and a secondary


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Normal response mode-NRM


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Asynchronous balanced mode-ABM


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HDLC Frame

Uses synchronous transmission

Transmissions are in the form of framesSingle frame format used


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HDLC Frames Types

Three types

1) Information frames (I -f rames):Transport user data and control information (ACK &

NAK)with piggybacking

2) Supervisory frames(S-frames):

only to transport control information. When nopiggybacking is used.

3) Unnumbered frames (U-frames):

U-f rames are used for carrying system

management.

Information carried by U-frames is intended for

the- l ink management (e.g. l ink set up, release)


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Frame format


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Frame fields

Flag field: The flag field is an 8-bit sequence

01111110that identifies both the start & the endof a frame.

Bit stuffing used to avoid confusion with data

containing flag sequence 01111110.

0 inserted after every sequence of five 1s


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Address Field

1) Identifies secondary station that transmitted or

will receive frame usually 8 bits long may beextended to multiples of 7 bits

2) leftmost bit indicates if is the last octet (1) or not

(0).3)Address 11111111 allows primary to broadcast

4) One byte can identi fy up to 128 stations(l bit is

reserved)


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Control field. The control field is a 1- or 2-byte

segment of the frame used for flow and error

control.

Information field. The information field contains

the user's data from the network layer or

management information. I ts length can vary

from one network to another.

FCS field. The frame check sequence (FCS) is the

HDLC error detection field. It can contain either a

2- or 4-byte CRC.


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Control field


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The first bit def ines the type. If the first bit of the controlfield is 0, thi s means the frame is an I -f rame.

The next 3 bits, called N(S),define the sequence number ofthe frame.

The last 3 bits, called N(R), correspond to theacknowledgment number when piggybackingis used.

The single bit betweenN(S) andN(R) is called the P/Fbit..It has meaning only when it is set (bit = 1) and can meanpoll or final.

Use of Poll/Final (P/F) bit depends on context, in commandframe P bit set to 1 to solicit (poll) response from peer

In response frame F bit set to 1 to indicate response tosoliciting command

Control Field for I-Frames


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Control Field for S-Frames

Used for f low and error control wheneverpiggybacking is either impossible or inappropr iate

The last 3 bits, called N(R), corresponds to the ACKnumber or NAK numberdepending on the type of

S-frame.

The 2 bits called codeis used to define the type of

S-frameitself. With 2 bits, we can have four types

of S-frames


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Types of S-frames

A) code-00-Receive ready (RR):acknowledges thereceipt of a safe & sound frame or group of frames. In this

case, the value N(R)field defines the acknowledgment

number.

B) code-10Receive not ready (RNR):

It acknowledges the receipt of a frame or group of

frames, and it announces that the receiver is busyand cannot receive more frames. I tacts as a kind of

congestion control mechanismby asking the sender

to slow down. its a RR frame with special feature


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C) code-01Reject (REJ):

I tis a NAK that can be used in Go-Back-N ARQto

improve the efficiency of the process by informingthe sender, before the sender time expires, that thelast f rame is lost or damaged. The value ofN(R) isthe negative acknowledgment number.

D) code-11Selective reject (SREJ):

This is a NAK frame used in Selective Repeat

ARQ. Note that the HDLC Protocol uses the termselective reject instead of selective repeat. Thevalue ofN(R) is the negative acknowledgmentnumber.


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Control Field for U-Frames

Unnumbered frames are used to exchange session

management and control information betweenconnected devices.

2-bit pref ix before the P/Fbit and a 3-bit suff ix

after the P/F bit. Together, these two segments

(5 bits) can be used to create up to 32 differenttypesof U-frames.


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HDLC Operation

Initialization

either sidemay request

by issuing

one of the

six set-modecommands

Data Transfer

with flowand error

control

using both I

and S-frames(RR, RNR,

REJ, SREJ)

Disconnect

when faultnoted or at

request of

higher-layer

user sends a

disconnect

(DISC) frame

Consists of exchange of I-frames, S-frames and U-frames

Involves three phases


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HDLC Operation Example


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high-level data link control(hdlc)

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