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Transmission Systems

High level Data Link Layer Protocol -

HDLC

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High level Data Link Control

Data Link Control – layer 2o Specifies flow and error control for

communicationo Arranges data into frames, supplemented by

control bitso Receiver checks control bits:

• No problem, strips them and passes data;

• Problem detected, communicate with the sender to correct the problem

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Data Link Protocols

A syn ch ron ou sP ro toco ls

C h arac te r O rien ted B it O rien ted

S yn ch ron ou sP ro toco ls

D ata L in k P ro toco ls

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Data Link Protocols

Asynchronous protocolso Feature Start & Stop Bits, variable-length gaps

Synchronous protocolso Interprets a transmission frame as a series of

characterso Control information is in the form of an existing

character encoding system (ASCII)

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Synchronous ProtocolsBit Oriented

SDLCo Synchronous Data Link protocols

HDLCo High Level Data link protocols

LAPso Link Access procedures

LANso Local Area Networks

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High-Level Data Link ControlHDLC

Specifications developed by the ISOSuperset of SDLC (used in IBM’s SNA)Specified as the OSI layer two protocolSupports half & full duplex over point-to-

point and multi-point links

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HDLC Station categories

Primary Station (a station than can issue commands)

Secondary Station (a station that issues responses to commands)

Combined Stations (a station can operate as either primary or secondary, issuing either requests, or responses)

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HDLC Link configurations

Unbalanced (top)o Master /Slave

Symmetrical o One physical station and

two logical functions

Balanced (bottom)o Both stations are the

combined type – point-to-point

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HDLC Data Transfer Modes

Normal Response Mode (NRM)o Unbalanced link configuration w/single

primary and multiple secondary stationso Secondary stations can only transfer data

when polled by the primary station Asynchronous balanced mode (ABM)

o Balanced link configuration w/combined stations

o Either station can initiate data transfer at any time

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HDLC Data Transfer Modes

Asynchronous response mode (ARM)o Unbalanced link configuration w/single

primary and multiple secondary stationso Secondary stations are allowed to transfer

data without a poll from the primary station

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HDLC Modes

NRM ARM ABM

StationType

Primary&

Secondary

Primary&

Secondary

Combined

Initiator Primary Either Any

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DLC Header DLC Trailer

General HDLC Frame Format

INFORMATION

Sandwich the information between aheader and trailer

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FCS

0111111001111110

ADDRESS CONTROL

INFORMATION

Initial Breakdown

FLAG FIELD

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HDLC Frame Types

Information (I) - carries user data and flow/error control information

Supervisory (S) - used to provide additional flow/error control functions

Unnumbered (U) used to provide for system managemento May carry information for management

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FLAG FIELD

Used to indicate beginning and end of the frame

Pattern = 01111110Zero bit stuffing

o inserting an extra zero whenever there are five consecutive 1s in the data

o receiver does not mistake the data for a flag

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ADDRESS

Address of the secondary station for this transmission

8 bit field, 7 bit address (if LSB is a 0, next 8 bits extend the address)

address is all 1’s - broadcast address

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CONTROL FIELD

Either 8 or 16 bit field - used for flow management

different for each type of frameo If start bit = 0 it is an information frameo If first two bits = “10”, it is a supervisory frameo If first two bits = “11”, it is an unnumbered frame

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Specific HDLCControl Bits: I, S, U

0 N(S) N(R)P/F

1 N(R)P/F0 S

1 M(2)P/F1 M(1)

N(S) = sequence number of I frame being sent

N(R) = sequence # of next expected I frame

P/F = poll/final bit

S = supervisory frame code

M(1) M(2) = unnnum. code

1 2 3 4 5 6 7 8 9

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Control Bits in S frames

Frame type 8765 4 321

Receive ready 1000 P/F RRR

Receive Not ready 1001 P/F RRR

Reject 1010 P/F RRR

Selective Reject 1011 P/F RRR

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Unnumbered Frames

SNRM – Set Normal Response Mode SARM – Set Asynchronous Response Mode SABM – Set Asynchronous Balanced Mode UP – unnumbered polling UA – unnumbered acknowledge DISC – disconnect RD – request disconnect DM – Disconnect mode

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Other fields

Information Variable length (usually

multiple of 8 bytes) I-frame = User Data S-frame = does not exist U-frame = management

data (network management)

Frame Check Sequence Uses the standard

CRC (16 bit) or CRC-32 (32 bit)

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HDLC Operation - Initialization

Alerts the other side that initialization is requested

Indicates which of the three modes (NRM, ARM, ABM) is to be used

Indicates whether a 3 or 7 bit sequence number is to be used during the exchange

Accomplished through the unnumbered control format

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Data transfer example - Peers

U-frame, SABM, P=1, establishes link in ABM

Station B returns UA, F=1 indicating agreement on mode and the P/F bits will no longer be used

Station A sends I frame, numbered 0 then another numbered 1

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Data transfer example - Peers Station B now sends it’s

own I frame but also ACKs I frames 0 and 1 from Station A

Station B follows with I frames 1 & 2, the ACK stays the same as no other data has been sent from Station A

Station A has no other data to send, but must ACK data from Station B, so an S-frame is sent, with RR, ACK 3

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HDLC Operation - Disconnect

Either sender or receiver can initiate a disconnect

sends a DISC framedisconnect is accepted with a reply of UA

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HDLC - summary

Extensive and flexible data link protocolMany subsequent link access procedures were

derivatives of this