Data Link Layer in Computer Networks: Framing, Error Detection and Correction, Study notes of Computer Systems Networking and Telecommunications

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ECE453 – Introduction toComputer Networks Lecture 4

Data Link Layer (I)

Lecture 4

Data Link Layer (I)

Design Issues

Provide a well-defined service interface Group bits (PHY) into frames (DL) Deal with transmission errorsDeal with transmission errors Regulate the flow of frames

Link Layer – The Logical & PhysicalFlows

Three Types of Services

Unacknowledged connectionlessservice Acknowledged connectionless serviceAcknowledged connectionless service Acknowledged connection-orientedservice

Framing

Character count Flag bytes with byte stuffing Starting and ending flags with bit stuffingstuffing

Framing – Byte Stuffing

Tightly tied to the used of 8-bit characters

Error Detection and Correction

Use redundancy

ARQ: Error detection and retransmission

FEC: Error correction (forward error

FEC: Error correction (forward error correction)

H-ARQ: combination ARQ and FEC

The Hamming Distance

Codewords (n bits): message (k bits) + checkbits (r bits) Hamming distance: the number of positions inwhich two codewords differ Hamming distance of the complete code (d_min):two codewords with the minimum Hammingdistance To

detect

t

errors, Hamming distance

of the code

:

(

t

+1)

To

correct

t

errors, Hamming distance

of the

code

: (

t

+1)

Generator of Linear Code

Each linear code has a generator matrixG. The output codeword of encoder isThe output codeword of encoder is

where s is the row vector of source information bits.

sG

C