Lecture Notes on Data Link Layer and Frames - Computer Networks | CMSC 417, Study notes of Computer Systems Networking and Telecommunications

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copyright 1997 Jeffrey K. Hollingsworth

Announcements

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Homework #2 was returned

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Reading^ –

Today: 3.1-3.

copyright 1997 Jeffrey K. Hollingsworth

Data Link Layer

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Goal: transmit error free frames over the physical link

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Sample Issues:^ –

how big is a frame?

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can I detect an error in sending the frame?

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what demarks the end of the frame?

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how to control access to a shared channel?

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Examples:^ –

Ethernet framing

copyright 1997 Jeffrey K. Hollingsworth

Other Link Functions

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Error Control^ –

may want to do sequence numbers and re-transmission

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this introduces overhead, but useful if probability of failure is high

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Flow Control^ –

provide rate matching between sender and receiver

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sender has rules about when it can send: credits, etc.

copyright 1997 Jeffrey K. Hollingsworth

Error Correcting Codes

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Idea: add redundant information to permit recovery^ –

this is the dual of data compression (remove redundancy)

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Hamming distance (n)^ –

number of bit positions that differ in two words

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key idea: need n single bit errors to go from one word to the other

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to detect d errors, need a hamming distance of d+1 from

any other valid

word.

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to recover d errors, need a hamming distance of 2d + 1

• any error of d bits is still closer to correct word

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Parity bit^ –

ensure that every packet has an odd (or even) # of 1’s

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permits detection of one 1 bit error

copyright 1997 Jeffrey K. Hollingsworth

Hamming Code Example

Cha r

AS CII

Ha mming

H

a^

m

I^

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Burst Errors^ –

can send hamming codes by column rather than row

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if use k rows, then can detect any burst error up to k bits

• uses kr bits to check a block km bits long