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ECE453 – Introduction to
Computer Networks
Lecture 4 – Data Link Layer (I)
Design Issues
Provide a well-defined service interface
Group bits (PHY) into frames (DL)
Deal with transmission errors
Regulate the flow of frames
phys. link
Link Layer: Setting the Context
twophysically connected devices:
host-router, router-router, host-host
unit of data:frame
Protocols implemented in “adapter”
application transport network link physical
network link physical
M M M M
Ht HnHt Hl HnHt HlHnHt M frame
data link protocol
adapter card
Link Layer – The Logical & Physical
Flows
Framing
Character count
Flag bytes with byte stuffing
Starting and ending flags with bit
stuffing
Framing – Byte Stuffing
Tightly tied to the used of 8-bit characters
Parity Bit
Even parity
Odd parity
A code with a single parity bit has a
Hamming distance of ???
It can be used to detect ??? errors
Example
The code has a Hamming distance of ??? It can detect ??? errors It can correct ??? errors
Arrival Original 0000000111 0000011111 0000000111 0000000000
The Hamming Code
Theorem: Given a code withm message bits
andr check bits (n=m+r) which allows all
single errors to be corrected, the lower limit
onr is (m+r+1)<=2r
Hamming code (the bits that are power of 2 are check bits, others are message bits, each check bit forces the parity of some collection of bits, including itself) Can only correct single bit error
Example - Hamming Code
How to Correct Burst Errors
Useskr check bits to make blocks ofkm
data bits immune to a single burst error
of lengthk or less
Error Detecting Code - I
x x x x x x x o x x x x x x x o x x x x x x x o x x x x x x x o x x x x x x x o x x x x x x x o x x x x x x x o o o o o o o o o
