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Data Link Layer – Part 1
V.T.Raja
Oregon State University
Data Link Layer – Part 1
Introduction
Data link layer is between the _______
layer and the __________ layer
Examples of data link layer protocols:
Most data link protocols perform three
major functions
Three Functions - Data Link Protocol
Media Access Control
When is MAC important?
Data flow
Simplex (One-way transmission)
Half-duplex (Two-way transmission, but only one
direction at a given time)
Full-duplex (Two-way transmission simultaneously)
MAC Types
Controlled Access
Polling
Roll call polling
Frequent roll call
‘Time-out period’
Hub polling
Token passing
Contention
Computers wait until the
circuit is free (i.e., no
other computers are
transmitting)
Example:
Which is better –
Contention or Controlled
Access?
Network Errors
Errors are not uniformly distributed, regardless of
error rate statistics
(e.g., Error rate: 1 in 500,000; 100 in every 50 million bits)
Normally errors occur in bursts
Burst errors
Two categories of network errors
Corrupted data (that have been changed)
Lost data
Sources/causes of network errors
Source of error
Line outages
White noise
Impulse noise
Cross-talk
Attenuation
What Causes it?
Storms, Accidents
Static (Movement of electrons)
Sudden increase in
electricity; voltage
fluctuations
MUX guardbands too small
Decrease in signal power
over distance
Error Prevention
Error Detection
It is possible to develop data transmission
methodologies that vibe very high error detection
and correction performance.
The only way to do error detection and correction
is to send extra data with each message.
In general, the larger the amount of error detection
data sent, the greater the ability to detect an error.
Efficiency of data throughput varies inversely as
the desired amount of error detection is increased.
Error Detection Techniques
There are three common error detection
methods.
Parity Checking
Even Parity
Odd Parity
Longitudinal Redundancy Checking (LRC)
Polynomial Checking
Checksum
Cyclic Redundancy
Parity Checking
One of the oldest and simplest method, parity checking
adds 1 additional bit to each byte in the message. The
value of this parity bit is dependent on the number of 1’s in
each byte transmitted. The parity bit is set to make the
total number of 1’s in the byte (including the parity bit)
either an even number (even parity) or an odd number (odd
parity).
Examples
Does parity checking work if two bits are erroneous?
Longitudinal Redundancy Checking (LRC)
LRC was developed to overcome the problem
with parity’s low probability of detection.
LRC adds one additional character, called the
block check character (BCC) to the end of the
entire message or packet of data.
The value of the BCC is calculated much like the
Parity bit, but for the entire message. Results in a
98% reliability rate.
Example
Polynomial Checking
Like LRC, polynomial checking adds 1 or more
characters to the end of the message based on a
mathematical algorithm.
With checksum, 1 byte is added to the end of the
message. It is obtained by summing the message
values,and dividing by 255. The remainder is the
checksum. (95% effective)
With CRC, 8, 16, 24 or 32 bits are added,
computed by calculating a remainder to a division
problem. (99.969% with 8-bit, 99.99% with 16
bit).