Chapter 6
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Transcript of Chapter 6
Chapter 6
Errors, Error Detection, and Error Control
Topics
Types of noise Error prevention Error detection
Simple parityLongitudinal parityCyclic redundancy checksum (CRC)
Error control
Introduction
Noise is always present. If a communications line experiences too much
noise, the signal will be lost or corrupted. Communication systems should check for
transmission errors. Once an error is detected, a system may
perform some action. Some systems perform no error control, but
simply let the data in error be discarded.
Types of Noise
White Noise Thermal or
Gaussian noise. Relatively constant. Disrupt the signal if
too strong. Use signal
regenerator or filter to remove it
Types of Noise
Impulse Noise Non-continuous One of the most
disruptive ones Destroy one or
more bits of information
Difficult to detect and remove
Types of Noise
CrosstalkUnwanted coupling between two different
signal paths.For example, hearing another conversation
while talking on the telephone.Relatively constant and can be reduced with
proper measures
Types of Noise
Echo The reflective
feedback of a transmitted signal
Most often occurs on coaxial cable.
Could interfere with the original signal.
Relatively constant, and can be significantly reduced.
Types of Noise
Jitter The result of small timing
irregularities Digital signals Occurs when repeated
over and over Could force systems to
slow down their transmission.
Types of Noise
Delay Distortion Occurs because the velocity of propagation of a
signal through a medium varies with the frequency of the signal
Can be reduced
Attenuation The continuous loss of a signal’s strength as it
travels through a medium
Error Prevention
Error Detection Techniques
Despite the best prevention techniques, errors may still happen
To detect an error, something extra has to be added to the data/signal.
Two techniques commonly used:
Simple Parity Checking
If performing even parity, add a parity bit such that an even number of 1s are maintained.
If performing odd parity, add a parity bit such that an odd number of 1s are maintained
Examples send 1001010 using even parity
send 1001011 using odd parity
Longitudinal Parity Checking
Longitudinal parity adds a parity bit to each character then adds a row of parity bits after a block of characters.
The row of parity bits is actually a parity bit for each “column” of characters.
The row parity bits plus the column parity bits add a great amount of redundancy to a block of characters
Catch of errors
What if the following?
Cyclic Redundancy Checksum (CRC) The CRC error detection method treats the packet of data to
be transmitted as a large polynomial. The transmitter takes the message polynomial and using
polynomial arithmetic, divides it by a given generating polynomial.
The quotient is discarded but the remainder is “attached” to the end of the message.
The message (with the remainder) is transmitted to the receiver.
The receiver divides the message and remainder by the same generating polynomial.
If a remainder not equal to zero results, there was an error during transmission.
Cyclic Redundancy Checksum (CRC Cost
Performance
Error Control
Once an error is detected, what is the receiver going to do?
1. Do nothing (Frame Relay)
2. Return an error message to the transmitter and request for resend
Stop-and-Wait ARQ Go-Back-N ARQ Selective-Reject ARQ
3. Fix the error with no further help from the transmitter