ERROR CONTROL, ERROR DETECTION

AND ERROR CORRECTION

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IntroductionNoise 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.

Transmission errors – are caused by electrical interference

from natural sources such as noise, lightning as well as from man-made sources such as motors, generators

and power lines.

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Transmission Error

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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.

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White NoiseAlso known as thermal or Gaussian noiseRelatively constant and can be reduced.If white noise gets to strong, it can completely disrupt the signal.

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Impulse NoiseOne of the most disruptive forms of noise.

Random spikes of power that can destroy one or more bits of information.

Difficult to remove from an analog signal because it may be hard to distinguish from the

original signal.Impulse noise can damage more bits if the bits

are closer together (transmitted at a faster rate).

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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.

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EchoThe reflective feedback of a transmitted

signal as the signal moves through a medium.

Most often occurs on coaxial cable.If echo bad enough, it could interfere

with original signal.Relatively constant, and can be

significantly reduced.

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JitterThe result of small timing irregularities

during the transmission of digital signals.Occurs when a digital signal is repeater

over and over.If serious enough, jitter forces systems to

slow down their transmission.Reduce jitter - shielding

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Delay DistortionOccurs because the velocity of

propagation of a signal through a medium varies with the frequency of the signal. Can be reduced using equalizers

AttenuationThe continuous loss of a signal’s

strength as it travels through a medium.

As signal travels, it become distorted.Changes shape

Successive bits may merge, making reception difficult

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Distance

When signal travels, it gets weaker.If too weak, cannot tell 1s and 0s.

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Distance

Unwanted signal from outside sourcesOften intermittent, difficult to diagnose

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SignalStrength

Signal

Interference

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Error PreventionTo prevent errors from happening, several

techniques may be applied:- Proper shielding of cables to reduce interference

- Telephone line conditioning or equalization - Replacing older media and equipment with new,

possibly digital components- Proper use of digital repeaters and analog

amplifiers- Observe the stated capacities of the media

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DATA COMMUNICATION ERRORS

Single - bit errors – when only one bit within a given data string is in error.

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Multiple – bit errors – when two or more nonconsecutive bits within a given data string are in error.

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Burst errors – when two or more consecutive bits within a given data string are in error.

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Error performance – is the rate in which error occur.

Bit error rate(BER) – actual historical record of a system’s error performance.

Error control can be divided into two general categories:

Error detection Error correction

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Error DetectionIs the process of monitoring data transmission and determining when errors have occurred.

Error detection uses the concept of redundancy, which means adding extra bits

for detecting errors at the destination.

Redundancy – a form of error detection in which it duplicates each data unit for the purpose of detecting errors.

Is an effective but rather costly means of detecting errors especially with long messages.

Redundancy checking – adding bits for the sole purpose of detecting errors.

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Redundancy

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Four types of redundancy Four types of redundancy checks are usedchecks are used

in data communicationsin data communications

Vertical redundancy checking Is the simplest error detection scheme. Generally referred to as character

parity or simply parity. It can detect single bit error. It can detect burst errors only if the

total number of errors is odd.

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Longitudinal redundancy checking A redundancy error detection scheme that

uses parity to determine if a transmission error has occurred within a message.

Sometimes called message parity. LRC increases the likelihood of detecting

burst errors. If two bits in one data units are damaged

and two bits in exactly the same positions in another data unit are also damaged, the LRC checker will not detect an error.

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VRC and LRC

Cyclic redundancy checking The most reliable redundancy checking

technique for error detection.

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Checksum Another relatively simple form of

redundancy error checking where each character has a numerical value assigned to it.

The checksum detects all errors involving an odd number of bits.

It detects most errors involving an even number of bits.

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If one or more bits of a segment are damaged and the corresponding bit or bits of opposite value in a second segment are also damaged, the sums of those columns will not change and the receiver will not detect a problem.

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Error correctionTwo methods

Retransmission after detecting error Forward error correction (FEC)

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RetransmissionWhen a receive station requests the transmit station to resend a message(or a portion of message) when the message is received in error.Often called automatic retransmission request(ARQ).Most reliable method of error correction

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Two basic types1. Discrete – uses acknowledgements to indicate the successful or unsuccessful reception of data.

Positive – the destination station responds with a positive acknowledgement when it receives an error-free message.

Negative - the destination station responds with a positive acknowledgement when it receives a message containing errors to call for a retransmission.

Retransmission after time-out if the sending station does not receive an

acknowledgement after a predetermined length of time(called a time-out), it retransmits the message.

2. Continuous – used when message are divided into smaller blocks or frames that are sequentially numbered and transmitted in succession, without waiting for acknowledgement between blocks.

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Forward Error Correction

Is the only error correction scheme that actually detects and corrects

transmission errors when they are received without requiring a

retransmission.

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Consider only a single-bit error in k bits of data k possibilities for an error One possibility for no error #possibilities = k + 1

Add r redundant bits to distinguish these possibilities; we need

2r k+1But the r bits are also transmitted along with data;

hence2r k+r+1

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Number ofNumber ofdata bitsdata bits

kk

Number of Number of redundancy bitsredundancy bits

rr

Total Total bitsbitskk + + rr

11 22 33

22 33 55

33 33 66

44 33 77

55 44 99

66 44 1010

77 44 1111

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Simple, powerful FEC Widely used in computer memory

Known as ECC memory

error-correcting bits

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Receiver receives 10010100101

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Minimum Hamming Distance is 3 It can correct at most 1 bit error It can detect at most 2 bit error But… not both!!! (Why?)

SECDED – Extended Hamming code with one extra parity bit Achieves minimum Hamming distance of 4 Can distinguish between one bit and two bit

errors

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