chapter 06 x

Chapter 6

Errors, Error Detection, and Error Control

Data Communications andComputer Networks: A Business Users Approach

Data Communications and Computer Networks Chapter 6

ObjectivesIdentify the different types of noise commonly found in computer networksSpecify the different error prevention techniques and be able to apply an error prevention technique to a type of noiseCompare the different error detection techniques in terms of efficiency and efficacyPerform simple parity and longitudinal parity calculations and enumerate their strengths and weaknesses


ObjectivesCite the advantages of cyclic redundancy checksum and specify what types of errors cyclic redundancy checksum will detectDifferentiate the three basic forms of error control and describe under what circumstances each may be usedFollow and example of stop-and-wait ARQ, go-back-N ARQ and selective reject ARQ


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.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.


Noise and ErrorsWhite noise (also known as thermal or Gaussian noise)Relatively constant and can be reduced.Depends on temperature. As temp. increases the noise level increasesIf white noise gets too strong, it can completely disrupt the signal.


Removing NoiseFrom a digital signal the signal is passed through a signal regeneratorFrom an analog signal the signal is passed through filters, not as effective as the signal regenerator


Noise and ErrorsImpulse 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).


Noise and ErrorsCrosstalkUnwanted coupling between two different signal paths.That is, electric or magnetic fields of one telecommunication signal affecting a signal in an adjacent circuitFor example, hearing another conversation while talking on the telephone.Relatively constant and can be reduced with proper measures.


Noise and ErrorsEchoThe reflective feedback of a transmitted signal as the signal moves through a mediumMost often occurs on coaxial cable.If echo bad enough, it could interfere with original signalRelatively constant, and can be significantly reduced.


Noise and ErrorsJitterThe 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.Steps can be taken to reduce jitter Proper shielding to reduce electromagnetic interference and crosstalkLimit number of times signal is repeated


Noise and ErrorsDelay distortionOccurs because the velocity of propagation of a signal through a medium varies with the frequency of the signal. Can be reduced by use of techniques that equalise transfer speeds of faster and slower signals

AttenuationThe continuous loss of a signals strength as it travels through a medium. Can be eliminated by amplifiers (analog signals) and repeaters (digital signals)


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


Error DetectionDespite the best prevention techniques, errors may still happen.To detect an error, something extra has to be added to the data/signal. This extra is an error detection code.Lets examine two basic techniques for detecting errors: parity checking, and cyclic redundancy checksum.


Parity ChecksSimple parity - 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.For example, if the character 1001010 is to be sent, using even parity, a parity bit = 1 would be added to the character.If the character 1001011 is to be sent, using even parity, a parity bit = 0 would be added to the character.


Parity ChecksLongitudinal 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.


Parity ChecksBoth simple parity and longitudinal parity do not catch all errors.Simple parity only catches odd numbers of bit errors.Longitudinal parity is better at catching errors but requires too many check bits added to a block of data.We need a better error detection method. What about cyclic redundancy checksum?


Cyclic Redundancy ChecksumThe 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.


Cyclic Redundancy ChecksumThe 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.If a remainder of zero results, there was no error during transmission.


Error ControlOnce an error is detected, what is the receiver going to do?1. Do nothing2. Return an error message to the transmitter3. Fix the error with no further help from the transmitter


Error ControlDo nothingSeems like a strange way to control errors but some newer systems such as frame relay perform this type of error control.

Return a message has three basic formats:1. Stop-and-wait ARQ (Automatic Repeat reQuest)2. Go-back-N ARQ3. Selective-reject ARQ


Error ControlStop-and-wait ARQ is the simplest of the error control protocols.A transmitter sends a frame then stops and waits for an acknowledgment.If a positive acknowledgment is received, the next frame is sent.If a negative acknowledgment is received, the same frame is transmitted again.


Error ControlGo-back-N ARQ is a more efficient protocol.It assumes that multiple frames are in transmission at one time.If a frame arrives in error, the receiver can ask the transmitter to go back to the Nth frame and retransmit it.After the Nth frame is retransmitted, the sender resends all subsequent frames.


Error ControlSelective-reject ARQ is the most efficient error control protocol.If a frame is received in error, the receiver asks the transmitter to resend ONLY the frame that was in error.Subsequent frames following the Nth frame are not retransmitted.Figure 6-10 shows a normal transmission of frames with no errors, while Figures 6-11 and 6-12 show examples of errors.


Error ControlFor a receiver to correct the error with no further help from the transmitter requires a large amount of redundant information accompanying the original data.This redundant information allows the receiver to determine the error and make corrections.This type of error control is often called forward error correction.


Error Detection and Error Control in ActionAsynchronous transfer mode (ATM) incorporates many types of error detections and error control.ATM inserts a CRC into the data frame (the cell).This CRC checks only the header and not the data.This CRC is also powerful enough to perform simple error correction on the header.A second layer of ATM applies a CRC to the data, with varying degrees of error control.

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