Errors, Error Detection, and Error Control

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Data Communications and Computer Networks A
Business Users Approach

• Chapter 6
• Errors, Error Detection, and Error Control

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Data Communications and Computer Networks
Chapter 6

• Objectives
• Identify the different types of noise commonly
  found in computer networks
• Specify the different error prevention techniques
  and be able to apply an error prevention
  technique to a type of noise
• Compare the different error detection techniques
  in terms of efficiency and efficacy
• Perform simple parity and longitudinal parity
  calculations and enumerate their strengths and
  weaknesses

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Data Communications and Computer Networks
Chapter 6

• Objectives
• Cite the advantages of cyclic redundancy checksum
  and specify what types of errors cyclic
  redundancy checksum will detect
• Differentiate the three basic forms of error
  control and describe under what circumstances
  each may be used
• Follow and example of stop-and-wait ARQ,
  go-back-N ARQ and selective reject ARQ

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Data Communications and Computer Networks
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• 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.

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Data Communications and Computer Networks
Chapter 6

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

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• Removing Noise
• From a digital signal the signal is passed
  through a signal regenerator
• From an analog signal the signal is passed
  through filters, not as effective as the signal
  regenerator

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Data Communications and Computer Networks
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• Noise and Errors
• Impulse noise
• One 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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Data Communications and Computer Networks
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• Noise and Errors
• Crosstalk
• Unwanted coupling between two different signal
  paths.
• That is, electric or magnetic fields of one
  telecommunication signal affecting a signal in an
  adjacent circuit
• For example, hearing another conversation while
  talking on the telephone.
• Relatively constant and can be reduced with
  proper measures.

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Data Communications and Computer Networks
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• Noise and Errors
• Echo
• The 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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Data Communications and Computer Networks
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• Noise and Errors
• Jitter
• The 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 crosstalk
• Limit number of times signal is repeated

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• Noise and Errors
• Delay distortion
• Occurs 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
• Attenuation
• The continuous loss of a signals strength as it
  travels through a medium. Can be eliminated by
  amplifiers (analog signals) and repeaters
  (digital signals)

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Data Communications and Computer Networks
Chapter 6

• Error Prevention
• To 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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• Error Detection
• Despite 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.

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Data Communications and Computer Networks
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• Parity Checks
• Simple 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.

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Data Communications and Computer Networks
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• Parity Checks
• 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.

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• Parity Checks
• Both 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?

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Data Communications and Computer Networks
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• Cyclic Redundancy Checksum
• 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.

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

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Error Control Once an error is detected, what is
the receiver going to do? 1. Do nothing 2. Return
an error message to the transmitter 3. Fix the
error with no further help from the transmitter
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Error Control Do nothing Seems 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 ARQ 3. Selective-reject ARQ
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• Error Control
• Stop-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.

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• Error Control
• Go-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.

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• Error Control
• Selective-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.

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• Error Control
• For 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.

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• Error Detection and Error Control in Action
• Asynchronous 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.