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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Last time

• Multiplexing
• Again at data link level
• This time - errors and error detection and error
  control

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Transmission Error
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OSI vs Internet Model

Error detection and control
Error detection and control
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Error detection in different layers

• Some error detection in other layers
• Transport layer in OSI
• Network layer in Internet model
• Focus on the data link layer
• Independent of the medium or physical layer

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Data Link Layer

• Responsible for taking the data and transforming
  it into a frame with header, control and address
  information.
• Physical path communication
• Error detection
• Error correction
• Resolve competing requests

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Overview

• Errors Happen
• Noise and Errors
• Error Prevention
• Error Detection Techniques
• Error Control
• 1) Do nothing
• 2) Return a message
• 3) Correct the error

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Data Communications and Computer Networks
Chapter 6

Introduction Noise is always present. If a
communications line experiences too much noise,
the signal will be lost or corrupted. Communicatio
n 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. If white noise gets to strong, it
can completely disrupt the signal.
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Data Communications and Computer Networks
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Data Communications and Computer Networks
Chapter 6

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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Data Communications and Computer Networks
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Data Communications and Computer Networks
Chapter 6

Noise and Errors - Crosstalk Unwanted 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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Data Communications and Computer Networks
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Data Communications and Computer Networks
Chapter 6

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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Data Communications and Computer Networks
Chapter 6

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. Reduce jitter - shielding
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Data Communications and Computer Networks
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Data Communications and Computer Networks
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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 using equalizers Attenuati
on The continuous loss of a signals strength as
it travels through a medium. Use less lossy
medium Use amplifiers

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Attenuation

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

Distance
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Distortion

• As signal travels, it become distorted.
• Changes shape
• Successive bits may merge, making reception
  difficult

Distance
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Interference

• Unwanted signal from outside sources
• Often intermittent, difficult to diagnose

Signal Strength
Signal
Interference
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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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Data Communications and Computer Networks
Chapter 6

Error Detection Data Level 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 cyclic redundancy checksum (CRC).
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Parity

• Value of parity bit is such that character has
  even (even parity) or odd (odd parity) number of
  ones
• Even number of bit errors goes undetected
• Simple Parity
• Longitudinal Redundancy Check/Vertical Redundancy
  Check (LRC/VRC)

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Parity Examples - Using Even Parity
Even Parity of 1s
LRC/VRC
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Data Communications and Computer Networks
Chapter 6

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
Chapter 6

Parity Checks What happens if the character
10010101 (parity bit is the last bit) and the
first two 0s accidentally become two 1s? Thus,
the following character is received
11110101. Will there be a parity error? Problem
Simple parity only detects odd numbers of bits in
error (50)
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Data Communications and Computer Networks
Chapter 6

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 (50 of
all errors) Longitudinal parity is better at
catching errors but requires too many check bits
added to a block of data. As such, these methods
are not that often used. However, a parity bit
exists in 1 byte of data. We need a better error
detection method. What about cyclic redundancy
checksum?

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Data Communications and Computer Networks
Chapter 6

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 (remainder (mod) arithmetic)
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Data Communications and Computer Networks
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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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Data Communications and Computer Networks
Chapter 6

r is the degree of the generating polynomial
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Data Communications and Computer Networks
Chapter 6

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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Flow Control

• Flow Control refers to mechanisms that make sure
  that the sending station cannot overwhelm the
  receiving station with data.
• Preventing buffer overflow
• Transmission time
• Time taken to emit all bits into medium
• Propagation time
• Time for a bit to traverse the link

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Stop-and-Wait Flow Control

• The simplest form of flow control is Stop and
  Wait Flow Control.
• Stop and Wait Flow Control works like this
• The sending station sends a frame of data and
  then waits for an acknowledgement from the other
  station before sending further data
• The other party can stop the flow of data by
  simply withholding an acknowledgement

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Stop-and-Wait Flow Control

• Source may not send new frame until receiver
  acknowledges the frame already sent
• Very inefficient, especially when a single
  message is broken into separate frames

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Stop-and-Wait Flow Control

• Stop and Wait Flow control works great if data is
  sent as a few large frames.
• However large frames are undesirable for the
  following reasons
• Large frame means one station occupies the link
  for a longer time (undesirable on a multipoint
  link)
• There is more chance of error in a large frame
  resulting in more lost data and more
  retransmission

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Stop and Wait Control

• Source transmits single frame
• Wait for ACK (Acknowledgement)
• If received frame damaged, discard it
• Transmitter has timeout
• If no ACK within timeout, retransmit
• If ACK damaged,transmitter will not recognize it
• Transmitter will retransmit
• Receive gets two copies of frame
• Use ACK0 and ACK1

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Error Control Steps

• Error Control consists of the following steps
  (not all are necessary)
• Error detection (Parity, CRC, etc. are used)
• Positive Acknowledgment (ACK) means no detected
  error
• Retransmission after time-out -- because a frame
  or an ACK might have been lost
• Negative acknowledgment (NAK) and retransmission
• Collectively all these mechanisms are called
  Automatic Repeat reQuest ARQ

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Data Communications and Computer Networks
Chapter 6

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. Assumption is higher mechanism will
detect and handle frame errors.
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Data Communications and Computer Networks
Chapter 6

Error Control Return a message has three basic
ARQ formats 1. Stop-and-wait ARQ 2. Go-back-N
ARQ 3. Selective-reject ARQ
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Data Communications and Computer Networks
Chapter 6

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
(ACK) is received, the next frame is sent. If a
negative acknowledgment (NAK) is received, the
same frame is transmitted again.
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Data Communications and Computer Networks
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Error Control Go-back-N ARQ and selective reject
are more efficient protocols. They assume that
multiple frames are in transmission at one time
(sliding window). A sliding window protocol
allows the transmitter to send up to the window
size frames before receiving any
acknowledgments. When a receiver does acknowledge
receipt, the returned ack contains the number of
the frame expected next.
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Data Communications and Computer Networks
Chapter 6

Error Control For a receiver to correct the error
with no further help from the transmitter
requires a large amount of redundant information
accompany 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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Data Communications and Computer Networks
Chapter 6
Error Control redundancy 011001 becomes 000
111 111 000 000 111 If 001 110 111 000 000 111 is
transmitted, what do we conclude? Applications? Sp
ace, valuable data Wastes BW

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Data Communications and Computer Networks
Chapter 6

Error Detection and Error Control in
Action Asynchronous transfer mode (ATM)
incorporates many types of error detection and
error control. ATM inserts a CRC into the data
frame (the cell), which 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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What we covered

• Types of errors and their prevention
• Error detection
• Parity 50
• CRC can detect nearly all errors
• Error correction
• 1. Do nothing
• 2. Return an error message to the transmitter
• 3. Fix the error with no further help from the
  transmitter