Objectives:

1
Session 4
2

• Objectives
• By the end of this session, the student will be
  able to
• 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
• Cite the advantages of cyclic redundancy
  checksum, and specify what types of errors cyclic
  redundancy checksum will detect
• Differentiate between the three basic forms of
  error control, and describe the circumstances
  under which each may be used
• Follow an example of Stop-and-wait ARQ, Go-Back-N
  ARQ, and selective-reject ARQ

3
White Noise
A.K.A. Thermal Noise or
Gaussian Noise
3
4
White Noise

• White Noise
• A relatively continuous noise like the static
  you hear when tuning between radio stations
• Always present to some degree, dependent on
  temperature.
• As temp increases noise increases (due to
  increased activity of electrons in medium)
• Can be reduced, not eliminated.
• To remove from digital signal pass through a
  signal regenerator before it overwhelms original
  signal
• To remove from analog signal pass signal
  through filters. Less reliable as you may filter
  out information inadvertently.

4
5
Impulse Noise
A.K.A. Noise Spike
5
6
Impulse Noise

• Impulse Noise
• Non-continuous noise.
• Difficult to detect as it occurs randomly
• The noise is an analog burst of energy.
• On analog signals, can be very difficult to
  detect/remove
• Eg. Clipping circuit to remove pops and crackles
  from old LPs sometimes removed the music as well
• Impulse noise on Digital signals can be
  'removed'. Dependant on duration of noise or
  speed of transmission

6
7
Impulse Noise

7
8
Crosstalk

• Crosstalk
• Unwanted coupling between two different signal
  paths
• Might be electrical (as in two sets of twisted
  pair wire (phone line))
• Might be electromagnetic unwanted signals
  picked up by microwave antennas.
• High humidity and wet weather can cause an
  increase in telephone crosstalk

8
9
Echo

• Echo
• Reflective feedback of a transmitted signal as it
  moves through a medium
• The same as a voice echoing in an empty room,
  signal moves through medium, hits end and bounces
  back- interfering with original signal
• Occurs at junctions where wires connect or at the
  open end of co-axial cables
• A filter is attached to the open of the cable to
  prevent the signal from travelling back on the
  medium. For coaxial cable LANs a filter is
  placed on the open ends to also eliminate
  incoming signals

9
10
Jitter
10
11
Jitter

• Jitter
• Small timing irregularities that become magnified
  during the transmission of digital signals
• The rise and fall of the digital signal becomes
  blurry
• Caused by electromagnetic interference,
  crosstalk, passing signal through too many
  repeaters, use of low quality devices
• Solutions- better shielding of cable, reduce
  electromagnetic interference and crosstalk.
  Reduce the number of times a signal is repeated.

11
12
Delay Distortion

• Delay Distortion
• The speed a signal travels through a medium
  varies with the frequency of the signal.
• Therefore, some frequencies will arrive at the
  destination before others

12
13
Error Prevention

• Error Prevention Techniques
• Install wiring with the proper shielding, to
  reduce electromagnetic interference and crosstalk
• Use telephone line conditioning or equalization
  (provided by the telephone company), in which
  filters are used to help reduce signal
  irregularities. For an additional charge, the
  telephone company will provide various levels of
  conditioning to leased lines. This conditioning
  provides a quieter line , which minimizes data
  transmission errors
• Replace older equipment with more modern, digital
  equipment although initially expensive, this
  technique is often the most cost-effective way to
  minimize transmission errors in the long run
• Use the proper number of digital repeaters and
  analog amplifiers to increase signal strength,
  thus decreasing the probability of errors
• Observe the stated capacities of a medium, not
  pushing the transmission speeds beyond their
  recommended limits, to reduce the possibility of
  errors. For example, twisted pair Category 5e
  cable should not be longer than the recommended
  10metre distance when transmitting at 10Mbps.

13
14
Error Prevention
14
15
Error Detection
15
16
CRC
Polynomial Arithmetic Message treated as a large
polynomial 1 .... 1 0 1 1 0 1 1 1 0 1 1 1 0 0 1
1 xn x15 x14 x13 x12 x11 x10 x9 x8 x7 x6 x5 x4 x3
x2 x1 x0 This polynomial is divided by a
'generating polynomial' to produce a quotient and
a remainder. The quotient is discarded The
remainder (as a bit string) is appended to the
message Generating Polynomials CRC-16
800516 CRC-CCITT 102116
16
17
CRC
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18
Error Control
Return a Message Stop-and-wait ARQ Go-Back-N
ARQ part of Sliding Window protocol Selective-R
eject ARQ part of Sliding Window
protocol ARQ Automatic Repeat reQuest
18
19
Stop-and-Wait ARQ
19
20
Sliding Window Protocols
0
7
1
6
2
5
4
3
20
21
Sliding Window Protocols

• Sliding Window Protocols
• Around since the 1970s.
• Allows a station to send multiple packets, before
  mandating an ACK.
• Window size of 7 chosen at the time due to
  processing speeds, and memory cost
• Window of 7 means that 7 packets could be
  outstanding before it had to stop and get the
  ACK.
• (extended sliding window of 127 created as well)
• Modern protocols have adjustable window sizes for
  tuning.
• In Sliding Window of size 7, packets are numbered
  0,1,2,3,4,6,7 (eight packet numbers)
• Only 7 can be unanswered. Since 2 data packets
  of the same number cannot be outstanding (eg. 2
  packets 4)
• So, if 4 packets have been sent, another 3 can be
  sent before it must wait.
• Acknowledgment is numbered with next EXPECTED
  packet number.

21
22
Sliding Window Protocols
22
23
Sliding Window Protocols
23
24
Sliding Window Protocols
24
25
Sliding Window Protocols
25
26
Sliding Window Protocols
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27
Forward Error Correction
C Check Bit (even parity) D Data Bit
C1
C3
D4
D5
D6
C0
D2
0
1
0
0
1
0
1
27
28
Forward Error Correction
C Check Bit (even parity) D Data Bit
C0 - Error C1 - OK C3 - Error 1 0 1 5 (5th bit
in error) EOE
C1
C3
D4
D5
D6
C0
D2
0
1
0
0
0
0
1
28
29
Forward Error Correction
C Check Bit (even parity) D Data Bit
C0 - OK C1 - OK C3 - Error 1 0 0 4 (4th bit in
error) EOO
C1
C3
D4
D5
D6
C0
D2
0
1
0
1
1
0
1
29