Session 4

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

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

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

Impulse Noise A.K.A. Noise Spike 5

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

Impulse Noise 7

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

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

Jitter 10

Jitter 11 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.

Delay Distortion 12 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

Error Prevention 13 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.

Error Prevention 14

Error Detection 15

CRC 16 Polynomial Arithmetic Message treated as a large polynomial: x n x 15 x 14 x 13 x 12 x 11 x 10 x 9 x 8 x 7 x 6 x 5 x 4 x 3 x 2 x 1 x 0 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 = CRC-CCITT=

CRC 17

Error Control 18 Return a Message Stop-and-wait ARQ Go-Back-N ARQpart of Sliding Window protocol Selective-Reject ARQpart of Sliding Window protocol **ARQ – Automatic Repeat reQuest

Stop-and-Wait ARQ 19

Sliding Window Protocols

Sliding Window Protocols 21 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.

Sliding Window Protocols 22

Sliding Window Protocols 23

Sliding Window Protocols 24

Sliding Window Protocols 25

Sliding Window Protocols 26

Forward Error Correction 27 C0C0 C1C1 D2D2 C3C3 D4D4 D5D5 D6D C – Check Bit (even parity) D – Data Bit

Forward Error Correction 28 C0C0 C1C1 D2D2 C3C3 D4D4 D5D5 D6D C – Check Bit (even parity) D – Data Bit C 0 -Error C 1 -OK C 3 -Error 1 0 1=5 (5 th bit in error) EOE

Forward Error Correction 29 C0C0 C1C1 D2D2 C3C3 D4D4 D5D5 D6D C – Check Bit (even parity) D – Data Bit C 0 -OK C 1 -OK C 3 -Error 1 0 0=4 (4 th bit in error) EOO