1 Chapter 6 Errors, Error Detection, and Error Control Data Communications and Computer Networks: A Business User’s Approach

2 Last time Multiplexing –Again at data link level This time - errors and error detection and error control

3 Transmission Error

4 OSI vs Internet Model Error detection and control Error detection and control

5 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

6 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

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

8 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. 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.

9 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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11 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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14 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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16 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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18 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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20 Data Communications and Computer Networks Chapter 6 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 Attenuation The continuous loss of a signal’s strength as it travels through a medium. Use less lossy medium Use amplifiers

21 Attenuation When signal travels, it gets weaker. –If too weak, cannot tell 1s and 0s. Distance

22 Distortion As signal travels, it become distorted. –Changes shape –Successive bits may merge, making reception difficult Distance

23 Interference Unwanted signal from outside sources –Often intermittent, difficult to diagnose Signal Strength Signal Interference

24 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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26 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. Let’s examine two basic techniques for detecting errors: parity checking cyclic redundancy checksum (CRC).

27 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)

28 Parity Examples - Using Even Parity Even Parity of 1’sLRC/VRC

29 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 is to be sent, using even parity, a parity bit = 1 would be added to the character. If the character is to be sent, using even parity, a parity bit = 0 would be added to the character.

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

31 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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34 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?

35 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)

36 Data Communications and Computer Networks Chapter 6 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.

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

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39 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

40 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

41 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

42 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

43 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

44 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

45 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

46 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.

47 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

48 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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50 Data Communications and Computer Networks Chapter 6 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.

51 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.

52 Data Communications and Computer Networks Chapter 6 Error Control – redundancy becomes If is transmitted, what do we conclude? Applications? Space, valuable data Wastes BW

53 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.

54 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