Data Communications and Computer Networks A Business Users Approach 8th Edition White
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Errors, Error Detection, and Error Control
Learning Objectives
After reading this chapter you should 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
Cite the advantages of arithmetic checksum, and specify what types of applications use arithmetic checksum.
Differentiate the basic forms of error control and describe under what circumstances each may be used
Follow an example of a Hamming self-correcting code Chapter Outline
1. Introduction
2. Noise and Errors
a. White noise
b. Impulse noise
c. Crosstalk
d. Echo
e. Jitter
f. Delay distortion
g. Attenuation
3. Error Prevention
4. Error Detection Techniques
a. Parity checks
Simple parity
Longitudinal parity
b. Arithmetic checksum
c. Cyclic redundancy checksum
5. Error Control
a. Toss the frame/packet
b. Return a message
Stop and wait error control
Sliding window error control
c. Correct the error
6. Error Detection In Action
7. Summary
Lecture Notes
Introduction
Given that noise is inevitable and errors happen, something needs to be done to detect error conditions. This chapter examines some of the more common error detection methods and compares them in terms of efficiency and efficacy. Once an error has been detected, what action should a receiver take? There are three options: toss the frame/packet, return an error message to the transmitter, or correct the error without help from the transmitter.
Noise and Errors
Transmitted data both analog and digital are susceptible to many types of noise and errors. Copper-based media have traditionally been plagued with many types of interference and noise. Satellite, microwave, and radio networks are also prone to interference and crosstalk. Even nearperfect fiber optic cables can introduce errors into a transmission system, though the probability of this happening is less than with the other types of media.
Error Prevention
Since there are so many forms of noise and errors, and since one form of noise or another is virtually a given, every data transmission system must include precautions to reduce noise and the possibility of errors. If you can reduce the possibility of noise before it happens, the transmitting station may not have to slow down its transmission stream. With proper error prevention techniques, many types of errors can be reduced.
Error Detection Techniques
Despite the best attempts at prevention, errors still occur. If an error is detected, it is typical to perform some type of request for retransmission. The error detection techniques themselves can be relatively simple or relatively elaborate. Generally, simple techniques do not provide the same degree of error checking as the more elaborate schemes. For example, the simplest error detection technique is simple parity. It adds a single bit to a character of data, but catches the fewest number of errors. At the other end of the spectrum is the most elaborate and most effective technique available today: cyclic redundancy checksum. Although it is more complex and typically adds 16 bits of error detection code to a block of data, it is the most effective error detection technique ever devised. In between is arithmetic checksum. It adds the transmitted characters, producing a sum, and appends this sum in some fashion to the end of the transmitted message. This technique is used in TCP and IP.
Error Control
Once an error is detected in the received data transmission stream, what is the receiver going to do about it? The receiver can do one of three things: toss the frame/packet (which is essentially doing nothing), return a message to the transmitter asking it to resend the data packet that was in error, or correct the error.
Error Detection and Error Control In Action
This In Action example examines a company that has to send data between two locations. To transmit data with error checking capabilities, you will examine two different error detection schemes and their probabilities of detecting errors.
Quick Quiz
1. What is the difference between error prevention, error detection, and error control?
Prevention uses methods to try to keep errors from happening, detection detects an error, and control is the response when a receiver detects an error.
2. What are the advantages of CRC over other error detection methods?
CRC catches almost all errors, only adds on the average 16 bits to the end of a message, and is relatively fast to calculate.
3. What are the three basic forms of error control?
Toss the frame/packet, return an error message to transmitter, or correct the error with no further information from transmitter.
Discussion Topics
1. Frame relay is a technology that does no error control. If a frame arrives garbled, it is dropped and frame relay does not inform the transmitter. Is this a reasonable approach?
2. Are there any other examples of famous projects that have failed because of relatively simple errors? (Hubble, Mars probes) Were these transmission errors or simply data and calculation errors?
3. Can you create a more advanced error control method than either stop and wait or sliding window?
Teaching Tips
1. Emphasize that CRC only adds r bits to the end of a message and yet catches almost all errors.
2. Be sure to clarify the differences between error detection, error control or correction, and forward error correction.
3. Try to enumerate a number of common transmission impairments and the type of noise that creates that impairment. For example, hearing another conversation while talking on the telephone: crosstalk. The hiss you hear when you put your ear next to a speaker that is on but not playing music: white noise.
Solutions to Review Questions
1. What is white noise and how does it affect a signal?
The constant hiss in the background. The more white noise, the harder it is to interpret the signal. Too much white noise may lead to signal loss.
2. What is impulse noise and why is it the most disruptive?
Spurious spikes of power that come out of nowhere. Can completely knock out multiple bits.
3. What is crosstalk and how does it affect a signal?
When the signal from one transmission bleeds into the signal of a second transmission. One signal may pick up another signal, those corrupting the original signal.
4. What is echo and how does it affect data transmission?
The bouncing back of a signal at the end of a wire can interfere with normal transmissions. May cause loss of signal.
5. What is jitter and why is it a digital signal problem?
When a digital signal is repeated over and over and the beginning and ending of the signal becomes less defined. Problem occurs with discretely increasing and decreasing signals.
6. Which of the noises introduced in this chapter are continuous and which are noncontinuous?
All are continuous except impulse.
7. Will proper shielding of a media increase or decrease the chance of errors?
Decrease.
8. What is the difference between even parity and odd parity?
Even parity: maintain an even number of 1s.
9. What is the ratio of check bits to data bits for simple parity?
One parity bit to every character (7 or 8 data bits typically).
10. What is the ratio of check bits to data bits for longitudinal parity?
One parity bit to every character plus one parity character to a block of data characters.
11. What types of errors will simple parity not detect?
An even-numbered bits of error.
12. What types of error will longitudinal parity not detect?
An even-numbered bits of error in exactly the same columns.
13. With arithmetic checksum, what is being added?
The ASCII (usually) values of the message to be transmitted.
14. What is a generating polynomial?
The polynomial that is used to divide the transmitted message and the received message and generates a remainder.
15. What types of errors will cyclic checksum not detect?
Very small percentage of errors >= r bits in length.
16. Frame relay practices which form of error control? Do nothing.
17. How many packets can be sent at one time using stop-and-wait error control?
One.
18. What is the function of an ACK in the stop-and-wait error control?
To acknowledge no errors found.
19. What is the function of a NAK in the stop-and-wait error control?
To respond that an error was found.
20. In communication systems what does timeout mean?
The receiver or transmitter has waited a fixed period of time and has not heard a response.
21. Why were the window sizes in early sliding window systems so small?
Because buffer size was limited; systems didn’t transmit high volumes of data at high speeds
22. What are two different ways to number sequences of data?
Count the packets and count the bytes
23. What condition must be met for error correction to be performed?
Long transfer delays, or high security needs
Suggested Solutions to Exercises
1. Which type of noise is the most difficult to remove from an analog signal? Why?
Impulse, because it is non-continuous and appears as an analog waveform, which might make it difficult to separate from the analog waveform of the data.
2. Which type of noise is the most difficult to remove from a digital signal? Why?
Jitter. Regeneration of the signal, which often reduces other forms of noise, can increase the amount of jitter.
3. Explain the relationship between twisted pair wires and crosstalk.
A signal traveling through one wire creates a magnetic field around that wire. If a second wire runs parallel to the first, a magnetic field passing through the second wire will induce a current in the second wire.
4. Which type of cable is most susceptible to echo?
Coaxial cable
5. Given the character 0110101, what bit will be added to support even parity?
6. Given the character 1010010, what bit will be added to support odd parity?
7. Generate the parity bits and longitudinal parity bits for even parity for the characters 0101010, 0011010, 0011110, 1111110, and 0000110.
8. Given the message “Hello, goodbye”, show the decimal arithmetic checksum that will be generated.
H=72; e=81; l=88; l=88; o=91; etc and then just add up the ASCII values.
9. List the types of errors that can escape a cyclic redundancy checksum system.
Error bursts > size r
10. List the types of error that cannot escape a cyclic redundancy checksum system.
Single bit errors, double bit errors, most errors < size r
11. Why do all the CRC generating polynomials end with a 1?
So that all odd numbers of bit errors can be detected.
12. In a stop-and-wait error control system, Station A sends packet 0, and it is lost. What happens next?
Station B responds with a NAK.
13. In a stop-and-wait error control system, Station A sends packet 0, it arrives without error, and an ACK is returned, but the ACK is lost. What happens next?
After a timeout, Station A will retransmit packet 0.
14. In a sliding window error control system in which each packet is numbered, Station A sends packets 4, 5, 6, and 7. Station B receives them and wants to acknowledge all of them. What does Station B send back to Station A?
An ACK with the number of the next packet expected
15. In a sliding window system with each byte numbered, Station A sends a packet with bytes numbered 801 to 900. If the packet arrives with no errors, how does Station B respond?
An ACK with the value 901
16. In a sliding window error control system, Station A sends a packet with bytes 501-700, followed immediately by a packet with bytes 701-900. Create a diagram of this error control scenario and show the response(s) that Station B will send if the second packet is lost in the network.
Receiver waits 500 ms for second packet to arrive. Since it doesn’t, the receiver returns an ACK with value 701.
17. In a sliding window error control system, Station A sends a packet with bytes 501-700, followed immediately by a packet with bytes 701-900. Create a diagram of this error control scenario and show the response(s) that Station B will send if both packets arrive but there is a checksum error in the second packet.
Receiver immediately returns an ACK with value 701.
18. In a sliding window error control system, Station A sends a packet with bytes 501-700, followed immediately by a packet with bytes 701-900. Create a diagram of this error control scenario and show the response(s) that Station B will send if both packets arrive but there is a checksum error in the second packet.
Same answer as in problem 16.
19. In a sliding window error control system, Station A sends three packets with bytes 0100, 101-200, and 201-300, respectively. The second packet with bytes 101-200 is held up somewhere in the network long enough that the third packet arrives before the second one. Create a diagram of this error control scenario and show the response(s) that Station B will send. Now assume that five seconds after Station B responds, the second packet shows up. What does Station B do now?
Receiver sends an ACK with value 101 and buffers packet 201-300. When packet with bytes 101-200 arrives, it is inserted into stream. If packet with bytes 201-300 shows up, it is tossed.
20. Assume a sliding window protocol has a 3-bit field to hold the window size. A three-bit value gives 8 combinations, from 0 to 7. Why then can a transmitter send out only a maximum of 7 frames at one time and not 8? Show an example.
If you sent all 8 frames 0 - 7, and none arrived, you might say that you are waiting for frame 0. But is that the frame 0 that was lost, or the next frame 0 after frame 7?
21. If a 7-bit sliding window size is used, how many packets can be sent before the transmitter has to stop and wait for an acknowledgment?
127 (not 128)
22. Is Stop-and-wait error control a half duplex protocol or a full duplex protocol?
Half duplex
23. Devise a code set for the digits 0 to 9 that has a Hamming distance of 2.
Many possible answers. Simplest would be to count from 0 to 9 using a 4-bit binary number and add a simple parity bit to the end of each.
24. A system is going to transmit the byte 10010100. Show the four check bits c8, c4, c2 and c1 that will be added to this byte. Check bits
25. The 12-bit string 010111110010 with embedded Hamming code bits (c8, c4, c2 and c1) has just arrived. Is there an error? If so, which bit is in error?
c8 = error, c4 = error, c2 = no error, c1 = no error; error bits = 1100, so bit number 12 is in error.
26. Construct a 12-bit Hamming code for the characters A and 3.
In ASCII, A = 01000001; in ASCII, 3 = 00110011
Thinking Outside the Box
1. To properly answer this problem, we really need to know the propagation time of the records through the medium, because a lot of time is going to be spent waiting for the stop-and-wait acknowledgment to be returned after every record. With a sliding window protocol and a window size of 7, you only need to send an acknowledgment after every 7 records. This should reduce the acknowledgment waiting time to 1/7 that of stop-and-wait.
2. This problem was solved in the Error Detection in Action section of this chapter.
3. TCP/IP uses a window size of 16 bits, so 65,536 bytes can be sent.
4. Yes, if another higher layer protocol can do the error control instead.