05Chapter05 - CHAPTER 5 Error Detection Networks must be...

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105 CHAPTER 5 Error Detection Networks must be able to transfer data from one device to another with complete accu- racy. A system that cannot guarantee that the data received by one device are identical to the data transmitted by another device is essentially useless. Yet anytime data are transmitted from source to destination, they can become corrupted in passage. In fact, it is more likely that some part of a message will be altered in transit than that the entire contents will arrive intact. Many factors, including line noise, can alter or wipe out one or more bits of a given data unit. Reliable systems must have a mechanism for detect- ing and correcting such errors. 5.1 TYPES OF ERRORS Whenever an electromagnetic signal flows from one point to another, it is subject to unpredictable interference from heat, magnetism, and other forms of electricity. This interference can change the shape or timing of the signal. If the signal is carry- ing encoded binary data, such changes can alter the meaning of the data. Figure 5.1 shows the two types of errors. In a single-bit error a 0 is changed to a 1 or a 1 to a 0. In a burst error multiple bits are changed. For example, a 0.01 s burst of impulse noise on a transmission with a data rate of 1200 bps might change all or some of 12 bits of information. Figure 5.1 Types of errors Errors Single-bit Burst
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CHAPTER 5 ERROR DETECTION Single-Bit Error The term single-bit error means that only one bit of a given data unit (such as a byte, character, data unit, or packet) is changed from 1 to 0 or from 0 to 1. Figure 5.2 shows the effect of a single-bit error on a data unit. To understand the impact of the change, imagine that each group of eight bits is an ASCII character with a 0 bit appended to the end. In the figure, 00000010 (ASCII STX ) was sent, meaning start of text , but 00001010 (ASCII LF ) was received, meaning line feed . For more informa- tion about ASCII code, see Appendix A. Single-bit errors are the least likely type of error in serial transmission. To see why, imagine a sender sends data at 1Mbps. This means that each bit lasts only 1/1,000,000 s or 1 μ s. For a single-bit error to occur, the noise must have a duration of only 1 μ s, which is very rare; noise normally lasts much longer than this. However, a single-bit error can happen if we are sending data using parallel trans- mission. For example, if eight wires are used to send all of the eight bits of a byte at the same time and one of the wires is noisy, one bit can be corrupted in each byte. Think of parallel transmission inside a computer, between CPU and memory, for example. Burst Error The term burst error means that two or more bits in the data unit have changed from 1 to 0 or from 0 to 1. Figure 5.3 shows the effect of a burst error on a data unit. In this case, 0100010001000011 was sent, but 0101110101000011 was received. Note that a Figure 5.2 Single-bit error Figure 5.3 Burst error of length five 0 0 0 0 1 0 0 0 0 0 1 0 Sent 0 changed to 1 0 1 0 0 1 0 0 Sent
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This note was uploaded on 01/07/2012 for the course CIS 067A taught by Professor Behrouzforouzan during the Fall '11 term at DeAnza College.

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05Chapter05 - CHAPTER 5 Error Detection Networks must be...

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