1. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.1 Computer Networks and Internets with Internet Applications, 4e By Douglas E. Comer Lecture PowerPoints By Lami Kaya, LKaya@ieee.org

2. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.2 Chapter 7 Packets Frames and Error Detection

3. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.3 Topics Covered 7.1 Introduction 7.2 The Concept Of Packets 7.3 Packets And Time-Division Multiplexing 7.4 Packets And Hardware Frames 7.5 Byte Stuffing 7.6 Transmission Errors 7.7 Parity Bits And Parity Checking 7.8 Probability, Mathematics, And Error Detection 7.9 Detecting Errors With Checksums 7.10 Detecting Errors With Cyclic Redundancy Checks 7.11 Combining Building Blocks 7.12 Burst Errors 7.13 Frame Format And Error Detection Mechanisms

4. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.4 7.1 Introduction This chapter describes a fundamental idea in NW discusses the concept of packets explains how a sender and receiver coordinate to transfer a packet shows how packets can be implemented in a character- oriented NW using a simple frame format explains transmission errors discusses mechanisms that NW use to detect errors Later chapters expand the packet concept and describe how particular NW technologies handle packets

5. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.5 7.2 The Concept Of Packets (1) Most NWs do not send data as continuous streams of bits –NWs divides data into small blocks called packets Computer NWs are often called packet NW or packet- switching NW because they use packet technology Two facts motivate the use of packets: –First, a sender and receiver need to coordinate transmission to ensure that data arrives correctly: Dividing the data into small blocks helps a sender and receiver determine which blocks arrive intact and which do not –Second, communication circuits and the associated modem hardware are expensive multiple computers can share connections and HW to reduce costs

6. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.6 7.2 The Concept Of Packets (2) Using small packets helps ensure fairness: To avoid having one computer hold a NW for an arbitrary time, modern NW enforce the use of packets –NW permits one computer to send a packet, then blocks that computer from sending again –Meanwhile the NW permits another computer to send a packet –A single computer can hold a shared resource only long enough to send a single packet –And must wait until other computers have a turn before sending a second packet

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8. 8 7.2 The Concept Of Packets (3) How long does a file transfer require? If a file contains 5 MB and the communication system can transfer 56,000 bps the transfer will require almost 12 minutes. –Any NW that grants an application exclusive use of shared resources will block other computers for intolerably long periods of time In contrast, consider the delays introduced when the NW in Figure 7.1 requires computers to divide data into packets that each contain 1000 bytes of data: –Suppose computer A begins sending data to D –after A begins sending, computer B needs to send data to C –After A finishes sending one packet, the NW will allow B to send one –because a packet contains only 8000 bits (1000x8) of data and the transfer rate 56,000 bps, a complete packet can be sent in only 0.143 s

9. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.9 7.3 Packets And Time-Division Multiplexing Conceptually, a NW that permits many sources to take turns accessing a shared resource –providing a form of time-division multiplexing (TDM) –Figure 7.2 illustrates the idea Dividing data into small packets ensures that all sources receive prompt service –because it prohibits one source from gaining exclusive access for an arbitrarily long time Allowing sources to take turns sending packets guarantees –that a source with a small amount of data will finish promptly

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11. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.11 7.4 Packets And Hardware Frames No universal agreement on the exact format of a packet Instead, each HW technology defines the details of packets and specifies how individual bits of the packet are transmitted –“frame” is a packet used with a NW the sender and receiver must agree on –how they will specify the beginning and end of each frame ASCII includes codes for unprintable characters: –We can use two of the unprintable ASCII characters to delimit a frame Figure 7.3 illustrates a frame The example framing scheme has disadvantages and advantages: –The chief disadvantage is overhead –The chief advantage is handling system failures and reboots

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13. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.13 7.5 Byte Stuffing (1) NW permits an application to transfer arbitrary characters An application may choose to send data that contains one or more occurrences of characters like soh and eot Transmitting such characters in the data portion of a frame causes problems –an eot to mean that the entire frame has arrived –an soh to mean a new frame has started to arrive Insert extra bits or bytes to change data for transmission, –the technique is known as “data stuffing” –“byte stuffing” and “character stuffing” refer to data stuffing used with character-oriented –“bit stuffing” refers to data stuffing used with bit-oriented

14. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.14 7.5 Byte Stuffing (2) Byte stuffing can solve the problem –by reserving a third character to mark occurrences of reserved characters in the data –Suppose the ASCII character esc (Hex =1Bh ) has been selected as the third character When any of the three special characters occur in data –the sender inserts esc and replaces the character Figure 7.4 summarizes the mapping Figure 7.5 shows an example data block and the resulting frame after byte stuffing has occurred

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17. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.17 7.6 Transmission Errors Lightning, power surges, and other EM interference –can introduce unwanted electrical currents A small change in the electrical signal can –cause the receiver to misinterpret one or more bits of the data Interference can –cause permanent damage to NW equipment (lightening) –changes the signal –completely destroy a signal –create the opposite effect although the sender does not transmit anything, a receiver might interpret the incoming interference as a valid sequence

18. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.18 7.7 Parity Bits And Parity Checking (1) Fortunately, new communication systems do not experience frequent interference –local communication circuits often operate for years without serious problems –on a long-distance communication circuit may be so small that modems can handle all problems automatically Despite the low probability of error –we should provide HW and SW mechanisms to detect and correct such problems

19. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.19 7.7 Parity Bits And Parity Checking (2) Parity checking requires the sender –to compute an additional bit –called a parity bit –and to attach it to each character before sending The receiver –removes the parity bit –performs the same computation as the sender –and verifies that the result agrees with the value of the parity bit The parity computation is chosen such that –if one of the bits in the character is damaged in transit, the receiver's computation will not agree with the parity bit –the receiver will report that an error occurred. There are two forms of parity –even or odd –both the sender and receiver must agree

20. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.20 7.8 Probability, Mathematics, And Error Detection Parity mechanism discussed above –works well to detect a single bit error, –but it cannot detect all possible errors There are several alternatives –In each mechanism, the sender transmits additional information along with the data –and the receiver uses the information to verify Differences among the mechanisms arise in three ways: –the size of the additional information –the computational complexity of the algorithm –and the number of bit errors that can be detected

21. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.21 7.9 Detecting Errors With Checksums (1) Many NW systems send a checksum along with each packet to help the receiver detect errors To compute a checksum –the sender treats the data as a sequence of binary integers and computes their sum –The data is not restricted to integer values it can contain characters, floating point numbers, or an image –The network system merely treats the data as a sequence of integers for purposes of computing a checksum Figure 7.6 illustrates a 16 -bit checksum computation The sender –treats each pair of characters as a 16 -bit integer –it computes the sum If the sum grows larger than 16 bits, the carry bits are added into the final sum

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23. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.23 7.9 Detecting Errors With Checksums (2) The chief advantages arise from the size and ease of computation. –Most NW that employ a 16-bit or 32-bit checksum –compute a single checksum for an entire packet The small size of the checksum means –the cost of transmitting the checksum is usually much smaller than the cost of transmitting the data –the computation required to create or verify a checksum is small Checksums have the disadvantage of not detecting all common errors Figure 7.7 shows that a checksum is not sufficient to detect a transmission error that reverses a bit in each of four data items –Despite the changes, a receiver will declare that the packet has a valid checksum –Reversing the value of the second bit in each data item produces the same checksum

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25. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.25 7.10 Detecting Errors With Cyclic Redundancy Checks How can a NW detect more errors without increasing the amount of additional information in each packet? –The answer lies in Cyclic Redundancy Check (CRC) techniques CRC techniques are also used to verify that data has been recorded correctly on storage devices –such as magnetic disks Figure 7.8 shows some HW components Figure 7.9 illustrates how bit values move during a shift operation and how the output changes

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28. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.28 7.11 Combining Building Blocks (1) Figure 7.10 illustrates how to compute a 16-bit CRC Several CRC algorithms exist –the algorithms differ in the number of bits allocated to each of the three shift registers and the initial values used Instead of computing a CRC over the incoming message and then comparing it to the incoming CRC –the receiver computes a CRC over the incoming message plus the incoming CRC –If all bits are received correctly, the computed value will be zero

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30. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.30 7.11 Combining Building Blocks (2) There are two reasons a CRC can detect more errors than a simple checksum First, because an input bit is shifted through all three registers, –a single bit of the message affects the resulting CRC in dramatic ways Second, because the hardware uses feedback in which the output from the leftmost shift register affects each exclusive or unit –the effect from a single bit of the message cycles through the shift registers more than one time Mathematically, a CRC uses a polynomial, usually expressed as powers of polynomial, to divide the message

31. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.31 7.12 Burst Errors Two case where is CRC especially First, HW failures sometimes cause a specific set of bits to be damaged –Ex, a character-oriented I/O device that is damaged might set the first two bits of every character to zero –Such errors are sometimes called vertical errors because they appear in a vertical column when the characters are arranged in rows Second, CRCs are especially useful for detecting errors that involve changes to a set of bits –called error “burst errors” interference may be caused by lightning, starting of an electric motor, etc

32. © 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.32 7.13 Frame Format And Error Detection Mechanisms NW usually associate error detection information with each frame –The sender calculates information such as a checksum or CRC –The receiver calculates the same value and compares it to the additional information that arrives in the frame Figure 7.11 illustrates a simple frame format Is byte stuffing required for the CRC as well? –The answer depends on the types of error detection required If characters can be lost, byte stuffing the CRC can add reliability

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