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Systems Architecture, Sixth Edition

Chapter 8 Data and Network Communication

Technology


Chapter Objectives

•  In this chapter, you will learn to: – Explain communication protocols – Compare methods of encoding and transmitting

data with analog or digital signals – Describe signals and the media used to transmit

digital signals – Describe wireless transmission technology and

compare wireless LAN standards

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Chapter Objectives (continued)

– Describe methods for using communication channels efficiently

– Explain methods of coordinating communication, including clock synchronization and error detection and correction

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Systems Architecture, Sixth Edition 4

FIGURE 8.1 Topics covered in this chapter Courtesy of Course Technology/Cengage Learning


Communication Protocols

•  Set of rules and conventions for communication •  Message: a unit of data or information

transmitted from a sender to a recipient •  Command message •  Common method of encoding, transmitting, and

interpreting these bits •  Complete communication protocol

– Complex combination of subsidiary protocols – Technologies to implement them

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FIGURE 8.2 Components of a communication protocol Courtesy of Course Technology/Cengage Learning


Encoding and Transmitting Bits

•  Carrier waves •  Modulation methods •  Data bits can be encoded into analog or digital

signals •  Signals

– Electrical, optical, or radio frequency – Capacity and errors

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Carrier Waves

•  A sine wave with encoded bits (transports bits from one place to another)

•  Characteristics of sine waves: amplitude, phase, frequency

•  Importance of waves in communications – Travel through space, wires, and fibers – Can have patterns encoded in them

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FIGURE 8.3 Characteristics of a sine wave Courtesy of Course Technology/Cengage Learning


Modulation Methods

•  Techniques used to encode bits in sine waves – Amplitude modulation (AM) – Frequency modulation (FM) – Phase-shift modulation – Multilevel coding

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FIGURE 8.6 The bit string 11010001 encoded in a carrier wave with amplitude modulation Courtesy of Course Technology/Cengage Learning


FIGURE 8.7 The bit string 11010001 encoded in a carrier wave with frequency modulation Courtesy of Course Technology/Cengage Learning


FIGURE 8.8 The bit string 11010001 encoded in a carrier wave with phase-shift modulation Courtesy of Course Technology/Cengage Learning


FIGURE 8.9 The bit string 11100100 encoded in a carrier wave with 2-bit multilevel coding Courtesy of Course Technology/Cengage Learning


Analog Signals

•  Uses full range of carrier wave characteristics to encode continuous data values

•  Can represent any data value within a continuum of values

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Digital Signals

•  Can contain one of a finite number of possible values

•  Types of digital signals – Binary – Trinary – Quadrary

•  Square wave: contains abrupt amplitude shifts between two different values

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FIGURE 8.10 The bit string 11010001 encoded in square waves (digital signals) Courtesy of Course Technology/Cengage Learning


FIGURE 8.12 A binary signaling method using voltage ranges Courtesy of Course Technology/Cengage Learning


Signal Capacity and Errors

•  Analog signals compared with digital signals – Carry more information – Are more susceptible to transmission error

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FIGURE 8.13 Margin of transmission error (voltage drop or surge) before the data value encoded in a digital binary signal is altered Courtesy of Course Technology/Cengage Learning


Transmission Media

•  Communication path that transports signals (e.g., copper wire, optical fiber)

•  Characteristics – Speed and capacity – Frequency – Bandwidth – Noise, distortion, and susceptibility to external

interference

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FIGURE 8.14 Elements of a communication channel Courtesy of Course Technology/Cengage Learning


Speed and Capacity

•  Interdependent •  Jointly described as data transfer rate •  Raw data transfer rate •  Effective data transfer rate

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Frequency and Bandwidth

•  Carrier wave frequency – Basic measure of data-carrying capacity (i.e.,

limits capacity) •  Bandwidth

– Difference between maximum and minimum frequencies of a signal

– High-bandwidth channels can carry multiple messages simultaneously

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FIGURE 8.16 The spectrum of electromagnetic frequency between 101 and 1019 Hz Courtesy of Course Technology/Cengage Learning


Signal-to-Noise (S/N) Ratio

•  Noise: unwanted signal components •  Measure of the difference between noise power

and signal power •  Effective data transfer rate can be much lower

than raw data transfer rate due to – Electromagnetic interference (EMI) – Attenuation – Distortion –  Internal or external noise

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FIGURE 8.17 S/N ratio as a function of distance for a channel Courtesy of Course Technology/Cengage Learning


Electrical Cabling

•  Transmits signals through copper wire •  Two types

– Twisted pair •  Relatively cheap; limited in bandwidth, S/N ratio,

and transmission speed – Axial (coaxial and twin-axial)

•  More expensive; offers higher bandwidth, greater S/N ratio, and lower distortion; resistant to EMI

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Optical Cabling

•  Provides very high bandwidth, little internally generated noise and distortion, immunity to EMI

•  Requires amplifiers and repeaters for long distances to increase signal strength and remove noise and distortion

•  Two types – Multimode – Single mode (higher transmission rates at greater

cost)

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Wireless Transmission

•  Uses shortwave radio frequency wave or light waves to transmit data through the atmosphere or space

•  Advantages – Relatively high bandwidth, avoidance of wired

infrastructure, simultaneous broadcast transmission

•  Disadvantages – Susceptibility to external interference, cost, high

demand for unused radio frequencies, security

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Channel Organization

•  Configuration and organization issues – Number of transmission wires or bandwidth

assigned to each channel – Assignment of those wires or frequencies to carry

specific signals – Sharing, or lack thereof, of channels among

multiple senders and receivers •  Three types: simplex, half-duplex, full duplex

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Channel Organization

Simple Uses one optical fiber or copper wire pair to transmit data in one direction only

Half-duplex Identical to a simplex channel but sends a control signal to reverse transmission direction

Full duplex Uses two fibers or wire pairs to support simultaneous transmission in both directions

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FIGURE 8.22 Configurations for simplex (a), half-duplex (b), and full-duplex (c) modes Courtesy of Course Technology/Cengage Learning


Parallel and Serial Transmission

Parallel Serial

•  Uses a separate transmission line for each bit position

•  Unreliable over distances greater than a few meters due to skew and crosstalk

•  Provides higher channel throughput

•  Relatively expensive

•  Uses a single line to send one bit at a time

•  Reliable over much longer distances

•  Lower wiring and cable cost

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FIGURE 8.23 Parallel transmission of a data byte (8 bits) Courtesy of Course Technology/Cengage Learning


FIGURE 8.24 Serial transmission of a data byte (8 bits) Courtesy of Course Technology/Cengage Learning


Channel Sharing

•  Uses available capacity by combining traffic of multiple users

•  For use when no single user or application needs a continuous supply of data transfer data capacity

•  Techniques – Circuit switching – Packet switching – Frequency division multiplexing

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Channel Sharing Techniques

Circuit switching •  Allocates an entire channel to a single user for duration of one data transfer operation

•  Only used where data transfer delay and available data transfer capacity must be within precise and predictable limits (e.g., telephone service)

Packet switching •  Allocates time on the channel by dividing many message streams into smaller units (packets) and intermixing them during transmission

Frequency division multiplexing (FDM)

•  Divides a broadband channel into several baseband channels (e.g., cable television)

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FIGURE 8.27 Packet switching—the most common form of TDM Courtesy of Course Technology/Cengage Learning


FIGURE 8.28 Channel sharing with FDM Courtesy of Course Technology/Cengage Learning


Communication Coordination

•  Sender and receiver must coordinate their approaches to various aspects of communication in a channel – Start and end times of bits or signal events – Error detection and correction – Encryption methods (or lack thereof)

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Clock Synchronization

•  Ensures that sender/receiver use same time periods and boundaries to encode/decode bit values

•  Synchronous transmission – Ensures that sender/receiver clocks are always

synchronized by sending continuous data streams

•  Asynchronous transmission – Relies on specific start and stop signals to

indicate beginning and end of a message unit

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FIGURE 8.32 Typical format for messages transmitted with synchronous character-framing methods Courtesy of Course Technology/Cengage Learning


FIGURE 8.33 Asynchronous character framing for serial transmission, including a start bit Courtesy of Course Technology/Cengage Learning


Error Detection and Correction

•  Error detection – Based on a form of redundant transmission –  Increasing redundancy increases chances of

error detection at the expense of reducing channel throughput

•  Common error detection methods – Parity checking – Block checking – Cyclical redundancy checking

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How Methods of Error Detection and Correction Vary

•  Size and content of redundant transmission •  Efficient use of the communication channel •  Probability that an error will be detected •  Probability that an error-free message will be

identified as an error •  Complexity of the error detection method

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Parity Checking

•  Also called vertical redundancy checking •  Can be based on even or odd bit counts •  Has a high Type I error rate •  Reliability issues

– Unreliable in channels subject to error bursts affecting many adjacent bits

– More reliable in channels with rare errors that are usually confined to widely spaced bits

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FIGURE 8.34 Sample parity bits Courtesy of Course Technology/Cengage Learning


Block Checking

•  Also called longitudinal redundancy checking (LRC)

•  Sending device counts number of 1-valued bits at each bit position within a block

•  Sender combines parity bits for each position into a block check character (BCC) and appends it to the end of the block

•  Receiver counts 1-valued bits in each position and derives its own BCC to compare with that transmitted by sender

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FIGURE 8.35 Block checking Courtesy of Course Technology/Cengage Learning


Cyclic Redundancy Checking (CRC)

•  Most widely used error detection technique •  Produces a BCC usually more than 8 bits long;

can be as large as 128 bits •  Much lower Type I and Type II error rates than

parity checking and LRC checking

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Summary

•  Communication protocols •  How bits are represented and transported

among computer systems and hardware components

•  Transmission media •  Channel organization •  Clock synchronization •  Detecting and correcting errors in data

transmission, reception, or interpretation

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