Systems Architecture, Sixth Edition
Chapter 8 Data and Network Communication
Technology
Systems Architecture, Sixth Edition
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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FIGURE 8.1 Topics covered in this chapter Courtesy of Course Technology/Cengage Learning
Systems Architecture, Sixth Edition
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
Systems Architecture, Sixth Edition
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
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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
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FIGURE 8.7 The bit string 11010001 encoded in a carrier wave with frequency modulation Courtesy of Course Technology/Cengage Learning
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FIGURE 8.8 The bit string 11010001 encoded in a carrier wave with phase-shift modulation Courtesy of Course Technology/Cengage Learning
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FIGURE 8.9 The bit string 11100100 encoded in a carrier wave with 2-bit multilevel coding Courtesy of Course Technology/Cengage Learning
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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
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FIGURE 8.12 A binary signaling method using voltage ranges Courtesy of Course Technology/Cengage Learning
Systems Architecture, Sixth Edition
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
Systems Architecture, Sixth Edition
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
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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
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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
Systems Architecture, Sixth Edition
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
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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
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FIGURE 8.24 Serial transmission of a data byte (8 bits) Courtesy of Course Technology/Cengage Learning
Systems Architecture, Sixth Edition
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
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FIGURE 8.28 Channel sharing with FDM Courtesy of Course Technology/Cengage Learning
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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
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FIGURE 8.33 Asynchronous character framing for serial transmission, including a start bit Courtesy of Course Technology/Cengage Learning
Systems Architecture, Sixth Edition
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
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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
Systems Architecture, Sixth Edition
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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