2. Chapter Goals Explain communication protocols
Describe signals and the media used to transmit digital signals
Compare and contrast methods of encoding and transmitting data using analog or digital signals
Describe methods for efficiently using communication channels
Describe methods for detecting and correcting data transmission errors
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3. Systems Architecture, Fifth Edition

4. Communication Protocols
Set of rules and conventions for communication
Message content and format
Bit encoding
Signal transmission
Transmission medium
Channel organization
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5. Communication Protocols (continued)
Include procedures for coordinating flow of data
Media access
Clock synchronization
Error detection and correction
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6. Systems Architecture, Fifth Edition

7. 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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8. 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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9. Systems Architecture, Fifth Edition

10. Modulation Methods Techniques used to encode bits in sine waves
Frequency modulation (FM)
Amplitude modulation (AM)
Phase shift modulation
Multilevel coding
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11. Amplitude modulation (AM) represents bit values as specific wave amplitudes.
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12. Frequency modulation (FM) represents bit values by varying carrier wave frequency while holding amplitude constant.
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13. Phase shift modulation makes a sudden shift in signal phase which can be detected and interpreted as data.
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14. Multilevel coding embeds multiple bit values within a single wave characteristic.
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15. Analog Signals
Use 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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16. Digital Signals Can contain one of a finite number of possible values
Pulse code modulation (PCM)
Binary data transmission via square waves
Square waves preferred over short distances
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17. Systems Architecture, Fifth Edition

18. A digital signaling scheme defines a range of wave characteristic values to represent each bit value.
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19. Signal Capacity and Errors
Analog signals compared with digital signals
Carry more information
Are more susceptible to transmission error
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20. Systems Architecture, Fifth Edition

21. Transmission Media
Communication path that transports signals (e.g., copper wire, optical fiber)
Characteristics
Raw data transfer rate (speed and capacity)
Bandwidth
Susceptibility to noise, distortion, external interference, and attenuation
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22. A communication channel consists of a sending device, receiving device, and the transmission medium that connects them.
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23. Speed and Capacity
Interdependent; jointly described as data transfer rate (raw versus effective data transfer rate)
Factors that account for transmission speed differences among media
Length of media
Ways in which multiple media segments are interconnected
Rate at which bits are encoded in signals and recognized by the receiver
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24. Frequency and Bandwidth
Carrier wave frequency
Fundamental 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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25. Range of electromagnetic frequencies; terms that describe subsets of that range
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26. Modulator-demodulator (modem) technology sends digital signals over voice-grade telephone channels by encoding them in an analog carrier wave. Current rates are as high as 56,000 bps.
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27. Signal-to-Noise (S/N) Ratio
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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28. S/N ration is positive for distances up to 5 kilometers
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29. 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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30. 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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31. Wireless Data Transmission
Uses shortwave radio or infrared 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, ability to be “overheard”
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32. 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: simple, half-duplex, full duplex
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33. 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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34. Systems Architecture, Fifth Edition

35. Parallel and Serial Transmission
Uses multiple lines to send several bits simultaneously
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
Cheaper to implement; uses fewer wires or wireless channels
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37. Systems Architecture, Fifth Edition

38. 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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39. 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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40. Packet switching: Packets are sent to their destination as channel capacity becomes available.
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41. FDM: Signals are transmitted within each subchannel at a fixed frequency or narrow frequency range.
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42. Clock Synchronization
Ensures that sender/receiver use same time periods and boundaries to encode/decode bit values
Asynchronous transmission
Relies on specific start and stop signals to indicate beginning and end of a message unit
Synchronous transmission
Ensures that sender/receiver clocks are always synchronized by sending continuous data streams
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43. Bit time boundaries are misaligned; receiver is unable to interpret several bits correctly because they contain two different signal levels.
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44. Synchronous transmission: Messages are transmitted in fixed-size byte groups called blocks.
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45. Asynchronous transmission appends one or more start bits to the beginning of each message.
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46. Error Detection and Correction
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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47. 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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48. 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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49. Systems Architecture, Fifth Edition

50. 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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51. An even parity bit is computed for each position of a block of 8 bytes
An even parity bit is computed for each position of a block of 8 bytes. The set of parity bits forms a BCC that is appended to the block for error detection.
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52. 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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53. 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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