Business Data Communications,
Fourth Edition
Chapter 5: Data Transmission
Learning Objectives
• Describe and differentiate between
amplitude modulation, frequency
modulation, phase modulation, and
quadrature amplitude modulation
• Give three examples each of simplex, halfduplex, and full-duplex data transmissions
• Describe the difference between serial and
parallel transmissions
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Learning Objectives
• Identify the differences between asynchronous
and synchronous transmission
• Describe how errors are detected, prevented,
and corrected on a data communications
network
• Define digital transmission and explain how it
works
• Describe the generic and specific forms of digital
subscriber lines
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Introduction
• Networks are made up of hardware and
software
• Electrical, optical and broadcast signals are
used to transmit data on networks
• Signal is changed to differentiate bits
• Error detection and correction techniques are
needed on a network
• Digital transmission differs from analog
transmission
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Analog Modulation
• Carrier signal is used to transmit data via
an electrical signal
• Modulation is the change made to a signal
• Amplitude Modulation (AM)
– Changes the height of the signal
• Frequency Modulation (FM)
– Changes the number of waves in the signal
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Analog Modulation
• Phase Modulation (PM)
– Phase shift indicates change in signal
– Direction of the wave changes
– Amplitude and frequency remain the same
• Quadrature Amplitude Modulation (QAM)
– Phase and amplitude are changed
– Improves transmission
– Cable modems use this modulation technique
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Digital Modulation
• Using Analog Signals to Transmit Digital
Data
– Amplitude-Shift Keying (ASK)
• Converts digital signals to be transmitted on an
analog line
– Frequency-Shift Keying (FSK)
• Modulating digital signals using FM
• GSM uses this
– Differential Phase-Shift Keying (DPSK)
• Modified version of phase shift keying
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Digital Modulation
• Using Digital Signals to Transmit Analog Data
– Pulse Amplitude Modulation (PAM)
• Varies height of the signal
• Used in early voice digitization
– Pulse Duration Modulation (PDM) or Pulse Width
Modulation (PWM)
• Duration of electrical pulse indicates changes
– Pulse Position Modulation (PPM)
• Varies spaces between pulses
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Digital Modulation
• Using Digital Signals to Transmit Analog
Data
– Pulse Code Modulation (PCM)
• Most common form of analog to digital modulation
• Four step process
–
–
–
–
Signal is sampled using PAM
Integer values assigned to signal
Values converted to binary
Signal is digitally encoded for transmission
• Nyquists Theorem
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Digital Modulation
• Using Digital Signals to Transmit Digital
Data
– Bits must be changed to digital signal for
transmission
– Unipolar encoding
• Positive or negative pulse used for zero or one
– Polar encoding
• Uses two voltage levels (+ and - ) for zero or one
– Bipolar encoding
• +, -, and zero voltage levels are used
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Transmission Direction
• Simplex Transmission
– Transmission occurs in only one
direction
– Examples
• Water faucet
• Airline displays in airport
• Stock trade tickers
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Transmission Direction
• Half-Duplex Transmission
– Transmission in either direction, but only one
way at a time
– Examples
• Mobile radios, such as CB, police, taxi
• Communication between modems
– Modem turnaround time
• Time it takes for the modem to switch from send to
receive mode and vice versa
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Transmission Direction
• Full-Duplex Transmission
– Transmission in both directions
simultaneously
– Examples
• Two-lane highway
• Communication between servers & clients
– Telephone system
• Echo occurs on international phone calls
• Echo suppressors used to eliminate echo, causes
line to become half-duplex
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Transmission Modes
• Serial
Transmission
– Data transmitted as
individual bits
– Examples
• TCP/IP (SLIP, PPP)
• Modems
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Transmission Modes
• Parallel Transmission
– An entire character is
transmitted at once
– Printers use this
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Synchronization
• Asynchronous Transmission
– Each character is transmitted individually
– Start/stop transmission
– Inefficient
• Transmitting 8 bits with two start/stop bits
• 8/10 = .8 or 80% efficiency
– DEC VT-100, PC modems use it
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Synchronization
• Synchronous Transmission
– Data is transmitted in blocks
– Each block is identified by a SYN character
– Efficiency
•
•
•
•
•
•
For 1 byte with 6 SYN characters (3 before, 3 after)
1/7 = .143 or 14.3%
For 10 bytes with 6 SYN characters
10/16 = .625 or 62.5%
For 100 bytes with 6 SYN characters
100/106 = .94 or 94%
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Errors
• Sources of Errors
– Impulse noise – electrical interference
– White noise – movement of electrons
– Attenuation – loss of signal
– Crosstalk – signals interfere
• Intermodulation noise – combination of frequencies
– Delay distortion – data arrives at different
times
– Line failure – loss of line
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Errors
• Error Detection
– Parity checking
• Vertical redundancy check
• Even versus odd parity
• Longitudinal redundancy check
–Block control character (BCC) created
• 50 % effective
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Errors
– Cyclic Redundancy Check (CRC)
• Uses polynomial equation
• More effective than parity checking
• CRC-16 is 100% effective
• Table 5-2
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Errors
• Error Correction
– Forward Error
Correction
• Errors corrected
with retransmission
• Hamming Code
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Errors
• Error Correction
– Error Detection with Retransmission
• Automatic repeat request (ARQ)
–Stop-and-wait ARQ
–Go-back-N ARQ
–Continuous ARQ
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Errors
• Error Correction
– Error Control Standards
• Microcom Networking Protocols (MNP)
– Built-in modem error controls
• V.42
– Uses MNP4 and link access protocol for modems (LAPM) error control
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Errors
• Error Prevention
– Improve shielding around cables
– Condition leased lines
– Reduce transmission speed
– Hang up and redial
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Digital Transmission
• Data is not converted to analog
• Integrated Services Digital Network (ISDN)
– Allows several types of transmission on a
single line
– Uses two 64 Kbps data lines, and one 16
Kbps control line
– More expensive than standard telephone line
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Digital Transmission
• Digital Subscriber Line (DSL)
– May send and receive data while talking on the
telephone
– Uses regular copper telephone lines
– Speeds
• Download – 1.5 to 8 Mbps
• Upload – 16 to 640 Kbps
• Local Multipoint Distribution Service (LMDS)
– Two way digital voice, data, Internet, video at 20 GHz
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Digital Transmission
• Coder/Decoder (CODEC)
– Changes digital signal to analog for
transmission on analog lines
– Also used for voice conversations on digital
lines
– Sampling of digital data results in a digital
signal
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Summary
• Data is transmitted on a
carrier wave
• Amplitude modulation
changes wave height
• Frequency modulation
changes number of waves
• Phase modulation changes
the direction of the wave
• Wireless transmission uses
frequency division multiple
access, time division
multiple access or code
division multiple access
Chapter 5: Data Transmission
• Simplex transmission is
one way only
• Half-duplex transmission
is both ways, one way at
a time
• Full-duplex transmission
is both ways,
simultaneously
• Asynchronous is
start/stop transmission
• Synchronous is
continuous transmission
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Summary
• Simplex transmission is
one way only
• Half-duplex transmission
is both ways, one way at
a time
• Full-duplex transmission
is both ways,
simultaneously
• Asynchronous is
start/stop transmission
• Synchronous is
continuous transmission
Chapter 5: Data Transmission
• Errors can be detected
using parity checking or
cyclic redundancy
checking
• Error correction can be
forward error correction
or retransmission
techniques
• ISDN and ADSL are
digital networks
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