Business data communications 5e by stallings chapter 16

Chapter 16:
Data Communication
Fundamentals
Business Data Communications, 5e


Data Communication
Components
• Data
– Analog: Continuous value data (sound, light,
temperature)
– Digital: Discrete value (text, integers, symbols)

• Signal
– Analog: Continuously varying electromagnetic wave
– Digital: Series of voltage pulses (square wave)

• Transmission
– Analog: Works the same for analog or digital signals
– Digital: Used only with digital signals

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Analog Data→Signal Options
• Analog data to analog signal
– Inexpensive, easy conversion (eg telephone)
– Data may be shifted to a different part of the

available spectrum (multiplexing)
– Used in traditional analog telephony

• Analog data to digital signal
– Requires a codec (encoder/decoder)
– Allows use of digital telephony, voice mail
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Digital Data→Signal Options
• Digital data to analog signal
– Requires modem (modulator/demodulator)
– Allows use of PSTN to send data
– Necessary when analog transmission is used

• Digital data to digital signal
– Requires CSU/DSU (channel service unit/data service
unit)
– Less expensive when large amounts of data are
involved
– More reliable because no conversion is involved
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Transmission Choices
• Analog transmission
– only transmits analog signals, without regard for data
content
– attenuation overcome with amplifiers
– signal is not evaluated or regenerated

• Digital transmission
– transmits analog or digital signals
– uses repeaters rather than amplifiers
– switching equipment evaluates and regenerates signal
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Data, Signal, and
Transmission Matrix

A
Data

D

D
A
A

D

Transmission
System

Signal
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Advantages of Digital
Transmission
•
•
•
•

The signal is exact
Signals can be checked for errors
Noise/interference are easily filtered out
A variety of services can be offered over
one line
• Higher bandwidth is possible with data
compression
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Why Use Analog Transmission?
•
•
•
•

Already in place
Significantly less expensive
Lower attenuation rates
Fully sufficient for transmission of voice
signals

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Analog Encoding
of Digital Data
• Data encoding and decoding technique to
represent data using the properties of
analog waves
• Modulation: the conversion of digital
signals to analog form
• Demodulation: the conversion of analog
data signals back to digital form
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Modem
• An acronym for modulator-demodulator
• Uses a constant-frequency signal known as
a carrier signal
• Converts a series of binary voltage pulses
into an analog signal by modulating the
carrier signal
• The receiving modem translates the analog
signal back into digital data
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Methods of Modulation
• Amplitude modulation (AM) or amplitude
shift keying (ASK)
• Frequency modulation (FM) or frequency
shift keying (FSK)
• Phase modulation or phase shift keying
(PSK)

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Amplitude Shift Keying (ASK)
• In radio transmission, known as amplitude
modulation (AM)
• The amplitude (or height) of the sine wave
varies to transmit the ones and zeros
• Major disadvantage is that telephone lines
are very susceptible to variations in
transmission quality that can affect
amplitude
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ASK Illustration

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0

0

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Frequency Shift Keying (FSK)
• In radio transmission, known as frequency
modulation (FM)
• Frequency of the carrier wave varies in
accordance with the signal to be sent
• Signal transmitted at constant amplitude
• More resistant to noise than ASK
• Less attractive because it requires more
analog bandwidth than ASK
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FSK Illustration

1

1

0
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Phase Shift Keying (PSK)
• Also known as phase modulation (PM)
• Frequency and amplitude of the carrier
signal are kept constant
• The carrier signal is shifted in phase
according to the input data stream
• Each phase can have a constant value, or
value can be based on whether or not phase
changes (differential keying)
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PSK Illustration

0

0

1

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Differential Phase Shift Keying
(DPSK)

0

1

1

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Voice Grade Modems

QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.

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Cable Modems
• Permits Internet access over cable television networks.
• ISP is at or linked by high-speed line to central cable
office
• Cables used for television delivery can also be used to
deliver data between subscriber and central location
• Upstream and downstream channels are shared among
multiple subscribers, time-division multiplexing
technique (see Chapter 17)
• Splitter is used to direct TV signals to a TV and the data
channel to a cable modem
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Cable Modem Layout

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Asymmetric Digital
Subscriber Line (ADSL)
•

•
•
•

New modem technology for high-speed digital transmission over
ordinary telephone wire.
Telephone central office can provide support for a number of ISPs,
At central office, a combined data/voice signal is transmitted over a
subscriber line
At subscriber’s site, twisted pair is split and routed to both a PC and
a telephone
– At the PC, an ADSL modem demodulates the data signal for the PC.
– At the telephone, a microfilter passes the 4-kHz voice signal.

•

The data and voice signals are combined on the twisted pair line
using frequency-division-multiplexing techniques (Chapter 17)

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DSL Modem Layout

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Digital Encoding
of Analog Data
• Evolution of telecommunications networks to digital
transmission and switching requires voice data in digital form
• Best-known technique for voice digitization is pulse-code
modulation (PCM)
• The sampling theorem: If a signal is sampled at regular
intervals of time and at a rate higher than twice the significant
signal frequency, the samples contain all the information of
the original signal.
• Good-quality voice transmission can be achieved with a data
rate of 8 kbps
• Some videoconference products support data rates as low as
64 kbps

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Converting Samples to Bits
• Quantizing
• Similar concept to pixelization
• Breaks wave into pieces, assigns a value in
a particular range
• 8-bit range allows for 256 possible sample

levels
• More bits means greater detail, fewer bits
means less detail
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