# Chapter 05 - Jerry Post

```Data Communications and
Computer Networks: A
Third Edition
Chapter 5:
Multiplexing: Sharing a Medium
Objectives
After reading this chapter, you should be able to:
•Describe frequency division multiplexing and list its
•Describe synchronous time division multiplexing
and list its applications, advantages, and
•Outline the basic multiplexing characteristics of
both T-1 and ISDN telephone systems
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Objectives (continued)
•Describe statistical time division multiplexing and list
•Cite the main characteristics of wavelength division
•Describe the basic characteristics of discrete multitone
•Cite the main characteristics of code division
•Apply a multiplexing technique to a typical business
situation
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Introduction
•Under the simplest conditions, a medium can carry
only one signal at any moment in time
•For multiple signals to share one medium, the
medium must somehow be divided, giving each
signal a portion of the total bandwidth
•The current techniques that can accomplish this
include frequency division multiplexing, time
division multiplexing, and wavelength division
multiplexing
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Frequency Division Multiplexing
•Assignment of non-overlapping frequency ranges to
each “user” or signal on a medium
•Thus, all signals are transmitted at the same time, each
using different frequencies
•A multiplexor
•Accepts inputs and assigns frequencies to each device
•Is attached to a high-speed communications line
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Frequency Division Multiplexing
(continued)
•Corresponding multiplexor, or demultiplexor
•Is on the end of the high-speed line
•Separates the multiplexed signals
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Frequency Division Multiplexing
(continued)
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Frequency Division Multiplexing
(continued)
•Analog signaling is used to transmit signals
AMPS cellular phone systems use frequency
division multiplexing
•Oldest multiplexing technique
•Involves analog signaling  more susceptible to noise
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Time Division Multiplexing
• Sharing signal is accomplished by dividing
available transmission time on a medium among
users
• Digital signaling is used exclusively
• Time division multiplexing comes in two basic
forms:
1. Synchronous time division multiplexing
2. Statistical, or asynchronous time division multiplexing
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Synchronous Time Division Multiplexing
•The original time division multiplexing
•Multiplexor
•Accepts input from attached devices in a round-robin
fashion
•Transmits data in a never ending pattern
•T-1 and ISDN telephone lines are common
examples of synchronous time division multiplexing
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Synchronous Time Division Multiplexing
(continued)
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Synchronous Time Division Multiplexing
(continued)
•If one device generates data at a faster rate than
other devices, then the multiplexor must either
•Sample incoming data stream from that device more
often than it samples other devices
•OR
•Buffer faster incoming stream
•If a device has nothing to transmit,
•Multiplexor must still insert a piece of data from that
device into the multiplexed stream
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Synchronous Time Division Multiplexing
(continued)
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Synchronous Time Division Multiplexing
(continued)
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Synchronous Time Division Multiplexing
(continued)
So that the receiver may stay synchronized with
the incoming data stream, the transmitting
multiplexor can insert alternating 1s and 0s into
the data stream
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T-1 Multiplexing
•T-1 multiplexor stream is a continuous series of frames
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ISDN Multiplexing
•ISDN multiplexor stream is also a continuous
stream of frames
•Each frame contains various control and sync info
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SONET/SDH Multiplexing
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Statistical Time Division Multiplexing
•Statistical multiplexor - transmits only the data from
active workstations
•If a workstation is not active, no space is wasted on the
multiplexed stream
•A statistical multiplexor
•Accepts incoming data streams
•Creates a frame containing only the data to be
transmitted
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Statistical Time Division Multiplexing
(continued)
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Statistical Time Division Multiplexing
(continued)
To identify each piece of data, an address is included
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Statistical Time Division Multiplexing
(continued)
If data is of variable size, length is also included
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Statistical Time Division Multiplexing
(continued)
More precisely, the transmitted frame contains a collection
of data groups
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Wavelength Division Multiplexing
•Wavelength division multiplexing multiplexes
multiple data streams onto a single fiber optic line
•Different wavelength lasers (called lambdas)
transmit the multiple signals
•Each signal carried on the fiber can be transmitted
at a different rate from the other signals
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Wavelength Division Multiplexing
(continued)
•Dense wavelength division multiplexing combines
many (30, 40, 50, 60, more?) onto one fiber
•Coarse wavelength division multiplexing combines
only a few lambdas
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Wavelength Division Multiplexing
(continued)
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Discrete Multitone (DMT)
•A multiplexing technique commonly found in
digital subscriber line (DSL) systems
•DMT combines hundreds of different signals, or
subchannels, into one stream
•Each subchannel is quadrature amplitude modulated
•recall - eight phase angles, four with double amplitudes
•Theoretically, 256 subchannels, each transmitting
60 kbps, yields 15.36 Mbps
•Unfortunately, there is noise
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Code Division Multiplexing
•Also known as code division multiple access
•Advanced technique that allows multiple devices to
transmit on the same frequencies at the same time
•Each mobile device is assigned unique 64-bit code
•To send a binary 1, mobile device transmits the
unique code
•To send a binary 0, mobile device transmits the
inverse of code
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Code Division Multiplexing (continued)
•Gets summed signal
•Interprets as a binary 1 if sum is near +64
•Interprets as a binary 0 if sum is near –64
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Code Division Multiplexing Example
•For simplicity, assume 8-chip spreading codes
•3 different mobiles use the following codes:
-Mobile A: 10111001
-Mobile B: 01101110
-Mobile C: 11001101
-Assume Mobile A sends a 1, B sends a 0, and C
sends a 1
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Code Division Multiplexing Example
(continued)
•Signal code: 1-chip = +N volt; 0-chip = -N volt
•Three signals transmitted:
•Mobile A sends a 1, or 10111001, or +-+++--+
•Mobile B sends a 0, or 10010001, or +--+---+
•Mobile C sends a 1, or 11001101, or ++--++-+
•Summed signal received by base station: +3, -1, -1,
+1, +1, -1, -3, +3
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Code Division Multiplexing Example
(continued)
Base station decode for Mobile A:
+3, -1, -1, +1, +1, -1, -3, +3
Mobile A’s code: +1, -1, +1, +1, +1, -1, -1, +1
Product result:
+3, +1, -1, +1, +1, +1, +3, +3
Sum of Product results: +12
Decode rule: For result near +8, data is binary 1
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Code Division Multiplexing Example
(continued)
Base station decode for Mobile B:
+3, -1, -1, +1, +1, -1, -3, +3
Mobile B’s code: -1, +1, +1, -1, +1, +1, +1, -1
Product result:
-3, -1, -1, -1, +1, -1, -3, -3
Sum of Product results: -12
Decode rule: For result near -8, data is binary 0
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Comparison of Multiplexing Techniques
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•XYZ Corporation has two buildings separated by a
distance of 300 meters
•A 3-inch diameter tunnel extends underground
between the two buildings
•Building A has a mainframe computer and Building
B has 66 terminals
•List some efficient techniques to link the two
buildings
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(continued)
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(continued)
•Possible Solutions:
•Connect each terminal to mainframe computer using
separate point-to-point lines
•Connect all terminals to mainframe computer using one
multipoint line
•Connect all terminal outputs and use microwave
transmissions to send data to the mainframe
•Collect all terminal outputs using multiplexing and send
data to mainframe computer using conducted line
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Summary
•Frequency division multiplexing
•Synchronous time division multiplexing
•Basic multiplexing characteristics of T-1 and ISDN
telephone systems
•Statistical time division multiplexing
•Wavelength division multiplexing
•Discrete multitone
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Summary (continued)
•Code division multiplexing
•Applying multiplexing techniques to typical