Chapter 5:

Multiplexing: Sharing a Medium

Data Communications andComputer Networks: A Business User’s ApproachThird Edition

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Objectives

After reading this chapter, you should be able to:

•Describe frequency division multiplexing and list its applications, advantages, and disadvantages

•Describe synchronous time division multiplexing and list its applications, advantages, and disadvantages

•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 its applications, advantages, and disadvantages

•Cite the main characteristics of wavelength division multiplexing and its advantages and disadvantages

•Describe the basic characteristics of discrete multitone

•Cite the main characteristics of code division multiplexing and its advantages and disadvantages

•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

•Broadcast radio and television, cable television, and 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 •Chip spreading 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)

•Receiver

•Gets summed signal

•Multiplies it by receiver code

•Adds up resulting values

•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:

Signal received: +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:

Signal received: +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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Business Multiplexing in Action

•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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Business Multiplexing in Action (continued)

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Business Multiplexing in Action (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 business situations