msis 4523 ch6.bandwidthutil

8/3/2019 MSIS 4523 Ch6.BandwidthUtil

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Data Communications SystemsCh 6: Bandwidth Utilization

JinKyu Lee, Ph.D.

[email protected]

Include the course code (MSIS4523) in every email subject!!


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Topics

Multiplexing Techniques

Commonly Used Forms of Multiplexing

Frequency Division Multiplexing

Time Division Multiplexing Synchronous vs. Statistical Wavelength Division Multiplexing

Discrete Multi-tone for DSL

Code Division Multiplexing


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What is Multiplexing

Sharing a common transmission medium simultaneouslybetween several data sources

Line sharing saves transmission costs

Allows efficient utilization of higher data rates

Means more cost-effective transmissions

Takes advantage of the fact that most individual datasources require relatively low data rates

Commonly Used

Forms of Multiplexing


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Frequency Division Multiplexing (FDM)

FDM is an analog multiplexing

technique that combines signals


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Frequency Division Multiplexing

Requires analog signaling & transmission Total bandwidth = sum of input bandwidths +

guard bands

Modulates signals so that each occupies a

different frequency band

Standard for radio broadcasting, analogtelephone network, cell phones, and television

(broadcast, cable, & satellite)


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The FDM Multiplexing Process


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The FDM Demultiplexing Process


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Assume that a voice channel occupies a bandwidth of 4KHz. We need to combine three voice channels into alink with a bandwidth of 12 KHz, from 20 to 32 KHz.Show the configuration using the frequency domainwithout the use of guard bands.

Shift (modulate) each of the three voice channels to adifferent bandwidth, as shown on the next chart

Solution

FDM Example 1


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Example 1 Solution


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Five channels, each with a 100-KHz bandwidth, are tobe multiplexed together. What is the minimumbandwidth of the link if there is a need for a guard bandof 10 KHz between the channels to prevent

interference?

Example 2

Solution


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Example 2 Solution

5 x 100 + 4 x 10 = 540 KHz

For five channels, we need at least four guard bands.This means that the required bandwidth is at least


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Four data channels (digital), each transmitting at 1Mbps, use a satellite channel 1 MHz wide. Design anappropriate configuration using FDM

Example 3

Solution


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Example 3 Solution

The satellite channel is analog. We divide it into fourchannels, each channel having a 250-KHz bandwidth. Eachdigital channel of 1 Mbps is modulated such that each 4 bitsare modulated to 1 Hz. One solution is 16-QAM modulation.


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Analog FDM Hierarchy


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ADSL Technology

ADSL Uses discrete multitone (DMT) to exploit the 1-

MHz capacity of existing twisted pair loops

There are three elements of the ADSL strategy

Reserve lowest 25 kHz for voice, known as POTS (Plain oldtelephone service)

Use echo cancellation or FDM to allocate a small upstream bandand a larger downstream band

Use FDM/DMT within the upstream and downstream bands,using DMT signaling


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Discrete Multitone (DMT)


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The Discrete Multitone (DMT) Method

Multiple carrier signals at different frequencies

Sends some of the bits on each channel

Transmission band (upstream or downstream) is dividedinto 4-kHz channels

Each channel is capable of carrying 60 kbps

256 downstream channels = 15.26 Mbps capacity

Provides 1.5 to 9 Mbps in practice

Test signals determine noise at each channel

Sends less bits over noisy channels

It then assigns more bits to better quality channels and fewer bitsto poorer quality channels


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ADSL/DMT Bandwidth Allocation

Each channel is 4 KHz wide, carryinga maximum of 60 Kbps. Only the

least noisy channels are used


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Wave Division Multiplexing (WDM)

WDM is an analog multiplexing technique tocombine optical signals. Optical prisms are

used for the multiplexing function

1 2 3

1

2

3

1

2

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1

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1

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22


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Synchronous Time-Division Multiplexing(TDM)

Used in digital transmission systems

Requires data rate of the medium to exceeddata rate of signals to be transmitted

Signals take turns over medium

Slices of data are organized into frames


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Time Division Multiplexing (TDM)

TDM is a digital multiplexingtechnique to combine data


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TDM Frame Structures

In a TDM, the data rate of the link is sum of thecomponent input data streams, plus any overhead


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Four channels are multiplexed using TDM. If eachchannel sends 100 bytes/s and we multiplex 1 byte perchannel, show the frame traveling on the link, the sizeof the frame, the duration of a frame, the frame rate,and the bit rate for the link.

Example 6

Solutionsize of the frame size of the frame

Frame rate = # of frames transmitted per sec.

Bit rate = bps

duration of a frame = 1/# of Frame


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A multiplexer combines four 100-Kbps channels using atime slot of 2 bits. Show the output with four arbitraryinputs. What is the frame rate? What is the frameduration? What is the bit rate? What is the bit duration?

Example 7

Solution

msize of the frame

duration of a frame = 1/# of Frames per sec

Frame rate = # of frames transmitted per sec.

Bit rate = bps


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Synchronous TDM and the PSTN

Basis of modern carrier telecommunications systems

US, Canada, Japan: DS-1(T-1), DS-3(T-3), ...

Europe, elsewhere: E1, E3, ...

ATM (DL Std) and SONET (MUX Std) everywhere

DS-1: Data rate of 1.544 Mbps

Original TDM system designed for voice

Uses PCM to digitize voice transmission at 8000 times/sec with 8bits/sample = 64kbps

24 channels x 8 bits/channel + 1 frame bit = 193 bits/frame with8000 frames/sec


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T-1 Frame Structure


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Multiplexing Telephone Lines on a T-1 Line


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North American Digital Signal (DS) Hierarchy


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DS and T Lines Rates

Service LineRate

(Mbps)Voice

Channels

DS-1 T-1 1.544 24

DS-2 T-2 6.312 96

DS-3 T-3 44.736 672

DS-4 T-4 274.176 4032


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European E Line Rates

E LineRate

(Mbps)Voice

Channels

E-1 2.048 30

E-2 8.448 120

E-3 34.368 480

E-4 139.264 1920


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Statistical Time Division Multiplexing(STDM)

Intelligent TDM

Data rate capacity required is well below the

sum of connected capacity Digital only, because it requires more complex

framing of data

Frame slots only to devices with full buffers

Uses memory buffers to avoid loss of data


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Statistical Time Division Multiplexing

Statistical multiplexor - transmits only the data from active

workstationsAccepts incoming data streams

Creates a frame containing only the data to be transmitted

If a workstation is not active, no space is wasted on the multiplexed stream


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Statistical Time Division Multiplexing (cont.)

To identify each piece of data, an address is included

If data is of variable size, length is also included


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Statistical Time Division Multiplexing (cont.)

More precisely, the transmitted frame contains a

collection of data groups


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Comparison of Multiplexing Techniques

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