Multi Mu tiplexing 01204325 Data Communications and Computer - - PowerPoint PPT Presentation

Mu Multi tiplexing

Chaipo Chaiporn J n Jaik aikae aeo De Department of f Computer Engineering Kasetsart Unive versity

01204325 Data Communications and Computer Networks

Based on lecture materials from Data Communications and Networking, 5th ed., Behrouz A. Forouzan, McGraw Hill, 2012.

Revised 2020-08-11

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Out Outline line

• Multiplexing over frequency
• Multiplexing over time
• Orthogonal frequency division multiplexing (OFDM)

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Sha Sharing M ng Medi dium um

M U X D E M U X

• A link is divided into channels

n signals (input) n signals (output) 1 link, n channels Multiplexer Demultiplexer

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Fr Frequenc equency Divis ivisio ion n Multiple ultiplexing ing (FD FDM)

• Medium BW > Channel BW
• Each signal is modulated to a different carrier frequency
• E.g., broadcast radio
• Channel allocated even if no data

An An analog multiplexing technique to combine signals

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Con Conceptu tual View w of

• f FDM

DM

Channel 3 Channel 2 Channel 1

f1 f2 f3

Frequency

T i m e

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FD FDM: M : Mul ultipl plexi xing P ng Proce cess

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FD FDM: D : Demul multipl plexi xing P ng Proce cess

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Gu Guard Bands

• Strips of unused bandwidth to prevent signals from
• verlapping

FDM

3 kHz

f f

3 kHz 3 kHz 3 kHz

FDM (no guard band) FDM (with guard band) f

3 kHz 3 kHz 3 kHz

f

3 kHz

f

3 kHz 3 kHz 3 kHz

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Ex Example – FD FDM Gu Guard B Bands

Five voice channels, each with 3-kHz bandwidth, are to be multiplexed together. If there is a need for a guard band of 1.5 kHz, what is the minimum bandwidth of the link?

f

3 kHz 3 kHz 3 kHz 3 kHz 3 kHz 1.5 kHz 1.5 kHz 1.5 kHz 1.5 kHz

21 kHz

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Ex Example – FD FDM w with QA QAM

Four digital-data channels, 1 Mbps each, use a satellite channel of 1 MHz. Design an appropriate configuration using FDM. (assume 1 Hz/baud)

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An Analog Hierarchy

F D M F D M F D M F D M

5 groups

…

10 supergropus 6 master groups group supergroup master group Jumbo group 4 kHz 4 kHz 4 kHz

…

12 voice channels

• Used by AT&T (from 1910s to early 1980s)

48 kHz 12 voice channels 240 kHz 60 voice channels 2.52 MHz 600 voice channels 16.984 MHz 3600 voice channels

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Multiplexer Demultiplexer

Wa Wavelength Division Multiplexing (WDM)

• WDM is a special case of FDM

l1 l2 l3 l1 l2 l3

l1+l2+l3

Fiber-optic cable

WDM WDM

l1 l2 l3 l1 l2 l3 l1+l2+l3

An An analog

• g multi

tiplexing technique to

• com
• mbine op
• pti

tical signals

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• Medium Data Rate > Signal Data Rate
• Multiple digital signals interleaved in time
• Time slots
• are preassigned to sources and fixed
• are allocated even if no data
• do not have to be evenly distributed among sources

Ti Time Division Multiplexing (TD (TDM)

T D M A B C A

B C

A

B C

A

B C

Frame Time slot

A A digital multiplexing technique to combine data

• ne unit

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Con Conceptu tual View w of

• f TDM

DM

Time F r e q u e n c y

Channel 3 Channel 2 Channel 1 Channel 3 Channel 2 Channel 1 Channel 3 Channel 2 Channel 1

2

3 1

M U X 1 2 3 2

3 1

2

3 1

1 2 3 D E M U X Data flow

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TD TDM Fr Frame mes

• A frame consists of one complete cycle of time slots

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Ex Example – TD TDM F Frame me

Four 1-Kbps connections are multiplexed together. A unit is 1 bit. Find: (1) the duration of 1 bit before multiplexing, (2) the duration of a frame, (3) the number of bits in a frame, (4) the duration of a time slot, and (5) the transmission rate of the link.

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Sy Synchronization

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Sy Synchronization

B C A 1

… 1 0 1 0 …

Synchronization pattern

• Multiplexer and demultiplexer must be synchronized
• Framing bits are used to provide synchronization

framing bit

B C A 0 B C A 1 B C A 0

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Ex Example – Sy Synchronization

• Identify all the framing bits from this bit sequence
• utput by a multiplexer
• 2 channels
• 1 unit = 2 bits

0 0 0 1 1 0 1 1 1 0 0 0 0 1 0 0 1 0 1 1

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Mu Multi ti-Le Level Mu Multi tiplexing

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TD TDM o

• f D

Different D Data Ra Rates

• Data rate from one source may be faster than the others
• More than one time slot can be assigned to certain sources

4 kbps 4 kbps 8 kbps 16 kbps A B C 1 frame B C A B

Assume 1 unit = 1 bit

MUX

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Di Digital Signal (DS (DS) ) Hierarchy

T D M T D M T D M

DS-1 DS-2 DS-3 DS-4 DS-0

T D M

DS-0

…

24 channels

1.544 Mbps 24 DS-0 6.312 Mbps 4 DS-1 44.376 Mbps 7 DS-2 274.176 Mbps 6 DS-3 64 kbps

1 kbps = 1,000 bps 1 Mbps = 1,000 kbps

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T L T Line nes a and A nd Ana nalog Si g Signa gnals

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T-1 1 Frame Stru tructu ture

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DS DS Servi vices s and T Lines

• DS-0, DS-1, etc, are services
• T lines are used to implement these services

Service Line Rate (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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E E Lines

• European variation of T lines
• Also used in Thailand

E Line Rate (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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In Inver erse e Mult ltip iple lexin xing

high-speed line high-speed line low-speed lines

Inverse MUX Inverse DEMUX

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OFD OFDM

• Orthogonal Frequency-Division Multiplexing
• Widely used in broadband/wideband digital

communications

• Digital TV
• ADSL
• Powerline communications
• Wireless networks: Wi-Fi, WiMAX
• 4G/5G mobile communications

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OFD OFDM C Concep ncept

• Sub-carriers are orthogonal to each other
• No cross-talk; guard bands not necessary

Source: http://connectedplanetonline.com

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200 300 400 500 600 700 800

Or Ortho hogonalit nality in OFD y in OFDM

Frequency (Hz)

t 20 ms

Modulated @500 Hz

t

Modulated @550 Hz

t

Modulated @600 Hz

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OFD OFDM Implemen Implementatio ion

Source: Wikipedia

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Summa Summary

• Multiplexing is a method of combining multiple narrow-

band signals into a wider-band signal

• FDM multiplexes signals into different frequencies
• TDM multiplexes signals into different time slots
• OFDM is a FDM method that employs a large number of

closely spaced orthogonal sub-carrier signals