Multiplexing and Multiplexing and SpreadingSpreading

01204325: Data 01204325: Data Communication and Communication and Computer NetworksComputer Networks

Asst. Prof. Chaiporn Jaikaeo, Ph.D.Asst. Prof. Chaiporn Jaikaeo, Ph.D.chaiporn.j@ku.ac.thchaiporn.j@ku.ac.th

http://www.cpe.ku.ac.th/~cpjhttp://www.cpe.ku.ac.th/~cpjComputer Engineering DepartmentComputer Engineering Department

Kasetsart University, Bangkok, ThailandKasetsart University, Bangkok, ThailandAdapted from lecture slides by Behrouz A. Forouzan© The McGraw-Hill Companies, Inc. All rights reserved

22

Sharing MediumSharing Medium

MUX

DEMUX

A link is divided into channelsA link is divided into channels

n signals(input)

n signals(output)

1 link, n channels

Multiplexer Demultiplexer

33

Frequency Division Multiplexing Frequency Division Multiplexing (FDM)(FDM)

Medium BW > Channel BWMedium BW > Channel BW Each signal is modulated to a Each signal is modulated to a

different carrier frequencydifferent carrier frequency E.g., broadcast radioE.g., broadcast radio Channel allocated even if no dataChannel allocated even if no data

An analog multiplexing technique to combine signalsAn analog multiplexing technique to combine signals

44

Conceptual View of FDMConceptual View of FDM

Channel 3

C

hannel 2

C

hannel 1

f1f2f3

FrequencyTime

55

FDM: Multiplexing ProcessFDM: Multiplexing Process

66

FDM: Demultiplexing FDM: Demultiplexing ProcessProcess

77

Guard BandsGuard Bands Strips of unused bandwidth to prevent signals Strips of unused bandwidth to prevent signals

from overlappingfrom overlapping

FDM

3 kHzf

FDM(no guard band)

FDM(with guard band)

f3 kHz 3 kHz 3 kHz

f3 kHz 3 kHz 3 kHz

f3 kHz

f3 kHz 3 kHz 3 kHz

88

FDM: FDM: Example 1Example 1Five voice channels, each with 3-kHz Five voice channels, each with 3-kHz bandwidth, are to be multiplexed bandwidth, are to be multiplexed together. together. If there is a need for a guard band of 1.5 If there is a need for a guard band of 1.5 kHz, what is the minimum bandwidth of kHz, what is the minimum bandwidth of the link?the link?

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

99

FDM: FDM: Example 2Example 2

Four digital-data channels, 1 Mbps each, use a satellite channel of 1 MHz. Design an appropriate configuration using FDM

Four digital-data channels, 1 Mbps each, use a satellite channel of 1 MHz. Design an appropriate configuration using FDM

1010

Analog HierarchyAnalog Hierarchy

FDM F

DM

FDM

FDM

5

gro

ups

…10

su

perg

ropu

s

6 m

ast

er

gro

ups

group

supergroup

mastergroup

Jumbogroup

4 kHz

4 kHz

4 kHz

…

12 v

oic

e c

hannels

Used by AT&TUsed by AT&T

48 kHz12 voice channels

240 kHz60 voice channels

2.52 MHz600 voice channels

16.984 MHz3600 voice channels

1111

The Advanced Mobile Phone System (AMPS) uses two bands. The first band of 824 to 849 MHz is used for sending, and 869 to 894 MHz is used for receiving. Each user has a bandwidth of 30 kHz in each direction. How many people can use their cellular phones simultaneously?

SolutionEach band is 25 MHz. If we divide 25 MHz by 30 kHz, we get 833.33. In reality, the band is divided into 832 channels. Of these, 42 channels are used for control, which means only 790 channels are available for cellular phone users.

ExampleExample

1212

Multiplexer Demultiplexer

Wavelength Division Multiplexing Wavelength Division Multiplexing (WDM)(WDM)

WDM is a special case of FDMWDM is a special case of FDM

1

2

3

1

2

3

1+2+3

Fiber-optic cable

WDM WDM

1

2

3

1

2

3

1+2+3

An analog multiplexing technique to combine optical signalsAn analog multiplexing technique to combine optical signals

1313

Medium Data Rate > Signal Data RateMedium Data Rate > Signal Data Rate Multiple digital signals interleaved in timeMultiple digital signals interleaved in time

Time slotsTime slots are preassigned to sources and fixedare preassigned to sources and fixed are allocated even if no dataare allocated even if no data do not have to be evenly distributed among sourcesdo not have to be evenly distributed among sources

Time Division Multiplexing Time Division Multiplexing (TDM)(TDM)

TDM

A

B

C

ABCABCABC

Frame

Time slot

A digital multiplexing technique to combine dataA digital multiplexing technique to combine data

one unit

1414

Conceptual View of TDMConceptual View of TDM

TimeFrequency

Ch

an

nel 3

Ch

an

nel 2

Ch

an

nel 1

Ch

an

nel 3

Ch

an

nel 2

Ch

an

nel 1

Ch

an

nel 3

Ch

an

nel 2

Ch

an

nel 1

23 1MUX

1

2

3

23 123 1

1

2

3

DEMUX

Data flow

1515

TDM FramesTDM Frames A frame consists of one complete A frame consists of one complete

cycle of time slotscycle of time slots

1616

TDM: TDM: ExampleExample

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

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

1717

Empty SlotEmpty Slot

1818

SynchronizationSynchronization

BC A 1

… 1 0 1 0 …… 1 0 1 0 …

Synchronization pattern

Multiplexer and demultiplexer must be Multiplexer and demultiplexer must be synchronizedsynchronized

Framing bitsFraming bits are used to provide are used to provide synchronizationsynchronization

BC A 0 BC A 1 BC A 0

framing bit

1919

Synchronization: Synchronization: ExampleExample Identify all the framing bits from this Identify all the framing bits from this

bit sequence output by a multiplexerbit sequence output by a multiplexer 2 channels2 channels 1 unit = 2 bits1 unit = 2 bits

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

2020

Multi-Level MultiplexingMulti-Level Multiplexing

2121

TDM of Different Data RatesTDM of Different Data Rates Data rate from one source may be faster than the Data rate from one source may be faster than the

othersothers More than one time slot can be assigned to certain More than one time slot can be assigned to certain

sourcessources4 kbps

4 kbps

8 kbps 16 kbpsMUX

A

B

C

1 frame

BC AB

Note: assume 1 unit = 1 bit

4 kbps

4 kbps

8 kbps4 kbps

4 kbps16 kbps

MUX

A

B

C

2222

TDM of Different Data RatesTDM of Different Data Rates

5 kbps

3 kbps8 kbpsA

B

AB A A A AB B

B B B

A A A A A

1 frame

AB A A A AB B

MUX

Note: assume 1 unit = 1 bit

2323

5 kbps

3 kbps

A

B

6 kbps

Bit PaddingBit Padding Different data rates may not be Different data rates may not be

integer multiples of each otherinteger multiples of each other Solution – Solution – Bit paddingBit padding

9 kbps

BitPadding

MUX

A AB A ABB AA AB

3000 cycles/s

Extra bit

1 ms

Note: assume 1 unit = 1 bit

A AB A ABB A

A AA AA

BBB

2424

Digital Signal (DS) HierarchyDigital Signal (DS) Hierarchy

TDM

TDM

TDM

DS-1

DS-2

DS-3

DS-4

DS-0

TDM

DS-0

…

24

channels

1.544 Mbps24 DS-0

6.312 Mbps4 DS-1

44.376 Mbps7 DS-2

274.176 Mbps6 DS-3

64 kbps

1 kbps = 1,000 bps1 Mbps = 1,000 kbps

2525

DS Services and T LinesDS Services and T Lines

ServiceService LineLineRate Rate

(Mbps)(Mbps)Voice Voice

ChannelsChannels

DS-1DS-1 T-1T-1 1.5441.544 2424

DS-2DS-2 T-2T-2 6.3126.312 9696

DS-3DS-3 T-3T-3 44.73644.736 672672

DS-4DS-4 T-4T-4 274.176274.176 40324032

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

servicesservices

2626

T Lines and Analog SignalsT Lines and Analog Signals

2727

T-1 Frame StructureT-1 Frame Structure

2828

E LinesE Lines European's version of T linesEuropean's version of T lines

Also used in ThailandAlso used in Thailand

E LineE LineRate Rate

(Mbps)(Mbps)VoiceVoice

ChannelsChannels

E-1E-1 2.0482.048 3030

E-2E-2 8.4488.448 120120

E-3E-3 34.36834.368 480480

E-4E-4 139.264139.264 19201920

2929

Inverse MultiplexingInverse Multiplexing

InverseDEMUX

InverseMUX

high-speed line high-speed line

low-speed lines

3030

Spread SpectrumSpread Spectrum Spread signal to use larger Spread signal to use larger

bandwidthbandwidth To prevent eavesdroppingTo prevent eavesdropping To reduce effect from interferenceTo reduce effect from interference

3131

Frequency-Hopping SSFrequency-Hopping SS "FHSS" – Frequency-Hopping Spread "FHSS" – Frequency-Hopping Spread

SpectrumSpectrum Used in Bluetooth technologyUsed in Bluetooth technology

3232

FHSS CyclesFHSS Cycles

3333

Direct-Sequence SSDirect-Sequence SS "DSSS" – Direct-Sequence Spread "DSSS" – Direct-Sequence Spread

SpectrumSpectrum Used in Wireless LANsUsed in Wireless LANs

3434

DSSS and InterferenceDSSS and Interference

Narrow Band Signal

Frequency

Amplitude

Narrow Band Interference

Spread Spectrum Signal

3535

DSSS ExampleDSSS Example