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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, [email protected]@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