tdc462 multiplexing

8/8/2019 TDC462 Multiplexing

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Data Communications

Multiplexing


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Multiplexing


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Frequency Division Multiplexing

FDM

Useful bandwidth of medium exceeds requiredbandwidth of channel

Each signal is modulated to a different carrierfrequency

Carrier frequencies separated so signals do notoverlap (guard bands)

e.g. broadcast radio

Channel allocated even if no data


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Frequency Division Multiplexing

Diagram


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FDM System


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FDM of Three Voiceband Signals


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Analog CarrierSystems

AT&T (USA)

Hierarchy of FDM schemes

Group

12 voice channels (4kHz each) = 48kHzRange 60kHz to 108kHz

Supergroup

60 channel

FDM of 5 group signals on carriers between 420kHzand 612 kHz

Mastergroup

10 supergroups


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Synchronous Time Division

Multiplexing

Data rate of medium exceeds data rate of digitalsignal to be transmitted

Multiple digital signals interleaved in time

May be at bit level of blocksTime slots preassigned to sources and fixed

Time slots allocated even if no data

Time slots do not have to be evenly distributedamongst sources


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Time Division Multiplexing


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TDM System


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TDM Link Control

No headers and trailers

Data link control protocols not needed

Flow control

Data rate of multiplexed line is fixedIf one channel receiver can not receive data, the othersmust carry on

The corresponding source must be quenched

This leaves empty slots

Error controlErrors are detected and handled by individual channel

systems


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Data Link Control on TDM


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Framing

No flag or SYNC characters bracketing TDMframes

Must provide synchronizing mechanism

Added digit framingOne control bit added to each TDM frame

Looks like another channel - control channel

Identifiable bit pattern used on control channel

e.g. alternating 01010101unlikely on a data channelCan compare incoming bit patterns on each channel

with sync pattern


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Pulse Stuffing

Problem - Synchronizing data sources

Clocks in different sources drifting

Data rates from different sources not related by

simple rational numberSolution - Pulse Stuffing

Outgoing data rate (excluding framing bits) higher thansum of incoming rates

Stuff extra dummy bits or pulses into each incomingsignal until it matches local clock

Stuffed pulses inserted at fixed locations in frame andremoved at demultiplexer


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TDM ofAnalog and Digital

Sources


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Digital CarrierSystems

Hierarchy ofTDM

USA/Canada/Japan use one system

ITU-T use a similar (but different) system

US system based on DS-1 formatMultiplexes 24 channels

Each frame has 8 bits per channel plus one

framing bit193 bits per frame


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Data Communications and Computer Networks

Chapter 5

The T-1 multiplexor stream is a continuous series of frames.


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Digital CarrierSystems (2)

For voice each channel contains one word ofdigitized data (PCM, 8000 samples per sec)

Data rate 8000x193 = 1.544Mbps

Five out of six frames have 8 bitPCM samples

Sixth frame is 7 bitPCM word plus signaling bit

Signaling bits form stream for each channel containingcontrol and routing info

Same format for digital data23 channels of data

7 bits per frame plus indicator bit for data or systems control

24th channel is sync


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Mixed Data

DS-1 can carry mixed voice and data signals

24 channels used

No sync byte

Can also interleave DS-1 channelsDs-2 is four DS-1 giving 6.312Mbps


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Basic ISDN Interface (1)

Digital data exchanged between subscriber andNTE - Full Duplex

Separate physical line for each direction

Pseudoternary coding scheme1=no voltage, 0=positive or negative 750mV +/-10%

Data rate 192kbps

Basic access is two 64kbps B channels and one16kbps D channel

This gives 144kbps multiplexed over 192kbps

Remaining capacity used for framing and sync


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Basic ISDN Interface (2)

B channel is basic user channel

Data

PCM voice

Separate logical 64kbps connections o differentdestinations

D channel used for control or data

LAPD frames

Each frame 48 bits long

One frame every 250Qs


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Frame Structure


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Sonet/SDH

Synchronous Optical Network (ANSI)

Synchronous Digital Hierarchy (ITU-T)

Compatible

Signal HierarchySynchronous Transport Signal level 1 (STS-1) or Optical

Carrier level 1 (OC-1)

51.84Mbps

Carry DS-3 or group of lower rate signals (DS1 DS1CDS2) plus ITU-T rates (e.g. 2.048Mbps)

Multiple STS-1 combined into STS-N signal

ITU-T lowest rate is 155.52Mbps (STM-1)


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SONET Frame Format


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SONET STS-1 Overhead Octets


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Statistical TDM

In Synchronous TDM many slots are wasted

Statistical TDM allocates time slots dynamicallybased on demand

Multiplexer scans input lines and collects data untilframe full

Data rate on line lower than aggregate rates ofinput lines


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Statistical TDM Frame Formats


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Performance

Output data rate less than aggregate input rates

May cause problems during peak periods

Buffer inputs

Keep buffer size to minimum to reduce delay


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BufferSize

and Delay


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Asymmetrical Digital

SubscriberLine

ADSL

Link between subscriber and network

Local loop

Uses currently installed twisted pair cableCan carry broader spectrum

1 MHz or more


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ADSL Design

Asymmetric

Greater capacity downstream than upstream

Frequency division multiplexing

Lowest 25kHz for voicePlain old telephone service (POTS)

Use echo cancellation or FDM to give two bands

Use FDM within bands

Range 5.5km


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ADSL Channel Configuration


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Discrete Multitone

DMT

Multiple carrier signals at different frequencies

Some bits on each channel

4kHz subchannelsSend test signal and use subchannels with better

signal to noise ratio

256 downstream subchannels at 4kHz (60kbps)

15.36MHz

Impairments bring this down to 1.5Mbps to 9Mbps


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DMT Transmitter


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Wavelength DivisionMultiplexing

Wavelength division multiplexing multiplexes multipledata streams onto a single fiber optic line.

Different wavelength lasers (called lambdas) transmitthe multiple signals.

Each signal carried on the fiber can be transmitted at a

different rate from the other signals.Dense WDM High number of lambdas

Coarse WDM A few lambdas


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Code Division MultiplexingAlso known as code division multiple access

An advanced technique that allows multiple devices to

transmit on the same frequencies at the same time.Each mobile device is assigned a unique 64-bit code

To send a binary 1, mobile device transmits the uniquecode

To send a binary 0, mobile device transmits the inverseof code


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Code Division Multiplexing

Receiver gets summed signal, multiplies it by receiver

code, adds up the resulting valuesInterprets 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 MultiplexingExample

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 a1


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Example

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 MultiplexingExample

Base station decode for Mobile A:

Signal received: +3,-1,-1,+1,+1,-1,-3,+3

Mobile As code: +1,-1,+1,+1,+1,-1,-1,+1

Product result: +3,+1,-1,+1,+1,+1,+3,+3

Sum ofProduct results: +12Decode rule: For result near +8, data is binary 1


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Example

Base station decode for Mobile B:

Signal received: +3,-1,-1,+1,+1,-1,-3,+3

Mobile Bs code: -1,+1,+1,-1,+1,+1,+1,-1

Product result: -3,-1,-1,-1,+1,-1,-3,-3

Sum ofProduct results: -12

Decode rule: For result near -8, data is binary 0


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Optical Spatial Division

Multiplexing

Improves network utilization of SONET networks

Fact data traffic is often bursty

Fact SONET is sync TDM

Sync TDM does not like bursty traffic

OSDM is not limited to multiples of 1.544 Mbpscontainers


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Orthogonal Frequency

Division Multiplexing

OFDM is a discrete multi-tone technology

Numerous signals of different frequencies are combinedto form a single signal for transmission

Before combining, each carrier is phase modulated torepresent bits

HomePlug technology modulates data bits on 84individual carriers ranging from 4 MHz 21 MHz


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Business Multiplexing InAction

XYZ Corporation has two buildings separated by a

distance of 300 meters.A 3-inch diameter tunnel extends underground betweenthe 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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Data Communications and Computer Networks

Chapter 5


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Possible SolutionsConnect each terminal to the mainframe computerusing separate point-to-point lines.

Connect all the terminals to the mainframe computerusing one multipoint line.

Connect all the terminal outputs and use microwavetransmissions to send the data to the mainframe.

Collect all the terminal outputs using multiplexing andsend the data to the mainframe computer using aconducted line.


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Review Questions

1. What is FDM used for? What are itsadvantages? Disadvantages?

2. If you FDM 20 channels together, each channel40,000 Hz, what is total bandwidth?

3. What is TDM used for? What are itsadvantages? Disadvantages?

4. If you sync TDM 40 voice channels together,what is the total data rate?

5. What is the advantage of statTDM? Anydisadvantages?


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Review Questions

6. In a T-1, how many frames are transmitted persecond? How many channels within oneframe? What is the sync bit used for?

7. In ISDN

, how many channels are transmittedon a basic rate service? What is the data rateof each of those channels? What is the Dchannel used for?

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