33 - Multiplexing

8/6/2019 33 - Multiplexing

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MultiplexingMultiplexing is a set of techniques that allows thesimultaneous transmission of multiple signalsacross a single data link .

There are three types of multiplexing: Frequency Division Multiplexing (FDM) Wave Division Multiplexing (WDM) Time Division Multiplexing (TDM)


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Categories of Multiplexing`


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Circuit Switching: TDMA and TDMA

FDMA

frequency

time

TDMA

frequency

time

4 usersExample:


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Frequency Division Multiplexing (FDM)

In FDM each signal modulates a different carrier frequency. The modulated carriers are combined toform a new signal that is then sent across the link.

Multiplexers modulate and combine signals whiledemultiplexers decompose and demodulate.

Guard bands keep the modulated signals fromoverlapping and interfering with one another.


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Wave Division Multiplexing (WDM)

Wave Division Multiplexing (WDM) is thesame as FDM, except that the multiplexing anddemultiplexing involve light signals

transmitted through fiber-optic channels.

A multiplexer can be made to combine severalinput beams of light, each containing a narrow

band of frequencies. A demultiplexer can also be made to reverse the process.


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DWDM principle


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Wavelength Division

Multiplexing uses different wavelengths on the same fibre is totally protocol independent (SDH, ATM,

Ethernet) known as Dense Wavelength Division Multiplex

(DWDM) when the wavelengths are close (afew nm.)

for DWDM, 40 or more wavelengths can beused on one fibre


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Time Division Multiplexing (TDM)In TDM, digital signals from n devices are interleavedwith one another, forming a frame of data (bits, bytes,or any other data unit).

In synchronous TDM, each frame contains at least onetime slot dedicated to each device. The order in whicheach device sends its data to the frame is unvarying. If adevice has no data to send, its time slot is sent empty.

In asynchronous TDM, the time-slot order of a framedepends on which devices have to send at that time.Asynchronous TDM adds device address to each time

slot.


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Multiplexing versus No Multiplexing


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FDM


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FDM Multiplexing Process, Time Domain


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Framing Bits


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Asynchronous TDM


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Multiplexing, Frequency Domain


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Demultiplexing, Frequency Domain


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TDM


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Synchronous TDM


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TDM, Multiplexing


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TDM, Demultiplexing


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Framing Bits

InterleavingIn synchronous TDM, each device has the opportunity to

send a specified amount of data (by bit, byte or any other unit) at a constant rate and in a fixed order. The switching

process from device to device is known as interleaving. In agiven system interleaved units will always be of the same

size.


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Data Rate


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Asynchronous TDM


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Frames and Addresses

a. Only three lines sending data


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Inverse Multiplexing

Inverse multiplexing takes the data streamfrom one high-speed line and breaks it into

portions that can be sent across several lower-

speed lines simultaneously, with no loss in thecollective data rate.

Inverse multiplexing splits a data stream fromone high speed line onto multiple lower speedlines.


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Multiplexing and Inverse Multiplexing


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Telephone Network


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Analog Leased Service


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Analog Hierarchy


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Switched/56 Service


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Digital Data Service (DDS)


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DS Hierarchy


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T lines are standard digital telephone carriers originallydesigned to transmit multiplexed voice channels (after

being digitized). Today, however, T lines are also usedto carry data between a residence or an organization

and the Internet. They provide a physical link betweennodes in a switched wide area network. T lines arecommercially available in two data rates: T-1 and T-3.Line Rate (Mbps) Voice Channels

T-1 1.544 24T-3 44.736 672

T Lines


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The data rate of a T-1 line is 1.544 Mbps.

Originally a T-1 line was used to multiplex24 voice channels. Each voice channel issampled and each sample digitized to 8 bits.

An extra bit added to providesynchronization. This makes the frame 193 bits in length. By sending 8000 frames per second, we get a data rate of 1.544 Mbps.When we use a T-1 line to connect to theInternet, we can use all or part of the capacityof the line to send digital data.

T-1 Lines


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Digital Subscriber Line (DSL) is a technology thatuses existing telecommunication networks such asthe local loop telephone lines to accomplish high-speed delivery of data, voice, video, and

multimedia.DSL technology used for residential connection toInternet is asymmetric DSL (ADSL). Telephonecompanies have installed high speed wide areanetworks to handle communications between their central offices. The link between the user and thenetwork is still an analog line.

DSL


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ADSL is asymmetric, which means it provides higher bit rates in downstream and(from telephone central office to the userssite) than the upstream direction.ADSL divides the bandwidth of the twisted

pair cable into three bands0 to 25 kHz25 to 200 kHz250 kHz to 1 MHz

ADSL


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Digital Subscriber Line (DSL)

Telephone companies originally transmitted within the0 to 4kHZ range to reduce crosstalk. Loading coilswere added within the subscriber loop to provide a

flatter transfer function to further improve voicetransmission within the 3kHZ band while increasing attenuation at the higher frequencies.ADSL (Asymmetric Digital Subscriber Line)

Uses existing twisted pair lines to provide higher bitrates that are possible with unloaded twisted pairs(i.e., no loading coils on subscriber loop.)


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DSL

the network transmits downstream at speedsranging from 1.536 Mbps to 6.144Mbps

asymmetric bidirectionaldigital transmissions

users transmit upstream at speeds ranging[higher frequencies] from 64 kbps to 640 kbps

0 to 4kHZ used for conventional analog telephone signals


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DSL

ITU-T G992.1 ADSL standard uses DiscreteMultitone (DMT) that divides the bandwidthinto a large number of small subchannels.

A splitter is required to separate voice signalsfrom the data signal.

The binary information is distributed among

the subchannels. Each subchannel uses QAM. DMT adapts to line conditions by avoidingsubchannels with poor SNR.


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T-1 Line

T 1 F


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T-1 Frame


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Fractional T-1 Line


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Types of Multiplexing

Frequency Division

Time Division PDH (Plesiochronous Digital Hierarchy)

SDH (Synchronous Digital Hierarchy)

ATM (Asynchronous Transfer Mode) Wavelength Division (for optical cables)


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PDH the data sources are nominally synchronous (to

within a few 10s of ppm of the nominal rate)

this makes the multiplexing process verycomplicated because of bit stuffing andstripping.and prone to transmission errors

every new data rate in the hierarchy needs acompletely new multiplexing definition


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SDH

the data sources are precisely synchronous

the multiplexing process is relatively simple

lower data rate tributaries can be extractedfrom the data stream without totaldemultiplexing (and similarly for inserting a

tributary) can easily make self-healing rings

the specification is future proof


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Wavelength Division

Multiplexing uses different wavelengths on the same fibre is totally protocol independent (SDH, ATM,

Ethernet) known as Dense Wavelength Division Multiplex

(DWDM) when the wavelengths are close (afew nm.)

for DWDM, 40 or more wavelengths can beused on one fibre


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DWDM principle


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DWDM system


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DWDM components

Tunable lasers Wavelength adaptors Diffraction gratings Thin film filters Bragg gratings Waveguide gratings


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SDH & DWDM combined

SDH and DWDM are complementary

SDH provides:

flexibility resilience in case of failure

DWDM provides: very high bandwidth

CONCLUSION: BANDWIDTH IS NO LONGER A PROBLEMON LONG-DISTANCE TRANSMISSION LINKS


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Examples of SDH/DWDM

systems TAT-14 (transatlantic cable) 8 fibre, dual bi-directional ring with protection ring 16 wavelengths of STM-64 per fibre pair

2.4 Tbit/s total capacity if fully equipped

FA-1: Flag Atlantic 1 (transatlantic cable) six fibres 40 wavelengths per fibre

10 Gbit/s SDH per wavelength 2.4 Tbit/s total capacity if fully equipped

(NB: 2.4 Tbit/s can carry 10,000,000 telephone circuits)


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Distribution technologies

CATVCommunity Access (or Cable) TV

ISDNIntegrated Services Digital Network

ADSLAsymmetric Digital Subscriber Line

Optical fibre

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33 - Multiplexing