TIME DIVISION MUX.docx

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Time-division multiplexing

Time-Division Multiplexing (TDM) is a type ofdigitalor (rarely)analogmultiplexingin which

two or more signals or bit streams are transferred apparently simultaneously as sub-channels in

one communication channel, but are physically taking turns on the channel. The time domain isdivided into several recurrent timeslots of fixed length, one for each sub-channel. A sample byte

or data block of sub-channel 1 is transmitted during timeslot 1, sub-channel 2 during timeslot 2,

etc. One TDMframeconsists of one timeslot per sub-channel. After the last sub-channel thecycle starts all over again with a new frame, starting with the second sample, byte or data block

from sub-channel 1, etc.

[edit] Application examples

ThePlesiochronous Digital Hierarchy(PDH) system, also known as thePCMsystem, fordigital transmission of several telephone calls over the same four-wire copper cable (T-

carrierorE-carrier) or fiber cable in the circuit switched digital telephone network

TheSDHandSynchronous Optical Networking(SONET) network transmissionstandards, that has suppressedPDH.

TheRIFF(WAV) audio standard interleaves left and right stereo signals on a per-samplebasis

The left-right channel splitting in use forStereoscopicLiquid Crystal shutter glassesTDM can be further extended into thetime division multiple access(TDMA) scheme, whereseveral stations connected to the same physical medium, for example sharing the samefrequency

channel, can communicate. Application examples include:

TheGSMtelephone system

[edit] TDM versus packet mode communication

In its primary form, TDM is used forcircuit modecommunication with a fixed number of

channels and constant bandwidth per channel.

What distinguishes time-division multiplexing fromstatistical multiplexingsuch aspacket modecommunication (also known as statistical time-domain multiplexing, see below) is that the

time-slots are recurrent in a fixed order and pre-allocated to the channels, rather than scheduledon a packet-by-packet basis. Statistical time-domain multiplexing resembles, but should not be

considered as, time division multiplexing
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Indynamic TDMA, ascheduling algorithmdynamically reserves a variable number of timeslots

in each frame to variable bit-rate data streams, based on the traffic demand of each data stream.

Dynamic TDMA is used in

HIPERLAN/2;

IEEE 802.16a.

[edit] History

For theSIGSALYencryptor of 1943, seePCM.

In1962, engineers from Bell Labs developed the first D1 Channel Banks, which combined 24

digitised voice calls over a 4-wire copper trunk between Bellcentral officeanalogue switches. Achannel bank sliced a 1.544 Mbit/s digital signal into 8,000 separate frames, each composed of

24 contiguous bytes. Each byte represented a single telephone call encoded into a constant bit

rate signal of 64 Kbit/s. Channel banks used a byte's fixed position (temporal alignment) in the

frame to determine which call it belonged to.[1]

[edit] Transmission using Time Division Multiplexing (TDM)

In circuit switched networks such as the Public Switched Telephone Network (PSTN) there

exists the need to transmit multiple subscribers calls along the same transmission medium.[2]

Toaccomplish this, network designers make use of TDM. TDM allows switches to create channels,

also known as tributaries, within a transmission stream.[2]

A standardDS0voice signal has a data

bit rate of 64 kbit/s, determined using Nyquists sampling criterion.[2][3]

TDM takes frames of the

voice signals and multiplexes them into a TDM frame which runs at a higher bandwidth. So ifthe TDM frame consists ofn voice frames, the bandwidth will be n*64 kbit/s.

[2]

Each voice sample timeslot in the TDM frame is called a channel .[2]

In European systems, TDMframes contain 30 digital voice channels, and in American systems, they contain 24 channels.

[2]

Both standards also contain extra bits (or bit timeslots) for signalling (seeSignaling System 7)

and synchronisation bits.[2]

Multiplexing more than 24 or 30 digital voice channels is called higher order multiplexing.[2]

Higher order multiplexing is accomplished by multiplexing the standard TDM frames.[2]

Forexample, a European 120 channel TDM frame is formed by multiplexing four standard 30

channel TDM frames.[2]

At each higher order multiplex, four TDM frames from the immediate

lower order are combined, creating multiplexes with a bandwidth ofn x 64 kbit/s, where n = 120,

480, 1920, etc.[2]

[edit] Synchronous Time Division Multiplexing (Sync TDM)

There are three types of (Sync TDM): T1, SONET/SDH (see below), and ISDN[4]

.

[edit] Synchronous Digital Hierarchy (SDH)
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Plesiochronous Digital Hierarchy (PDH) was developed as a standard for multiplexing higher

order frames.[2][3]

PDH created larger numbers of channels by multiplexing the standard

Europeans 30 channel TDM frames.[2]

This solution worked for a while; however PDH suffered

from several inherent drawbacks which ultimately resulted in the development of theSynchronous Digital Hierarchy (SDH). The requirements which drove the development of SDH

were as follows:

[2][3]

Be synchronousAll clocks in the system must align with a reference clock. Be service-orientedSDH must route traffic from End Exchange to End Exchange

without worrying about exchanges in between, where the bandwidth can be reserved at a

fixed level for a fixed period of time.

Allow frames of any size to be removed or inserted into an SDH frame of any size. Easily manageable with the capability of transferring management data across links. Provide high levels of recovery from faults. Provide high data rates by multiplexing any size frame, limited only by technology. Give reduced bit rate errors.

SDH has become the primary transmission protocol in most PSTN networks.[2][3]

It was

developed to allow streams 1.544 Mbit/s and above to be multiplexed, so as to create larger SDH

frames known as Synchronous Transport Modules (STM).[2]

The STM-1 frame consists ofsmaller streams that are multiplexed to create a 155.52 Mbit/s frame.

[2][3]SDH can also multiplex

packet based frames such as Ethernet, PPP and ATM.[2]

While SDH is considered to be a transmission protocol (Layer 1 in theOSI Reference Model), it

also performs some switching functions, as stated in the third bullet point requirement listed

above.[2]

The most common SDH Networking functions are as follows:

SDH CrossconnectThe SDH Crossconnect is the SDH version of a Time-Space-Timecrosspoint switch. It connects any channel on any of its inputs to any channel on any ofits outputs. The SDH Crossconnect is used in Transit Exchanges, where all inputs and

outputs are connected to other exchanges.[2]

SDH Add-Drop MultiplexerThe SDH Add-Drop Multiplexer (ADM) can add orremove any multiplexed frame down to 1.544Mb. Below this level, standard TDM can be

performed. SDH ADMs can also perform the task of an SDH Crossconnect and are used

in End Exchanges where the channels from subscribers are connected to the core PSTNnetwork.

[2]

SDH Network functions are connected using high-speed Optic Fibre. Optic Fibre uses light

pulses to transmit data and is therefore extremely fast.[2]

Modern optic fibre transmission makesuse ofWavelength Division Multiplexing(WDM) where signals transmitted across the fibre are

transmitted at different wavelengths, creating additional channels for transmission.[2][3]

This

increases the speed and capacity of the link, which in turn reduces both unit and total costs.[2]

[edit] Statistical Time-division Multiplexing (Stat TDM)
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STDM is an advanced version of TDM in which both the address of the terminal and the data

itself are transmitted together for better routing. Using STDM allows bandwidth to be split over

1 line. Many college and corporate campuses use this type of TDM to logically distribute

bandwidth.

If there is one 10MBit line coming into the building, STDM can be used to provide 178 terminalswith a dedicated 56k connection (178 * 56k = 9.96Mb). A more common use however is to only

grant the bandwidth when that much is needed. STDM does not reserve a time slot for each

terminal, rather it assigns a slot when the terminal is requiring data to be sent or received.

This is also called Asynchronous Time-division Multiplexing[4]

(ATDM), in an alternative

nomenclature in which "STDM" or "Synchronous Time Division Multiplexing" designates theolder method that uses fixed time slots.
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