A Functional Description of a TDM

8/3/2019 A Functional Description of a TDM

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Optical Communication

Ali Mohammed Hassn MuafaTC 08-19

A functional description Of a TDM-PON and aWMD-PONTime-division multiplexing-PONShort for Time Division Multiplexing, a type of multiplexing that combines data

streams by assigning each stream a different time slot in a set. TDM repeatedly

transmits a fixed sequence of time slots over a single transmission channel. Within T-

Carrier systems, such as T-1 andT-3, TDM combines Pulse Code Modulated (PCM)

streams created for each conversation or data stream.

Time Division Multiplexing works by the multiplexor collecting and storing the

incoming transmissions from all of the slow lines connected to it and allocating a time

slice on the fast link to each in turn. The messages are sent down the high speed linkone after the other. Each transmission when received can be separated according to

the time slice allocated.

Theoretically, the available speed of the fast link should at least be equal to the total

of all of the slow speeds coming into the multiplexor so that its maximum capacity is

not exceeded.

Two ways of implementing TDM are:

1. Synchronous TDM2. Asynchronous TDM3.

Synchronous Multiplexing:
http://www.webopedia.com/TERM/M/multiplexing.htmlhttp://www.webopedia.com/TERM/T/T_1_carrier.htmlhttp://www.webopedia.com/TERM/T/T_3_carrier.htmlhttp://www.webopedia.com/TERM/P/PCM.htmlhttp://www.webopedia.com/TERM/P/PCM.htmlhttp://www.webopedia.com/TERM/T/T_3_carrier.htmlhttp://www.webopedia.com/TERM/T/T_1_carrier.htmlhttp://www.webopedia.com/TERM/M/multiplexing.html


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Synchronous TDM works by the muliplexor giving exactly the same amount of time to

each device connected to it. This time slice is allocated even if a device has nothing to

transmit. This is wasteful in that there will be many times when allocated time slots

are not being used. Therefore, the use of Synchronous TDM does not guarantee

maximum line usage and efficiency.

Synchronous TDM is used in T1 and E1 connections.

Asynchronous Multiplexing:Asynchronous TDM is a more flexible method of TDM. With Asynchronous TDM the

length of time allocated is not fixed for each device but time is given to devices that

have data to transmit.

This version of TDM works by tagging each frame with an identification number to

note which device it belongs to. This may require more processing by the multiplexor

and take longer, however, the time saved by efficient and effective bandwidth

utilization makes it worthwhile.

Asynchronous TDM allows more devices than there is physical bandwidth for.

This type of TDM is used in Asynchronous Transfer Mode (ATM) networks.

Wavelength Division Multiplexing PON1. Introduction to WDM-PON

It seems to be generally agreed that passive optical network(PON) is the best

architecture for delivering broadband services. PONs being commercially available

today are classified into two categories in terms of the multiple access technologies;one TDM-PON and the other WDM-PON. As good examples of TDM-PON, A/B-

PON, E-PON, and G-PON are well known and already playing a key role in

broadband access network services. However, it is quite likely that the TDM-PONs

today can not support the bandwidth-exhausting multimedia services like IP-

television, HD-quality VOD which are surely coming soon. Besides, TDM-PONs are

never economical from the network investment point of view. In other words, TDM-

PONs have not fully taken advantage of the optical fiber bandwidth, which is actually

infinite. On the other hand, WDM-PON currently available offer enough bandwidth

not only for present but also for future multimedia broadband services and fully

utilize the optical fiber bandwidth.

1.1. What is WDM-PON?

The network architecture of WDM-PON is basically the same as that of TDM-PON.

What is different is that WDM-PON assigns a wavelength to each subscriber while

TDM-PON assigns a time-slot. Therefore, WDM-PON can be regarded as an

aggregation of point-to-point connections between each subscriber and the central

office. Fig. 1 illustrates the typical WDM-PON architecture.


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[Fig. 1] WDM-PON Network Architecture

1.2. WDM-PON that Service Providers Want

For WDM-PON to be deployed in a large scale, it should offer at least the following

features to service providers; 1) "Colorless OLT and ONU" : ONU or ONT should

be able to operate in a colorless manner. That is, WDM-PON can be handled as

simply and easily as TDM-PON like E-PON or G-PON. Otherwise, WDM-PON

management would increase the operating as well as inventory costs.

2) "Working even in poor fiber plants" : Not like high quality fiber plant for core

networks, the access network infrastructure is a little poor Usually the access network

has problems caused by high optical loss and uncontrolled reflections, etc. Even in

such environment, WDM-PON should work well!

3) "Cost-effective" : Cost-effectiveness is important, maybe the most important factor

that service providers are considering. So, WDM-PON should be cost-competitive,

being compared with its TDM-PON alternative like E-PON or G-PON.

Utilization of WDM technology allows a network carrier to increase the bandwidth of

existing fiber without having to lay new fiber cables. BTw are using two types of

WDM:-

Coarse WDM (CWDM) - Usually 8, but up to 16 channels on a single fiber. Dense WDM (DWDM) - Usually 80, but up to 160 channels per optical fiber.

Needs more expensive transmitter & receiver modules.

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A Functional Description of a TDM