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TDM vs WDM-Difference between TDM and WDM

This page on TDM versus WDM describes difference between TDM and WDM multiplexing types. Both are multiplexing techniques but used for different purposes and applications. TDM refers to time division multiplexing which uses single RF carrier to transmit the time multiplexed packet or frame. WDM refers to wavelength division multiplexing which uses different optical frequencies at various wavelengths to transmit the packet or frame of data separately over multiple channels after they are broken up into small blocks.

WDM-Wavelength Division Multiplexing

This technique increases capacity of the optical fiber medium. It assigns unique frequencies of light within certain band to different incoming optical signals. Here all the channels are transmitted at various different frequencies and hence the channels can be selected using a tuner of desired bandwidth.

In other words, in WDM each channel has its different color of light. Refer WDM basics for more information.

There is another variant of WDM referred as DWDM (Dense Wavelength Division Multiplexing) in which wavelengths are closely spaced compare to WDM. This will increase the greater overall capacity of the system. Refer DWDM tutorial and difference between CWDM and DWDM for more.

WDM takes multiple optical signals, maps them to individual wavelengths, and multiplexes the wavelengths over a single fiber.

TDM vs WDM

As shown in the figure-1, in TDM, multiple channels can be combined time wise and transmitted over single fiber using one wavelength. In WDM, multiple time multiplexed channels can be transmitted over single fiber using different multiple wavelengths.

TDM-Time Division Multiplexing

In TDM, data packets are time multiplexed and are transmitted using the same radio carrier frequency. Hence if say, there are 2 channels, each operating at 4.8 Kbps and they are time multiplexed, then resulting capacity of these 2 channels is aggregate of two channels i.e. 9.6 Kbps. Refer TDM basics and comparison with FDM.

The example of this technique is SONET TDM. This technique takes synchronous and asynchronous signals and multiplexes them to a single higher bit rate for transmission at a single wavelength over fiber. The source signals may have to be converted from electrical to optical or from optical to electrical and back to optical before being multiplexed as required.

Basic difference between TDM and WDM

Following are the key difference between TDM and WDM systems.

• In TDM, resulting capacity is the aggregate or sum of all the input signals/channels. In WDM, each signal is transmitted independent of the others and hence each channel will have its own dedicated bandwidth.

• In WDM, all signals will arrive at the same time while in TDM they will arrive one after the other. This is because in TDM the signals are broken up and multiplexed time wise before transmission. This is same as TDMA frame, where in data from various stations are multiplexed and then transmitted.

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