TDM vs. WDM: Key Differences in Multiplexing Techniques

This article explores the differences between Time Division Multiplexing (TDM) and Wavelength Division Multiplexing (WDM), two multiplexing techniques used for different purposes and in different applications.

What is TDM?

TDM, or Time Division Multiplexing, utilizes a single Radio Frequency (RF) carrier to transmit time-multiplexed packets or frames. Think of it as slicing data streams into time slots and interleaving them onto a single channel.

What is WDM?

WDM, or Wavelength Division Multiplexing, employs different optical frequencies (wavelengths) to transmit data packets or frames. Data is broken into small blocks and transmitted separately over multiple channels, each with its own unique wavelength.

WDM: Boosting Optical Fiber Capacity

WDM is a powerful technique for increasing the capacity of optical fiber mediums. It works by assigning unique light frequencies within a specific band to different incoming optical signals. Each channel operates at a distinct frequency, allowing for selection using a tuner with the desired bandwidth. In essence, each channel in WDM has its own “color” of light.

DWDM: Dense Wavelength Division Multiplexing

A variant of WDM, known as Dense Wavelength Division Multiplexing (DWDM), offers even higher capacity. In DWDM, wavelengths are spaced much closer together compared to WDM, allowing for a greater overall system capacity.

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

Figure 1: Illustrating TDM and WDM concepts.

As shown in Figure 1, TDM combines multiple channels time-wise for transmission over a single fiber using one wavelength. In contrast, WDM allows multiple time-multiplexed channels to be transmitted over a single fiber using different wavelengths.

TDM in Detail: Time Division Multiplexing

In TDM, data packets are time-multiplexed and transmitted using the same radio carrier frequency. For example, if two channels, each operating at 4.8 Kbps, are time-multiplexed, the resulting capacity becomes the aggregate of the two: 9.6 Kbps.

A common example of TDM is SONET TDM. This technique takes synchronous and asynchronous signals and multiplexes them into a single higher bit rate for transmission over fiber at a single wavelength. Source signals might need conversion from electrical to optical, or vice-versa, before being multiplexed, depending on the system requirements.

Key Differences Between TDM and WDM

Here’s a breakdown of the main differences between TDM and WDM:

• Capacity: In TDM, the resulting capacity is the sum of all input signals/channels. In WDM, each signal is transmitted independently, with each channel having its dedicated bandwidth.
• Signal Arrival: In WDM, all signals arrive simultaneously. In TDM, they arrive one after the other because the signals are broken up and multiplexed in the time domain before transmission. This concept is similar to a TDMA frame, where data from various stations are multiplexed and then transmitted.