Understanding Wavelength Division Multiplexing (WDM)

Transferring information from one location to another can be accomplished using various transmission technologies and mediums. These include copper cables, free space, and fiber optic cables. Given a single medium, the maximum data rate achievable is often limited. To overcome this, we employ various techniques to transmit information simultaneously, collectively known as multiplexing techniques. Examples include TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access), CDMA (Code Division Multiple Access), and OFDMA (Orthogonal Frequency Division Multiple Access).

WDM, or Wavelength Division Multiplexing, is another such multiplexing technique. It shares similarities with FDM (Frequency Division Multiplexing) due to their mathematical relationship:

$\text{Wavelength} = \frac{C}{\text{Frequency}}$

where $C$ is the speed of light.

The key difference is that WDM multiplexes light signals and transmits them through fiber optic cables. This leads to a higher data rate capacity, as different light signals, each with a unique wavelength, carry distinct data streams. Essentially, narrower band signals are combined to form a wide band light signal.

The core principle of WDM is the use of different optical carriers, often referred to as “colors,” to transmit various signals through an optical fiber. Here, the optic fiber can be visualized as a multi-lane highway.

WDM is agnostic to the type of traffic being carried. Any type of data can travel over the fiber optic cable, including ATM packets, SDH (Synchronous Digital Hierarchy), and IP data. The combination of SONET/SDH’s functional capabilities and DWDM’s (Dense Wavelength Division Multiplexing) enormous bandwidth has spurred the development of 32 and 96 channel WDM systems. These systems are capable of transmitting data at rates ranging from 320 Gbps to 1.2 Terabit per second.

When the number of wavelength channels exceeds 20, WDM is typically referred to as DWDM, or Dense WDM.