What is Wavelength Division Multiplexing (WDM)?

At-a-Glance

  • WDM Increases the capacity of fibre using the prism principle, whereby different colours (wavelengths) use the same fibre simultaneously. There are up to 80 wavelengths
  • Each coloured channel behaves as an independent connection. Hence we use the term ‘virtual fibre’
  • Each wavelength can independently carry a different service type, such as Gigabit broadband, data centre connection, leased line connection or services for another user
  • Each wavelength channel can carry enormous bandwidths of up to 100Gb/s, or small ones in the kilobit range, with no effect on neighbouring channels
  • DWDM can operate on both a fibre pair or on a single fibre
  • Wavelengths can be routed between different locations along a fibre, enabling branches, rings and other network layouts
  • DWDM and CWDM are both types of WDM

WDM is the acronym for wavelength division multiplexing. This may sound complex, but in reality, it is simple.

You may know that white light is made up of different colours (wavelengths) and these can be separated using a prism. WDM makes use of this principle to carry many light signals at the same time. As this principle works in both directions, it is possible for several communication paths to be combined and separated, and to be carried on the same fibre at the same time, each using a different wavelength channel.

WDM dramatically reduces the number of separate fibres required to carry connections from one place to another and increases the bandwidth capacity available on each fibre so that many more services or customers can be supported on the same fibre at the same time.

What is DWDM?

In practical terms, the prisms are replaced by specially created filters that combine and separate the individual connections coming from different fibre communication devices. WDM also enables individual connections to be routed to different places using a device called an optical add-and-drop multiplexer which enables only some wavelengths within the fibre to be accessed at each location.

Virtual Fibre

Each individual WDM channel behaves like a separate independent fibre and can carry either small bandwidths of a few kb/s or enormous bandwidths of 10Gb/s or 100Gb/s.

WDM connections can stretch into long distances of hundreds and even thousands of kilometres. This is because wavelength sensitive splitting multiplexing devices do not (as in GPON) waste power by sending it down the wrong path. Further, WDM channels may be amplified if distances require to distances of typically 80-250km per hop.

There are two main types of WDM – CWDM or Coarse WDM, where there are usually 8 channels available and DWDM or Dense WDM, where 40 or more channels are available although this distinction is less valid recently due to the falling cost of the devices used, making CWDM less cost-efficient of the two.

In their elementary form, WDM devices are passive. This simply means there are no electronic parts used, which also means there are also no electrical power or cooling. Feature rich DWDM systems include demarcation devices, wavelength conversion, service monitoring, sub-wavelength multiplexing devices and amplifiers.