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Understanding Wavelength Division Multiplexing

Wavelength division multiplexing (WDM) is a fiber optic technology that significantly increases the amount of data a single fiber cable can transmit. By sending multiple signals at different wavelengths (or colors) of laser light through a fiber cable, wavelength division multiplexing technology can increase the number of signals being sent simultaneously, while also allowing bidirectional communications over a single strand of fiber.​ This means, that WDM can maximise the usefulness of fiber and can generate substantial savings.

A WDM system consists of four elements as illustrated in the below diagram: a transceiver; a WDM multiplexer; a patch cord and dark fiber:

  • Transceivers are wavelength-specific lasers that convert data signals from SAN and IP switches to optical signals that can be transmitted into the fiber. Each data stream is converted into a signal with a light wavelength that is a unique color. Due to the physical properties of light, channels cannot interfere with each other. All WDM wavelengths are therefore independent. Creating virtual fiber channels in this way means that the number of fibers required are reduced by the factor of the wavelengths used. It also allows new channels to be connected as needed, without disrupting the existing traffic services. Since each channel is transparent to the speed and type of data, any mix of SAN, WAN, voice and video services can be transported simultaneously over a single fiber or fiberpair.

  • The WDM multiplexer (passive mux), optimizes the use of the fiber, gathering all the data streams together to be transported simultaneously over a single fiber. At the other end of the fiber, the streams are demultiplexed, i.e. separated into different channels again. Since they're usually positioned at the end points in a network, multiplexers are often referred to as terminal muxes. When connecting two sites, a multiplexer is positioned at each site, creating a point-to-point connection. In many cases, networks have additional sites where connectivity is required of some form, but not for all types of traffic. Here optical add drop multiplexers (OADMs) are used to extract the desired wavelengths needed for the specific site while bypassing the traffic types not needed. In this way, more versatile ring, distribution and access networks can be built.
  • The patch cable is the glue that joins the transceiver and the multiplexer together. LC connector cords are popular, as they connect the output of the transceiver to the input on the multiplexer.

  • A pre-requisite for any WDM system is access to a dark fiber network. The most common way of transporting optical traffic over an architecture is by using a fiber pair. One of the fibers is used for transmitting the data and the other is used for receiving the data. This allows the maximum amount of traffic to be transported. At times only a single fiber is available. Because different light colors travel on different wavelengths, a WDM system can be built regardless. One wavelength is used to send data and a second one to receive it.

The Difference between CWDM and DWDM

The two key WDM technologies are Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM). Which solution is best suited to a given environment depends on the network and user requirements.

CWDM supports up to 18 wavelength channels transmitted through a fiber at the same time. To achieve this, the different wavelengths of each channel are 20nm apart. DWDM meanwhile supports up to 80 simultaneous wavelength channels, with each of the channels only 0.8nm apart.

CWDM technology offers a convenient and cost-efficient solution for shorter distances of up to 70 kilometers. For distances between 40 and 70 kilometers, CWDM tends to be limited to supporting eight channels. Unlike CWDM, DWDM connections can be amplified and can therefore be used for transmitting data much longer distances.

The sweet spot for CWDM is up to 10 Gigabit Ethernet and 16G Fibre Channel. And it is quite unlikely capacities will increase beyond this in the future. DWDM however is able to handle higher speed protocols up to 100Gbps per channel, making it a more suitable choice for higher speed protocols.

Traditionally CWDM components have been lower in cost, making it more popular than DWDM. Now the price for both solutions is comparable. With higher speeds, more channel capacity, longer distances and passive networking, DWDM is the technology of choice for green field installations.

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