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Wavelength Division Multiplexing (WDM) Increases Network Capacity

May 03, 2016

Wavelength Division Multiplexing (WDM) Increases Network Capacity

WDM is a method of separating or combining multiple wavelengths out of or into a single fiber strand with each wavelength carrying a different signal. Using optical filters lets a certain range of wavelengths pass through, while another range is allowed. Thin-film filter technology (TFF) is often used to achieve this effect. Multiple thin layers are stacked and interference effects are created by sequential reflections on the interface between the layers. This lets light reflect for certain wavelengths and pass through for others.

The capacity of a network can be increased cost effectively by using WDM. Two types of WDM are commonly used:

  • Dense Wave Division Multiplexing (DWDM) devices are mainly used when more wavelengths are required between sites and when the network extends over a very long distance. Forty wavelength channels from 1530 nm to 1570 nm are distributed in the C-band. To increase capacity, DWDM can be overlaid on a CWDM infrastructure.
  • Coarse Wave Division Multiplexing (CWDM) has 18 different wavelength channels standard, spaced 20 nanometers (nm) apart between 1270 nm and 1610 nm. Most systems only use the top eight channels between from 1470 nm and 1610 nm. CWDM systems have the advantage that they can always be upgraded at a later stage. This limits the initial installation costs. The requirements on the lasers is not severe due to the wide channel spacing, allowing less expensive lasers without any temperature control to be used.

The insertion loss of DWDM and CWDM is typically lower than that of optical splitters. This increases the reach of a network from a centralized office substantially. As every customer has wavelength(s) assigned to them, this provides better security and makes eavesdropping virtually impossible.

WDMs can be utilized in different ways:

Add/drop vs mux/demux.

A multiplexer, also known as a mux, combines several wavelength channels on one fiber, while a de-multiplexer (demux) separates them at the other side. A mux/demux configuration is very useful to increase a fiber’s end-to-end capacity. A mux is normally located at a central office, while demuxes are placed in either a splice closure or cabinet. From there the fibers are routed in a star-shaped topology to their ultimate destination.

An alternative to separating the wavelengths at one side, individual wavelengths can be added or dropped at various points across the line. This process does not affect other wavelengths. This is often preferable when the distance between sites is long or they are grouped in a circular structure.

One or two fibers?

An alternative to sending signals at different wavelengths through the same fiber is to use two different fibers. Many CWDM systems use two fibers where one is used for upstream signals and the other for downstream. In this configuration, each customer uses two fibers and one wavelength. Each customer will have two wavelengths if they use a single fiber.

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  • This sounds like another spectacular innovation. I think. I think I have the basic idea of how the fibers can be used to increase capacity. In this case though, I may be better off finding a video that explains it.

    Craig DeJong on
  • What the difference is between Wi-Fi and cellular connection? A cellular network transmits cellular data to your phone while Wi-Fi transmits data to your phone or computer through a Wi-Fi transmitter. A cellular network uses ultra-high frequencies (300–3000 MHz) for communications which allows for transmission over a larger distance. A Wi-Fi network uses what’s known as Super-High frequencies (3-30 Ghz) and allows higher bandwidths at shorter ranges. Hope this helps. I know this is a little technical but I hope it helps.

    Scott Becker on
  • It sounds like there are constant advances in cell phone technology. I wish I was as knowledgeable and am confused about what the difference is between WiFi and cellular connection. Anyone care to help?

    Bobby Burns on
  • It’s remarkable how smart engineers and other designers are. You have (what sounds like) a complex system designed to make sure you get strong cell phone signals throughout an area. However, when problems arise, what do you do? Well, apparently you can work with multiple fibers. What are wavelength division multiplexing advantages and disadvantages? I’m not sure, but I feel confident knowing someone with expertise in this area can solve any problems that come up.

    Pete J. Scott on
  • My limited understanding is that the fiber cables are used in DAS design to make sure facilities have consistently strong signals for cell phones, including the ability to make calls and do so without them dropping. Are these multiplexers part of the layout is a DAS installation? It sounds like they give some flexibility in DAS construction for facilities.

    Roy Sanders on

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