8 Passive WDM Facts and Project Considerations

As bandwidth requirements continue to grow (video and voice connections, additional critical applications, big data and storage all making their claims on the network), the need for additional links between sites becomes inevitable, so how can you keep fiber project costs down to a minimum and meet these changing demands?

Passive Wavelength Division Multiplexing (WDM), which has been a fiber technique of choice for telcos for decades, has seen technological improvements, no longer requires large, complex systems and is not expensive as it once was. Different WDM configurations are now available that are suitable (and cost-effective!) for enterprises, datacenters and government operators, as well as the large-scale service providers.

If you are hesitating implementing passive WDM in your next project because you still see it as complicated or have yet to see the benefits of maximising bandwidth, whilst also cutting costs…read on!

The WDM basics…

01. What is Wave Division Multiplexing (WDM)?

Passive WDM basically packs a punch by transporting different data streams (via differing light wavelengths) through one strand of fiber. Compare that to the single beam of light normally used to transport one data stream, on one wavelength, over one strand, and you can easily see the improvements that it brings. Multiple streams of data can be sent over the same fiber by assigning each data stream its own wavelength, with no interference between the streams. Data streams ranging from 100 Mbps to 400 Gbps can be sent over their own independent channels while allowing for expansion and the addition of more channels.

02. When to use Coarse WDM (CWDM) or Dense WDM (DWDM)

These two categories of WDM, are both effective in solving increasing bandwidth capacity, but have different channel configurations and come with their advantages and disadvantages, depending on the environment you are planning to use them in and your network challenges. The key differences being that Coarse Wavelength Division Multiplexing (CWDM) is lower density, with a shorter reach than Dense Wavelength Division Multiplexing (DWDM). It is typically deployed in spans of up to 80km or less where lower capacity is not an issue, but where lower cost and no power consumption / availability is important. CWDM can support up to 18 channels on a single fiber, with the current maximum speed being 100 Gbps.

DWDM by contrast provides higher density, higher bandwidth and with more accurate lasers. In cases when passive solutions cannot deliver the required reach DWDM can be amplified to give much longer reach, but inevitably at a higher cost and technical complexity. However, they can be used side-by-side in the same system simultaneously, to provide extra flexibility and redundancy when needed.

With DWDM it is possible to fit 40, 80 or up to 96 channels on the same fiber pair, enabling huge amounts of data to be pushed through the higher number of wavelengths available. With transceivers that are currently available, DWDM speeds can reach 400 Gbps per wavelength. DWDM is the best choice for long-distance communications from 120km and beyond with the ability to use optical amplifiers, and when boosted by Erbium Doped-Fiber Amplifiers (EDFAs) they are able to carry high quantities of data across distances of up to thousands of kilometers.

Read more on the difference between CWDM and DWDM in our recent blog >

03. How WDM Transceivers work

The role of the transceiver is to convert electrical signals from host equipment into optical signals to be transmitted via fiber. Each signal is converted to a specific light wavelength with a unique color. New channels (using new wavelengths) can be added, without affecting existing traffic, and a mix of data transported at different speeds can be transported over a single fiber of fiber pair simultaneously. Of course your selection of CWDM or DWDM for your transceiver should match that of your multiplexer. Connection of the WDM transceivers to the WDM multiplexer is via LC or CS cabling, the WDM transceivers being housed in a switch or router. Find out more about WDM transceivers here >

04. Mux and Demux – The beating heart of WDM

Once the data signals have been converted to specific wavelengths, the WDM multiplexer (also known as a mux) is the real heart of the system, taking all the individual wavelengths and transmitting them onto a single fiber. At the other end of the fiber the demultiplexer (demux) separates the different wavelengths out into the individual channels.

Single strand and dual strand configuration muxes can be used. A typical passive WDM multiplexer contains both a mux and a demux – so you would typically only need one ‘multiplexer’ box either end of your standard single fiber or fiber pair, installation along with WDM transceivers. A straightforward and simple scenario.

The ever-evolving technology of today allows us to build multiplexers that allow for up to 96 channels to simultaneously be carrier over the same fiber pair.

Getting the most from WDM in your future projects…

05. Increasing optical density cost-effectively

Optical traffic is most commonly transported using a fiber pair, with one fiber transmitting and the other used for receiving data. With WDM, even if you only have a single strand of fiber available, it can carry multiple streams of data in both directions, allowing up to 9 CWDM channels to be carried on that single fiber (and in DWDM but this is not common). A dual fiber CWDM multiplexer, however, can allow up to 18 channels over one fiber pair, delivering excellent channel capacity expansion. Today, up to 96 DWDM channels can be carried simultaneously over the same fiber pair, with a single 1 U ultra-high-density solution.

With standard, high density, next-generation high density and ultra-high density solutions on the market, there are options to suit all types of bandwidth appetites, with not a huge difference in cost between them, giving you the option to also consider saving rack space as part of your next project as well!

06. How much rack space can you save?

The higher the density WDM multiplexing solution you choose the more channels you can fit inside a single 1U chassis. For example, you could have 3 x 40 channel DWDM multiplexer cassettes in one 1U chassis. As each cassette is dedicated to a fiber pair, if you are a telecom provider you can, for example, organise your fiber for the north, east and west in one chassis. Put another way – you could reach out to 3 sites with 40 channels each, simply and cost effectively, and save space. Remember that with WDM multiplexing you can send data in both directions, so ideal if you are looking to save in your network or need to allocate more to key applications and services.

07. Handling scalability and flexibility simply

The advantages WDM brings for operators, being able to divide and dedicate channels between many more customers, as well as for organisations, dividing channels between departments and coping with increased numbers of applications, video and big data processing, are clear. Couple this increased density, with an array of different modular solutions, which can be quickly slotted in, in addition to the traditional fixed 1U solutions, and you have the ability to scale and expand capacity incredibly quickly and simply, now and in preparation for future needs – bringing on new sites or site expansions as-and-when needed. Upgrading (along with some much-needed rack-space housekeeping and spring cleaning) is easy.