• What is WDM?
• Who Uses WDM?
• How is WDM Deployed?
• WDM Links

What is WDM?
Wave Division Multiplexing (WDM) describes the concept of combining several streams of data onto the same physical fiber-optic cabling.  This capacity increase is achieved by relying on one of the fundamental principles of physics.  Light of different wavelengths does not interfere.  The main idea is to use several different wavelengths (or frequencies) of light, with each wavelength carrying a different stream of data.
 
This feat is accomplished via several components.  First, the transmitted data must be sent on a particular carrier wavelength.  Typical fiber-optic systems use three distinct wavelengths, 850nm, 1310nm, and 1550nm.  If the signal is already optical, at one of these wavelengths, it must be converted to a wavelength within the WDM spectrum.  Several independent signals will typically be converted each to a separate carrier wavelength within the spectrum.  These signals then are combined via an optical combiner (basically a carefully constructed piece of glass) such that most of the power of all the signals is transferred onto a single fiber.  On the other end, the light is split using a splitter (another carefully constructed piece of glass) into many channels.  Each of these channels is passed through a filter to select only the particular wavelength of interest.  Finally, each filtered wavelength is sent to a separate receiver, sometimes located on different devices, where it is converted back to the original format (either copper, or some other non-WDM wavelength).
 
There are two types of WDM systems in common use, providing Coarse (CWDM) and Dense (DWDM) granularity of wavelengths.  CWDM systems typically provide up to 8 or 16 wavelengths, separated by 20nm, from 1310nm to 1630nm.  Some DWDM systems provide up to 144 wavelengths, typically with 2nm spacing, roughly over the same range of wavelengths.


Who Uses WDM?
WDM (either CWDM or DWDM) is commonly used for one of two purposes.  The original and primary purpose of WDM technology is capacity-enhancement.  In this scenario, many streams of data are multiplexed onto a small number of fiber-optic cables.  This dramatically increases the bandwidth carried per fiber.  In an extreme case, sub-oceanic cabling today sometimes runs 144 channels of OC-192.  At 10Gbps per channel, the total bandwidth on each individual fiber is 1.44 Terabits (that's 12,000,000,000,000 bits per second).  Of course, in many scenarios, this level of bandwidth is unnecessary, but it is common to run several streams of Gigabit Ethernet over a single fiber-pair when fiber-optic cabling starts to run out.  In many cases it is simply not cost-effective, or even possible, to deploy more fiber.  In these cases, WDM technology is the only option left when the bandwidth inevitably needs a booster shot.
 
The second purpose for WDM technology came about more recently as more and more customers began to require high-speed network interconnections between facilities.  This usage is commonly referred to as "Wavelength Services".  A carrier (or Utility company acting as a carrier) has the option of providing a full wavelength, point-to-point, for a customer with multiple physical locations.  For example, a large corporation with two buildings on opposite ends of town may want to run a Gigabit Ethernet connection between the facilities.  The carrier can either deploy a Gigabit Ethernet infrastructure, or can deploy a WDM infrastructure.  In the former case, future customers will also generally be required to deploy Gigabit Ethernet.  By using WDM instead, other customers can easily select OC-3 or OC-12, or even FibreChannel as the protocol to connect their facilities.  Of course, a Gigabit Ethernet deployment is relatively inexpensive, and is often used to provide services from site-to-site around a metro area, but using WDM, the carrier doesn't need to worry about which particular kind of technology is used, which allows a more flexible service offering.


How is WDM Deployed?
There are several pieces to a full WDM deployment, and many possible configurations, depending on what kind of network is required.  In the simplest case, multiple channels of Gigabit Ethernet can be connected directly from a switch or router (or several switches or routers) to a WDM system.  The WDM systems will take the channels and convert them to a single fiber-pair.  Then, on the other end of the fiber (perhaps as much as 70km distant), an identical WDM system converts back to normal Gigabit Ethernet.
 
When providing wavelength services, more components are typically needed.  First, to connect to a customer or end-point, a transponder is typically used.  This device converts the wavelength of the data to and from an acceptable WDM wavelength.  Sometimes transponders connect to the end-system via copper cabling, but typically they use multimode fiber-optic connections.  An add-drop multiplexer module couples the data together in the outbound direction, and decouples and filters inbound data.  Often, several multiplexers are combined to couple in many channels.  Multiplexers may combine many wavelengths in a single module, or may even be for a single wavelength at a time, depending on the needs of a particular location.  This 'multicolored' signal may then be sent in a linear or ring topology.  In either topology, at each location, one or more colors are added or dropped.  The rest of the colors are passed through without being affected (except for some small attenuation).  The WDM solution provides a point-to-point connection by 'adding' the color in one location, and 'dropping' it at the other location.  In a ring topology, each signal can travel either way around the ring, which provides a fault-tolerance mechanism.  In the event of a ring-cut, the system reverts to a linear topology with no redundancy.
 
One key issue to be addressed in any WDM system is attenuation.  Single WDM links can exceed 70km, but to go past that distance, one must either terminate and regenerate each color, or deploy an Erbium-Doped Fiber Amplifier (EDFA), which provides a linear gain across the entire WDM spectrum.  As these devices add cost to the network, it is always important to understand the distances and attenuation of the various splitters, combiners, and add-drop multiplexers in the network.
 
MRV provides a complete range of WDM products ranging from simple fiber-optimizers and dual-channel systems all the way to a full CWDM/DWDM system capable of transporting 48 channels, each up to 2.5Gbps.  Additionally, MRV provides unique "Colored GBICs" and "OADM Cables".  A colored GBIC is an optical plug-in module for switches and routers that generates optical signal at a particular WDM wavelength (they are ordered by wavelength number).  This product eliminates the need for a separate transponder module by providing the data in an already acceptable form for WDM systems to multiplex.  The OADM (Optical Add-Drop Multiplexer) Cable takes this concept one step further.  Instead of requiring a whole WDM system, MRV provides a simple optical patch cable that operates as a single channel add-drop multiplexer.  When combined with the Colored GBICs, this allows a WDM network to be created directly with existing switches or routers, without the need for any other WDM-specific systems.


WDM Links

• FiberDriver-LD Modular CWDM/DWDM
• FiberDriver Fixed Form WDM
• FiberDriver Modular CWDM
• Colored GBICs and OADM Cables
• FiberDriver-LD in a Banking Network
• Utilities Network Solutions