Wavelength Division Multiplexing is a technique that allows for the transport of multiple frequencies (or wavelengths) to be transmitted over the same optical networking fiber simultaneously. This is accomplished through the use of equipment like optical transmitters or transceivers with outputs tuned to individual and specific wavelengths so that there are distinct and non-overlapping transmission channels.
Coarse Wavelength Division Multiplexing (CWDM) uses wavelengths between 1260nm and 1670nm (the O, E, S, C, L and U transmission bands) and allows up to 18 individual channels to be created within this region, carrying any combination of voice, data or video with channels spaced 20nm apart. CWDM is a cost-effective solution for relatively low-bandwidth deployments. However, because CWDM signals cannot be amplified, there are no broadband optical amplifiers capable of supporting this range and distances are limited to 80km.
A Dense Wavelength Division Multiplexing (DWDM) solution takes WDM to the next level by decreasing channel spacing to 0.8nm or less and shrinking the operational wavelength range. This can produce 80 or more channels or lanes of traffic, opening the door to more high speed, high bandwidth applications.
Amazingly, all DWDM wavelengths reside within the narrow 1525nm to 1565nm region known as the C-Band. This area is utilized due to the relatively low (0.25dB/km) signal loss (fiber attenuation) compared to lower wavelengths found in the O or E-bands, for example. As a result of the narrow channel spacing, higher-precision lasers and filtering processes are required to maintain channel integrity and minimize interference.
DWDM Architecture
Passive DWDM network architecture begins with a transponder or transceiver accepting data inputs of various traffic types and protocols. This transponder performs the essential function of mapping input data onto individual wavelengths. Each wavelength is fed to an optical multiplexer (MUX) which filters and combines multiple signals into a single output port for transmission over the main/core/common DWDM fiber. At the receiving end, wavelengths can then be separated to isolate the individual channels by using an optical demultiplexer (De-MUX). Each channel is then routed to the appropriate client-side output through an additional wavelength matched transponder.
Because DWDM technology overlaps the CWDM frequency band, a “hybrid” solution can also be selected. This type of system leaves the CWDM MUX and deMUX hardware in place, inserting DWDM wavelengths on top of existing channels in the 1530 to 1550nm range, creating up to 28 additional channels. This type of hybrid system can provide a significant capacity boost without requiring new fiber installation or wholesale infrastructure changes for a company.
An Optical Add Drop Multiplexer (OADM) is an optional component of DWDM architecture that can be added to either passive or active networks to facilitate the addition or subtraction of a specified wavelength from a mid-stream location on the main/core/common DWDM fiber. Bidirectional architecture includes transmitters and receivers at both ends of the circuit as well as combination MUX/De-MUX devices.
For long-haul networks, DWDM architecture gains complexity with the addition of active system components needed to compensate for optical losses that will make signal reception and data recovery impossible. An Erbium Doped Fiber Amplifier (EDFA) can be used as a booster or launch amplifier to boost the optical power levels just as they leave the MUX, while a pre-amplifier performs the same function prior to entering the DeMUX. Additional inline amplifiers might also be included. Passive networks, without EDFA, minimize this complexity.
