As an unprecedented opportunity to dramatically increase the bandwidth capacity, WDM (Wavelength Division Multiplexing) technology is an ideal solution to get more bandwidth and lower cost in nowaday telecommunications networks. By virtue of fame, WDM becomes a household word now. Yet, most of the time, we only know what is “WDM” but do not really know WDM technology. Actually, there are various of terminologies used in WDM that are always a headache for us. Now, let’s see what are they.
WDM Includes CWDM and DWDM
WDM (Wavelength Division Multiplexing)
A technology that multiplexes a number of optical carrier signals onto a single optical fiber by using different optical wavelengths (i.e., colors) of laser light. It breaks white light passing through fiber optic cable into all the colors of the spectrum, much like light passed through a prism creates a rainbow. Every wavelength carries an individual signal that does not interfere with the other wavelengths.
CWDM (Coarse Wavelength Division Multiplexing)
CWDM is a specific WDM technology defined by the ITU (International Telecommunication Union) in ITU-T G.694.2 spectral grids, using the wavelengths from 1270 nm to 1610 nm within a 20nm channel spacing. It is a technology of choice for cost efficiently transporting large amounts of data traffic in telecoms or enterprise networks.
DWDM (Dense Wavelength Division Multiplexing)
DWDM is a specific WDM technology also defined by the ITU but in ITU-T G.694.1 spectral grids. The grid is specified as frequency in THz, anchored at 193.1 THz, with a variety of specified channel spacing from 12.5 GHz to 200 GHz, among which 100 GHz is common. In practice, DWDM frequency is usually converted to wavelength. DWDM typically has the capability to transport up to 80 channels (wavelengths) in what is known as the Conventional band (C-band) spectrum, with all 80 channels in the 1550 nm region.
WDM Transmission System
Single Fiber Transmission
Single fiber, namely bi-directional communication on one single fiber. This system utilizes two identical sets of wavelengths for both directions over a single fiber. Individual channels residing on the single fiber system may propagate in either direction.
Dual Fiber Transmission
Dual fiber, namely comprised of two single fibers, one fiber is used for the transmit direction and the other is used for the receive direction. In dual fiber transmission system, the same wavelength is normally used in both the transmit and receive directions. The second fiber may serve as a backup fiber as in a redundant system, or it may provide an optical path in the opposite direction.
Upstream (Return) & Downstream (Forward)
The direction of a communication signal can be refered using these two terminologies. The downstream direction is defined as communication originating at a service provider and sent to the service user. Upstream is in the opposite direction.
WDM Topology
Network Topologies
WDM products bring higher efficiency to fiber networks through multiple channel usage of fiber. Networks are identified by their fiber layout or topology. Network topologies such as Mesh, Ring, P2P (Point-to-Point), and P2MP (Point-to-Multipoint) will sometimes use WDM products particularly designed for the network. So, it is important to understand the intended network use when selecting WDM products. Entire networks are often comprised of several kinds of sub-network topologies.
Ring Topology
In metropolitan area networks, infrastructures are generally organized over a ring topology. Ring topology is a type of network topology consisting of a closed loop. Fiber ring networks are comprised of a series of fiber spans that terminate at network nodes spread throughout the loop. Each node in the ring will connect to two, and only two, adjacent nodes. Ring networks are often dual fiber systems. Contrast ring topology with an unclosed, end-to-end or point-to-point fiber span.
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