WDM stands for Wavelength Division Multiplexing. It is a technology which combines two or more kinds of optical signals at different wavelengths and transmits them on one optical fiber. It is well known that white light consists of multiple colors of light, so a beam of white light passing through a prism creates a rainbow. WDM technology is much like this. It separates light with all the colors in the spectrum and transmits them on one optical fiber. Light at different wavelengths carries different signals and does not interfere with each other during the transmission. The following figure may help to learn about WDM.
CWDM stands for Coarse Wavelength Division Multiplexing. It is a special technology defined by the ITU (International Telecommunication Union) in ITU-T G.694.2 spectral grids. It uses the wavelengths from 1270 nm to 1610 nm with a channel spacing of 20 nm. CWDM is suitable for use in metropolitan applications and cable television networks.
DWDM stands for Dense Wavelength Division Multiplexing. It is also a special technology defined by the ITU in ITU-T G.694.1 frequency grid. DWDM has a reference frequency fixed at 193.10 THz, with channel spacing varied from 12.5 GHz to 200 GHz, and a channel spacing of 100 GHz is common. In practical application, DWDM frequency is usually converted to wavelength. DWDM can transmit at most 80 channels (wavelengths) in the Conventional band (C-band) spectrum, and all 80 channels can transmit at the wavelength of 1550 nm at the same time.
There is a figure showing the comparison between CWDM and DWDM.
Single Fiber Bidirectional Transmission
Single fiber bidirectional transmission refers to bidirectional communication on one strand of fiber. It utilizes two sets of same wavelengths for bidirectional transmission on one optical fiber. In single fiber bidirectional transmission system, each channel can realize bidirectional transmission.
Dual Fiber One-direction Transmission
Dual fiber refers to communication on two fiber. One fiber is used for communication on transmitted direction, and the other fiber is used for communication on received direction. In dual fiber one-direction transmission system, the same wavelength is usually used for both transmitted direction and received direction. In redundant system, the second fiber can be used as a backup fiber or it can provide an optical path in the opposite direction.
Upstream & Downstream
The transmission direction of signals can be expressed in upstream or downstream. The upstream direction refers to that the communication is sent from the service user to the service supplier, while the downstream direction is in the opposite direction.
By using multiple channels on optical fiber, the products of WDM bring higher efficiency to fiber optical network. An entire network usually consists of several different kinds of sub-network topologies. The network is invisible, but it can be identified by fiber cabling or topology. Sometimes, particular WDM products will be used in some networks with topology, such as Mesh, Ring, P2P (Point-to-Point), and P2MP (Point-to-Multipoint). Therefore, it is necessary to know about the type of network when choosing WDM product.
In metropolitan area networks, the infrastructures are usually structured with ring topology. The network with ring topology is a closed loop consisting of a series of optical fiber spans. And those spans are terminated at the nodes in the loop. Each node is merely connected with two adjacent nodes through the optical fiber span. Ring network usually adopts dual fiber bidirectional system.
In network topology, node is the termination of single branch or multiple branches of the network. WDM network is composed of a set of nodes which are connected through optical fiber (physical topology). After establishing the light link between the nodes, a logical topology cover the entire network. Using WDM technology in optical fiber can make one node become several serving area which can expand the customer base and available bandwidth.
WDM Technology brings unprecedented increasing of bandwidth capacity and it becomes an ideal solution for more bandwidth and lower cost in modern telecommunication network. Therefore, some basic terminologies above are useful.