The basic principle behind the working of all kinds of WDM techniques is the same. Several signals are transmitted in a single fiber at the same time but are just separated by different wavelengths.

The channels associated with CWDM are spaced 20 nanometers (10 to the power -9) apart this is higher compared to the DWDM technique in which the channels are spaced at 0.4 nm. Because of these relatively wide-spaced channels, the maintenance cost is very low and non- cooled lasers can be used. The emission in CWDM occurs on eight different channels at different wavelengths ranging from 1470 nm to 1610 nm.  It is in the C-Bands that these channels are present. It can accommodate even up to 18 channels with wavelengths from 1270 nm.

The CWDM energy emitted from the lasers is also spread over a wide range compared to DWDM. The tolerance level of CWDM is + or – 3 nm which is comparatively preferable than DWDM. The power consumed by CWDM is less compared to the latter. The laser used in CWDM is of low precision. One considerable drawback of this technique is that the realizable distance between any two nodes is lesser compared to DWDM.

It is considered an intermediate technology which is believed to meet the demands associated with the ever-growing fiber network. The insertion loss associated with an eight channel CWDM device is 2 decibels per end. The isolation loss is lesser in the case of CWDM de-multiplexers compared to the insertion loss of the same. The case is totally reverse in the case of CWDM multiplexers that have better isolation loss and a higher isolation loss. This isolation loss need not be taken in to account while dealing with unidirectional applications as these multiplexers combines the outputs of several transmitters.

Let us discuss the basic function of CWDMs. Initially it performs filtration (of light) thereby using only the desired wavelengths. To combine various signals on a single fiber, they perform multiplexing operation. The reverse operation is de-multiplexing in which the signals are separated from that fiber as the output. The International Telecommunications Union defines the wavelengths that are used by coarse WDM.

Full Duplex operation is one in which sending and receiving signals can be carried out simultaneously. The wavelength assigned for transmitting signals is 1530 nm whereas for receiving signals, it is 1550-nm. CWDM offers an application called Add/Drop module in which the received signal can be dropped from a link and a new signal can be added or multiplexed on to the link.

A standard example of CWDM system is Ethernet LX-4 which operates at 10 Gbits/s. CWDM is suitable for metropolitan applications and cable TV networks. A recent development in CDWM technique is the introduction of GBIC and SFP. One implementation of coarse WDM that makes use of no electrical power is Passive CWDM.

With much new advancement in the field of wavelength multiplexing techniques, CWDM still has a long way to go in the field of telecommunication.

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