We have seen that it is possible to pack many channels into a SONET/SDH network, using the principle of time division multiplexing (TDM). However, available technology puts an upper limit to the realizable band width. OC-48/STM-16 with a speed of 2.488 Mbit/sec is most popular today. More expensive OC-192/STM-64 with a 10 Gigabit/sec is available. A practical upper limit using developing technology is 40 Gbps.
An alternative is to assign different frequencies to different channels, multiplex them for carrying information over fibers and finally demultiplex at the receiver end. The wavelength division multiplexing (WDM) is the same as frequency division, excepting that the terminology is used for optical frequencies.
Features of WDM:
1.
Bandwidth: The fact that one can use different wavelengths over the same channel increases bandwidth capacity
enormously. Most WDM systems work in the C-band around 1550 nm. ITU has specified a standard channel separation grid from 191.1 THz to 196.5 THz separated by 100 GHz. In practice, channels separated by 50 GHz are used. (In terms of wavelengths, it corresponds to the range 1526 nm to 1570 nm with a separation of about 0.8 nm. However, in WDM, the channels are equispaced in frequency and not in wavelengths.) Older systems which were spaced at 200 GHz are known as WDM whereas systems with denser packing such as give above are called Dense WDM (DWDM) . Still older ITU specification, referred to now a days as CWDM (Coarse- WDM) specified a 20 nm spacing in the wavelength range 1270 to 1610 nm.
The number of channels (alls called s) could be reaching up to 128 so that a single fiber supporting, say OC-48, can give a bandwidth of over 300 Mbit/sec. Modern systems (for instance at OC-192) can easily pack channels to give a bandwidth of 400 Gbps. A similar calculation for at OC-768 can reach up to 1.6 Terrabits/sec.
2.
Since WDM carries each signal independently of other signal, each channel has a dedicated bandwidth. Signals arrive at
the destination at the same time and not in different time slots as is the case with TDM.
s) could be reaching up to 128 so that a single fiber supporting, say OC-48, can give a bandwidth of over 300 Mbit/sec. Modern systems (for instance 
at OC-192) can easily pack channels to give a bandwidth of 400 Gbps. A similar calculation for 
at OC-768 can reach up to 1.6 Terrabits/sec. 
