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Digital communications
Digital communications

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4.3 Optical networking

DWDM improves the utilisation of optical fibre for point-to-point links, but a further step in exploiting the potential of optical fibre comes from optical networking in which routeing or switching is done optically.

Optical networking is in its infancy, but the concept of the optical layer based upon wavelength channels is emerging. The optical layer effectively sits below the SDH layer in the network, and provides wavelength channels from one location to another.

An analogy can be drawn between networking with SDH and the optical network, where wavelength channels in the optical layer perform the equivalent function to containers in SDH. Then, as SDH transports containers across a network, so the optical layer transports wavelengths. A DWDM transmission link is the equivalent of an STM-n.

Activity 16

Can you think what function would be required of optical equivalents of cross-connects and add/drop multiplexers?


Add/drop multiplexers would have to extract and replace individual wavelength channels, as in Figure 28 (a). Cross-connects would have to interconnect wavelength channels between different DWDM links, as in Figure 28(b).

Figure 28
Figure 28 Optical cross-connects and add/drop multiplexer.

Activity 17

The space switching shown in Figure 28(b) can be done with MEMS switches (Figure 25), but the resulting cross-connect will not be as flexible as it might first appear. Can you see in what way it is limited, and what more is needed for full flexibility?


Incoming wavelengths have to go to the same wavelength channels in the output, as it stands. For a fully flexible switch, wavelength conversion is needed. (It is like being able to do time-multiplexed space switching of PCM channels, but not being able to do time switching.)

It is possible to perform wavelength conversion by going via an electrical signal – detecting the optical signal at one wavelength and generating another optical signal at a different wavelength. More flexibility then comes from having wavelength-tunable lasers. An ‘all-optical’ device would be preferable, however, and experimental devices exist that can do this, but wavelength converters are perhaps the least-developed devices that are required for a universal optical layer.