Three basic devices used in the SONET system are shown in Fig. Functions of the three devices are mentioned below:

• Synchronous Transport Signal (STS) multiplexer/demultiplexer: It either multiplexes signal from multiple sources into a STS signal or demultiplexes an STS signal into different destination signals.

• Regenerator: It is a repeater that takes a received optical signal and regenerates it. It functions in the data link layer.

• Add/drop Multiplexer: Can add signals coming from different sources into a given path or remove a desired signal from a path and redirect it without any error.

A number of electrical signals are fed into an STS multiplexer, where they are combined into a single optical signal. Regenerator recreates the optical signal without noise it has picked up in transit. Add/Drop multiplexer reorganize these signals.

A section is an optical link, connecting two neighboring devices: multiplexer to multiplexer, multiplexer to regenerator, or regenerator to regenerator.

A line is a portion of network between two multiplexers: STS to add/drop multiplexer, two add/drop multiplexer, or two STS multiplexer.

A Path is the end-to-end portion of the network between two STS multiplexers, as shown in Fig.