Three basic devices used in the SONET system are shown in Fig. 4.3.1. Functions of the three devices are mentioned below:

·         Synchronous Transport Signal (STS) multiplexer/demultiplexer: It either multiplexes signal from multiple sources into an 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.

                                          Devices used in the SONET system

Section, Line and paths

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 neighbouring 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.

                                    Section, line and path in a SONET network

SONET Network Elements

Terminal Multiplexer

The path terminating element (PTE), an entry-level path-terminating terminal multiplexer, acts as a concentrator of DS–1s as well as other tributary signals. Its simplest deployment would involve two terminal multiplexers linked by fiber with or without a regenerator in the link. This implementation represents the simplest SONET link (a section, line, and path all in one link; see Figure)

                                  Terminal Multiplexer

Regenerator

A regenerator is needed when, due to the long distance between multiplexers, the signal level in the fiber becomes too low. The regenerator clocks itself off of the received signal and replaces the section overhead bytes before retransmitting the signal. The line overhead, payload, and POH are not altered.

                  Regenerator

Add/Drop Multiplexer (ADM)

A single-stage multiplexer/demultiplexer can multiplex various inputs into an OC-N signal. It can add signals coming from different sources into a given path or remove a desired signal from a path and redirect it without demultiplexing the entire signal, as shown in Fig. Instead of relying on timing and bit positions, add/drop multiplexer uses header information such as addresses and pointers to identify individual streams.

In rural applications, an ADM can be de consolidating traffic from widely separated locations. Several ADMs can also be configured as a survivable ring. SONET enables drop and repeat (also known as drop and continue)—a key capability in both telephony and cable TV applications. With drop and repeat, a signal terminates at one node, is duplicated (repeated), and is then sent to the next and subsequent nodes.

The add/drop multiplexer provides interfaces between the different network signals and SONET signals. Single-stage multiplexing can multiplex/demultiplex one or more tributary (DS–1) signals into/from an STS–N signal. It can be used in terminal sites, intermediate (add/drop) sites, or hub configurations. At an add/drop site, it can drop lower-rate signals to be transported on different facilities, or it can add lower-rate signals into the higher-rate STS–N signal. The rest of the traffic simply continues straight through.

                        Add/drop Multiplexer

Wideband Digital Cross-Connects

A SONET cross-connect accepts various optical carrier rates, accesses the STS–1 signals, and switches at this level. It is ideally used at a SONET hub as shown in Fig. One major difference between a cross-connect and an add/drop multiplexer is that a cross-connect may be used to interconnect a much larger number of STS– 1s. The broadband cross-connect can be used for grooming (consolidating or segregating) of STS–1s or for broadband traffic management. For example, it may be used to segregate high-bandwidth from low bandwidth traffic and send it separately to the high-bandwidth (e.g., video) switch and a low-bandwidth (voice) switch. It is the synchronous equivalent of a DS–3 digital cross-connect and supports hubbed network. Architecture.

            Wideband digital cross-connect