SDH was designed to allow for flexibility in the creation of products for electronically routing telecommunications traffic. The key products are as follows:

• optical-line systems
• radio-relay systems
• terminal multiplexers
• add-drop multiplexers (ADM)
• hub multiplexers
• digital cross-connect switches

A generic network using these products is shown in Figure 2.

Figure 2. SDH Network Application

Optical-line systems, and to a lesser extent radio-relay systems, provide the transmission-bearer backbone for the SDH network. Terminal multiplexers provide access to the SDH network for various types of traffic using traditional interfaces such as 2 Mbps G.703 or in data-oriented forms such as fiber distributed data interface (FDDI) via an appropriate bridge or router.

ADM can offer the same facilities as terminal multiplexers, but they also can provide low-cost access to a portion of the traffic passing along a bearer. Most designs of ADM are suitable for incorporation in rings to provide increased service flexibility in both urban and rural areas (spans between ADMs are typically 60 km). ADM ring design also employs alternative routing for maximum availability to overcome fiber cuts and equipment failures. A group of ADMs, such as in a ring, can be managed as an entity for distributed bandwidth management. The routing function of a typical ADM is outlined in Figure 3.

Figure 3. The Routing Function of a Typical ADM

Hub multiplexers provide flexibility for interconnecting traffic between bearers, usually optical fibers. A hub multiplex is connected as a star, and traffic can be consolidated or services managed while standby bearers between hubs provide alternate routing for restoration. Several rings of ADMs can converge on a single hub, providing interconnection of traffic between those rings and connection into the existing network.

Some designs of ADM also can be used as hub multiplexers, or they can combine the two functions to optimize network topology between ring and star for each application while still using a common base of equipment. A single unit can act as an ADM on a ring while serving as a hub multiplex for a number of fiber spurs off the ring, with each spur supporting a major business user.

A cross-connect allows nonblocking connections between any of its ports. An SDH cross-connect performs this function for SDH virtual containers (VC) that is, when connecting a PDH signal, the SDH cross-connect also connects the associated SDH path overhead (POH) for network management. In contrast with telephony exchanges (central offices [COs] in North America), which respond primarily to individual customer demands, cross-connects are the major flexibility points for network management.