The role of a transmission network is to connect points. This can be done in two ways: via a fixed point-to-point connection (exemplified by asynchronous digital hierarchy) or via a flexible network that allows connection changes and fast responses to new connection requests (see Figure 5).

Figure 5. Point-to-Point and Flexible Connections

The equipment formerly implemented a point-to-point connection called a terminal multiplexer (MUX) or line system (see Figure 6). A terminal MUX offers fixed connections between end-user termination points. Note that this was the only type of equipment available for asynchronous transmission, the technological predecessor to SONET.

Figure 6. Line System

The other type of connection is made through a flexible network. This can be implemented using cross-connects or bus structures.

A digital cross-connect (DCS) is a piece of equipment that provides flexible connections between its termination points. Similarly, a bus structure provides flexible connections between the termination points of the elements making up the network. The bus route has “bus stops” known as add/drop multiplexers (ADMs) that provide the traffic with the flexibility to jump on or off or remain in place (see Figure 7).

Figure 7. Line System

Network survivability is a key issue addressed by SONET. Networks are designed to cater for node and link failure.

In the case of a point-to-point network, link resilience can be introduced by duplicating the link. Maximum protection is achieved if these two links are separately routed.

In the case of flexible network implemented with a cross-connect, the cross-connect is the major point of vulnerability, and a design with several cross-connects may be needed to achieve the degree of availability needed. However, with a bus structure, resilience can be achieved by adding an extra link to the network to make up a ring (see Figure 7). Now two alternative routes can be offered for a connection.