The SONET multiplexer, an entry level path terminating terminal multiplexer, acts as a concentrator of DS1s as well as other tributaries. This implementation represents the simplest SONET configuration.
In this configuration, the SONET path and the Service path (DS1 or DS3 links end-to-end) are identical and this synchronous island can exist within an asynchronous network world.
A point-to-multipoint (linear add/drop) architecture includes adding and dropping circuits along the way. The SONET ADM (add/drop multiplexer) is a unique network element specifically designed for this task. It avoids the current cumbersome network architecture of demultiplexing, cross-connecting, adding and dropping channels, and than remultiplexing. The ADM is typically placed along a SONET link to facilitate adding and dropping tributary channels at intermediate points in the network.
The hub network architecture accommodates unexpected growth and change more easily than simple point-to-point networks. A hub (Figure 28) concentrates traffic at a central site and allows easy reprovisioning of the circuits.
The SONET building block for a ring architecture is the ADM. Multiple ADMs can be put into a ring configuration for either bi-directional or uni-directional traffic. The main advantage of the ring topology is its survivability; if a fiber cable is cut, the multiplexers have the intelligence to send the services affected via an alternate path through the ring without interruption.