Add/drop multiplexer

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Add/drop multiplexer

The SONET add/drop multiplexer (ADM) is the workhorse of SONET networks. Essentially a form of digital cross-connect system (DCS), ADMs can be placed in series along a SONET transmission system so that either non-SONET or SONET lower-speed streams can be added to, or dropped from, higher-speed transmissions system. ADMs operate bidirectionally and can terminate circuits from either direction. Typical ADM implementations employ protection switching capabilities, providing an active (A) fiber and a standby (B) fiber, making ADMs the key component of a SONET ring.

ADMs are typically equipped with time slot interchangers (TSI) to enable DCS functions between channels, carried as part of the STS payload. When optioned in the add/drop mode, ADMs eliminate the need for back-to-back multiplexers (a common solution in T-carrier networks) to break out component signals.

Because ADMs interface to both conventional electrical networks (e.g., T-carrier) and optical networks (e.g., SONET), they need a low-speed electrical interface and a high-speed optical interface. They also require remote and local operations interfaces, a data communications channel (DCC) for Section Layer overhead functions, local nonvolatile random access memory (NVRAM) for backup purposes, and frame and facility maintenance capabilities.

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