AddDrop Multiplexer (ADM) is the basic unit for composing a Synchronous Digital Transport Network. A SONET/SDH network can be made up of such Network Equipments (NE) as ADM
Terminal Multiplexer (TM)
Synchronous Digital Cross Connection (SDXC) and Network Management System (NMS) with much flexibility. The optical Aggregate Unit is the most important part of ADM. This paper introduce the composition of ADM and the reference design of OAU board.\=