Synchronous Optical NETwork (SONET) is a newly adopted transmission standard proposed byBeilcore in 1985. The main purposes of SONET are the midspan meets (transmission equipment provided by different vendor can be connected directly), management of the entire network in term of the embedded operation channel, and supporting the application interfaces.SONET Network Element (NE) is the basic equipment in SONET which consists of Terminal Multiplexer (TM), adddrop multiplexer, and digital crossconnect system etc. This paper presents a general guide to design the software architecture of the SONET NE. The object-oriented and layering concepts are heavily used as design rules, which in the future will provide a better approach for designing a product family of the SONET NEs, i.e., reuse the objects. Furthermore, maintenance, testing, and debugging of the software can be easily achieved.