S1C, DS2, DS3, or OC-M (M < N) interfaces. There is non-path- terminating information payloads from each incoming OC-N signal which are passed the SONET ADM and transmitted by the OC-N interface at the other side. Figure 19.14 also shows a synchronization interface for local switch application with external timing and an operations interface module (OIM) that provides local technician orderwire, 6 local alarm, and an interface to remote operations systems. A controller is part of each SONET ADM, which maintains and controls the ADM functions, to connect to local or remote 6 Orderwire is a voice, teleprinter, or PC circuit for coordinating setup and maintenance activities among technicians. 19.3 SYNCHRONOUS DIGITAL HIERARCHY Synchronization Timing references interface Technician DSX and OC interface inputs/outputs Common Alarms equipment interface Network management termination Optical fiber Notes: DSX = DS1-DS3 and OC = optical carrier, SONET OA & M = Operations, administration and maintenance Figure 19.15 An ADM in a terminal configuration. technician interfaces and to connect to required and optional operation links that permit maintenance, provisioning, and testing. Figure 19.15 shows an example of an ADM in the terminal mode of operation with DS1 interfaces. In this case, the ADM multiplexes up to Nx(28 DS1)7 or equivalent signals into an OC-N bit stream. Timing for this terminal configuration is taken from either an external synchronization source, the received OC-N signal (called loop timing), or its own local clock depending on the network application. 19.3 SYNCHRONOUS DIGITAL HIERARCHY 19.3.1 Introduction SDH was a European/CCITI development, whereas SONET was a North American devel- opment. They are very similar. One major difference is their initial line rate. STS-1/OC-1 has an initial line rate of 51.84 Mbps; and SDH level 1 has a bit rate of 155,520 Mbps. These rates are the basic building blocks of each system. SONET's STS-3/OC-3 line rate is the same as SDH STM-1 of 155.520 Mbps. Another difference 