ng bits to the bit stream as part of the mapping process. An example might be a DS3 tributary signal at a nominal rate of 44.736 Mbps to be synchronized with a payload capacity of 49.54 Mbps provided by an STS-1 SPE. The 19.2 SONET SYNCHRONIZATION TRIBUTARY PAYLOAD SYNCHRONOUS SIGNAL PAYLOAD CAPACITY PAYLOAD ENVELOPE MAPPING STS-1 SPE MAPPED DS3 at 44.74 Mbps at 49.54 Mbps at 50.11 Mbps STUFF BITS OVERHEAD Figure 19.12 The SPE assembly process. (From Ref. 2, courtesy of Hewlett-Packard.) DESYNCHRONIZATION Synchronous Payload Tributary Payload Envelope Capacity Payload Signal STS--1 SPE Demapping at 50.11 Mbps Mapped DS3 at 44.74 Mbps at 49.54 Mbps Desynchronizing Phase-Locked Loop Overhead Figure 19.13 The SPE disassembly process. (From Ref. 2, courtesy of Hewlett-Packard.) addition of path overhead completes the assembly process of the STS-1 SPE and increases the bit rate of the composite signal to 50.11 Mbps. The SPE assembly process is shown graphically in Figure 19.12. At the terminus or drop point of the network, the original DS3 payload must be recovered as in our example. The process of SPE Disassembly is shown in Figure 19.13. The term used here is payload demapping. The demapping process desynchronizes the tributary signal from the composite SPE signal by stripping off the path overhead and the added stuff bits. In the example, an STS-1 SPE with a mapped DS3 payload arrives at the tributary disassembly location with a signal rate of 50.11 Mbps. The stripping process results in a discontinuous signal representing the transported DS3 signal with an average signal rate of 44.74 Mbps. The timing discontinuities are reduced by means of a desynchronizing phase-locked loop, which then produces a continuous DS3 signal at the required average transmission rate (Refs. 1, 2). 19.2.2.8 Line Rates for Standard SONET Interface Signals. Table 19.1 shows the standard line transmission rates for OC-N (OC = optical carrier) and STS-N. 19.2.3 Add-Drop Multiplexer The SONET ADM multiplexes one or more DS-n signals into the 