w streams can be transported by a SONET system by adding new service adapters at the edge of the SONET network. All inputs are converted to an STS-1 signal. Several synchronous STS-1s are multiplexed together in either a single- or two-stage process to form an electrical STS-n signal (n 1). STS multiplexing is performed at the byte interleave synchronous multiplexer. Traditional SONET Architectures DS1 (28 max per STS-1) Figure 3-7 Service Adapters (Not of interest for (map to VTs) Multiplexing aspect Electrical of SONET. broadband applications) to Optical Converter DS3 (1 max per STS-1) Service Adapters (map to STS-1) (Byte Interleave (Limited interest for Synchronous Mux) Rate = Clock = broadband applications) N X 51.84 Mbps N X 51.84 Mbps STS-Nc (concatenated STS-1s) Timing Service Adapters IP (POS) 10 GbE MPLS/ATM Figure 3-8 depicts the architectural evolution of private-line-like services since the mid-1990s. The first generation consisted of ATM over SONET, the second generation consisted of IP over SONET, and the third generation uses 10 GbE WAN PHY over SONET. 3.2.2 SONET Frame Format SONET uses a basic transmission rate of STS-1, which is equivalent to 51.84 Mbps. Higher-level signals are multiples of the base rate. For exam- ple, STS-3 is three times the rate of STS-1 (3x51.84=155.52 Mbps). An STS-12 rate is 12x51.84=622.08 Mbps. SDH is built on 155.52-Mbps mod- ules and multiples thereof. STS-1 Building Block The frame format of the STS-1 signal is shown in Figure 3-9, which expands upon Figure 3-6. The frame is comprised of two main areas: the transport overhead and the synchronous payload enve- lope (SPE). The SPE can also be partitioned into two parts: the STS path overhead and the payload. The signal is transmitted byte by byte beginning with byte 1, reading left to right from row 1 to row 9. The entire frame is transmitted in 125 microseconds. Once the payload is multiplexed into the SPE, it can be transported and switched through SONET without having to be examined and possibly demultiplexed at i