SONET/SDH multiplexing techniques are built around a fixed size row and column structure that contains overhead (OH) and a synchronous payload envelope (SPE). The SPE is designed to carry synchronous transport streams (STS). The dimensions of the SONET/SDH building block are shown in Table 4-2.

SONET/SDH multiplexing frame structure is shown in Figure 4-4. Notice the structure is built around the same 125-microsecond, 8-kHz timing and synchronization structure as PDH multiplexing and framing. The difference however, is that STS really is synchronous not asynchronous, plesiochronous, or non-synchronous because unlike the others, SONET/SDH clocking is orders of magnitude more accurate than PDH clocking, and multiplexing is byte-interleaved instead of bit-interleaved.

Figure 4-4: SONET/SDH STS-1 Multiplex Frame Structure

Think of the intersection of each row and column as defining a single byte. Given 8 kHz and 125-microsecond framing, the basic STS bit rate calculated as follows:

Overhead and the associated envelope and transport stream are fixed in size and duration, resulting in a fixed bitrate. Overhead is further subdivided into channels and functions used to configure, manage, monitor, and control the line and section elements of the link. Another overhead is path oriented and provides end-to-end management and monitoring capability across tandem connections...