onous 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 TABLE 4-2 Frame Duration: 125 Microseconds Frame Repetition Rate: 8,000 Frames/Second Synchronous Transport Stream (STS-1) Bit Rate: 51.84Mbs (6.48MB/Octets/second) Overhead Columns Section Sub-Total Payload: Total STS-1: PRACTICAL IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS 51.84 125 Microseconds XXXXX 90 Columns Section Synchronous Payload FIGURE 4-4 SONET/SDH STS-1 Multiplex Frame Structure more accurate than PDH clocking, and multiplexing is byte- interleaved instead of bit-interleaved. 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. An- other overhead is path oriented and provides end-to-end manage- ment and monitoring capability across tandem connections passing through terminals and extending to customer premises when an STS facility is built from multiple links. The SONET/SDH structure is extendable from one stream to 192 streams. Each STS consists of 86 columns by nine rows or bytes of payload and one column by nine rows of bytes for path overhead. Section and line overhead are unique to each link. Path overhead 4 NETWORK TECHNOLOGY AND METHODOLOGY extends across all links in a path made up of multiple STS passed through a terminal intact.