s an optical carrier (OC). The format of the OC signal is based on the corre- sponding STS signal format. The two lowest levels in the SONET hierar- chy also have defined electrical signals: EC-1 and EC-3. Both of these sig- nals facilitate the use of test instruments as well as the interconnection of transmission equipment. Table 6.1 summarizes the SONET digital hierarchy. In examining the entries in Table 6.1, note that in trade publications it is quite common for a reference to STS-48 and STS-192 in the form of OC-48 and OC-192 to be rounded to data transmission rates of 2.5 and 10 Gbits/s, respective- ly. Similarly, OC-768, which is presently in field trials, is commonly referred to as a 40-Gbit/s transmission link. If you periodically read trade literature, you may come across an OC term with a trailing "c," such as OC-192c. This term is usually employed to indicate a "clear channel" that provides a data transmission rate at the well-known optical signal rate. THE STS-1 FRAME SONET's fundamental signal, STS-1, operates at 51.84 Mbits/s, whereas, as noted in Table 6.1, higher-level signals operate at 3x multiples of that rate. Although the STS-1 frame represents a sequence of serially transmit- ted bits, most publications employ a two-dimensional matrix to illustrate the format of the frame. This two-dimensional matrix facilitates the TABLE 6.1 Abstract signal Buildup Optical signal Electrical signal Bit rate, The SONET Digital Mbits/s Hierarchy STS-1 51.84 STS-3 3 X STS-1 155.52 STS-12 12 X STS-1 OC-12 622.08 STS-48 48 X STS-1 OC-48 2488.32 STS-192 192 X STS-1 OC-192 9953.28 STS-768 STS-1 OC-768 39813.12 Chapter Six schematic representation of overhead byte locations and will also be used by this author. Thus, Figure 6.5 depicts the STS-1 frame as a two- dimensional series of rows and columns, also indicating the section and line overhead areas. The omission of a path overhead area from Figure 6.5 is intentional. The rationale for this omission is that the path overhead area literally "floats" within the pay