 transmission. 19.2.2 Synchronous Signal Structure SONET is based on a synchronous signal comprised of 8-bit octets, which are organized into a frame structure. The frame can be represented by a two-dimensional map comprising N rows and M columns, where each box SO derived contains one octet (or byte). The upper left-hand corner of the rectangular map representing a frame contains an identifiable marker to tell the receiver it is the start of frame. SONET consists of a basic, first-level, structure called STS-1, which is discussed in the following. The definition of the first level also defines the entire hierarchy of SONET signals because higher-level SONET signals are obtained by synchronously multiplexing the lower-level modules. When lower-level modules are multiplexed together, the result is denoted as STS-N (STS stands for synchronous transport signal), where N is an integer. The resulting format then can be converted to an OC-N (OC stands for optical carrier) or STS-N electrical signal. There is an integer multiple relationship between the rate of the basic module STS-1 and the OC-N electrical signals (i.e., the rate of an OC-N is equal to N times the rate of an STS-1). Only OC-1, -3, -12, -24, -48, and -192 are supported by today's SONET. 19.2.2.1 Basic Building Block Structure. The STS-1 frame is shown in Figure 19.1. STS-1 is the basic module and building block of SONET. It is a specific sequence of 810 octets3 (6480 bits), which includes various overhead octets and an envelope capacity for transporting payloads. STS-1 is depicted as a 90-column, 9-row structure with a frame period of 125 usec (i.e., 8000 frames per second). STS-1 has a bit rate of 51.840 Mbps. Consider Figure 19.1. The order of transmission of octets is row-by-row, from left to right. In each octet of STS-1 the most significant bit is transmitted first. The reference publications use the term byte, meaning, in this context, an 8-bit sequence. We prefer to use the term octet. The reason is that some argue that byte is ambiguous with con