d of 783 bytes per STS-1 frame. PAYLOAD The payload area of an STS-1 frame consists of 783 bytes. Although not shown in Fig. 6.5, the payload area is actually subdivided. The major portion of the payload area is used for the actual payload, while 9 bytes within the general payload area are used for path overhead. Thus, the net payload area available per STS-1 frame is actually 774 bytes. The gross payload area is organized into a floating position within the STS-1 frame. Since the payload area floats, a mechanism is necessary to denote the actual position of the payload within the frame. This mechanism is provided by the use of two line overhead bytes: H1 and H2. In SONET terminology the total 783 bytes in the gross payload area, including 774 bytes of payload data and 9 bytes of path overhead data, is referred to as a synchronous payload envelope (SPE). The floating of the SPE provides a mechanism for overcoming differences in synchronization between different T- and E-carrier transmission facilities. An example of a synchronous payload envelope floating within an STS-1 frame is shown in Fig. 6.6. Actually, the STS SPE can begin any- where within an STS-1 envelope. Figure 6.6 shows an SPE beginning in one STS-1 frame and ending in the next frame. The line overhead bytes (H1 and H2) represent an offset in bytes within the gross payload area. The H1 and H2 bytes are allocated to a pointer that indicates the offset in bytes between the pointer and the first byte of the STS SPE. The H3 Chapter Six byte is used to compensate for clocking differences; that is, when a source clock is fast with respect to the STS-1 clock, the H3 byte can be used, permitting one payload byte from the SPE to be placed into the H3 byte. When this occurs, special coding is placed into byte locations H1 and H2. As you examine the SPE positioning within the STS-1 frame shown in Fig. 6.6, note that the first path overhead byte has the designator J1. Because the payload wraps around to the next row, the second path over- head byte, designated B