ayload capacity in the STS-1 frame is only 84 columns. FRAME N FRAME N + 1 Pointer bytes Transparent Overhead Synchronous Frame N Payload Pointer Envelope bytes [Frame N] Transparent Overhead Frame N + 1 Figure 19.4 STS-1 SPE typically located in STS-1 frames. (From Ref. 2, courtesy of Hewlett-Packard.) The STS POH (path overhead) is associated with each payload and is used to commu- nicate various pieces of information from the point where the payload is mapped into the STS-1 SPE to the point where it is delivered. Among the pieces of information carried in the POH are alarm and performance data. 19.2.2.2 STS-N Frames. Figure 19.5 illustrates the structure of an STS-N frame. The frame consists of a specific sequence of NX810 octets. The STS-N frame is formed by octet-interleave STS-1 and STS-M (<N) modules. The transport overhead of the individual STS-1 and STS-M modules are frame-aligned before interleaving, but the associated STS SPEs are not required to be aligned because each STS-1 has a payload pointer to indicate the location of the SPE or to indicate concatenation. 19.2 SONET N x 90 Columns x 3 Columns 9 Rows 125 us Transport STS-N Envelope Capacity Overhead Figure 19.5 STS-N frame. 19.2.2.3 STS Concatenation. Superrate payloads require multiple STS-1 SPEs. FDDI and some B-ISDN payloads fall into this category. Concatenation means the linking together. An STS-Nc module is formed by linking N constituent STS-1s together in a fixed phase alignment. The superrate payload is then mapped into the resulting STS-Nc SPE for transport. Such STS-Nc SPE requires an OC-N4 or an STS-N electrical signal. Concatenation indicators contained in the second through the Nth STS payload pointer are used to show that the STS-1s of an STS-Nc are linked together. There are NX783 octets in an STS-Nc. Such an STS-Nc arrangement is illustrated in Figure 19.6 and is depicted as an NX87 column by 9-row structure. Because of the linkage, only one set of STS POH is required in the STS-Nc SPE. Here the STS POH always appears in the firs