to above, is known as a Synchronous Payload Envelope (SPE). Another distinction also makes a distinction between the STS logical signal and its optical manifesta- tion at a Network Node Interface (NNI), in which case it is referred to as an Optical Carrier (OC). North American transmission systems are usually referred to by their transmission rates in OCs (e.g., OC-3 or OC-48), and in practice they are always constructed to use the same transmission rates as SDH systems, as illustrated in Table 10.1. In fact, the higher the transmission rates involved, the more closely the SONET and corresponding SDH systems come to resemble each other. There are also some more detailed differences. For example, overhead bytes are defined slightly differently for the two systems. The similarity between an OC-3 and an STM-1 is very close but not exact. The difference is to be found in two bits within the pointer bytes of the SDH, known as the "SS" bits (for an explanation of the pointer mechanism, see The Concept of Pointers section). In SDH the default value is 11, and in SONET the default value is 00. Although the STS-M bitrates are exact multiples of the STS-1 bitrates, and STM-N bitrates are exact multiples of the STM-1 bitrate, when multiplexing takes place each syn- chronous transport module is broken down to recover the VCs, and the outgoing STS-Ms or STM-Ns are reassembled with new overheads. Figure 10.1 shows the frame and header structure. Both systems enable the performance of traffic paths to be monitored across the network and network management to be automated. A key driver for their introduction was that a The Basis of SONET and SDH 125 us 125 us Information Payload Information Payload 1 frame = 2430 bytes = 81 + 2349 bytes Frame header = 81 bytes known as the SOH (not one continuous block, Information payload = 2349 bytes; but actually distributed throughout each byte represents a the information payload) 64-kbit/s channel Fig. 10.1 Basis of the SDH frame structure. PTO could have a consistent mechanism for partiti