is placed in the RSOH of the following STM-1 frame. Data communication channel (D1, D2, D3 bytes): The 192 kbit/s data communication channel (DCC) provides the capability to transfer network management and main- tenance information between regenerator section terminating equipment (RSTE). Orderwire channel (E1 byte): The E1 byte is used to provide a local orderwire channel for voice communications between regenerators and remote terminal locations. User communication channel (F1 byte): The F1 byte is intended to provide the net- work operator with a channel that is terminated at each regenerator location and can carry proprietary communications. The information transmitted on this chan- nel can be passed unmodified through a regenerator or can be overwritten by data generated by the regenerator. AU pointers (H1, H2, H3 bytes): The AU pointer bytes are used to enable the transfer of STM-1 frames within STM-N frames, and therefore are processed by MS ter- minating equipment. Separate pointers are provided for each STM-1 frame in an STM-N frame. A.3 Multiplexer Section Overhead The functions of the various bytes carried in the STM-1 multiplexer section overhead (MSOH) are described in the following. Parity check (B2 bytes): A 24-bit wide BIP checksum is calculated over all the bits in the STM-1 frame (except those in the RSOH). The computed checksum is placed in the MSOH of the following STM-1 frame. Protection switching (K1, K2 bytes): The K1 and K2 bytes carry the information needed to activate/deactivate the switching between the main and protection paths on an MS. They are also the vehicle of the automatic protection switching Appendix A: SDH/SONET Signaling (APS) protocol, managing, in real time, both dedicated and shared protection in SDH networks. Data communication channel (D4 to D12 bytes): Bytes D4 to D12 provide a 576kbit/s DCC between MS terminating equipment. This channel is used to carry network administration and maintenance information. Orderwire channel (E2 byte): The E2 byte is used to provide a lo