path overhead B3 path BIP-8 Path error monitoring BIP-8. C2 STS path signal STS path signal label Use as defined in ANSI T1.105. label F2 path user Path user channel Required for use by PTE. channel G1 path status Path status One byte is allocated to convey back to an originating STS PTE the path terminating status and performance. H4 multiframe Multiframe indicator Only required for VT-structured indicator payloads. J1 STS path trace STS path trace The content of the message is not constrained by this standard, since it is assumed to be user programmable at both the transmit and receive ends. N1 tandem Tandem connection maintenance/ connection path data channel maintenance/path data channel Z3-Z4 growth Growth Two bytes are allocated for future as-yet-undefined purposes. R = required; A = application-specific function; O = optional of the signals heading from the CI to the network may lack traceability to a PRS. In that case, the signals at all interface points heading in the direc- tion from the CI to the network must be within +20 ppm of the nominal rate specified for each interface. Traditional SONET Architectures Figure 3-28 Line overhead. Frame N Frame N+1 125 microseconds SPE boundary SOH & 250 microseconds Overhead byte Function Usage Coding Line overhead B2 line BIP-8 Line error monitoring BIP-8 for STS-1 on each STS-1 of STS-N. BIP-8 for STS-Nc (BIP-24 for STS-3c). D4-D12 line data Line data communications channel communication channel (Data E2 orderwire Orderwire H1-H2 pointer Pointer These bytes will be provided in all STS-1 signals within an STS-N signal. Concatenation is handled as defined in ANSI T1.105. H3 pointer action Pointer action These bytes will be provided in all STS-1 signals within an STS-N signal. K1, K2 APS Automatic protection switch If APS is used, will operate as channel defined in ANSI T1.105.01. MO STS-1 line REI STS-1 line REI STS-1 line only. M1 STS-N line REI STS-N line REI In a SONET signal at rates at or above STS-3, one byte, the M1 byte, is allocated for a line REI function