al operation, the customer installation transmits a signal that has a bit rate accuracy equal to that of the received signal by either of the following methods: Locking the frequency of its transmitted signal clock to the long-term average of the incoming signal. (This is often referred to as loop timing.) Providing equal signal bit rate accuracy from another PRS. SONET Path Figure 3-25 Example of SONET Zero, one, or equipment more STEs associated with the NI. N-PTE N-STE N-LTE CI-LTE CI-PTE PTE = Path Terminator Equipment LTE = Line Terminating Equipment STE = Section Terminating Equipment Traditional SONET Architectures Figure 3-26 Section overhead. Frame N Frame N+1 125 microseconds SPE boundary SOH & LOH 250 microseconds Overhead byte Function Usage Coding Section overhead A1 framing Frame alignment 11110110 A2 framing Frame alignment 00101000 B1 section BIP-8 Section error monitoring BIP-8 D1-D3 section data Section data communications channel communication channel (Data E1 orderwire Orderwire F1 section user Section user channel The F1 byte is defined only for channel STS-1 number 1 of an STS-N signal. JO section trace Section trace Defined only for STS-1 number one in an STS-N signal. When the Section Trace function is not supported or if no value has been programmed, then 01 Hex is transmitted. Z0 section growth Section growth One byte is defined in each STS-1 for future growth except for STS-1 number 1 (which is defined as J0). R = required; A = application-specific function; O = optional When in a maintenance state (that is, a synchronization failure condi- tion), the timing of the signals from the network may lack traceability to a PRS. In that case, the signals at all interface points heading in the direction from the network to the CI need to be within +20 ppm of the nominal rate specified for each interface. Also, when in a maintenance state, the timing Chapter 3 Figure 3-27 Path overhead. Frame N Frame N+1 125 microseconds SPE boundary SOH & 250 microseconds Overhead byte Function Usage Coding STS 