. The M1 byte is located in the third STS-1 in order of appearance in the byte interleaved STS-N frame. S1 synchronization Synchronization messaging Defined only for STS-1 number messaging one in an STS-N signal. Z1 growth Line growth One byte is defined in each STS-1 for future growth except for STS-1 number 1. Z2 growth Line growth In SONET signals at rates above STS-1 and below STS-192, one Z2 byte is defined in each STS-1 except the third STS-1 for future- growth interleaved STS-N frames. R = required; A = application-specific function; o = optional Chapter 3 Figure 3-29 VT overhead. Frame N Frame N+1 125 microseconds SPE boundary SOH & 250 microseconds Overhead byte Function Usage Coding VT path overhead J2 VT path trace VT path trace For further study. Path error monitoring, path signal Use as defined in ANSI T1.105. label, and remote defect monitoring Z6 VT path growth Growth Allocated for future as-yet- undefined purposes. Z7 VT path growth Growth and remote defect monitoring Use as defined in ANSI T1. 1155. VT line overhead M1 line quality Line quality One byte is allocated every superframe (16 kbit/s) for the line quality functions. Line quality functions include the VT1.5 line BIP-2, two-bit line REI, one-bit RDI-LV, one framing check bit, and two reserved bits. S1 synchronization Synchronization message One byte per superframe message (16 kbps) is allocated for a synchronization message function. Bits 5-8 of this byte will be allocated to carry the same four-bit synchronization message codes as defined for other SONET transport interfaces. Bit 2 is a framing check bit. Bits 1, 3, and 4 of this byte are reserved. R = required; o = optional Although the previous example represents minimum requirements on synchronization for SONET NIs, it does not address all of the complexities associated with network synchronization and interfaces. For further infor- Traditional SONET Architectures mation, refer to ANSI T1.105.09. Some of the aspects of SONET synchro- nization and timing that are dealt with in ANSI T1.