 (MS) overhead-carried in overhead rows 5-9. Regenerator section (RS) overhead-carried in overhead rows 1-3. AU (administration unit) pointers-carried in overhead row 4. Path overhead (POH)-carried in the first column of a VC-4. The POH carried in the VC-4 is called high-order POH. This allocation is shown in Figure A.4. Appendix A: SDH/SONET Signaling 2430 bytes/frame Serial signal stream 155.52 Mbit/s STM-1 virtual container (VC-4) Section 9 rows overhead Container capacity = 150.34 Mbit/s Payload capacity = 149.76 Mbit/s 9 columns 260 columns 1 column 2430 bytes/frame X 8 bits/byte X 8000 frames/s = 155.52 Mbit/s FIGURE A.3 Specific dimensions and composition of the STM1 frame. Section overhead Path overhead Framing Framing Framing Framing Framing Framing Path trace Regenerator Order- section BIP-8 BIP-8 overhead (rows 1-3) Signal label AU pointers Pointer Pointer Pointer Pointer Pointer Path status (row 4) BIP-24 User channel Multiframe Multiplex section overhead (rows 5-9) Order- Bytes reserved for future use FIGURE A.4 Specific overhead allocations in the STM1 section overhead. Appendix A: SDH/SONET Signaling A.2 Regenerator Section Overhead The regenerator section overhead (RSOH) is terminated and renewed by every SDH equip- ment that has the role of regenerating the signal (e.g., line terminals, add-drop multiplexers, digital cross connects). The functions of the various bytes carried in the STM 1 RSOH are described as follows. Framing (A1, A2 bytes): The six framing bytes carry the framing pattern and are used to indicate the start of an STM 1 frame. Channel identifier (C1 byte): The C1 byte is used to identify STM-1 frames within a higher-level SDH frame (STM-N, where the standardized values of N are 4, 16, etc.). The byte carries the binary representation of the STM-1 frame number in the STM-N frame. Parity check (B1 byte): An 8-bit wide bit-interleaved parity (BIP-8) checksum is cal- culated over all the bits in the STM-1 frame to permit error monitoring over the RS. The computed even-parity checksum 