cal orderwire chan- nel for voice communications between MS terminating equipment. Alarm signals: Alarm information is included as part of the MSOH. These func- tions are explained in Section A.5. A.4 VC-4 POH Functions The POH is contained within the VC portion of the STM-1 frame. The POH data of the VC-4 occupies all the 9 bytes of the first column. The functions of the various bytes carried in the VC-4 POH are described hereafter. Path trace message (J1 byte): The J1 byte is used to repetitively transmit a 64-byte string (message). The message is transmitted 1 byte per VC-4 frame. A unique message is assigned to each path in an SDH network. Therefore, the path trace message can be used to check continuity between any location on a transmission path and the path source. Parity check (B3 byte): A BIP-8 even checksum, used for error performance monitor- ing on the path, is calculated over all the bits of the previous VC-4. The computed value is placed in the B3 byte. Signal label (C2 byte): The signal label byte, C2, indicates the structure of the VC-4 container. The signal label can assume 256 values; however two of these values are of particular importance: 1. The all "O"s code represents the VC-4 unequipped state (i.e., the VC-4 does not carry any tributary signals) 2. The code "00000001" represents the VC-4 equipped state The G1 byte is used to send status and performance monitoring information from the receive side of the path terminating equipment to the path originating equip- ment. This allows the status and performance of a path to be monitored from either end, or at any point along the path. Multiframe indication (H4 byte): The H4 byte is used as a payload multiframe indica- tor to provide support for complex payload structures, for example, payload struc- tures carrying multiple tributary units (TUs-see Chapter 3, Section 3.2.6). If, for example, the TU overhead is distributed over four TU frames, these four frames form a TU multiframe structure. The H4 byte then indicates which frame of the TU multifr