Signal Label received from network. For VC-3: The SONET standard defines the C2 byte as the path signal label. The purpose of this byte is to communicate the payload type being encapsulated by the SONET Framing OverHead. It allows a single interface to transport multiple payload types simultaneously. For VC-12, VT-1.5: VT path overhead V5 byte allocates bits 5-7 for VT Path Signal Label, to indicate the content of the VT SPE. 000 - Unequipped 001 - Equipped - non specific payload 010 - Asynchronous Mapping 011 - Bit Synchronous Mapping (no longer used) 100 - Byte Synchronous Mapping (TU11 or TU12) 101 - Extended Signal Label for Virtual Concatenation Applications (FCD-155 Default) 110 - Test Signal (0.181) (ITU) 111 - VC AIS (ITU)