with the following meaning (read from right to left): Bits DCBA Meaning Signaling network management message Signaling network testing and maintenance Spare Telephone user part ISDN user part Data user part (call- and circuit-related message) Data user part (facility registration and cancellation) Remainder (8 sequences) Spare Signaling information field Service User information Routing information subfields First bit label octet transmitted 5 octets 1 octet Subservice Service field indicator 12 bits 14 bits 14 bits 4 bits 4 bits (standard telephone label structure) Figure 14.10 Signaling information field (SIF) preceded by the service information octet (SIO). The sequence runs from right to left with the least significant bit transmitted first. DPC, destination point code; OPC, originating point code; CIC, circuit identification code. CCITT SIGNALING SYSTEM NO. 7 The SIO directs the signaling message to the proper layer 4 entity, whether SCCP or user part. This is called message distribution. The subservice indicator contains the network bits C and D and two spare bits, A and B. The network indicator is used by signaling message-handling functions determining the relevant version of the user part. If the network indicator is set at 00 or 01, the two spare bits, coded 00, are available for possible future needs. If these two bits are coded 10 or 11, the two spare bits are for national use, such as message priority as an optional flow procedure. The network indicator provides discrimination between international and national usage (bits D and C). The routing label forms part of every signaling message: To select the proper signaling route To identify the particular transaction by the user part (the call) to which the mes- sage pertains The label format is shown in Figure 14.10. The DPC is the destination point code (14 bits) which indicates the signaling point for which the message is intended. The originating point code (OPC) indicates the source signaling point. The circuit identification code (CIC) indicates th