Circuit Identification Codes

One of the effects of moving call signaling from CAS to Common Channel Signaling (CCS) is that the signaling and voice are now traveling on two separate paths through the network. Before the introduction of SS7 signaling, the signaling and voice component of a call were always transported on the same physical facility. In the case of robbed-bit signaling, they are even transported on the same digital time slot of that facility.

The separation of signaling and voice create the need for a means of associating the two entities. ISUP uses a Circuit Identification Code (CIC) to identify each voice circuit. For example, each of the 24 channels of a T1 span (or 30 channels of an E1 span) has a CIC associated with it. When ISUP messages are sent between nodes, they always include the CIC to which they pertain. Otherwise, the receiving end would have no way to determine the circuit to which the incoming message should be applied. Because the CIC identifies a bearer circuit between two nodes, the node at each end of the trunk must define the same CIC for the same physical voice channel.

ITU defines a 12-bit CIC, allowing up to 4096 circuits to be defined. ANSI uses a larger CIC value of 14 bits, allowing for up to 16,384 circuits.

Figure 8-4 shows an ISUP message from SSP A that is routed through the STP to SSP B. For simplicity, only one STP is shown. In the message, CIC 100 identifies the physical circuit between SSP A and B to which the message applies. Administrative provisioning at each of the nodes associates each time slot of the digital trunk span with a CIC. As shown in the figure, Trunk 1, time slot (TS) 1 is defined at each SSP as CIC 100. Trunk 1, time slot 2 is defined as CIC 101, and so on.

DPC to CIC Association

Since each ISUP message is ultimately transported by MTP, an association must be created between the circuit and the SS7 network destination. This association is created through provisioning at the SSP, by linking a trunk group to a routeset or DPC.

The CIC must be unique to each DPC that the SSP defines. A CIC can be used again within the same SSP, as long as it is not duplicated for the same DPC. This means that you might see CIC 0 used many times throughout an SS7 network, and even multiple times at the same SSP. It is the combination of DPC and CIC that uniquely identifies the circuit. Figure 8-5 shows an example of three SSPs that are interconnected by ISUP trunks. SSP B uses the same CIC numbers for identifying trunks to SSP A and SSP C. For example, notice that it has two trunks using CIC 25 and two trunks using CIC 26. Since SSP A and SSP C are separate destinations, each with their own unique routeset defined at SSP B, the DPC/CIC combination still uniquely identifies each circuit. SSP B can, in fact, have many other duplicate CIC numbers associated with different DPCs.

Figure 8-5. Combination of DPC/CIC Provide Unique Circuit ID

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Unidentified Circuit Codes

When a message is received with a CIC that is not defined at the receiving node, an Unequipped Circuit Code (UCIC) message is sent in response. The UCIC message's CIC field contains the unidentified code. The UCIC message is used only in national networks.