Signaling in telecommunication networks can be categorized into two classes: in-band signaling; and, out of band signaling. In-band signaling is also known as channel associated signaling (CAS). In this class, signaling messages are sent on the same physical channel that carries bearer traffic. Signaling System R1, Signaling System R2 and Signaling System C5 are examples of in-band signaling.
Out-of-band signaling is also known as common channel signaling (CCS). In this class, signaling messages are sent on a separate physical channel from the bearer traffic. CCS uses a common link to carry signaling information for a number of bearer channels or trunks. This form of signaling is cheaper, has faster connect times, and is more flexible than CAS. The two main signaling systems in this category are Signaling System 7 (SS7) and Primary Rate Interface (PRI). The SS7 network acts as a backbone for the Public Switched Telephone Network (PSTN) as it allows for efficient call setup and teardown. The SS7 network interconnects thousands of service providers under one common signaling network. SS7 networks are also used to interconnect cellular Mobile Switching Centers (MSCs).
An SS7 network contains Signaling End Points (SEP) that send and receive SS7 messages and signaling links. There are three kinds of signaling end points: a Service Switching Point (SSP) or Central Office Switch, a Signal Transfer Point (STP) and a Service Control Point (SCP). An SSP is a voice switch that processes and switches voice-band traffic. An STP is responsible for transferring SS7 messages between SSPs or between SSPs and SCPs. An SCP acts as an interface between a telecommunication database and the SS7 networks. Each signaling end point in an SS7 network is identified by a unique numeric point code. All SS7 messages transmitted on the SS7 network carry origination point codes (OPC) and destination point codes (DPC) for routing purposes.
ISDN User Part (ISUP) and Telephone User Part (TUP) are protocols residing at Level 4 of the SS7 stack. ISUP includes signaling messages and procedures for controlling inter-exchange calls between two analog subscribers, two ISDN users, and an ISDN user and an analog subscriber. TUP, a predecessor of ISUP, provides similar functions but only between two analog subscribers. The separation of signaling and bearer channels creates the need for associating two entities. ISUP and TUP use 16-bit circuit identification code (CIC) to identify each bearer channel. When ISUP messages related to bearer channel signaling are sent between two switches, they always include the pertaining CIC. Since bearer channels are bi-directional, each end of the switch must associate the same CIC with the same physical bearer channel. A service provider controls the association pairing a CIC with an individual bearer channel. The service provider assigns a CIC value to an individual bearer channel during network planning and provisioning according to applicable predetermined rules. There is no standard procedure that is followed for CIC association. However, the association must be unique for each DPC. A CIC value can be used again within the same SSP as long as it is not duplicated for the same DPC.
Technical specifications for SS7 are published by the International Telecommunication Union (ITU). Specific documents related to this disclosure include:
ITU Q.35 Technical characteristics of tones for the telephone service,
ITU Q.761 Signaling system No. 7—ISDN user part functional description,
ITU Q.762 Signaling System No. 7—ISDN User Part general functions of messages and signals,
ITU Q.763 Signaling System No. 7—ISDN user part formats and codes,
ITU Q.764 Signaling System No. 7—ISDN user part signaling procedures,
ITU Q.784.1 ISUP Basic Call Test Specification,
ITU Q.723 TUP—Formats and codes, and
ITU Q.724 TUP—Signaling procedures.
A third party monitoring an SS7 link may not know in advance which channels within a link contain signaling links, and which channels contain bearer channels. Further, the third party may not know in advance the mapping used to associate individual signaling messages on a signal link with specific bearer channels. As discussed above, ISUP and TUP messages contain a CIC code. Each CIC code is statically mapped to a bearer channel or time slot on a specific link. Once the third party has identified a pairing between a CIC with a bearer channel, the third party can associate signaling messages over the signaling link with activity on the paired bearer channel. Therefore, there is a need by a third party monitoring traffic on an SS7 link to associate CIC codes with bearer channels.