Intelligent Network

The Intelligent Network technology is at the heart of PSTN service provision. The whole range of convergence products and services (such as Internet call-waiting, click-to-dial, and universal mailbox, just to mention a few) rely on and expand this technology.

The Intelligent Network standards are published by ITU in the Q.12xy series. The IN architecture, in accordance with open distributed processing (ODP) principles, is viewed in terms of planes: the service plane is concerned only with the service description in terms of service features; the global functional plane deals with the service-independent building blocks (SIBs); the distributed functional plane addresses the elements of the architecture involved in the IN message exchange in terms of functional entities (FEs) (that is, the objects that are not associated with any box) and information flows (IFs), which model the message exchange among FEs; and the physical plane defines the actual boxes, called physical entities (PEs), and maps the FEs to PEs.


As with most other ITU-T standards, the IN Recommendations are being adopted by regional standards bodies for use in their respective countries; however, we address only the ITU-T standards. You can find a detailed description of the IN standards up to CS-2 in Faynberg et al. (1997), but the text of the ITU-T Recommendations and their regional counterparts is, naturally, the ultimate reference.


Although the IN standards have provided in both IN CS-1 and CS-2 an effective model for service creation by specifying the so-called service-independent building blocks (SIBs),[16] the standardization of SIBs stopped after CS-2. Because standardization efforts ended, and the existing SIBs were not designed for interworking the PSTN and the Internet, we do not cover them.


The functional architecture and the mechanism for triggering the interactions between its elements are essential topics related to capability sets, addressed in the sections that follow. Figure 1 describes a subset of the presently standardized FEs and their interconnections.

Figure 1: IN capability set 1 (CS-1) functional architecture

Source: ITU-T Recommendation Q.1211

The IN FEs are grouped according to their role in supporting IN: FEs involved in service execution and FEs involved in service creation and management.


The service execution FEs are:

• Call control agent function (CCAF). Provides user access capabilities. It may be viewed as a proxy for a telephone (or ISDN terminal) through which a user interacts with the network.

• Call control function (CCF). Provides the basic switching capabilities available in any (IN or non-IN) switching system. These include the capabilities to establish, manipulate, and release calls and connections. It is the CCF that provides the trigger capabilities; however, another object called the service switching function (SSF) is needed to support the recognition of triggers as well as interactions with the service control. The SSF and CCF are supposed to be colocated (that is, they cannot be placed in different PEs).

• Service control function (SCF). Executes service logic. It provides capabilities to influence call processing by requesting the SSF/CCF and other service execution FEs to perform specified actions. Implicitly, the SCF provides mechanisms for introducing new services and service features independent of switching systems. It is therefore the function that interworks (via applicable gateways) with the IP hosts in support of joint service control. Two main principles of both CS-1 and CS-2 standards are single-endedness (that is, the service logic is aware of only one relation with the SSF/CCF for the purpose of a given terminating or originating call process) and single point of control (that is, only one instance of service logic may be in contact with the SSF/CCF for the purpose of a given terminating or originating call process).

• Specialized resource function (SRF). Provides a set of real-time capabilities, which Recommendation Q.1204 calls specialized. These capabilities include playing announcements and collecting user input [either dual-tone-multi-frequency (DTMF) or voice, depending on the facilities]. The SRF is also responsible for conference bridging, fax support, and certain types of protocol conversion as well as text-to-speech (and vice versa) conversion. The SRF is crucial to supporting services like click-to-fax. To this end, the SRF also interworks with IP hosts, although—unlike the SCF—it supports the delivery of a service rather than the control of it.

• Service data function (SDF). Provides generic database capabilities to either the SCF or another SDF.

The following three service creation and management FEs are defined in ITU-T Recommendation Q.1204:

• Service creation environment function (SCEF). Responsible for developing (programming) and testing service logic, which is then sent to the service management function (SMF).

• Service management function (SMF). Deploys the service logic (originally developed within the SCEF) to the service execution FEs, and otherwise administers these FEs by supplying user-defined parameters for customization of the service and collecting from them the billing information and service execution statistics.

• Service management agent function (SMAF). Acts as a computer terminal that provides the user interface to the SMF.

These entities serve to complete the architecture and reflect the industry development. No associated protocols have been defined by ITU-T.


As you may recall, the fundamental idea of IN is to open the basic switching process (run by switches) to external influence. This is done by defining and standardizing the call model.

Figure 2 depicts the CS-2 Basic Call State Model (BCSM), which is the latest released standard. BCSM models both the originating and terminating basic call processes, which are depicted in Figures 2 and 3, respectively. In both processes, the primary states of calls as seen by a switch, depicted within rectangles are termed points in call (PICs). In addition, certain transitions lead to other states called detection points (DPs). It is at DPs that the switch may interrupt its processing by sending a message to the SCF. Each DP may be either armed or unarmed. As far as IN is concerned, being armed is the first essential prerequisite for being active, for only when a DP is armed is the external service logic (within the SCF) informed that the DP has been encountered.

Figure 3: Originating CS-2 BCSM.

Source: ITU-T Recommendation Q.1224

A DP may be armed either statically (from the SMF, as the result of the service feature provisioning) or dynamically (by the SCF). If it is statically armed, the DP remains armed until the SMF disarms it—as long as the service that needs it is to be offered; if it is dynamically armed, the DP will remain armed for no longer than the duration of a particular SCF-to-SSF relationship. A statically armed DP is called a trigger detection point (TDP); a dynamically armed DP is called an event detection point (EDP). The DP nomenclature is illustrated in Figure 5.

Figure 4: Terminating CS-2 BCSM.

Source: ITU-T Recommendation Q.1224

Figure 5: Detection point processing.

Source: ITU-T Recommendation Q.1214

The DP nomenclature is essential for understanding how IN can interacts with the IP networks. When a service originates from the Internet, the DPs in the PSTN may be armed only dynamically. When, on the other hand, a service originates from the PSTN, all DPs that can invoke it must be armed beforehand (but other DPs can be armed dynamically in the process of service delivery). These cases are respectively addressed by the IETF PSTN/ Internet INTernetworking (pint) working group (www.ietf.org/html.charters/ pint-charter.html ) and the Service in the PSTN/IN Requesting Internet Service (spirits) working group (www.ietf.org/html.charters/spirits-charter.html ) in cooperation with ITU-T SG 11.


Figure 5 depicts the distribution of the IN functional entities among the subset of physical entities in CS-1. (For purposes of our discussion, we have reduced this subset to the bare minimum.) The physical entities are:

• Service switching point (SSP). A switch that provides access to IN capabilities.

• Service control point (SCP). A general-purpose computer that has access to the SS No. 7 network for communicating with SSPs and IPs.

• Service data point (SDP). Contains only the SDF. The SCP can access data in an SDP either directly or through a signaling network.

• Intelligent peripheral (IP). Its function is primarily to support the SRF. However, it may also include the SSF/CCF to provide external access to resources.

• Adjunct (AD). Functionally equivalent to an SCP, but connected to a single switch via a high-speed network, not the SS No. 7 network.

• Service node (SN). Similar to an AD, but in addition to performing a role of an SCP, it can perform the role of an IP. The SN connects to switches via the ISDN interface. (As you will see in Part Three, present implementations often include the SCP and small SSP as part of SN offers; however, those SCPs and SSPs act independently rather than as part of the group of standard SN functions.

Figure 6: IN CS-1 physical architecture (after Q.1215).

Source: ITU-T Recommendation Q.1215

The Intelligent Network Application Part (INAP) protocol is defined as a TC user.