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1 Overview

The IN concepts, architecture and protocols were developed originally as standards by the ITU-T which is the standardization commitee of the International Telecommunication Union. The aim of the IN was to enhance the core telephony services offered by traditional telecommunications networks, which usually amounted to making and receiving voice calls, sometimes with call divert. This would then provide a way for operators to build services in addition to those already present on a standard telephone exchange. Examples of the kind of services which could be offered are:

• Toll free calls
• Prepaid calling
• Private number plans
• Call screening

A complete description of the IN emerged in a set of ITU-T standards named Q.1210 to Q.1219, or Capability Set One (CS-1) as they became known. The standards defined a complete architecture including the architectural view, state machines, physical implementation and protocols. They were universally embraced by telecom suppliers and operators, although many variants were derived for use in different parts of the world (see Variants below).

Following the success of CS-1, further enhancements followed in the form of CS-2. Although the standards were completed, they were not as widely implemented as CS-1, partly because of the increasing power of the variants, but also partly because they addressed issues which pushed traditional telephone exchanges to their limits.

2 Key Concepts

The main concepts surrounding IN services or architecture are:

• Service Switching Point (SSP) This is co-located with the telephone exchange itself, and acts as the trigger point for further services to be invoked during a call. The SSP implements the Basic Call State Machine (BCSM) which is a Finite state machine that represents an abstract view of a call from beginning to end (off hook; dialling; answer; no answer; busy; hang up etc). As each state is traversed, the exchange encounters Detection Points (DPs) at which the SSP may invoke a query to the SCP to wait for further instructions on how to proceed. This query is usually called a trigger. The exact criteria used to determine whether or not a call triggers are determined by the operator, but typically might be based on the subscriber calling or the dialled number.

• Service Control Point (SCP) This is a separate set of platforms that receive queries from the SSP. The SCP contains service logic which implements the behaviour desired by the operator. During service logic processing, additional data required to process the call may be obtained from the SDF. The logic on the SCP is created using the SCE.

• Service Data Function (SDF) This is a database that contains additional subscriber data, or other data required to process a call. For example, the subscribers prepaid credit which is remaining may be an item stored in the SDF to be queried in real time during the call. The SDF may be a separate platform, or is sometimes co-located with the SCP.

• Service Creation Environment (SCE) This is the development environment used to create the services present on the SCP. Although the standards permit any type of environment, it is fairly rare to see low level languages like C used. Instead, proprietary graphical languages have been used to enable telecom engineers to create services directly.

• Specialized Resource Function (SRF) or Intelligent Peripheral (IP) This is a node which conects to both the SSP and the SCP and delivers additional special resources into the call, for example play voice announcements or collect DTMF tones from the user.