# Fusion of Telecommunications and IT Services Boosted by Application Programming Interfaces
## 2 Developing Telecommunication Services--From INAP to CAMEL  
In legacy PSTN networks, digitalization of telephone exchanges opened the possibility of implementing various telephone call-related services. Originally, call-related services were integrated into the code of the digital exchanges, i.e., in the stored program itself. In the first 2G and 3G systems, the Mobile Switching Center (MSC) played the role of the digital telephone exchange. With the introduction of Release 4 networks, the MSS (MSC Server) took over the function of the MSC in combined 2G/3G networks (Figure 1). Similarly to the fixed networks, at the beginning of mobile telephony, call-related services were implemented by the switches too. These services were hardcoded to the business logic (BL) of the 2G/3G switches across the entire network. With the arrival of Intelligent Networks, services have been centralized to a dedicated platform [36; 37; 38; 39]. The service logic (SL) was decoupled from the business logic of the telephone exchange into an external entity: the Service Control Function (SCF). As shown in Figure 2, the SCF had access to the Service Data Function (SDF), a database that could be managed by the Service Management Function (SMF). The telephone exchanges and Intelligent Networks were communicating with each other using the SS7 (Signaling System No. 7) stack. The telephone exchange can report certain call events (e.g., call initiations, call answers, and call disconnects) to the SCF platform. So, SCF can instruct the switch on the outcome of the call, e.g., continue, release, or redirect a call. SCF can initiate user interaction, send additional charging-related data, and can also control the call duration for prepaid subscribers. Typical telecommunication service use cases include, for example, the following:  
* Prepaid services, where payment precedes the use of the service. The purchased credit is used to pay for communications services when the service is used. When the credit is almost fully consumed, then the subscriber is warned by a tone within the call, and when it is fully consumed, further access is denied by the cellular network or by the Intelligent Network;
* Interactive voice response menus;
* Sequential alerting of devices of a small office;
* Private numbering plans for enterprises. In the application layer of the interface between the switch and the SCF, two main alternatives have become widespread. The Intelligent Networks Application Part (INAP) has been standardized for fixed telephony and later adapted also for mobile networks [36, 37, 38, 39]. To avoid incompatibility of different vendor-specific implementations and to address roaming and mobility, the ETSI (European Telecommunications Standards Institute) and 3 GPP (3 rd Generation Partnership Project) specified a new service invocation interface for mobile networks. The Customized Applications for Mobile networks Enhanced Logic (CAMEL) became standardized by 3 GPP and ETSI. The CAMEL Application Part (CAP) has evolved from the INAP to meet the needs of mobile networks and extended the IN framework to GSM and 3G networks, as shown in Figure 3.  
Figure 3: 2G/3G mobile service logic over CAMEL with decoupled business and service logic [36]. Figure 2: General IN architecture with decoupled business and service logic [36]. The CAMEL framework provided tools for Mobile Network Operators (MNOs) to define additional features for standard GSM/UMTS (Universal Mobile Telecommunications System) services. The protocols are described in a series of ETSI technical specifications: CAMEL specifications have been published in four consecutive phases [40, 41, 42, 43, 44]. Each new phase builds upon the capabilities of the previous one. The standards have been issued first for GSM NSS (Network Switching Subsystems). In fact, the first two phases were defined before 3G networks existed. The standards added IN services to GSM networks, but they were applicable to 2.5G (2G with Enhanced Data rates for GSM Evolution, EDGE) and 3G networks later too.