# Fusion of Telecommunications and IT Services Boosted by Application Programming Interfaces
## 4 Fusion of Telco and IT Services  
### Evolution Path of API Services in Telecommunications  
A brief overview of Application Programming Interface-based services is given in this section by reviewing several papers. The selected papers cover the evolution path of APIs over several years. In the same timeframe, the telecommunication landscape has been evolving too, starting from legacy 2G/3G to actual 4G/5G and future 6G mobile systems. In an early paper, dating back to 2001, the authors discussed the possibility of using Open APIs for mobile networks [51]. In those years, content distribution to mobile terminals became available through the increased data rates offered by EDGE and WCDMA (Wideband Code Division Multiple Access) technology [3]. One mobile network vendor (Ericsson) provided a platform that enabled OEM (Original Equipment Manufacturer) vendors to develop applications and graphical user interfaces on top of their platform. This way, mobile phone terminal manufacturers gained the possibility to differentiate their products from other vendors. In 2003, [PERSON] [52] provided an overview of the Parlay/OSA (Open Services Architecture) API, an open interface that was created by a consortium of 65 IT and telecom industry companies. Parlay/OSA APIs have been used to develop services for both mobile data and fixed networks. Furthermore, the APIs targeted services for the next-generation converged networks based on the Internet Protocol (IP), as expected at that time. Among others, the main advantages have been using technology-independent APIs built on open standards. Developers could use multiple programming languages (e.g., C, C++, or Java). New value-added services became available, which promised revenue for MNOs and CSPs. Various domains were targeted by the new services, e.g., mobile location, terminal capability, connection management, and charging. In 2008, it was shown by [53] how the convergence of traditional telephone networks with the Internet started benefiting from the convenient user interface and relatively big storage size of personal computers, which the usual telephone devices did not have at that time. SOAP- and REST (Representational State Transfer)-based APIs were shown in that paper for building call and messaging services in which callers could control their own calls. It was shown that REST is a good alternative to SOAP when developing Open APIs. Several services are detailed, such as third-party invitations for mini-conferencing, click to dial, call recording, short and voice messaging, and playing audio on demand during a call. [PERSON] and [PERSON] discussed already in 2011 how cloud computing and IT created new possibilities towards 'Open Telco' by changing the legacy telecommunication landscape [12]. Their paper underlined the importance of Open APIs in this evolution. Two steps are discussed: how operators can consider Open Telco and Networking as a Service (NaaS) concepts. On the one hand, network operators could use Open APIs to provide network assets to developers for accelerating application development and creating a new application ecosystem. On the other hand, operators could start to use all kinds of cloud computing services in their own infrastructure, including Infrastructure (IaaS), Platform (PaaS) and Software as a Service (SaaS). The IaaS layer offers networking, computation, and storage services provided by the cloud. The PaaS layer has close links to Business and Operations Support Systems (BSS/OSS). The SaaS layer matches the service delivery platforms. In another paper dating back to 2011, [PERSON] et al. [34] concluded that the opening and sharing of Internet and telecom services was a must to satisfy subscribers. The authors stated that according to the long tail theory, 20% of the telecom services satisfied 80% of the subscribers. However, subscribers' demands were not fully aligned with the operators' services. To overcome these limitations, IMS could provide multimedia services over Internet Protocol (using REST APIs or RPC-based methods, e.g., SOAP).