IMS was invented as a platform to provide value-added services over mobile networks and enable mobile network operators to achieve ‘all-IP’ communications. IMS has also come to be widely used in fixed networks as a unified architecture that eases heterogeneous terminals’ access to IP voice and multimedia applications.
Despite the benefits of IMS, its implementation in fixed networks is a complex undertaking, for reasons including:
- Different regulations: Fixed network operators are required to comply with certain national/regional regulations that include stipulations based on legacy technologies, e.g. the obligation to provide a minimum set of services in line with the concept of “universal service”. When implementing such services using the IMS platform, these services place additional requirements on existing IMS standards.
- Different IMS implementations: For interconnection between two operators, service limitations may arise when establishing a phone call from one operator to another. For example, some IMS implementations do not support the P-Early Media Header and the PSTN XML schema. In this regard, ISDN services cannot be provided between interconnected operators due to limitations of the transmission of ISDN Specific parameters. As a result, there are some restrictions of rendering voice announcements and double ringing tones.
Fixed network operators must therefore tune their IMS platform to such requirements and upload necessary updates (patches) into the embedded IMS software. This is a very costly, time-consuming process, which must be performed by every operator implementing the IMS platform.