 The Design Model Layer 5.6.3 Methodology: The Realization Layer Using the Methodology to Change the Existing Service 5.7.1 Changing the Existing Service by Specialization 5.7.2 Changing the Initial Specification by Adding Authentication 5.8 Discussion and Related Work 5.9 Conclusion References 5.1 Introduction During the past two decades, the telecommunications environment has evolved from centralized dedicated telephony networks to a distributed environment with a vast number of service providers, third-party vendors, and a range of network tech- nologies. In addition to the traditional telephony services, there is now a range of different services, Internet-based services and Web Services, with a strong drive for convergence, to deliver all services over a single IP-based core network. In this evolving, convergent environment, there is a need for rapid development and deployment of services. As the service environment changes, the services are also required to be adaptable. This is because, of what makes sense at the time of deploy- ment, many would not be valid after some years of use. For example, for a multime- dia over IP (MMoIP) service, in order for the provider to be able to ascertain the identity of and the number of users accessing a service at a specific time, some level of authentication may be required. Initially, the operator may for instance choose a username password-based solution, as this seems to make sense at the time of deploy- ment. However, a period of usage may demonstrate that this authentication is too weak, and with a significant number of unauthorized accesses where a large number of calls are made with the end user not known or incorrectly identified. As a result, there may be widespread fraud and misuse of the service, with a need to upgrade the service with stronger authentication to counter these threats. In order to avoid dis- rupting the service, ideally this upgrade should be deployed dynamically. Indeed it may be desirable for the service to be monitored by a management syste