f existing IT applications by creating services that can be re-used across the enterprise or the telecom network. Such architecture thus minimizes the need for extensive rewriting of code each time a new business or telecom service is required or each time a business process is altered. Figure A1.1 illustrates a process flow which partly uses sub-processes from an external source, called web service provider. The principle can for example be called: Software as a service Services on demand Service enablers on demand Business processes as a service (more advanced development) A web service is a software application identified by a uniform resource identifier (URI), whose interfaces and binding are capable of being defined, described and discovered by XML artefacts and which supports direct interactions with other software applications using XML-based messages via Internet-based protocols. Web services make software/services/service enablers/business processes accessible to any service provider to support activities in a process. Web services are the tools that enable businesses to quickly and effectively integrate internal systems and share information and logic across the enterprise. A client is software that makes use of a web service, acting as its 'user' or 'customer'. A message is the basic unit of communication between a web service and a client; data to be communicated to or from a web service as a single logical transmission. In order to make use of a web service, it is necessary to know what information is expected and in what form. Therefore, the interface needs to be explained for use by remote parties. The structure of the messages accepted and/or generated needs to be described. An example can be seen in Figure A1.2. A number of services are defined in the web and by the different service providers and enterprises. For a specific application own services as well as services belonging to service providers may be used. Those services may use other services in other domains. For example a billing service