bility. The patient may register by communicating directly with the elec- tronic tele-consultation service desk, or if the patient is in need of special assistance to register, a human receptionist may be contacted. The receptionist then gets the registration information from the patient, and provides this information to the electronic tele-consultation service desk. We will use the tele-consultation service to exemplify the model-driven, policy-enabled approach. For the purpose of clarity in presentation, we will consider a simplified view of the service involving a doctor, a receptionist, a patient, and the electronic tele-consultation service desk. We will also use the tele-consultation service to illustrate the steps involved in the method- ology, in particular, explaining how to change the existing tele-consultation service by adding authentication to the composed service and by adding a choice between three specializations of the call collaboration used in the existing service. 5.4 Service Models this section, we introduce some basic terminology, exemplified using the tele- consultation service. Then we introduce the global PESM diagrams and composi- tion policies which are modeled at the service layer. 5.4.1 Collaborations and Roles A composite service is described at the service model layer using a UML 2.x col- laboration. Such an overview diagram describes the distributed parts involved in a service and which roles are to be played by the different service parts, providing a means of statically and structurally representing the binding of collaboration uses involved in a composed service to the actual service parts. Figure 5.2 shows the col- laboration overview diagram for the basic tele-consultation service. An elementary collaboration is a service collaboration with exactly two elemen- tary roles that collaborate on an interface to provide a service or a service feature [2]. An elementary role is the smallest modeling element. These are predefined behavioral specifications (elementary role state machin