d important properties for the service. It comprises a) the set of interfaces that describe the interactions that take place between the various entities responsible for performing a particular task, b) service level specifications (SLSs) between the entities and c) behavioural specifications between entities. Thus, reference points are a prescriptive part of the specifications which can be used to test an implementation's conformance. A reference point is needed to enable the conforming entities to evolve independently of each other. Figure 15.7 shows a reference point and an interface example. It also contains SLS and behavioural specifications. This example covers QoS, security, signalling, mobility management, and network management. Interconnection l3 Network management interface Service level specification and l3 Policy management interface behavioural specifications Reference point l3 Mobility management interface l1 Call control signalling interface 12 Authentication, authorization and accounting interface Figure 15.7 Example of properties in a reference point 15.5 SERVICE LEVEL AGREEMENTS In this paragraph the expressions SLA and (Service) interconnection agreement are used. SLA has a wider interpretation. Let us start with the interconnection agreement, which is more of a peer-to-peer agreement. See Figure 15.8. An interconnection agreement should contain at least the following: Commercial criteria: Make sure you know how interconnection impacts your business. Service criteria: Make sure that the interconnection agreement includes agreement on service provision and QoS in order to support your business objectives. Technical criteria: Select your points of interconnection to suit your business idea in the best possible way. In a real interconnection situation, the two interconnecting operators may have to include both national and sometimes regional (e.g. EU) laws and regulations into the agreement. Commercial criteria Technical criteria Service criteria Figure 15.8 Main parts of a service interconnectio