on. 5 Protocol Network Modelling 5.3.2 Discrimination Levels Figure 5.13 depicts the discrimination model. Logical node Reference point protocol SE2 protocol SE1 protocol Level 2 discriminator Discrimination userData protocol Discrimination userData Level 1 discriminator protocol (optional) SE Service element SAH SAP handler Fig. 5.13 The discrimination model; discrimination level terminology The model says that a logical node must implement the interface to the CSP. This function is modelled as a SAP handler (SAH). The SAH divides the logical node into the SAH and a service element (SE0) that is defined by the reference point protocol. The SAH handles the interface between the logical node and the transport-service providing CSP. The SAH handles no protocol, i.e. it does not exist in the D-dimension. The service element of the logical node can be further partitioned into a level 1 discrimination function and a number of service elements that are called level 1 service elements (SE1). The protocol that defines the level 1 discriminator han- dles messages that consist of a level 1 discrimination identifier (Did), and userData. The discrimination parameter identifies to which service element the userData belongs. The discrimination protocol is optional, since there are alter- natives other than specifying a discrimination protocol to provide discrimination on level 1. Each SE1 element can be further partitioned into a level 2 discriminator func- tion and a number of elements that are called level 2 service elements (SE2). The protocol structure of Fig. 5.13 implies that whether or not an SE1 is parti- tioned into SE2s can vary between different SE1 types in the same logical node. In that case, we have an inhomogenous reference point protocol. A reference point protocol often shows a two-level structure, consisting of an integrator that provides both level 1 and level 2 discrimination, and a flat structure of service elements. Whether a service element represents an SE1 or an SE2 depends on to which of these service