n APi, as depicted. Since the network layer should not know anything about the layer structure above, there must exist a special element that interfaces the CNL server and performs discrimination by APi. Contrary to the CEi discriminator, this element must handle a protocol that transfers access-point identifiers between ter- minating layer elements. This is done by the transport-layer protocol in the OSI RM. Contrary to the CON case, the using layer elements can operate according to different protocols (P1 and P2 in the figure). Thus, these elements can be parts of different layers that can exist simultaneously in the network. Layers that use a layer offering discrimination by APi can use this service in two principle ways: 1. All layer elements of the same layer are associated with the same APi value. We say that such an APi is a layer APi. 2. Layer elements of the same layer can be associated to different APi-value. Con- sequently, such an APi is a layer-element APi. Layer APi: ayer-element APi: on APi=X on APi=X on APi=X on APi=Y destAPi=Y {destAPi=X, {destAPi=Y, origAPi=X, LE1->X X->LE2 LE1->X Y->LE2 Fig. 2.9 Two approaches of using access-point identifiers (APi). LE...=layer element; X,Y=access-point identifiers 2.2 Discrimination (Connectivity Layers Only) Figure 2.9 models implications of these approaches: In case of layer APi, the association between a particular layer element and the APi value (by which it is referred to in messages) is a value that is defined by the discriminating layer. This approach can be used in network systems where there are few using layers which all are part of the same network system. Examples of such discriminating layers are the IP layer, the SCCP layer (in SS7) and LAPD. Discriminating layers that support layer APi are recognized by the fact that they define APi values in their protocol specifications, and that the APi value-range is rather small. A benefit with this approach is that a using layer element need not refer the APi of the destination element when requesting the