2.3.3.3 Discrimination In Connectivity Layers We will end this chapter by discussing some other discrimination problems in con- nectivity layers. In an AMLn model, the actor is normally the element that handles users' layer interfaces (if any), while agents always handle used connectivity inter- faces (CSPs) (see left of Fig. 2.48). However, in connectivity layers, users' layer interfaces may be distributed to both the actor and the agent, as we have seen in the ISDN model of Fig. 2.43. Figure 2.48 indicates this in the model to the right. Users' layer interfaces, separated by e.g., APi Distributed layer interfaces Layer element {APi,..} {ITi, APi, } Actor Agent Actor Agent Used layer interface Fig. 2.48 Distribution of users' layer interfaces This creates an additional discrimination problem: it requires an agent to be able to distinguish between layer interfaces and actor-agent interfaces, i.e., another application of the interface type identifier (ITi). The ITi parameter need only one bit and tells the receiving agent if the other discriminating parameter refers a layer 2 Layer Structures interface (in which case it is an APi) or an actor-agent interface (in which case it might be an ALi, an ASi, or an Li). In standards you will seldom find an explicitly-defined ITi since designers of network systems normally do not define any actor-agent separation. In case it hap- pens, the ITi is probably hidden in a complex type of parameter that defines the APi for both agent and actor, as well as other forms of discrimination. Such an example is the PTI parameter in the ATM switching layer (see Sect. 6.4.3). Sometimes no explicit discrimination parameters are needed because message parameters are bound to certain positions and of fixed length. Examples exist in the access part of the ISDN switching layer that was discussed before. Figure 2.49 shows how data in BRA frames are discriminated at termination points. Call handling BlorB2orB1+B2 Media resources A BRA frame 8 bits 1 bit 8 bits 1 bit 8 bits 1 bit 8 bits 1 bit act