 routing table in a switching layer8). The purpose of this analysis is to identify which layer (L1 or L2) to use for further transport, and (in case) which network address and access point of that layer to refer. In this case it selects L2 and orders its L2-layer ele- ment (a) to send the message data to network address X. By "local" we mean an address that exists in the (derived) node where the layer element runs. In such a node, a processing platform exists to support communication between adjacent layer ele- ments through local addresses. 8 In Chap. 3 and Appendix C, we define a number of AMLn tables that can be used for specifying network configuration properties. The T-ROs, T-AP and T-LI in Fig. 2.4 are such tables. 2 Layer Structures When Z receives the data from X, it interprets the message from the routing ele- ment. It first checks the network address. Since it is its own address, it knows that it must terminate the data, i.e., hand it over to one of its users. It reads the access-point identifier 2 and uses some address translation tables (T-AP/LI in Fig. 2.4) to identify the local address of B. It then sends the data to B, which ends this scenario. Let's summarize these findings: The internal structure of a layer is an intralayer structure, showing layer ele- ments, which protocols they handle, and which layer interfaces they provide and Layer elements of different layers communicate over layer interfaces. There are basically two types of interfaces: interfaces that offer some type of connectivity services (i.e., transporting data between two using layer elements) and interfaces that offer some other type of service (e.g., setting up connections, translating addresses, accessing information, controlling a network resource, etc.). Actual layer interfaces may be one or the other, or a mix of both. The functionality of a network system can be described as an interlayer struc- ture. Some layers are connectivity layers, others are control layers. A layer element can offer connectivity services to several la