es the connectivity services of layer L3 to transport the message data to B. L3 is a layer that provides multiple layer interfaces at its layer ele- ments. A must therefore refer the layer interface to B (identified here by access- point identifier 2), when communicating with its L3 element (X). However, access-point identifiers, by definition, are not layer-wide addresses. They are only used by a layer element (in this scenario by Z) to discriminate between using layer elements, once the data has arrived at a terminating layer element. The same APi value may be handled in several or all L3 layer elements. In some network systems (e.g., in LAPD, "link access procedure on the D-chan- nel," part of ISDN), the same APi values are assigned to all using layer elements of the same layer, but this is not necessarily a rule. Thus, access-point identifiers are not enough to determine the receiving user layer element (B). The layer element (Z) that uses the APi for finding the actual layer interface must also be addressed by a network address (Z). Thus, A orders its L3 element (X) to send the message data to network address Z and access- point 2, which together constitute a unique identification of B. The scenario also indicates how layer L3 (a switching layer) operates: The L3 element X has no direct connection to Z. It has, however, access to a layer L1 that can transport data to different L1 network addresses, amongst them b, that is an L1-layer element used by an L3-routing-layer element. Thus X cre- ates an L3 message to the routing element. This message contains, besides the data from A, the L3 network address Z and the access-point identifier 2. X then orders its L1-layer element (a) to send this message data to address b. The L3 routing element is a layer element that uses multiple layer interfaces, each interface belonging to a separate layer (L1 and L2). The routing element reads the network address Z and performs a routing analysis, using routing tables (T-ROs in Fig. 2.4; the suffix "s" indicates that this is a