ition (also emphasized by the ISDN plane concept and the symbols we have defined) in no way restricts the "layer" concept to layers that offer connectivity only. For example, layer L2 in Fig. 2.3 does not offer any connectivity services. The use of our specialized point sym- bols helps in identifying the type of layer. An important feature of many layers is that they support layer interfaces to more than one using layer and use layer interfaces of more than one layer, as depicted in Fig. 2.4. This structure defines six layers. Network Tables that support address translation between access-point identifiers and local addresses in nodes Access-point identifier T-ROs T-AP/LI L1&L2 Fig. 2.4 The access point. X,Y,Z=network addresses in layer L3;a,b=network addresses in layer L1; a,B,x=network addresses in layer L2; T-ROs=routing tables This creates layer structures that are far more complex and less hierarchical than the impression given by the OSI RM layer structure. Note that the layer dimension (L) is not linear, since L4 through L6 are layers on the same "level," and SO are L1 and L2. When a layer provides multiple layer interfaces by the same layer element (e.g., X and Z in Fig. 2.4), messages will include an access-point identifier (APi) that refers a using layer element. The terminating layer element will translate the APi value to a local address that identifies the using layer element. In this regard, the Note that there is no consensus in existing standards on this term: OSI RM and LAPD call it "service access point identifier" (SAPI), IP calls it "protocol number," TCP calls it "port number," MTP calls it "service indicator" (SI), SCCP calls it "subsystem number" (SSN), etc. 2.1 Concepts Based on the OSI RM terminating element must manage tables (T-AP/LI) that translate APi values to local addresses. The scenario in Fig. 2.4 tells the following story: A layer element A of layer L5 sends a message to layer element B. This part of the scenario is described in the protocol specification of L5. In reality, A us