yer elements by identifying them with different access-point identifiers in messages. A routing- and switching-layer element can make use of more than one layer interface that offers connectivity services. The process of transferring a message that has arrived over one used layer interface to another is commonly called relaying. If an analysis of a network address in the message is needed, it is com- monly called routing analysis and the relaying activity switching. When a layer element of a switching layer receives a message that refers its own network address, it terminates the message. In network system standards, a protocol specification is normally associated with a single layer. According to our understanding of layer interface, if messages of the protocol contains an access-point identifier, those layer elements that are referred to by that identifier belong to a layer other than the one defined by the pro- tocol. However, if we analyze existing ITU-T protocols with respect to access- point identifier, we find that it is more the rule than an exception that a protocol specification comprises several layers. One example is the LAPD protocol, which is described in a single protocol specification. Figure 2.5 shows the interlayer structure that this standard reveals. From the AMLn point of view, the LAPD protocol specification does not describe a single layer, but a stratum that comprises two separate layers (L1 and L2 in the model). The reason is that the LAPD-transport layer performs discrimination among 64 possible layer interfaces, identified by an access-point identifier called SAPI. The protocols of these layers depend only on the properties of the service in the layer interface, i.e., they are independent of how the LAPD transport protocol looks. 2.1 Concepts Based on the OSI RM LAPD users Management system MEI=0..14.16..255 MEI agent layer Layer- management TEI assignment MEI=15 discrimination) layer SAPI=0..62 SAPI=63 LAPD transport (SAPI discrimination) Message protocol Abstract protocol BRA PRA Fig.