Link- and transmission Link- and transmission- layer element layer element Fig. 2.8 Structuring OSI-layer elements depends on the type of connectivity service. APi=access- point identifier; CEi=connection-endpoint identifier; Ci=connection identifier; CNL=connection- less; CON=connection-oriented dest...=destination ...; NA=network address; orig...=originat- ing...; P. .=protocols 10 In the telephony world, the expression "on-demand service" is often used when a user must order the connectivity service before he can use it. 2 Layer Structures The OSI RM network layer that offers CON services (to the left) terminates multiple channels, with each channel identified by a connection identifier (Ci) in the message. The network layer delivers the user data, together with a (local) con- nection-endpoint identifier (CEi) value, to the layer element, which may use this fea- ture to distribute different streams to instances of the layer element (or many streams to the same instance). There must therefore exist a special element that interfaces the CON server and performs discrimination by CEi. In case CEis are established on-demand, this element can only support instances, i.e., elements that operate according to the same protocol (P1 in the figure). Note that the discrimina- tion element need not handle any protocol for being able to perform its task, which creates a semantic contradiction: if the interface between the discrimination ele- ment and the OSI CON layer is a layer interface, the discrimination element cannot be part of the layer (by OSI RM definition) since it handles no part of the using layer's protocol. In AMLn, this contradiction is resolved by the actor-agent separa- tion (see Fig. 2.47). The OSI RM network layer that offers CNL services (to the right in Fig. 2.8) performs no discrimination. It terminates a single stream of messages, where each message includes a destination network address (destNA) and an originating net- work address (origNA). Discrimination of using layer elements must therefore rely o