ce (based on 2048 kbit/s) for the Support of Access Network (AN) Virtual Circuit or Virtual Channel A channel in a connection-oriented packet switching network, such as X.25, TR and ATM Virtual Path An aggregation of VCs Wide Area Network X.25 NA X.25 Network Addresses ITU-T X.121, International Numbering Plan for Public Data Networks Appendix B: SAG and SAC Operations B1 Invoke Operations (in SAG) A SAG ("specified agent interface") defines the operations and primitives that are used over actor-agent interfaces (AAI) in control layers. The abstract primi- tives of these operations are meta-primitives since they carry information about other primitives (which are the primitives between the element state machines, ESM, of actors). SAG primitives are defined as part of an agent-layer specification. AAIs are local interfaces in layer elements. Neither these interfaces nor their meta-primitives are therefore visible on the network level. AAIs and meta-primi- tives are, however, often visible in implementors' solution models. The benefit of defining AAIs and SAGs is that a model of a layer can be separated into two mod- els: a layer protocol machine (LPM) and a layer state machine (LSM) with an unambiguous mapping between these machines. SAG primitives should therefore always be defined, whether or not the intention is to implement them. AMLn defines two meta-primitives: r-invokeOrder(...) and I-invokeReport(...). These primitives have identical sets of parameters. The sets of parameters differ somewhat between two types of agent layers: agent layers that offer no association handling, and agent layers that do. Different AAI/SAG symbols are used in models for these two types (see the figure below). Using-layer element Layer element Message j-invRep(...) Agent SAG without associations: protocol Actor r-invOrd(...) Used-layer Using-layer element element Layer element Message Association primitives Agent protocol SAG with associations: Actor I-invRep(...) r-invOrd(... Used-layer element In the following section we apply 