S, and performs the association procedure (over the broadcast channel TEI = 127 on the actual LAPD route). Once the association is estab- lished, the LE agent uses the tables discussed previously for relaying informa- tion between the LE actor and a particular TE actor. If we take a look at the DSS1 L3 protocol specification, we do find several parameters in the SETUP that are ISDN/PSTN network addresses, however. One is called "calling party subscriber number" and the other "called party subscriber number". It is important to understand that these have nothing to do with establish- ing associations between actors in TEs and LEs, but are parameters of the abstract protocol of the DSS1 L3 LSM: The calling party subscriber number is used by a TE user to see who is calling (sometimes referred to as the service "A-number display"). The called party subscriber number is used to support interworking between the operator network and an ES that acts as a gateway (a PABX) of a local sub- scriber network. Hundreds of TEs may exist in this network, each identified by a separate ISDN/PSTN network address. It is the responsibility of the ES to route calls inside this network. B3 Operations in SAC A SAC ("specified actor interface") defines the operations and primitives that are used over actor-agent interfaces (AAI) in connectivity layers, i.e., a layers that transport data between layer interfaces. The abstract primitives of these operations are not meta-primitives since a connectivity layer has no LSM to support. The interface symbol used for SACs therefore looks different to SAG symbols. SAC operations are defined as part of an actor specification. Connectivity layers either realize link routes (such as LAPD), or perform multi- plexing, cross-connecting, or switching (different types of switching included). The actors are either relaying only, terminating only, or both. An actor that is relay- ing is called common actor (i.e., connects several agents). A SAC defines two conceptual operations: relay and terminate, with the 