3 agent layer is obviously not a common agent layer, i.e., it does not support multiple actor layers. Consequently, when a DSS1 L3 actor in a TE invokes its agent, there is no need for referencing any actor address (AA in AMLn). The call-handling actor in the LE can, however, communicate with all TE actors (it may have to deal with hundreds of sessions simultaneously). It must there- fore, in its invokes in the actor-agent interface, be able to identify individual ESs by some kind of actor address (only an actor suffix, As, since there is only one actor layer). Furthermore, since there might exist several TEs in an ES, and since any of these TEs can take a call offer, or initiate a call request, a particular call must be associated to a particular TE by some kind of association identifier (ASi in AMLn) as well. The DSS1 L3 protocol specification does not foresee any need to repeat an invoke before a previous one has been dealt with. Consequently, no invocation identifier is needed. Thus, in summary, all we need is an ASi parameter in the invokes. The mapping of the ASi on a DSS1 L3 message parameter is easily identified as the callRe ("call reference") parameter. An ASi value is, according to AMLn, established as a result of an association procedure. We deal with this issue for DSS1 L3 first in Sect. B2. Suffice here to accept that a callRef exists and is used in LE invokes of the call for addressing actors in TEs. According to the standard, a callRef is unique only within a particular ES. It is, however, reasonable to assume that the LE actor should not be concerned with parameters for identifying ESs, and therefore use an ASi that is unique within the whole DSS1 L3 layer. This requires the LE agent to translate a "global ASi value" that the LE actor knows to a "local ASi value" (i.e. a callRef) that an ES knows. Using some of the tables we have discussed for agent layers (see also Appendix C) we can define the DSS1 L3 agent in an LE as in the model below. DSS1 L3 agent ASik>callRef r-invOrd(ASi,...) Call- { cal