ontrary to the common agent case (where the agent defines these primitives). An example of a common actor (a multiplexing actor) is shown in Fig. 2.39. This is a partial model of the V5 multiplexing standard. A multiplexing layer element Packet terminateOrder(..) handler Packer {Li,...} Packer actor handling handling relayOrder(..) agent agent AAlik>Li 64 kbps multiplexing layer Fig. 2.39 Agents and a common actor in a multiplexing layer element. SAC=specified actor inter- face; Li=link identifier This common actor connects many identical agents to the left and a single agent to the right. Its role is to multiplex LAPD frames that arrive on different BRA links (to the left) on a single 64-kbps link to the right (and the other way around). The protocol between multiplexing layer elements will therefore include a link identi- fier (Li) that identifies which of the BRA links is the source or the sink for user- data. The actor must be able to associate a link identifier (a parameter of the proto- col between multiplexing layer elements) to a particular LAPD actor-agent inter- face. It therefore manages a table (T-L) that provides the translation between a link identifier and an actor-agent interface identifier (i.e., Li-AAli). Since data that arrive at a common actor are not terminated by the actor, but are relayed by it to another actor-agent interface, we define the event of a piece of data arriving at a common actor as the primitive relayOrder(...). The corresponding primitive from the common actor to an agent is called terminateOrder(...) since the agent is supposed to terminate the relayed data somewhere. These primitives define the specified actor interface (SAC). The parameters of these primitives are identi- cal: if the interface supports several streams in parallel, there are two parameters: user data (UD) and some type of channel identifier (CHi). If the interface sup- ports just one stream, the user data is the only parameter. The same modeling principle is applied to switching layers (see Fig. 2.40). 2.3 