 according to the channelldentifica- tion, the network selects an available channel associated with the callingPartyNumber." Fig. 2.17 A small part of the DSS1 L3 protocol specification The action part obviously describes what the actor in the layer element does, while the message description defines implicitly what the agent must do, as mod- eled in Fig. 2.18. DSS1 L3-layer element in an LE Actor Agent -SETUP describes describes Many LAPD layer interfaces "The actor shall check that the service messageTyp- requested matches the services subscribed by callReference, the caller." bearerCapability {speech, 3.1 kHz audio, ...} channelldentification {none, B1, B2, any} OPT "If the caller accepts any channel, the network calledPartyNumber, selects an available channel associated with the callingPartyNumber, caller." Fig. 2.18 The actor and the agent in the DSS1 L3-layer element in a local exchange (LE) The scenario for handling a SETUP reception in this model is divided into two description parts: 1. The bits of the message arrive at the agent over a layer interface of a LAPD layer. The agent must identify all the parameters of the SETUP, using the encod- ing rules defined by the protocol specification. Each value will also be repre- sented as a local data-type value, according to the local processing environment of the layer element. The agent must then check the validity of the message. For example, some parameters are mandatory, some are optional, some may have a 2 Layer Structures default value associated. Other things to check is that all values are within defined value ranges. Any errors or inconsistencies in these respects will not start any other action in the layer element than a negative response message (DISCONNECT) to the calling terminal, i.e., the actor will not at all be involved. 2. If the message is valid, the agent delivers parameters of the actor-actor event to the actor, which takes actions according to the description to the left. These actions depend of course on the values that were carried by t