 a number of message types as parts of the specification. A message type is a structured data type, which means that it defines a structure of parameters and their types, value range, and how to encode them into bits. One of the messages in DSS1 L3 is called SETUP. This message is the first that is sent between an ISDN terminal (TE) and a local exchange (LE) of the network when either the terminal requests a service from the local exchange (normally an "outgo- ing call"), or the local exchange offers a service (e.g., an "incoming call") from another terminal to (one of) its own terminals. The existing protocol specification defines both the message type and the actions that are expected from the LE or the terminal when receiving a SETUP. Figure 2.17 shows the reception of a SETUP at an LE and a very small part of what the DSS1 L3 specification says about that (also slightly transcribed by the author). We are only scratching the surface of the message type specifications of DSS1 L3 here (more than 400 pages). As a matter of fact, the bulk of the DSS1 L3 speci- fication deals with specifying all the details of messages. The actions associated to the SETUP message in Fig. 2.17 was constructed by two text fragments that were found (through tedious readings) in different places in the protocol specification. 13 Since the OSI RM does not define the actor-agent separation in layer elements, we regard as "OSI layer" any layer that does not define this distinction. 2.3 Agents and Actors DSS1 L3- DSS1 L3- layer element layer element in an LE SETUP in TE Many LAPD Single LAPD layer interfaces layer interface DSS1 L3 about the action in LE: DSS1 L3 about the SETUP message: "The LE shall check that the bearer service messageType, requested in the bearerCapability matches callReference, with the bearer services subscribed by the bearerCapability {speech, 3.1 kHz audio, ...} callingPartyNumber." channelldentification {none, B1, B2, any} OPT calledPartyNumber, "If the sender of the message accepts any callingPartyNumber, channel