e Sect. 6.4) solves control signaling differently: spe- cial VCs are used for carrying data for control layers, which is why the ATM proto- col specification does not define control layer messages. The ATM switching layer is therefore insensitive to changes in the ATM control protocol. In reality, the ATM switching layer is (in line with the telecom tradition) defined as a "switching resource layer" that is controlled from a separate control layer (defined as part of the broadband-ISDN network system, B-ISDN). Let's now return to X.25 and the model of the terminating X.25 nodes, where X.25 users access the VC service. Figure 2.46 shows an X.25 terminating node that serves a single user. 2 Layer Structures VC user VC requester clear receive VC control actor invoke X.25 L3 setOrder (CEi+>VCi) relay Agent terminate {VCi, ...} Terminating actor An X.25 VC terminating node Fig. 2.46 An X.25 VC terminating node. T-CO=connection table; XNA=X.25 network address The terminating node needs neither of the tables we defined for the switching node. It may, however, store its own network address (XNA), and the switching actor may contain a table (called T-CO) that records which VCs are in use by the user. This table may also translate between a local CEi and a corresponding Ci (VCi is the Ci in X.25). A common layer interface is used both as a control point and as a connectivity service point (as for most packet-switching systems). Since X.25 can terminate a large number of VCs in a terminating node, the node may also be used for support- ing many users concurrently, in which case the terminating actor must also perform discrimination between used-layer interfaces. This is exactly the "discrimination by CEi" case that characterizes the OSI RM-CON service, as was discussed in Sect. 2.2.2. Figure 2.47 shows how the CON service is defined in OSI RM and how it looks when we apply the actor-agent separation principle. CON service according to OSI RM CON service according to AMLn Layer element instance transport- layer If CEi=x If C