 is shown to the left in the model. It exhibits a TR connection and four protocols, separated into three levels with two different protocols on the top level: one for process control data and one for user data. Note, however, that the X.25 standard defines all protocols except the LAPB as the X.25 network layer protocol: The LAPB level is what OSI RM calls a link layer. LAPB is an error handling protocol (based on HDLC, High-level Data Link Control). Error detection is done through a checksum that comprises the complete LAPB-PDU. Error cor- rection is performed through re-sending. The checksum and some other parame- ters of less interest are the LAPB-header. Correct LAPB-SDUs are delivered to the X.25 discriminator. The correspond- ing PDU consist of a message discrimination parameter and an X.25 packet. The discriminator identifies whether the packet contains userdata or process control data. In the X.25 standard, this parameter is defined as the control bit in the third octet in the network layer PDU. This bit is set to 1 for all packets that are handled by the network process controller, and to 0 for all user-data packets. Since discrimination has nothing to do with the role of the network process con- trol systems, I have instead defined a discrimination protocol that contains two parameters. The discriminator uses the information of the message discrimina- tion parameter and distributes the packet either to the packet switch or to the X.25-DNCP. From a modelling point of view, the interfaces between the dis- criminator and its users are permanent associations, identified by the association identifiers 0 and 1. Figure 3.63 shows the structure of the PDU of process control packets. VCN stands for virtual circuit number. Note that addresses in X.25 are referred to as called address and calling address (the third variant for denoting SAPIs SO far). 3.4 Modelling Telecom Resource Systems The information in the PDU is used by the X.25-DNCP to select an outgoing route with help from the routing analyser. The route 