rop- erties (QoS) for the SDU. There is also a single protocol defining PDUs ("message" and "datagram" are alternative names). The switching elements are normally called router. A CNL-SAP must have a unique SAPI, in accordance with the OSI RM view. CNLUs may "own" their SAPI (suitable for terminal systems that are permanently connected to the packet network) or just borrow them from the packet network when needed it (terminal systems that temporarily connect to the network). The scenario in Fig. 3.36 comprises what happens both on the incom- ing and the outgoing side of the router. A CNL-CSP CNL-UDT CNL-UDT CNL-UDT Router CN-connection CNL-UDT sendCnlOrder(destSAPI, SDU, qos) 5 recCnlRepCnf(origSAPI, destSAPI, SDU, qos) 3 {origSAPI SAPI, destSAPI SAPI, sendCnIOrd(origSAPI, destSAPI, SDU, qos) userData1, qos QoS} {origSAPI SAPI, destSAPI SAPI, userData2, qos QoS} Fig. 3.36 A scenario for connection-less communication 1. The CNLU sends data (SDU) by referring to a destination SAPI (destSAPI. The CNLU also provides quality requirements (qos) in the send event. 3.4 Modelling Telecom Resource Systems 2. The CNL-UDT may have to fragment the SDU, depending on the size of data that can be handled by the actual packet network. 3. It then creates a PDU (datagram, message) that is sent to the CNL packet net- work. 4. The receiving CNL-UDT may perform error checking (which, in the case of IP, only comprises header information, i.e. the parameters origSAPI, destSAPI and qos). 5. If the header is correct, the complete datagram is provided for routing and switching. If the header is in error, the datagram is discarded. 6. Routing implies selecting a suitable outgoing link. The router considers the destSAPI, the requirements on qos and the size of fragments on different outgo- ing links. 7. The data are sent to the CNL-UDT for the selected outgoing link. 8. The CNL-UDT may assemble consecutive SDUs (if the outgoing link allows larger fragments), or it may further fragment an SDU (if the size of fragments on the outgoing link are