CON network (voice) protocol or terminal system Packet data CON network protocol, L3 CON-UDT (PS-data) Packet data Packet handler protocol, L2 in CS network TE-TPC Teleservice Access Protocol CS network 64 kbps SAPI 0 CS network Fig. 3.34 Terminal system that provides voice services over a packet-switching network, using a circuit-switching network for access The scenario for establishing the voice service in this case is the sum of the two previous scenarios: First a CN connection must be established to the CNU, and a packet network node must be instantiated in the CNU. This is handled by a scenario where TEU, TEC and TE-TPC are involved. When the CN connection exists, the TEC informs the CONU which teleservice element (TE-UDT) must be instantiated. Note that this teleservice protocol is not the same as the voice protocol for circuit-switched services. Due to the spe- cific QoS properties of packet data transfer, a special protocol must be used to handle the variations in transfer delay that occur in packet-switched networks. The CONU then takes over and orders a CON connection from the CON-UDT. When the CON connection exists, the CONU informs the TEC. Finally, the TEC informs the TEU that the voice service can be used. Some packet networks (e.g. X.25, IP, MTP) offer connection-less (CNL) serv- ices (also called "datagram services"). The corresponding logical network model is quite different, since there are no CON connections to establish before the service can be used, and consequently, a process control system for connection handling is not needed. The model appears as in Fig. 3.35. 3 Logical Network Modelling Terminal System Packet network A CNL-CSP CNL-UDT Datagram protocol CNL-UDT Router CN connection CNL network Fig. 3.35 Logical network for CNL services (datagram services) In this case, there is only one type of network service interface to consider, a CNL-SAP. The CNL-SAP can be used at any time for sending CNL-SDUs, refer- ring a destination address (SAPI) and, in case of IP, requirements on transfer p