n this case is that packet transfer (contrary to CN connections) introduces a variable packet-transfer delay, which can be devastat- ing to the quality of, e.g. voice communication. To design packet-switching net- works SO that a reasonable quality of service can be maintained for isochronous services is, therefore, a major concern in packet technology. To date, packet technology for isochronous services cannot compete with circuit-switched net- works, when requirements for both cost and quality are considered. 3.4 Modelling Telecom Resource Systems Since channels in packet-transport links do not represent a permanent division of link capacity, such channels are often denoted as virtual. This concept is nor- mally associated only with connection-oriented services. Transport links in packet- switching networks fall into two main classes: CON links and CNL links. In CON links, channels are identified by channel identifiers (permanent connec- tion identifiers). This identifier is called the virtual circuit number in X.25 net- works. In ATM networks, individual channels are identified by a virtual channel identifier within a virtual path, identified by a virtual path identifier. Before a channel can be made available for packet transfer, the channel must have been reserved, and paths must have been set up through routers the entire way between the user's terminal systems (similar to how CN connections are established in circuit-switching networks). When a packet is transferred though a router, no routing analysis need be done. The packet switch in the router only substitutes connection identifiers between the incoming and outgoing links. In CNL links, each packet identifies the sending user and the receiving user, i.e. includes the originationSAPI and the destinationSAPI (called source address and destination address in IP, originating point code and destination point code in MTP). These types of packets are often called datagram. The combina- tion of sender and receiver SAPI constitutes the connection identifier ove