Within the switched category there are two subcategories, circuit- and packet-switched networks as shown in Figure 6.4, both of which are used for data transmission. Figure 6.4 also shows some sample networks and what switching principles they use. Data Communications Data transmission through the network Leased line Packet switched Circuit switched networks networks Network examples: Virtual Packet by packet Telephone Circuits routing Network Cellular telephone Frame Networks Relay Packet switched Internet cellular networks Figure 6.4 Leased lines and circuit- and packet-switched networks. 6.2.1 Circuit Switching Circuit-switched networks provide fixed bandwidth and very short and fixed delay. It is the primary technology for voice telephone, video telephone and video conferencing. The disadvantage is that it is inflexible for data commu- nications where the demand for transmission data rate is far from constant but varies extensively over short time scales. Some older generation data networks used the circuit switching princi- ple. In the beginning a circuit-switched connection is dialed up by the data source. The routing is based on the destination subscriber number given when the circuit is established. The connection is released after the commu- nication is over (see Figure 6.5). During a conversation, the data capacity of the connection is fixed and it is reserved only for this conversation regardless of whether the data capacity is used or not. At the end of the call, the circuit is released. ISDN as well as the telephone network use the circuit-switching principle. 6.2.2 Packet Switching Packet-switched networks are specially designed for data communication. The source data are split into packets containing route or destination identi- fications. The packets are routed toward the destination by packet-switching nodes on the path through the network. The major drawback of the packet- Introduction to Telecommunications Network Engineering switched technology is that it usually cannot provide a service for a