pplica- tions that require constant and low delay. There are two basic types of packet-switched networks as illustrated in Figure 6.5: virtual circuits and datagram transmission. In the case of virtual circuits, the virtual connection is established at the beginning of each conversation or it is permanently set up and every packet belonging to a certain connection is transmitted via the same established route. The main difference between circuit-switched physical circuits and virtual circuits is that many users share the capacity of the transmission lines and channels between network nodes if virtual instead of physical circuits are used. At a certain moment active users may use all the available capacity if other users are not transmitting anything. The complete address information is not needed in the packets when the connection is established. Only a short connection identifier is included in each packet to define the virtual circuit to which the packet belongs. The operation of switched virtual circuits is explained in more detail in Section 6.2.4. The circuit is first Circuit switched data transfer established, then Physical circuit data is transferred and in the end the circuit is released. The capacity of the circuit is not Signaling available for other users. Data transfer Examples: telephone network, Signaling Packet switched data transfer In true packet switched Optional routes data communication there for packets is no dedicated connection between communicating devices. Each packet includes complete destination address Address and is sent and routed independently. One example is the Internet. Packet switched data transfer with virtual circuits Permanent virtual circuit exists (or virtual circuit is established Virtual circuit for conversation). All data is transferred via the same path. (Signaling (In the end the virtual circuit is Data transfer released.) Each packet includes circuit identification. Capacity Signaling) between nodes is shared by all users. Examples: frame relay, ATM Figure 6.5 Circuit