A virtual-circuit network is a cross between a circuit-switched network and a datagram network.
It has some characteristics of both.
1. As in a circuit-switched network, there are setup and teardown phases in addition to the data transfer phase.
2. Resources can be allocated during the setup phase, as in a circuit-switched network, or on demand, as in a datagram network.
3. As in a datagram network, data are packetized and each packet carries an address in the header. However, the address in the header has local jurisdiction not end-to-end jurisdiction. The reader may ask how the intermediate switches know where to send the packet if there is no final destination address carried by a packet.
4. As in a circuit-switched network, all packets follow the same path established during the connection.
5. A virtual-circuit network is normally implemented in the data link layer, while a circuit-Switched network is implemented in the physical layer and a datagram network in the network layer.
Figure 1.50 is an example of a virtual-circuit network. The network has switches that allow traffic from sources to destinations. A source or destination can be a computer, packet switch, bridge, or any other device that connects other networks.
In a virtual-circuit network, two types of addressing are involved: global and local (virtual-circuit identifier).
A source or a destination needs to have a global address-an address that can be unique in the scope of the network or internationally if the network is part of an international network.
The identifier that is actually used for data transfer is called the virtual-circuit identifier (VCl). A VCl, unlike a global address, is a small number that has only switch scope; it is used by a frame between two switches. When a frame arrives at a switch, it has a VCI; when it leaves, it has a different VCl.
2. Circuit-Switched Technology in WANs
Virtual-circuit networks are used in switched WANs such as Frame Relay and ATM networks. The data link layer of these technologies is well suited to the virtual-circuit technology.
Switching at the data link layer in a switched WAN is normally implemented by using virtual-circuit techniques.
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