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Chapter: Computer Networks : Physical Layer

Virtual-Circuit Networks

1. Addressing 2. Circuit-Switched Technology in WANs

Virtual-Circuit Networks:


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.


1. Addressing


In a virtual-circuit network, two types of addressing are involved: global and local (virtual-circuit identifier).

Global Addressing


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.


Virtual-Circuit Identifier


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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