ATM BASICS - Chapter 3 ppt

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Chapter 3

Virtual Circuits

The actual service offered by means of ATM technology is connection-ori￾ented. Although connections in ATM are implemented internally with pack￾et switching, the end user is provided with virtual circuits. From this chap￾ter the reader may learn different types of virtual circuits that are in use in

contemporary ATM networks. The chapter presents the general concept of

permanent and switched connections as well discusses methods for setting

them up, maintaining and releasing them.

As it was presented earlier, the basic B-ISDN services require a compromise

between pure packet switching and pure circuit switching. The actual ser￾vice offered within an ATM network is connection oriented, but internally it

is implemented with packet switching, not circuit switching. The term ‘con￾nection’ is used to name the logical circuit that is set-up between end-users

having physical links to ATM network. A virtual connection is a concatena￾tion of logical links between neighboring ATM switches. It is not defined by

means of a time slot or a physical pair of wires but it is defined by table

entries inside ATM switches. The term ‘virtual’ is an equivalent word for

logical as far as ATM circuits are discussed. Today, a number of connection

types are offered: permanent virtual connections, switched virtual connec￾tions, and soft-permanent virtual connections.