Circuit Switching Concepts

Circuit switching technology appeared long before packet switching. It appeared to support Telephone calls first, then was used for computer communication.

In the past, a dedicated physical circuit was formed by Switchboard Operators each time a phone A wants to communicate with phone B. The switchboard operator connects the ingress circuit to the egress circuit to form a dedicated switched circuit. Then switchboard operators were replaced with Central Office switches that automatically “switch” between circuits.

There was the concept of virtual private circuits, which are dedicated circuits established over physical shared circuits in the operator’s network. The term virtual comes from the fact that they do not physically exist from end to end.

In Traditional telephony, each voice call is transported over a 64kbps channel. Voice calls are carried over trunks or “big fat pipes” whose data rates can be up to 10Gbps. These trunks are usually high-density copper cables or fiber optic cables.

Circuit switching:

  • has three stages: connection establishment, communication, connection teardown
  • maintain a state per flow, or per circuit, which creates the burden of disposing of the circuit once the communication is closed.

Some limitations of circuit switching technology:

  • Inefficient use of expensive links
  • circuit switches have fixed data rates, despite the fact that computer communications are bursty. Sometime we need to use the full capacity of the circuit, and sometimes not.
  • necessity to manage the state of each circuit, at connection establishment and at connection teardown

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