Structure of the Telephone System

The telephone system model is organized as a highly redundant, multilevel hierarchy. It comprises of the following components −

  • Telephone of the subscriber or end user

  • End office − Local central office directly connected to end user at a distance of 1 – 10km.

  • Local loop − A two-way connection between the telephone and the end office.

  • Toll office − switching centres which are called tandem offices when located within the same local area.

  • Toll connecting trunk − Lines that connect end offices with toll offices.

  • Intermediate switching offices − Interconnected non-hierarchical switching offices for connecting toll offices.

  • Inter toll trunk − Very high bandwidth channels that connect either two toll offices via intermediate switching offices.

The model can be diagrammatically represented as follows −

Communication for different Transmissions

  • Both caller and callee are attached to same end office −
  • In this case, a direct electrical connection is set up between the local loops of the subscribers by the switching mechanism of the end office.

  • Caller and callee are attached to different end offices −
  • In this case, the end office of the caller sets up a connection with one or more connected toll offices, which then performs the switching job. This again has two cases −

    • If the end offices of the caller and the callee have a common toll office, then the toll office establishes a connection within itself.

    • If there are no common toll office between the caller and callee, then a path is established between the different toll offices, through intermediate switching office via intertoll trunks.

Transmission Media Used

  • Local loop − analog twisted pair cables.
  • Toll connecting trunk − fibre optic links
  • Between switching offices − fibre optic cables and microwaves

Updated on: 03-Aug-2019

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