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