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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
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