PSTN Switched Network

PSTN is an example of circuit switched network. The hierarchy of PSTN is shown below, 

PSTN hierarchy

1. Regional offices (class 1)

2. Sectional offices (class 2)

3. Primary offices (class 3)

4. Toll offices (class 4)

5. End offices (class 5)

Local loops

The local loop, a twisted-pair cable that connects the subscriber telephone to the nearest end office or local central office when used for voice, has a bandwidth of 4000 Hz.

Trunks

Trunks are transmission media that handle the communication between offices. Transmission is usually through satellite links or optical fibers.

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