A-level Physics/Application of Physics

Public Switch Telephone Network
The Public Switched Telephone Network, or PSTN, is the worldwide network of lines used to carry telephone calls. It is a "switched" network because a connection is made between caller and receiver before any communication begins. Originally, a physical circuit (formed via a number of switches) had to be formed between the caller and receiver, limiting the capacity of the network. This problem was resolved with the development of time division multiplexing.

The first telephone exchanges (where the circuit switches were located) were operated manually by the telephone operator. This was a slow process. Over time, the human operators were replaced with electromechanical switches. These were in turn superseded by electronic switches.

The telephone network can be visualised as a "hub and spokes" arrangement. When the call is made, if it is local to the exchange, then the signal is carried along a spoke to the local exchange (i.e., the hub) then routed down another spoke to the receiver. If the call is a "trunk" call (i.e., not local to the exchange) then the local exchange routes the call to the trunk exchange for onward delivery to the exchange local to the receiver. For international calls, the local exchange will route to an international gateway.

Mobile Phone
A mobile phone is a handset consisting of a radio transmitter and receiver. A number of different frequencies and encoding techniques are used (hence the need for dual, tri and quad band phones if worldwide usage is required). The handset is constantly in touch with the nearest base station via a low power signal. When a call is made or received, the handset establishes stronger radio wave link with the nearby base station. Each base station is linked to the cellular exchange by a communication cable. The cellular exchange allows entry to the PSTN.

The increasing use of mobile phones does not make it possible for each mobile phone to have its own carrier frequency as the range of carrier frequencies for linking between mobile phones and base stations is limited. Hence the same carrier frequency must be used by many mobile phones at the same time (repeated cells). This is done using UHF which has a limited terrestrial range and using low power transmitters. Each base station has an omnidirectional antenna and the transmitted radio wave are powered so as to have a range approximately equal to the radius of the cell (a few Km) The advantage of operating on UHF is that the aerial of the mobile phone will be very short. Neighbouring cells will typically use different range of carrier frequencies so as to prevent interference.