Telephone System

The telephone networks, referred to as the USA POTS - Plain Old telefhone System - or PSTN-Public Switched Network telefhone were initially scaled to traffic unique voice, so that the bandwidth of a circuit between two subscribers, two telephone extensions was calculated in view of the compromise between the economy of trunks between switching centers and intelligibility of human speech.

Thus, although this bandwidth varies between different networks, typically has a value between 3000 and 4000 Hz, well below the actual transmission capacity of a copper mesh used in the telephone plant. In addition, the relationship between signal and noise existing in any medium of transmission has also been tailored to that commitment, usually ranging between 30 dB and 35 dB rarely.

Structure of the Telephone System

Currently, the telephone system is organized as a hierarchy of different levels and extremely redundant.
Each phone has two copper wires coming out of the device and connect directly to the train station end, or the central location, closer to the telephone company.

Connections via two cables between the subscriber's phone and final season on the market are known as local loop.
Currently the local loops are formed by twisted pair cables. However, in the past, the most common were the cables without insulation 25 cm apart from each other on telephone poles.
Among the switching stations, the use of cable railing, and especially micoondas optical fiber is quite frequent.
  
If a subscriber station connected to a certain final call to a subscriber of the same station, the switching mechanism within the station will set up a direct electrical connection between two local loops.
This connection remains intact during the call.

If the phone call is connected to another end station, another procedure should be taken. Each end station contains a number of output lines, called trunks of long-distance connection to one or more stations switching stations called long distance.
If the end stations of the transmitter and receiver have a long distance trunk connection connected to the same long-distance station, the connection may be established within the long-distance station.
If the transmitter and receiver do not share the same long-distance station, the path will be established at a higher point of the hierarchy.

There are mainline stations, local and regional authorities form a network through which the long distance stations are connected.
Distance stations, major, regional communicate with each other through long-distance trunks high bandwidth.

The telephone systems were initially implemented using only analog transmission technologies, both among the subscribers and the telephone exchanges as the internal connections between the plants.
Thus, the physical circuits established between the subscribers were multiplexed in analog trunks between switching centers corresponding using FDM techniques.
This switching centers are being gradually replaced by more modern equipment, using digital technology, though the sign on extensions of subscribers continues to be analog in nature, the communication between the central uses techniques of multiplexing and transmission of digital data.