1.4 The Plain Old Telephone System (POTS)

“This ‘telephone’ has too many shortcomings to be seriously considered as a means of communication. The device is inherently of no value to us.” (Western Union Internal Memo, 1876)

The Plain Old Telephone System (POTS) or the Public Switched Telephone Network (PTSN) to give it its grander title refers to the publicly available dial-up telephone network offered by British Telecom (BT) and other providers in the United Kingdom. It is an interconnected network of switching centres and connections to subscribers, which offers voice dial-up between any two subscribers. Connection to overseas countries is also possible.

As telephone communications evolved, switching centres were used to connect users together. A pair of overhead wires (the local loop) was run from the switching centre to each subscriber, and the wires were terminated on a switchboard. At the switchboard a telephone operator made manual connections between subscribers.

As the number of subscribers increased, the number of operators required became a problem and the amount of cabling and interconnection became unmanageable. The development of automatic exchanges (using electro-mechanical devices) simplified the interconnection by placing it under the control of the subscriber.

Cabling problems were overcome by the use of multi-strand underground cable, consisting of 500 to 1000 pairs. Each switching centre (called a central exchange or metropolitan exchange) was interconnected to the next, and located at population centres to minimise the amount of cable required to reach all subscribers.

Initially, the only connection mechanism available was via dial-up lines provided over the PSTN. These were suitable for voice use, but unsuitable for digital data. They were also prone to noise from the electromechanical switches used to interconnect subscribers, and they placed limits on the speed at which data could be transmitted. Higher grade circuits were offered to data subscribers. These circuits were an end-to-end connection with no switching involved. These are called leased lines, and are suitable for data communications at a number of different speeds.

Next: Other Data Transmission Systems