Telephony is the use of telecommunication devices to send and receive voice data over distances by means of connecting telephones.
In the early days, telephones were connected directly to each other in pairs. This provided obvious limitations - you had to have a dedicated physical line, connecting your telephone with another telephone, in the place where you wished to communicate with. This limitation was overcome by the introduction of the telephone exchange. The telephone exchange enabled many local telephones within a small area to be connected up. Long distance telephone calls were made possible by the linking up of these various telephone exchanges. The Public Switched Telephone Network (PSTN) refers to the global network of connected telephone exchanges.
Prior to the 1960s all telephone communication was analog. Now nearly all telephony communication is digital. Digital telephony superseded analog telephony because it is more reliable, communication is clearer, and it is more cost effective to implement and maintain. Whilst digital telephony was initially developed for voice calls, it was soon realised that it provided an excellent and expedient way for transmitting data; ISDN was one of the first implementations of data being sent over digital telephone lines.
Telephony also encompasses mobile phone and Voice Over Internet Protocol (VoIP) communication. Mobile phones communicate wirelessly with broadcasting masks to connect to other mobile phones on a cellular network. VoIP is the sending and receiving of voice data, using data packets, over the Internet. These data packets are routed through the Internet in the same way as conventional data. VoIP is very cost effective (cheap calls), as you do not pay the expensive tolls associated with the making of conventional calls over the PSTN network.
Satellite
phones enable potentially global phone coverage by means of communicating
directly with orbiting satellites.
Satellite phones are relatively new in terms of telephony technology and
as such the cost associated with them is quite high, both in terms of hardware
and the calls themselves.