Brief Overview
Telephones were originally connected directly together in pairs. Each user would have a separate telephone wired to various places he might wish to reach. This became inconvenient when people wanted to communicate with many other people using telephones, so the telephone exchange was invented. Each telephone could then be connected to other local ones, thus inventing the local loop and the telephone call. Soon, nearby exchanges were connected by trunk lines, and eventually distant ones were as well.
In modern times, most telephones are plugged into telephone jacks. The jacks are connected by inside wiring to a drop wire which connects the building to a cable. Cables usually bring a large number of drop wires from all over a district access network to one wire center or telephone exchange. When the user of a telephone wants to make a telephone call, equipment at the exchange examines the dialed telephone number and connects that telephone line to another in the same wire center, or to a trunk to a distant exchange. Most of the exchanges in the world are connected to each other, forming the public switched telephone network (PSTN). By the end of the 20th century almost all were stored program control exchanges.
After the middle of the 20th century, fax and data became important secondary users of the network created to carry voices, and late in the century, parts of the network were upgraded with ISDN and DSL to improve handling of such traffic.
Today, telephony has been digitized and has merged into digital telephony, the use of digital technology in the provision of telephone services and systems. Telephone calls can be provided digitally, but are restricted to cases in which the last mile is digital, or where the conversion between digital and analog signals takes place inside the telephone. This advancement has reduced costs in communication- and improved the quality of voice services. This has been found useful for new network services (ISDN) that transfer data speedily over telephone lines.
Another important concept that has been merged into telephony is computer telephony integration, which enables computers to know about and control phone functions such as making and receiving voice, fax, and data calls with telephone directory services and caller identification. The integration of telephone software and computer systems is a major development in the evolution of the automated office. The term is used in describing the computerized services of call centers, such as those that direct your phone call to the right department at a business you're calling. It's also sometimes used to describe the ability to use your personal computer to initiate and manage phone calls (in which case you can think of your computer as your personal call center). CTI is not a new concept and has been used in the past in large telephone networks, but only dedicated call centers could justify the costs of the required equipment installation. Primary telephone service providers are offering information services such as automatic number identification, which is a telephone service architecture that separates CTI services from call switching and will make it easier to add new services. Dialed Number Identification Service (DNIS) on a scale is wide enough for its implementation to bring real value to business or residential telephone usage. A new generation of applications (middleware) is being developed as a result of standardization and availability of low cost computer telephony links.
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