Redirected from Telephony
The telephone is a telecommunications device designed to transmit speech by means of electric signals. It was invented by Antonio Meucci[?] around 1860, who called it teletrophone, as recently recognized by the US Congress in the resolution 269 on June 15th, 2002. Before that resolution it was generally attributed to Alexander Graham Bell. The first recorded public demonstration of Meucci's invention took place in 1860, and had a description of it published in New York's Italian language newspaper.
The history of additional inventions and improvements includes the carbon microphone (later replaced by the electret[?] microphone now used in almost all telephone transmitters), the manual switchboard, the rotary dial, the automatic telephone exchange, the computerized telephone switch, Touch Tone(R) dialing (DTMF), the digitization of sound using different coding techniques including pulse code modulation or PCM (which is commonly used for .WAV files and on compact disks). Newer systems include ISDN, DSL, cell phone (mobile) systems, digital cell phone systems, and the third generation cell phone[?] systems that promise to allow high-speed packet data transfer.
The industry was early on divided into telephone equipment manufacturers and telephone network operators (telcos), the latter often holding a national monopoly. In the United States, the Bell System was vertically integrated: it fully or partially owned the telephone companies that provided service to about 80% of the telephones in the country and also owned Western Electric, which manufactured or purchased virtually all the equipment and supplies used by the local telephone companies. The Bell System divested itself of the local telephone companies in 1984 in order to settle an antitrust suit brought against it by the United States Department of Justice.
Telephone Operating Companies Some Telco names (in alphabetic order) include: AT&T, BC TEL, Bell, Bell Canada, British Telecom, Cable and Wireless[?], Deutsche Telekom, GTE, ITT, NTL, NTT, SBC Communications[?], Telefonica, Teleglobe[?], Telewest, Telstra, Telia, TELUS, Verizon
The network that connects most phones together is known as the PSTN (public switched telephone network). Phone lines are usually copper wires which form a circuit between the subscriber and the exchange, although some recent installations may use optical fiber for part of the distance. An analog line typically uses frequencies of 0-3.5 kHz, with frequencies higher than this filtered at the exchange. The analog speech signals are carried over the digital backbone network as a stream of digitally encoded samples at a sample rate of 8 kHz. The frequences above 4 kHz can be utilized for DSL connections.
A line is a single voice communications circuit between the subscriber and the central switching office. A trunk is a single circuit between central offices and may be analog or digital and is transmitted via copper, microwave, or fiber optics. A trunk group[?] is a grouping of identical trunk circuits between two specific central offices.
Automatic telephone systems generally use numeric addresses, more commonly known as telephone numbers. The addressing system often distinguishes local, long-distance and international calls. Local calls are initiated by dialling the local number. A long-distance number is indicated by a long-distance prefix (CCITT recommends "0") followed by area code and a number local to that area. International phone calls require an international prefix (CCITT recommends "00") followed by area code and local number. US and Canadian phone systems use "1" as the long distance prefix and "011" for international prefix. See country calling codes for access codes to international telephone services.
Larger companies and organizations often employ a PABX (Private Automatic Branch Exchange). This is a telephone switch that defines its own local phone number range, which is commonly embedded in a public local phone number range. Some of the largest companies now even have their own internal telephone networks across the country, or even throughout the world, with limited gateways into the PSTN.
Most PSTN systems use analog communication between individual phones and the local switch. If digital communication is used for an individual phone, the system used is usually ISDN (Integrated Services Digital Network).
Between switches in the PSTN, most signalling is now digital using Signalling System 7 ("SS7").
Most wireless phone systems are cell-structured. Wireless communication is used between the handsets and the cell. Communication between cells can be wireless, or over ground cables. When an active handset moves from one cell to another, the call is automatically transferred to the next cell without interrupting the call.
There are now multiple standards for common carrier wireless telephony, often with multiple incompatable standards used in the same nation: