The history of additional inventions and improvements include the carbon microphone?, the manual switchboard, the rotary dial, the automatic telephone exchange, the computerized telephone exchange, 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 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. Some well-known company names (in alphabetic order) include: AT&T, Bell, Bell Labs, British Telecom, [Deutsche Telekom]?, Ericsson, GTE, ITT?, Lucent, Motorola, Nokia, Nortel, Siemens?, Telefonica, Telia, Verizon
The network that connects most phones together is known as the PSTN (public switched telephone network).
Automatic telephone systems generally use numeric addresses, more commonly known as a telephone number. 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. North American phone systems use "1" for long distance prefix and "011" for international prefix. See country calling codes for access codes to international telephone services.
Larger companies and organizations 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 signalling between individual phones and the local switch. If digital signalling 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.
Most wireless phone systems are cell-structured. Wireless communication is used between the handsets and the cell. Communication between cells can be wireless, or using 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:
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