Cellular Mobile Communication Cellular, or mobile phones, originally used in cars, airliners, and passenger trains, but increasingly becoming ubiquitous, are basically low-power radio-telephones. Calls go through radio transmitters that are located within small geographical units called cells. Because each cell’s signals are too weak to interfere with those of other cells operating on the same frequencies, more channels can be used than would be possible with high-power radio frequency transmission. Narrow-band frequency modulation (FM) is the most common mode of transmission, and each message is assigned a carrier unique to the cell from which it is transmitted. Since the cellular phone was first tested in 1978, the cellular market in Britain alone had grown at a rapid rate to over 40 million users by 2001. In Japan, where by 2001 penetration was as high as 45 per cent (57 million users), the growing capabilities of cellular phones (see Cellular Radio) also meant that the number of people using mobiles with Internet access was set to reach 10 million. Voice Mail Voice mail allows incoming messages to be recorded for later playback when the call is not answered. In advanced forms of voice mail the user may record a message to be sent later in the day. For residential service voice mail can either be purchased from the telephone company as an exchange-based service or it is available by purchasing an answering machine. This usually contains a regular telephone set along with a recording, playback, and automatic ring detection capability. If an incoming call is answered at any telephone on the line before a pre-set number of rings, the answering machine does nothing. However, after the pre-set number of rings, the answering machine goes off hook and plays a pre-recorded message stating that the owner cannot answer the phone now and inviting the caller to leave a message to be recorded. The answering machine’s owner is alerted to the presence of a recorded message by a light or audible “beep” and can retrieve the message later. Most answering machines and all exchange-based services also allow the owner to retrieve recorded messages from a remote location by dialing a code after the machine has answered. Technological Trends Replacement of transoceanic coaxial cables by fiber-optic cables has continued through the 1990s. Advances in integrated-circuit technology and semiconductors have made it possible to design and market telephones that not only produce high-fidelity speech quality, but also offer a host of features such as pre-stored numbers, call forwarding, call waiting, and caller identification. Cellular telephony has grown dramatically, and cellular phones are now offered as standard equipment in many cars.