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A revolution in communications is taking place. Telephones, formerly connected through underground wires, are increasingly becoming hand held devices that transmit via radio waves. Soon people could be reached anywhere on the globe via a network of satellites. |
Conversely, television, originally broadcast on the same frequencies that cellular phones now use, is set to become a digital medium, providing hundreds of different channels piped into homes along optical fibers - hair-thin strands of glass that carry enormous amounts of information at the speed of light.
A cellular phone conversation is accompanied by unheard digital exchanges (a series of "I" and "O" representing data) as the handset and the computers that control the network process the call.
The cells the phones are named after are a mosaic of hexagonal areas, each with a transmitter/receiver or base station at its center.
More people can use cellular phones than other radio phone systems because the signals used are very weak, so frequencies used in one cell may be reused in another a short distance away.
Every 15 minutes, each base station beams out a message asking all the handsets within its cell to "report in." This enables the central computers to know where to route a call when a handset is phoned.
The fast-growing number of fax machines, computer modems, and new telephone users demands transmission lines that can handle far greater numbers of calls than traditional copper. This demand is being met by fiber-optic cables, which carry digital messages in the form of rapid bursts of intense laser light.
Capable of carrying hundreds of thousands of simultaneous phonecalls down a pair of glass strands, in addition to fax messages, computer data, and television signals, fiber-optic cables are revolutionizing global communication and home entertainment.
Some cable operators already offer a huge choice of channels, interactive games, and even on-demand video films.
Conversations are more intelligible when the two parties can see one another. However, video phones, which make this possible by simultaneously transmitting pictures and speech, are still not widely used.
This is because transmitting a complete video signal requires the sending of more than 200 million bits (units of information) a second - 4000 times more than existing cables can handle.
Accepting lower picture quality and using compression, a technique by which redundant or repeated bits of data are omitted, the signal can be reduced to 64,000 bits per second. Even this is beyond the capacity of ordinary telephone lines, so current videophones can send only crude, still pictures.
One model, sending data at 14,400 bits per second, takes five seconds to send one still picture.
Настоящая революция происходит на наших глазах в сфере коммуникаций. Телефоны, которые ранее соединялись между собой посредством кабельных подземных коммуникаций, становятся мобильными устройствами, связывающимися с помощью радиоволн. В недалеком будущем люди смогут связаться с любой точкой нашей планеты через сеть спутников.
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