Прочитайте текст и выполните задания Cell-phone channels 1. A single cell in an analog cell-phone sy...: ответ на тест 1077968 - Английский язык

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Cell-phone channels
1. A single cell in an analog cell-phone system uses one-seventh of the available duplex voice channels. That is, each cell (of the seven on a hexagonal grid) is using one-seventh of the available channels so it has a unique set of frequencies and there are no collisions. A cell-phone carrier typically gets 832 radio frequencies to use in a city. Each cell phone uses two frequencies per call – a duplex channel – so there are typically 395 voice channels per carrier.
2. Therefore, each cell has about 56 voice channels available. In other words, in any cell, 56 people can be talking on their cell phone at one time. Analog cellular systems are considered first-generation mobile technology, or 1G. With digital transmission methods (2G), the number of available channels increases. For example, a TDMA-based digital system (more on TDMA later) can carry three times as many calls as an analog system, so each cell has about 168 channels available.
3. Cell phones have low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts). The base station is also transmitting at low power. Low-power transmitters have two advantages. The transmissions of a base station and the phones within its cell do not make it very far outside that cell. The power consumption of the cell phone, which is normally battery-operated, is relatively low. Low power means small batteries, and this is what has made handheld cellular phones possible.
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Cell-phone channels
1. A single cell in an analog cell-phone system uses one-seventh of the available duplex voice channels. That is, each cell (of the seven on a hexagonal grid) is using one-seventh of the available channels so it has a unique set of frequencies and there are no collisions. A cell-phone carrier typically gets 832 radio frequencies to use in a city. Each cell phone uses two frequencies per call – a duplex channel – so there are typically 395 voice channels per carrier.
2. Therefore, each cell has about 56 voice channels available. In other words, in any cell, 56 people can be talking on their cell phone at one time. Analog cellular systems are considered first-generation mobile technology, or 1G. With digital transmission methods (2G), the number of available channels increases. For example, a TDMA-based digital system (more on TDMA later) can carry three times as many calls as an analog system, so each cell has about 168 channels available.
3. Cell phones have low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts). The base station is also transmitting at low power. Low-power transmitters have two advantages. The transmissions of a base station and the phones within its cell do not make it very far outside that cell. The power consumption of the cell phone, which is normally battery-operated, is relatively low. Low power means small batteries, and this is what has made handheld cellular phones possible.
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Lower power transmitters …

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GPS
1. Our ancestors had to go to pretty extreme measures to keep from getting lost. They erected monumental landmarks, laboriously drafted detailed maps and learned to read the stars in the night sky. Things are much, much easier today. For less than $100, you can get a pocket-sized gadget that will tell you exactly where you are on Earth at any moment. As long as you have a GPS receiver and a clear view of the sky, you'll never be lost again.
2. The Global Positioning System is vast, expensive and involves a lot of technical ingenuity, but the fundamental concepts at work are quite simple and intuitive. When people talk about «a GPS», they usually mean a GPS receiver. The Global Positioning System (GPS) is actually a constellation of 27 Earth-orbiting satellites (24 in operation and three extras in case one fails). The U.S. military developed and implemented this satellite network as a military navigation system, but soon opened it up to everybody else.
3. Each of these 3,000- to 4,000-pound solar-powered satellites circles the globe at about 12,000 miles (19,300 km), making two complete rotations every day. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites «visible» in the sky. A GPS receiver's job is to locate four or more of these satellites, figure out the distance to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration.
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Radio scanner
1. The air around you is bursting with radio waves. You know that you can flip on the AM/FM radio in your car and receive dozens of stations. You can flip on a CB radio and receive 40 more. You can flip on a TV and receive numerous broadcast channels. Cell phones can send and receive hundreds of frequencies. And this is just the tip of the radio spectrum iceberg. Literally tens of thousands of other radio broadcasts and conversations are zipping past you – police officers, firefighters, ambulance drivers, paramedics, sanitation workers, space shuttle astronauts, race car drivers, and even babies with their monitors are transmitting radio waves all around you at this very moment.
2. To tap into this ocean of electromagnetic dialogue and hear what all of these people are talking about, all you need is a scanner. A scanner is basically a radio receiver capable of receiving multiple signals. Generally, scanners pick up signals in the VHF to UHF range. Radio scanners are very portable and affordable. Scanners typically operate in three modes: scan, manual scan, and search.
3. In scan mode the receiver constantly changes frequencies in a set order looking for a frequency that has someone transmitting. The frequencies can be preprogrammed on some models or manually set on practically all models. In manual scan mode, the user taps a button or turns a dial to manually step through preprogrammed frequencies one frequency at a time. In search mode, the receiver is set to search between two sets of frequencies within a given band. This mode is useful when a user does not know a frequency, but wants to know what frequencies are active in a given area. If the frequency the scanner stops at during a search is interesting, the user can store that frequency in the radio scanner and use it in scan mode.
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Lidar
1. These days, more and more police departments are using laser speed detectors. The basic element in a laser speed gun, also called a lidar gun (for light detection and ranging), is concentrated light. The lidar gun clocks the time it takes a burst of infrared light to reach a car, bounce off and return back to the starting point. By multiplying this time by the speed of light, the lidar system determines how far away the object is.
2. Unlike traditional police radar, lidar does not measure change in wave frequency. Instead, it sends out many infrared laser bursts in a short period of time to collect multiple distances. By comparing these different distance samples, the system can calculate how fast the car is moving. These guns may take several hundred samples in less than half a second, so they are extremely accurate. As it turns out, conventional radar is relatively easy to detect. The simplest radar detector is just a basic radio receiver, something like the one you use to pick up FM and AM radio stations.
3. The air is full of radio signals – they’re used for everything from television broadcasts to garage door openers – so for a receiver to be at all useful, it must pick up only signals in a certain range. The receiver in a radio is designed to pick up signals in the AM and FM frequency spectrum, whereas the receiver in a radar detector is tuned to the frequency range used by police radar guns. Periodically, the frequency range used by the police is expanded, and speedsters everywhere have to invest in new detection equipment.
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The History of Answering Machines
1. Valdemar Poulsen, the Danish telephone engineer and inventor, patented what he called a telegraphone in 1898. The telegraphone was the first practical apparatus for magnetic sound recording and reproduction. It was an ingenious apparatus for recording telephone conversations. It recorded, on a wire, the varying magnetic fields produced by a sound. The magnetized wire could then be used to play back the sound. That was the first automatic answering machine
2. Mr. Willy Müller invented the first automatic answering machine in 1935. This answering machine was a three-foot-tall machine popular with Orthodox Jews who were forbidden to answer the phone on the Sabbath. The Ansafone, created by inventor Dr. Kazuo Hashimoto for Phonetel, was the first answering machine sold in the USA, beginning in 1960.
3. CASIO COMMUNICATIONS created the modern telephone answering device (TAD) industry as we know it today by introducing the first commercially viable answering machine a quarter of a century ago. In 1971, PhoneMate introduced one of the first commercially viable answering machines, the Model 400. The unit weighs 10 pounds, screens calls and holds 20 messages on a reel-to-reel tape. An earphone enables private message retrieval. The first digital tad was invented by Dr. Kazuo Hashimoto of Japan in mid-1983. US patent 4,616,110 entitled Automatic Digital Telephone Answering.
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The first automatic answering machine created in 1935 was …