ÁNALISIS E INTERPRETACIÓN DEL RD 1591/2009 QUE REGULA LOS PRODUCTOS

a radio receiver adjusts the local oscillator frequency. In order for the receiver to amplify the signal and feed it to circuits that demodulate it to separate the signal wave from the carrier wave, the incoming signals must be above the threshold of sensitivity of the receiver and tuned to the frequency of the signal.

Some radio devices act as both transmitter and receiver for radio signals. These devices are known as transceivers.When a responding signal is sent back to the originating radio, the radio transceiver changes modes from reception to transmission and back again. Cisco Aironet Access Points and bridges, as

transceivers, have this characteristic.Transceivers change modes from transmission to reception over and over again.They will do this many thousands of times per second.Though transceivers allow you to transmit and receive with the same device, thus reducing the size and cost of radios; in wireless networking, this capability introduces latency, a delay in communications. It is idiosyncratic to radio communications and negatively affects data throughput, albeit minimally.

Frequency

AC is the type of electric current generally used to produce electromagnetic fields. As you have seen (in Figure 2.2), AC alternates, or cycles over time.This cycling over a period of time is referred to as amplitude.The amplitude oscillates from zero to some maximum and back again.The number of times the cycle is repeated in one second is called the frequency. AC frequencies can range from a single cycle in thousands of years to quadrillions of cycles per second. Remember Heinrich Hertz, he is the one who invented the spark coil for generating and detecting radio waves.The unit of measurement for frequency is called a Hertz, after Heinrich. In fact, radio waves were originally called Hertzian waves. A Hertz

is usually defined as one cycle per second, or one wave per second.The fre- quency unit or Hertz is normally abbreviated Hz. Because frequencies can be very large, the standard units of quantities used in science and commonly seen in the data world are used to annotate them. For example, 1000 Hz equals 1 KHz (kilohertz), 1000 KHz equals 1 MHz (megahertz), 1000 MHz equals 1 GHz (gigahertz), and so on.

At any given instance, a radio wave will have an amplitude variation similar to that of its time variation. Picture the waves produced by a pebble dropped into a still pond. One of the waves traveling on the pond represents a radio wave, the height of that wave represents the amplitude and the speed at which that wave travels represents the time variation.The distance from the top of one wave to the next is known as the wavelength.The frequency of an electromagnetic field (RF field) is directly related to its wavelength. By specifying the frequency of a radio wave (f ) in megahertz and the wavelength (w) in meters, the two are inter- related mathematically, according to the following formula:

w= 300/f

In the car radio example, the radio is tuned to 96.3 MHz.This is the signal frequency of the radio station transmitter we want to “listen to.” At 96.3 MHz, the signal has a wavelength of about 3 meters, or about 10 feet.This same for- mula applies if the wavelength is specified in millimeters (mm) and the frequency is given in gigahertz.Therefore a Cisco Aironet AP that transmits a signal at 2.4 GHz would have an approximate wavelength of 120 mm, or a little less than 5 inches. Remember, all radio waves travel at the speed of light, so a radio wave with a shorter wavelength will cross a specific point in space (like an antenna) more times than a radio wave with a long wavelength.

In general, as the frequency of a radio gets higher the corresponding wave- length of the electromagnetic field gets shorter. At 9 KHz, the free space wave- length is approximately 33 kilometers (km) or 21 miles (mi). At the highest radio frequencies, the electromagnetic wavelengths measure approximately one mil- limeter (1 mm). As the frequency is increased beyond that of the RF spectrum, electromagnetic energy takes the form of various types of light and energy such as infrared light (IR), visible light, ultraviolet light (UV), X-rays, and gamma rays.

Electromagnetic radiation, as radio waves, can be generated and used at fre- quencies higher than 10 KHz. A considerable segment of the electromagnetic radiation spectrum is available for use, extending from about 9 KHz, the lowest allocated wireless communications frequency, to thousands of gigahertz, with the upper ends of the frequency spectrum consisting of gamma and cosmic rays.

Many types of wireless devices make use of radio waves. Radio and television broadcast stations, cordless and cellular telephone, two-way radio systems and satellite communications are but a few. Other wireless devices make use of the visible light and infrared portions of the frequency spectrum.These areas of the spectrum have electromagnetic wavelengths that are shorter than those in RF fields. Examples include most television remote controls, some cordless computer keyboards and mice, and many laptop computers.Table 2.1 depicts the eight bands of the frequency spectrum used in the United States Frequency Allocation, displaying frequency and bandwidth ranges.These frequency allocations vary slightly from country to country.

Table 2.1The United States Frequency Allocation Chart

Free-Space

Designation Frequencies Wavelengths

Very Low Frequency (VLF) 9 KHz–30 KHz 33 km–10 km Low Frequency (LF) 30 KHz–300 KHz 10 km–1 km Medium Frequency (MF) 300 KHz–3 MHz 1 km–100 m High Frequency (HF) 3 MHz–30 MHz 100 m–10 m Very High Frequency (VHF) 30 MHz–300 MHz 10 m–1 m Ultra High Frequency (UHF) 300 MHz–3 GHz 1 m–100 mm Super High Frequency (SHF) 3 GHz–30 GHz 100 mm–10 mm Extremely High Frequency (EHF) 30 GHz–300 GHz 10 mm–1 mm

The radio frequency (RF) spectrum is divided into several ranges, or bands. Most bands represent an increase of frequency corresponding to an order of mag- nitude of a power of 10.The exception to this is the extreme low end of the fre- quency spectrum.Table 2.2 shows examples of the classes of devices assigned to each frequency.

Table 2.2Example Device Classes by Frequency Allocation

Designation Examples

Very Low Frequency Radio navigation devices for marine vessels, mil- itary communication with nuclear submarines (maritime mobile)

Low Frequency Marine and aeronautical radio navigation and location devices

Medium Frequency Marine and aeronautical radio beacons, distress beacons, AM radio broadcasting, and maritime radio voice communications

High Frequency Amateur radio and satellite communications, radio astronomy, and space research

Very High Frequency Amateur radio and satellite, FM radio broad- casting, TV broadcasting (Channels 2–13), radio astronomy, mobile satellite communications Ultra High Frequency Fixed satellite communications, meteorological

satellite communications, amateur radio, TV broadcasting (Channels 14–36 and 38–69), WLANs, land mobile communications (cell phones, cordless phones, etc.), radio

astronomy, and aeronautical radio navigation Super High Frequency Inter-satellite communications, WLANs, weather

radars, land mobile communications Extremely High Frequency Space research, Earth exploration satellites,

amateur radio and satellite communications, radio astronomy, fixed and mobile satellite communications