Wireless Networks Had Significant Impact In Wwii Computer Science Essay

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Wireless networks have had a significant impact on the world as far back as World War II. Through the use of wireless networks, information could be sent overseas or behind enemy lines easily, efficiently and more reliably. Since then, wireless networks have continued to develop and their uses have grown significantly.

2.0 Wireless Technology

We need a power source and a transmitting antenna, and also a receiving antenna to which we can connect the thing to be powered (the electrical load, or just the load). The power source will deliver a high power signal to the antenna. This will create an electrostatic field around the antenna that changes as the signal to it changes. This will create electromagnetic waves that will travel out from the antenna and through the air.

The receiving antenna will be in the path of these waves, and the waves will pass by it and "sweep" it with their moving electromagnetic fields. This will induce a signal in the receiving antenna proportional to the energy that the antenna captures. This signal will cause current flow that will power the load.

The problem with this system is that it is challenging to "direct" and "confine" the transmitted signal to optimize how much of it gets to the receiving antenna. Additionally, distance causes loss, and there will be a lot of loss over longer distances. The more the distance the signal travels, the much more the loss. But the system works to a limited degree

3.0 How Information is transferred

Radio waves can be made to carry information by varying a combination of the amplitude, frequency and phase of the wave within a frequency band. When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the electrons of the conducting material. This effect (the skin effect) is used in antennas.

4.0 AM and FM Waves

AM stands for amplitude modulation. AM radio ranges from 535 to 1705kHz (kilohertz, or thousands of cycles per-second of electromagnetic energy).

How far an AM station's signal travels depends on such things as the station's frequency (channel), the power of the transmitter in watts, the nature of the transmitting antenna, how conductive the soil is around the antenna (damp soil is good; sand and rocks aren't), and, a thing called ionospheric refraction. The ionosphere (see illustration below) is a layer of heavily charged ion molecules above the earth's atmosphere.

Ionospheric refraction is a big issue, because AM radio waves can end up hundreds and even thousands of miles away from where they started, and in the process interfere with all other stations on the same frequency. But, as we'll see in a later module on international shortwave, ionospheric refraction can be good, because it makes possible long-distance communication.

Note that for AM radio stations the ground wave (in light blue above) doesn't go very far. This means numerous stations can be put on the same frequency without interfering with each other -assuming they are far enough apart. (Keep in mind that this drawing can't be anywhere near close to scale and show these things.) The ionosphere is much more effective in reflecting these radio waves at night. (Incidentally, technically, it's refracting, not reflecting, but the effect is somewhat the same.)

FM (frequency modulated) radio and TV waves don't act in the same way as AM radio waves. For starters, they are on a higher frequency in the RF spectrum

(The name RF, for radio frequency, was obviously named for radio, but when TV came along they just stuck with the name).

The FM radio band goes from 88 to 108 MHz (megahertz, or millions of cycles per second). FM stations must be 200kHz apart at these frequencies, which means that there's room for 200 FM stations on the FM band. But, unlike AM radio stations, FM stations don't end up being assigned frequencies with nice round numbers like 820 or 1240. Thus, an FM station may be at 88.7 on the dial. You may have noticed that FM stations don't reduce power or sign off the air at sunset. Because of their higher frequency ionospheric refraction doesn't appreciably affect FM or TV signals.

For the most part, FM and TV signals are line-of-sight. Although this means that FM stations don't interfere with each other, this characteristic creates a couple of other problems.

First, these waves go in a straight line and don't bend around the earth as AM ground waves do. Thus, they can quickly disappear into space.

So, the farther away from the FM or TV station you are, the higher you have to have an antenna to receive the FM or TV signal. Note that the earth is round and, therefore, these signals will literally leave the earth after 50 miles or so. And, there's another problem. Since FM and TV signals are line-of-sight, they can be stopped or reflected by things like mountains and buildings. In the case of solid objects like buildings, reflections create ghost images in TV pictures and that "swishing sound" when you listen to FM radio while driving around tall structures. Of course, the higher the FM or TV transmitter antennas are the greater area they will cover -which explains why these antennas are commonly very tall, or placed on the top of mountains. Note also from the drawing above that FM and TV signals tend to go through the ionosphere rather than being refracted form it. Again, this means that no matter what the station's power, it's signal will at some point leave the earth.

5.0 Basic Differences Between AM and FM

The term modulation refers to how sound is encoded on a radio wave called a carrier wave; or, more accurately, how the sound affects the carrier wave so that the original sound can later be detected by a radio receiver. In the top-left of this drawing the RF energy (carrier wave) is not modulated by any sound. There would be silence on your radio receiver.

Sound transmitted by an AM radio station affects the carrier wave by changing the amplitude (height) of the carrier wave, as shown on the left. Unfortunately, this type of modulation is subject to static interference from such things as household appliances -and especially from lightening storms.

AM also limits the loud-to-soft range of sounds that can be reproduced (called dynamic range) and the high-to-low sound frequency range (called frequency response, to be explained below).

FM radio, however, it's virtually immune to any type of external interference, it has a greater dynamic range, and it can handle sounds of higher and lower frequencies. This is why music, with its much greater frequency range than the human voice, sounds better on FM radio.

Note on the left that when the carrier wave of FM radio is modulated with sound that the distance between the waves, or the frequency of the carrier wave, changes. Thus, AM radio works by changing the amplitude of the carrier wave and FM radio works by changing the frequency of the carrier wave.

6.0 Antenna

An antenna (or aerial) is a transducer designed to transmit or receive electromagnetic waves. In other words, antennas convert electromagnetic radiation into electrical current. Antennas generally deal in the transmission and reception of radio waves, and are a necessary part of all radio equipment.

For a receiving antenna, this is at the (radio) receiver rather than at the antenna. Tuning is done by adjusting the length of an electrically long linear antenna to alter the electrical resonance of the antenna.

Antenna tuning is done by adjusting an inductance or capacitance combined with the active antenna (but distinct and separate from the active antenna). The inductance or capacitance provides the reactance which combines with the inherent reactance of the active antenna to establish a resonance in a circuit including the active antenna. The established resonance being at a frequency other than the natural electrical resonant frequency of the active antenna. Adjustment of the inductance or capacitance changes this resonance.

Antennas designed specifically for reception might be optimized for noise rejection capabilities. An antenna shield is a conductive or low reluctance structure (such as a wire, plate or grid) which is adapted to be placed in the vicinity of an antenna to reduce, as by dissipation through a resistance or by conduction to ground, undesired electromagnetic radiation, or electric or magnetic fields, which are directed toward the active antenna from an external source or which emanate from the active antenna. Other methods to optimize for noise rejection can be done by selecting a narrow bandwidth so that noise from other frequencies is rejected, or selecting a specific radiation pattern to reject noise from a specific direction, or by selecting a polarization different from the noise polarization, or by selecting an antenna that favors either the electric or magnetic field.

For instance, an antenna to be used for reception of low frequencies (below about ten megahertz) will be subject to both man-made noise from motors and other machinery, and from natural sources such as lightning. Successfully rejecting these forms of noise is an important antenna feature. A small coil of wire with many turns is more able to reject such noise than a vertical antenna. However, the vertical will radiate much more effectively on transmit, where extraneous signals are not a concern.

The ferrite rod antenna is a form of RF antenna design that is almost universally used in portable transistor broadcast receivers as well as many hi-fi tuners where reception on the long, medium and possibly the short wave bands is required.

As the name suggests the antenna consists of a rod made of ferrite, an iron based magnetic material. A coil is would around the ferrite rod and this is brought to resonance using a variable tuning capacitor contained within the radio circuitry itself and in this way the antenna can be tuned to resonance. As the antenna is tuned it usually forms the RF tuning circuit for the receiver, enabling both functions to be combined within the same components, thereby reducing the number of components and hence the cost of the set.

Typical ferrite rod antenna assembly used in a portable radio

The ferrite rod antenna operates using the high permeability of the ferrite material and in its basic form this may be thought of as "concentrating" the magnetic component of the radio waves. This is brought about by the high permeability -of the ferrite.

The fact that this RF antenna uses the magnetic component of the radio signals in this way means that the antenna is directive. It operates best only when the magnetic lines of force fall in line with the antenna. This occurs when it is at right angles to the direction of the transmitter. This means that the antenna has a null position where the signal level is at a minimum when the antenna is in line with the direction of the transmitter.

Operation of a ferrite rod antenna

7.0 Application

7.1 Remote control

It's no good the remote control just sending out a burst of random infrared. Clearly if your remote control has 20 or more buttons on it, it must have a way of sending out at least this many signals. When you press one of the buttons, the remote generates a systematic series of on/off infrared pulses that signal a binary code (a way of representing any kind of information using only zeros and ones, which computers use). So a short pulse of infrared could signal a 1 and no pulse could signal a 0. Sending many infrared pulses, one after another, allows your remote to send whole strings of zeros and ones. One code (maybe it's 101101) might mean "volume up", while another (perhaps 11110111) could mean "mute sound."

As well as sending out pulses that tell the TV what you want it to do, the remote also sends a short code that identifies the product you're trying to control (for example, a specific make and model of TV). That ensures your remote operates only the TV, not the video, and not any other TVs that happen to be nearby. Generally, this means each remote control unit can operate only one appliance made by only one manufacturer.

7.2 Radar dish

The radar dish, or antenna, transmits pulses of radio waves or microwaves which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the transmitter. The time it takes for the reflected waves to return to the dish enables a computer to calculate how far away the object is, its radial velocity and other characteristics.

The radar's frequency, pulse form, polarization, signal processing, and antenna determine what it can observe.Electromagnetic waves reflect (scatter) from any large change in the dielectric constant or diamagnetic constants. This means that a solid object in air or a vacuum, or other significant change in atomic density between the object and what is surrounding it, will usually scatter radar (radio) waves. This is particularly true for electrically conductive materials, such as metal and carbon fiber, making radar particularly well suited to the detection of aircraft and ships. Radar absorbing material, containing resistive and sometimes magnetic substances, is used on military vehicles to reduce radar reflection. This is the radio equivalent of painting something a dark color so that it cannot be seen through normal means.

Radar waves scatter in a variety of ways depending on the size (wavelength) of the radio wave and the shape of the target. If the wavelength is much shorter than the target's size, the wave will bounce off in a way similar to the way light is reflected by a mirror. If the wavelength is much longer than the size of the target, the wave is polarized (positive and negative charges are separated), like a dipole antenna. When the two length scales are comparable, there may be resonances. Early radars used very long wavelengths that were larger than the targets and received a vague signal, whereas some modern systems use shorter wavelengths (a few centimeters or shorter) that can image objects as small as a loaf of bread.

7.3 Cellular radio network

GSM (Global System for Mobile Communications) is a cellular network, which means that mobile phones connect to it by searching for cells in the immediate vicinity. There are five different cell sizes in a GSM network-macro, micro, pico, femto and umbrella cells. The coverage area of each cell varies according to the implementation environment.

Cell horizontal radius varies depending on antenna height, antenna gain and propagation conditions from a couple of hundred meters to several tens of kilometres. The longest distance the GSM specification supports in practical use is 35 kilometres (22 mi). There are also several implementations of the concept of an extended cell. where the cell radius could be double or even more, depending on the antenna system, the type of terrain and the timing advance.

Indoor coverage is also supported by GSM and may be achieved by using an indoor picocell base station, or an indoor repeater with distributed indoor antennas fed through power splitters, to deliver the radio signals from an antenna outdoors to the separate indoor distributed antenna system. These are typically deployed when a lot of call capacity is needed indoors; for example, in shopping centers or airports. However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of the radio signals from any nearby cell.

The modulation used in GSM is Gaussian minimum-shift keying (GMSK), a kind of continuous-phase frequency shift keying. In GMSK, the signal to be modulated onto the carrier is first smoothed with a Gaussian low-pass filter prior to being fed to a frequency modulator, which greatly reduces the interference to neighboring channels (adjacent-channel interference).

GSM networks operate in a number of different carrier frequency ranges (separated into GSM frequency ranges for 2G and UMTS frequency bands for 3G), with most 2G GSM networks operating in the 900 MHz or 1800 MHz bands. Where these bands were already allocated, the 850 MHz and 1900 MHz bands were used instead (for example in Canada and the United States). Most 3G networks in Europe operate in the 2100 MHz frequency band.

Regardless of the frequency selected by an operator, it is divided into timeslots for individual phones to use. This allows eight full-rate or sixteen half-rate speech channels per radio frequency. These eight radio timeslots (or eight burst periods) are grouped into a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate for all 8 channels is 270.833 kbit/s, and the frame duration is 4.615 ms.

GSM has used a variety of voice codecs to squeeze 3.1 kHz audio into between 6.5 and 13 kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called Half Rate (6.5 kbit/s) and Full Rate (13 kbit/s). These used a system based upon linear predictive coding (LPC). In addition to being efficient with bitrates, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal.

7.4 Bluetooth

Bluetooth wireless technology is a short-range communications technology intended to replace the cables connecting portable and/or fixed devices while maintaining high levels of security. The key features of Bluetooth technology are robustness, low power, and low cost. The Bluetooth Specification defines a uniform structure for a wide range of devices to connect and communicate with each other.

Bluetooth technology operates in the unlicensed industrial, scientific and medical (ISM) band at 2.4 to 2.485 GHz, using a spread spectrum, frequency hopping, full-duplex signal at a nominal rate of 1600 hops/sec. The 2.4 GHz ISM band is available and unlicensed in most countries.

Bluetooth technology's s adaptive frequency hopping (AFH) capability was designed to reduce interference between wireless technologies sharing the 2.4 GHz spectrum. AFH works within the spectrum to take advantage of the available frequency. This is done by the technology detecting other devices in the spectrum and avoiding the frequencies they are using. This adaptive hopping among 79 frequencies at 1 MHz intervals gives a high degree of interference immunity and also allows for more efficient transmission within the spectrum. For users of Bluetooth technology this hopping provides greater performance even when other technologies are being used along with Bluetooth technology.

8.0 Conclusion

Wireless networks are a product of convenience for society. Using a wireless network allows an individual to access the internet when he is not connected to a computer with an internet cable. Wireless Networks can be used by devices such as cell phones, laptops, and handheld computers. This technology is actually make the transmission of data easy. However, it will cause data less secure and the electromagnetic waves could be damaging to our health. So that, we should use the technology wisely in order to reduce the side effect of wireless telecommunication system.