UnGuided or also called wireless or wireless communication, transport electromagnetic waves without using a physical conductor. Instead, signals are radiated through the air (or, in a few cases, water) and therefore, are available to anyone with a device capable of accepting them.
The electromagnetic spectrum
When the electrons move, they create electromagnetic waves that can propagate through free space (even in a vacuum). The British physicist James Clerk Maxwell predicted these waves in 1865 and the German physicist Heinrich Hertz produced and observed it for the first time in 1887. The number of oscillations per second of an electromagnetic wave is its frequency, f, and is measured in Hz (in honor of Heinrich Herz). The distance between two consecutive maxima (or minimums) is called wavelength and is designated universally with the Greek letter λ (lambda).
By connecting an antenna of the appropriate size to an electrical circuit, electromagnetic waves can be spread efficiently and picked up by a receiver at a distance — all wireless communication based on this principle.
In a vacuum, all electromagnetic waves travel at the same speed, no matter what their frequency. This speed, usually called the speed of light, c, is about 3 X 108 m / sec, or about 1 foot (30 cm) per nanosecond. In copper or fiber, the velocity drops to almost 2/3 of this value and becomes slightly frequency-dependent. The speed of light is the maximum speed limit. No object or signal can become faster than light.
The fundamental relationship between f, λ, and c (in the vacuum is):
λf = c
Where c is a constant, if we know f we can find λ and vice versa. For example, 1 MHZ waves have a wavelength of 300 m, and 1 cm waves have a frequency of 30 GHz.
We’ll be covering the following topics in this tutorial:
Radio waves are easy to generate, can travel long distances and penetrate buildings without problems, so they are used a lot in communication, both indoors and outdoors. Radio waves are also omnidirectional, which means that they travel in all directions from the source, so the transmitter and receiver do not have to align with care physically.
The properties of radio waves depending on the frequency. At low frequencies, the radio waves cross the obstacles well, but the power drastically reduced with the distance to the source, approximately in proportion 1 / r3 in the air. At high frequencies, radio waves tend to travel in a straight line and bounce off obstacles. The rain also absorbs them. At all frequencies, radio waves are subject to interference by motors and other electrical equipment.
Because of the radio’s ability to travel long distances, interference between users is a problem. For this reason, governments strictly legislate the use of radio transmitters.
In the VLF, LF and MF bands, radio waves follow the terrain, as shown in figure (a). These waves can be detected perhaps at 1000 km at the lower frequencies, and less at higher frequencies. The AM radio broadcast uses the MF band, and that’s why the AM radio stations in Boston can not be heard easily in New York. Radio waves in these bands easily cross buildings, and that is why portable radios work indoors. The main problem when using these bands for data communication is the relatively low bandwidth they offer.
Figure (a) In the VLF, VF and MF bands, the radio waves follow the curvature of the earth (b) in the HF band, the waves bounce off the ionosphere.
In the HF and VHF bands, waves at ground level tend to be absorbed by the earth. However, the waves that reach the ionosphere, a layer of charged particles that surround the earth at the height of 100 to 500 km, are refracted and sent back to our planet, as shown in Figure (b). In certain atmospheric conditions, the signals may bounce several times. Amateur radio operators use these bands for long-distance conversation. The army also communicates in the HF and VHF bands.
Above 100 MHz, the waves travel in a straight line and, therefore, can focus in a narrow beam. Concentrating the energy in a small beam with a satellite dish (such as the familiar satellite television dish) produces a much higher signal about noise, but the transmitting and receiving antennas must very well align with each other. Also, this directionality allows multiple transmitters aligned in a row to communicate with multiple receivers in rows, without interference. Before fiber optics, these microwaves formed the heart of the long-distance telephone transmission system for decades. The name of the long-distance telecommunications company MCI comes from Microwave Communications, Inc. because its entire system initially based on microwave towers (since then it has modernized the central portions of its network using fibers).
Since microwaves travel in a straight line, if the towers are far apart, parts of the earth get in the way. Consequently, periodic repeaters are needed. The higher the towers, the more separated they can be. The distance between the repeater rises very roughly with the square root of the height of the towers. With towers of 100 m of height, the repeaters can be spaced 80 km away. See figure.
Unlike radio waves at lower frequencies, microwaves do not pass through buildings well. Also, even though the beam may be well focused on the transmitter, there is some divergence in space. Some waves can be refracted in the lower atmospheric layers and take a little longer to arrive than direct waves. Deferred waves can get out of phase with the direct wave and thus cancel the signal. This effect is called multipath fading and is often a severe problem that depends on the weather and frequency.
In short, microwave communication used for both long-distance telephone communication, cell phones, television distribution, and other uses, which the spectrum has become very scarce. This technology has many advantages over fiber. The main one is that you do not need right of way; it is enough to buy a small piece of land every 50km and build a microwave tower in it to bypass the telephone system and communicate directly.
Microwaves are also relatively inexpensive. Erecting two simple towers (perhaps just large poles with four stop cables) and putting antennas on each can cost less than burying 50km of fiber through a congested urban area or on a mountain, and it can also be cheaper than renting Fiber from the telephone company, especially if the telephone company still does not fully pay for the copper it removed when it installed the fiber.
Infrared and millimeter waves
Unguided infrared and millimeter waves widely used for short-range communication. All remote controls for televisions, video recorders, and stereos use infrared communication. These controls are relatively directional, cheap and easy to build, but have one major drawback: do not go through solid objects (try to stand between your remote control and your TV and see if it still works). In general, as we move the longwave radio into visible light, the waves behave more and more like light and less and less like radio.
On the other hand, the fact that infrared waves do not pass well through solid walls is also an advantage. It means that an infrared system in a building room does not interfere with a similar system in adjacent rooms. Furthermore, the security of infrared systems against espionage is better than that of radio systems precisely for this reason. For this reason, it is not necessary to obtain a license from the government to operate an infrared system, in contrast to the radio systems that must be licensed.
These properties have made the infrared an exciting candidate for LANs wireless and interior. For example, the computers and offices of a building can equip with transmitters and infrared receivers that are relatively unfocused (that is, they are omnidirectional). This way the laptops capable of using infrared can be in the local LAN without having to connect to it physically. When several people show up for a meeting with their portable machines, they have to sit in the conference room to be connected entirely, without having to the plugin. Infrared communication can not be used outdoors because the sun shines with equal intensity in the infrared as a visible spectrum.
Transmission by light waves (laser beam)
Optical signage without guides has used for centuries. Paul Revere used binary optical signage from the old North church just before his famous trip. A more modern application is to connect the LANs of two buildings using lasers mounted on their roofs. Optical signalling coherent with lasers and inherently unidirectional, so that each building needs its laser and its photodetector. This scheme offers very high bandwidth and a little cost. It is also relatively easy to install and, unlike microwaves, it does not require a license from the FCC (Federal Communications Commission, Federal Communications Commission).
The advantage of the laser, a very narrow beam, is also a weakness here. To aim a laser beam of 1mm in width to a target of 1mm to 500 meters away requires the aim of a modern Oakley Annie. In general, lenses are added to the system to defocus the beam slightly.
One disadvantage is that lasers can not penetrate rain or dense fog, but it usually works well on sunny days.
Satellite transmissions are much more like direct-view microwave transmissions in which the stations are satellites that are orbiting the earth. The principle is the same as with terrestrial microwaves, except that there is a satellite acting as a super high antenna and as a repeater (see Figure). Although the signals that transmitted via satellite still have to travel in a straight line, the limitations imposed on the distance by the curvature of the earth are minimal. In this way, the relay satellites allow microwave signals to transmitted across continents and oceans as a single jump.
Satellite microwaves can provide transmission capacity and from any location on earth, no matter how remote it may be. This advantage makes quality communications available in undeveloped parts of the world without the need to make significant investments in inland infrastructure. Of course, the satellites themselves are costly, but renting time or frequency from one of them can be relatively cheap.
Line of sight propagation requires that the sending and receiving antennas are fixed / static concerning the location of the others at all times (one antenna must be able to see the other). For this reason, a satellite that moves faster or slower than the rotation of the earth is useful only for short periods (in the same way that a stopped clock is only accurate twice a day). To ensure constant communication, the satellite must move at the same speed as the earth so that it appears to fix at a certain point. These satellites are called geosynchronous.
Because the orbital velocity depends on the distance from the planet, there is only one orbit that can be geosynchronous. This orbit occurs in the equatorial plane and is approximately 36,000 kilometres from the surface of the earth.
However, a single geosynchronous satellite can not cover the entire earth. A satellite in orbit has a line of sight contact with a large number of stations, but the curvature of the earth still means that a large part of the planet can not yet see. Therefore, it is necessary to have a minimum of three satellites equidistant from each other in geosynchronous orbit to provide a complete global transmission. The figure shows three satellites, spaced 120 degrees apart in a geosynchronous orbit around the equator. It is a view from the north pole.
Frequency bands for satellite communication
The frequencies reserved for microwave communication via satellite are in the range of gigahertz (GHz). Each satellite sends and receives two different bands. The transmission from the ground to the satellite is called downlink.
Cellular telephony was designed to provide stable communications connections between two mobile devices or between a mobile unit and a stationary unit (ground). A server provider must be able to locate and follow the caller, assigning a channel to the call and transferring the signal from one channel to another as the device moves outside the range of one channel and within the range of another.
For this tracking to be possible, each cell service area divided into small regions called cells. Each cell contains an antenna and is controlled by a small central, called cell power station. In turn, each plant controlled by a switching centre called the mobile telephone switching office (MTSO). The MTSO coordinates the communications between all the cell power plants and the telephone exchange. It is a computer centre that is responsible for connecting calls and recording information about the call and billing.
The traditional cellular transmission is analogue. To minimise noise, frequency modulation (FM) used between mobile phones and cell exchange. The FCC assigns two bands for cellular use. The band between 824 and 849 Mhz carries all the communications that start on mobile devices. The band between 869 and 894 Mhz transports communications that start from landlines. The carrier frequencies distributed every 30Khz, which allows each band to support up to 833 carriers.
To make a call from a mobile phone, the user enters a 7 or 10 digit code (a phone number) and press the send button. At that moment, the mobile phone sweeps the band, searching for a start channel with a strong signal and sends the data (telephone number) to the nearest cell exchange using that channel. The cell exchange relays the data to the MTSO. The MTSO sends the data to the central telephone exchange. If the recipient of the call is available, a connection established, and the results returned to the MTSO. At that time, the MTSO assigns an unused voice channel to the call, and the connection established. The mobile phone automatically adjusts its tuning for the new channel and begins voice transmission.
When a landline phone makes a call to a mobile phone, the telephone exchange sends the number to the MTSO. The MTSO locates the mobile phone by sending questions to each cell in a process called paging. Once the mobile device has found, the MTSO transmits a call signal, and when the mobile device responds, it allocates a voice channel, allowing the transmissions to begin.
It may happen that during a conversation, the mobile device moves from one cell to another. When it does, the signal can weaken. To solve this problem, the MTSO monitors the level of the signal every few seconds. If the signal strength decreases, the MTSO looks for a new cell that can better accommodate that communication. At that moment, the MTSO changes the channel that carries the call (transfers the signal from the old channel to a new one). The transfers carried out so smoothly that most of the time, they are transparent to the users.
Analogue cellular (FM) services based on a standard called analogue cellular circuit-switched (ACSC). To transmit digital data using an ACSC service, it is necessary to have a modem with a maximum speed of 9,600 to 19,200 bps.
However, since 1993, several service providers have started using a cell phone standard called cellular digital data packets (CDPD). CDPD provides a low-speed digital service through the existing cellular network. It based on the OSI model, which study in the second unit of the module.
To use existing digital services, such as 56kbps switching services, CDPD uses what is called a tri-sector. A tri-sector is a communication of three cells, each of which uses 19.2 kbps, which allows a total of 57.6 Kbps (which can accommodate on a 56K switched line eliminating some overload). Following this scheme, the United States has been divided into 12,000 trisects. There is one router for every 60 trisects.