Currently, there are three basic ways to transmit information: electricity, radio signals and beams of light. The term transmission is used to describe the process of sending information. All three types of transmission flow through something. Media is the term used to refer to what they flow through, something that is used to physically connect devices that communicate--this could be fiber-optic cable (beams of light), copper wire (electricity) or the air (radio waves). The 'media' for each type of transmission has different characteristics. Those characteristics will affect the transmission of the signal in one way or another. Please remember that radio and light are both energy transmissions, and are both part of the electromagnetic spectrum. Reduced to simplest terms, the only difference between radio waves and light, is the frequency of the wave, and that we can see some frequencies of light if it is in our visual range. Our bodies don't have senses that can perceive radio waves.
ELECTRICITY
Electricity is usually transmitted over copper wires (though any metal could be used). The phone system uses copper wire to transmit voice and data. Ethernet networks also use copper wire to connect devices. Electricity transmitted on a copper wire faces resistance from the copper and this resistance will distort the signal. This distortion, or noise reduces the maximum capacity of the wire according to the Nyquist theorem. Since all telecommunications technology over copper wires today uses direct current (DC), the noise and attenuation caused by copper's resistance to electricity is a serious limitation. Total distances for direct current communication is usually measured in thousands of feet, not miles. This weakening of the electric current does several things to the data carried by the electric signal. First, the farther the signal travels, the weaker it gets because some of the electricity is turned into heat by the resistance of the copper. This imposes a limit to how far a signal can be transmitted. Second, the copper wire can cause what is called attenuation. Parts of the signal simply get chopped off and are gone, changing the signal and causing information to be lost. Still, for short range communication, copper wire is usually the best choice. You see electricity used for Serial, Universal Serial Bus (USB), Parallel, Ethernet, FastEthernet, Token Ring, and even for Copper Data Distributed Interface (CDDI), a variation on FDDI.
Media for electricity can be coaxial cable, shielded or unsheilded twisted pair, or straight-ribbon phone wire (sometimes called satin).
RADIO
Radio communication links can be divided into two very broad groups: terrestrial, and non-terrestrial. Terrestrial radio systems are usually used for telephone, cellular, GCM and PCS phones, television, pagers, Citizens Band radios, Wi-Fi ethernet, Bluetooth and generally anything else that bills itself as wireless that has any appreciable range. Radio communication is possible between points on the globe that are within line of sight from each other. Most nonmetallic objects are radio-permeable, which means that radio signals can penetrate people, buildings, cars and other objects not made entirely of metal. So line of site is not entirely correct. A building between two radio towers doesn't block their transmission, but the earth does, so no two points can be 'over the horizon' from each other.
Non-terrestrial systems are used to communicate with satellites. These systems typically use microwave radio signals, powerful amplifiers and large dishes to focus the radio signal on a specific satellite. Satellites then can retransmit the information to another point on the globe that is not within line of site of the transmitting earth station. This circumvents the earthbound line of site problem.
Radio waves have two basic problems. They face line of sight problems, and absorption problems. Terrestrial systems cannot be placed so far apart that the curvature of the earth gets in the way of their signals. Terrestrial systems can sometimes 'bounce' signals off the atmosphere, but this is a rare occurrence and can't be depended upon. Furthermore, since the entirety of a terrestrial system's transmission passes through the earth's atmosphere near the surface where the atmosphere is densest, absorption becomes a major problem. Any weather system in between can reduce the operating effectiveness of a radio signal. Water affects radio waves. Fog, rain or snow all absorb some of the signal and will reduce the effective power of a transmitted signal. This is referred to as 'absorption'. Some radio frequencies are less susceptible to absorption than others.
The transmission media for radio communications is usually the air and coaxial cable.
LIGHT
There are three forms of light-based communication in use today.
- Laser
- Broadcast Infrared
- Fiber Optics
Laser based systems fire a tightly focused, high-powered laser beam from one point to another. The beam passes through the air, and is blocked by physical objects in it's path. Lasers can also be blocked or deflected by water in the atmosphere (fog and rain). All light can be bent by differences in atmospheric density caused by temperature differences. A hot building roof can deflect a signal aimed at the dish sitting on that roof during cooler weather. A cold front passing through the area can also deflect a laser signal.
Broadcast Infrared is commonplace today. An infrared signal is transmitted through the air in a general direction. Your television remote uses it, your infrared port on your Personal Data Assistant uses it, your computer even has an infrared port. Rather than focus the beam at a specific target, an infrared signal is transmitted in all directions or in one general direction. Because the beam is spread out, it is of lower power, and dissipates quickly. Effective transmission distances are usually measured in feet, and not more than a few hundred feet usually.
As you know from using your remote, distance is a limitation. Another limitation is line of sight. If anything gets between you and the TV, the remote doesn't always work, but it does work if you point the remote at a wall and 'bounce' the signal. However, water absorbs infrared, and infrared is essentially heat radiation, so it is affected by air that has large shifts in temperature also. If you've ever seen the heat waves rising off black pavement, you know what I am talking about. The hot air is less dense and light passes through more easily than the cooler air that has a tendency to deflect or diffract the light.
The transmission media for laser and infrared systems is usually the air.
Fiber Optics doesn't suffer from either line of sight or absorption under normal circumstances. Fiber optic systems use a laser beam fired into a special glass or plastic fiber with a transparent core. The fiber is designed to allow light to be transmitted along it's length. It is specially made to reflect light that reaches the outer edge of it's core, and gently deflect the light wave back to the center of the fiber. Thus, the light signal can be bent around corners. The fiber is sealed, thus preventing water from getting in the way. These fibers are tiny. Several fibers together are the width of a human hair, and each fiber can carry thousands of connections. Fiber optics are used most freqently by telecommunications companies. Most fiber optic cable is buried in the ground, so water sometimes creeps in and causes attenuation, but this is increasingly rare. Because fiber optics are so reliable, and difficult to disrupt (it practically takes a backhoe to stop it), they are the most popular form of long distance communication in use today.
The transmission media is the fiber optics itself. Fiber optic media comes in single mode and multi-mode. Multi-mode can handle a much wider range of light frequencies, but has a shorter range. Single-mode fiber has a far greater reach, but is limted in the light frequencies it can carry.