TRANSMISSION FREQUENCIES

Transmission media make possible the transmission of the electronic signals from one computer to another. These electronic signals express data values in the form of binary (on/off ) impulses, which are the basis for all computer information (represented as 1s and 0s). These signals are transmitted between the devices on the network, using some form of transmission media (such as cables or radio) until they reach the desired destination computer. All signals transmitted between computers consist of some form of electromagnetic (EM) waveform, ranging from radio frequencies through microwaves and infrared light. Different media are used to transmit the signals, depending on the frequency of the EM wave form. Figure illustrates the range of electromagnetic waveforms (known as the electromagnetic spectrum) and their associated fre-quencies.

The electromagnetic spectrum consists of several categories of wave forms, including radio frequency waves, microwave transmissions, and infrared light. The frequency of a wave is dependent upon the number of waves or oscillations that occur during a period of time. An example that all people can relate to is the difference between a high-pitched sound, such as a whistle, and a low-pitch sound such as a fog horn. A high- pitched sound has a very high frequency; in other words, numerous cycles of oscillation (or waves) occur each second. Whereas, a low frequency sound, such as the fog horn, is based on relatively few cycles or waves per second (see Figure ). Although sound is not an example of electromagnetic energy (it's mechanical energy), the principles are similar.

Radio frequency waves are often used for LAN signaling. Radio fre- quencies can be transmitted across electrical cables(twisted-pair or coaxial) or by radio broadcast. Microwave transmissions can be used for tightly focused transmis- sions between two points. Microwaves are used to communicate between earth stations and satellites, for example, and they are also used for line-of-sight transmissions on the earth's surface. In addition, microwaves can be used in low-power forms to broadcast signals from a transmitter to many receivers. Cellular phone networks are examples of systems that use low-power microwave signals to broadcast signals. Infrared light is ideal for many types of network communications.

Infrared light can be transmitted across relatively short distances and can be either beamed between two points or broadcast from one point to many receivers. Infrared and higher frequencies of light also can be transmitted through ?ber-optic cables. A typical television remote control uses infrared transmission. The next sections examine the major factors you should consider when evaluating what type of transmission media should be implemented.