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Transmission Frequencies
Transmission media make possible the transmission of the electronic signals from one computer to another computer. These electronic signals express data values in the form of binary (on/off) impulses. The signals are transmitted through the network using a combination of electronic devices (such as network boards and hubs) and transmission media (such as cables and 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 microwave and infrared light. Different media are used to transmit the signals, depending on the frequency of the EM waveform. Figure 3.1 illustrates the range of electromagnetic waveforms (known as the electromagnetic spectrum) and their associated frequencies.

The electromagnetic spectrum consists of several categories of waveforms, including radio frequency waves, microwave transmissions, and infrared light.

Radio frequency waves often are used for LAN signaling. Radio frequencies can be transmitted across electrical cables (twisted-pair or coaxial) or by using radio broadcast transmission.

Microwave transmissions can be used for tightly focused transmissions between two points. Microwaves are used to communicate between Earth stations and satellites, for example, and they also are 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 fiber-optic cables.

The next sections examine examples of network transmission media and describe the advantages and disadvantages of each media type.

Further Information