transmission media

transmission media

In a data transmISSIOn system, the transmission medium is the physical path
between transmitter and receiver. Transmission media can be classified as guided or
unguided. In both cases, communication is in the form of electromagnetic waves.
With guided media, the waves are guided along a solid medium, such as copper
twisted pair, copper coaxial cable, or optical fiber. The atmosphere and outer space
are examples of unguided media, which provide a means of transmitting electromagnetic
signals but do not guide them; this form of transmission is usually referred
to as wireless transmission.
The characteristics and quality of a data transmission are determined both
by the characteristics of the medium and the characteristics of the signal. In the
case of guided media, the medium itself is usually more important in determining
the limitations of transmission. For unguided media, the bandwidth of the signal
produced by the transmitting antenna is usually more important than the
medium in determining transmission characteristics. One key property of signals
transmitted by antenna is directionality. In general, signals at lower frequencies
are omnidirectional; that is, the signal propagates in all directions from
the antenna. At higher frequencies, it is possible to focus the signal into a directional
beam.
Figure 2.10 depicts the electromagnetic spectrum and indicates the frequencies
at which various guided media and unguided transmission techniques operate.
In the remainder of this section, we provide a brief overview of unguided, or wireless,
media.
For unguided media, transmission and reception are achieved by means
of an antenna. For transmission, the antenna radiates electromagnetic energy
into the medium (usually air), and for reception, the antenna picks up electromagnetic
waves from the surrounding medium. There are basically two types of
configurations for wireless transmission: directional and omnidirectional. For
the directional configuration, the transmitting antenna puts out a focused electromagnetic
beam; the transmitting and receiving antennas must therefore
be carefully aligned. In the omnidirectional case, the transmitted signal spreads
out in all directions and can be received by many antennas.

Three general ranges of frequencies are of interest in our discussion of wireless
transmission. Frequencies in the range of about 1 GHz (gigahertz = 109 Hz) to
100 GHz are referred to as microwave frequencies. At these frequencies, highly
directional beams are possible, and microwave is quite suitable for point-to-point
transmission. Microwave is also used for satellite communications. Frequencies in
the range 30 MHz to 1 GHz are suitable for omnidirectional applications. We refer
to this range as the radio range.
Another important frequency range, for local applications, is the infrared portion
of the spectrum. This covers, roughly, from 3 X 1011 to 2 X 1014 Hz. Infrared is
useful in local point-to-point and multipoint applications within confined areas, such
as a single room.