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Wireless Communications with LANs
For some of the reasons described earlier in this chapter, it is often advantageous for a network to include some wireless nodes. Typically, though, the wireless nodes will be part of what is otherwise a traditional, cable-based network.

An access point is a stationary transceiver connected to the cable-based LAN that enables the cordless PC to communicate with the network. The access point acts as a conduit for the wireless PC. The process is initiated when the wireless PC sends a signal to the access point; from there, the signal reaches the network. The truly wireless communication, therefore, is the communication from the wireless PC to the access point. An access point transceiver is one of several ways to achieve wireless networking. Some of the others are described in later sections.

You can classify wireless LAN communications according to transmission method. The four most common LAN wireless transmission methods are as follows:
Characteristics of Radio Transmission

Designing a radio system to have the ideal characteristics for an application requires plenty of design tradeoffs. This is because the characteristics of radio transmissions change dramatically with frequency. Low-frequency radio, for example, supports limited data rates but has the significant advantage that it frequently can communicate past the horizon. Shortwave operators are familiar with this phenomenon, and they commonly can monitor transmissions from the other side of the earth.

As frequency increases, transmissions become increasingly line-of-site. AM radio broadcast frequencies, for example, range from kilohertz to low-megahertz. Perhaps you have picked up an AM radio station from several states away late at night, which can occur because AM radio transmissions can bounce off the atmosphere’s ozone layer. Some of the lowest-frequency AM radio transmissions can actually travel along the ground in a phenom-enon called ground waves. Some transmissions can bounce a considerable distance. Conversely, FM transmissions seldom can be received past the horizon—in fact, you can seldom clearly receive an FM broadcast beyond a range of 100 miles. This is partly a function of power, but the primary cause of the range limitation is the inability of FM frequencies to go beyond the horizon. On a line-of-sight basis, however, high-frequency transmissions attenuate less rapidly than low frequency transmissions.

Lower-frequency radio waves can penetrate solid materials to a greater degree than higher frequencies. Very low radio frequencies, for example, can be used to communicate with submerged submarines, although the data rates are extremely slow. Penetration capability also is a function of power—higher-power transmissions penetrate building walls more effectively than lower-power transmissions.

The following sections look briefly at these important wireless transmission methods.

Further Information