Going Wi-Fi: A Practical Guide to Planning and Building an 802.11 Network

Overview

Equipment that adheres to the specifications as set out in the 802.11 series of standards can use the unlicensed ISM and U-NII frequency bands only if certain rules are observed regarding transmitted power and signal modulation. For instance the original 802.11 standard was crafted for use in the ISM 2.4 GHz to 2.485 GHz band, because there was (and is) no licensing requirement so long as the transmitted power of the 802.11 equipment is no more than one Watt and the transmitter uses a "Spread Spectrum" transmission technique. The one Watt power restriction serves to limit the range where one radio may interfere with another. The spread spectrum requirement is intended to make the WLAN signal appear as background noise to a narrowband or narrow spectrum receiver.

The term "signal modulation" refers to techniques whereby data is superimposed on or "encoded" into a carrier signal wave by means of a process, which is referred to as "modulation." This modulation process changes the signal wave so that it varies in its signal pattern. All "signal modulation" is accomplished in either of two main ways: analog and digital.

All 802.11 technologies use digital modulation. In fact, there are a number of digital modulation techniques used by the 802.11 series. The most important though are spread spectrum, especially Direct Sequence Spread Spectrum (DSSS), Complementary Code Keying (CCK), and Orthogonal Frequency Division Multiplexing (OFDM). We will now take a closer look at each of these three signal modulation techniques.