IEEE created the 802.11 standard to define how radio frequency (RF) in the unlicensed industrial, scientific and medical (ISM) frequency bands is used for the Physical layer and the MAC sub-layer of wireless links. RF bands are allocated by the International Telecommunications Union-Radio (ITU-R). The designated bands for ISM communities are 900Mhz, 2.4Ghz and 5Ghz.

The first release of the 802.11 standard defined that the maximum data rate should be of 1-2Mb/s using the 2.4Ghz band. Because the demand for speed is high, new standards were developed. Today, we have 802.11a, 802.11b, 802.11g and, the in-development, 802.11n.

When you refer to one of these standards, you actually refer to a specific speed, a specific radio frequency. 802.11a and 802.11g are both able for speeds up to 54Mb/s, but they work on different radio frequencies. 802.11a is using the 5.7Ghz band, while 802.11g is using the 2.4Ghz band. The 802.11b is the slowest standard in terms of data rates. The maximum data rate is 11Mb/s using the 2.4Ghz band. The 802.11n standard is not finished yet, but is expected to allow a maximum data rate of 248Mb/s using two streams.

The data rates in WLANs are affected by the modulation technique. There are two modulation techniques you should be aware of in your preparation for the CCNA exam: Direct Sequence Spread Spectrum (DSSS) and Orthogonal Frequency Division Multiplexing (OFDM). ODFM allows for faster data rates, while DSSS is simpler and less expensive to implement than ODFM.

802.11a uses the ODFM modulation technique. Devices using 802.11a are able to reach speeds of up to 54Mb/s and use the 5Ghz band which makes it less likely to experience interference, than devices that operate at 2.4Ghz. However, using the 5Ghz band has some disadvantages. Higher RFs are easily absorbed by obstacles, such as walls, leading to poor performance. Also, the higher the frequency, the smaller the covered range. Some countries also have restrictions for high frequencies and you may not be allowed to use the 5Ghz band.

802.11b is using the 2.4Ghz band and provides speeds of 1, 2, 5.5 and 11Mb/s. The modulation technique used is DSSS.

802.11g is able to reach higher speeds using the same 2.4Ghz band due to the modulation technique used, ODFM. IEEE 802.11g is backward compatible with 802.11b and supports data rates of 6, 9, 12, 18, 24, 48 and 54Mb/s in addition to those supported by 802.11b.
Both, 802.11b and 802.11g, have advantages and disadvantages using the 2.4Ghz band. The main advantage when you use the 2.4Ghz band is the better range you get. On the other side, because there are too many consumer devices already using this band, you may encounter interferences.

802.11n is still in development and its main purpose is to improve WLAN data rates and range without requiring additional power or RF band allocation. 802.11n equipments will use multiple radios and antennas, each broadcasting on the same frequency to establish multiple streams. The multiple input/multiple output (MIMO) technology divides a high data-rate stream in multiple lower rate streams. The theoretical maximum data rate for two streams is 248Mb/s. The 802.11n standard is expected to be ratified by in November 2009, but this date is subject of change.

The WLAN standards are influenced by three organizations: ITU-R, IEEE, and Wi-Fi Alliance.

ITU-R is the organization handling the allocation of RF spectrum and satellite orbits.

IEEE is the organization developing the 802 standards for LAN and MAN and many more other standards.

The Wi-Fi Alliance is an association of vendors with the objective of improving the interoperability between products coming from different vendors.

We hope you found this lesson helpful as an introduction to Wireless LAN networks, as required for the Cisco CCNA exam. In future lessons, we will study different WLAN topics, like WLAN security and WLAN operation modes.