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Wireless LAN Standards

802.11 wireless LAN is an IEEE standard that defines 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.

When 802.11 was first released, it prescribed 1 - 2 Mb/s data rates in the 2.4 GHz band. At that time, wired LANs were operating at 10 Mb/s so the new wireless technology was not enthusiastically adopted. Since then, wireless LAN standards have continuously improved with the release of IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, and draft 802.11n.

Typically, the choice of which WLAN standard to use is based on data rates. For instance, 802.11a and g can support up to 54 Mb/s, while 802.11b supports up to a maximum of 11 Mb/s, making 802.11b the "slow" standard, and 802.11 a and g the preferred ones. A fourth WLAN draft, 802.11n, exceeds the currently available data rates. The IEEE 802.11n should be ratified by September 2008. The figure compares the ratified IEEE 802.11a, b, and g standards.

Click the Table button in the figure to see details about each standard.

The data rates of different wireless LAN standards, are affected by something called a modulation technique. The two modulation techniques that you will reference in this course are Direct Sequence Spread Spectrum (DSSS) and Orthogonal Frequency Division Multiplexing (OFDM). You do not need to know how these techniques work for this course, but you should be aware that when a standard uses OFDM, it will have faster data rates. Also, DSSS is simpler than OFDM, so it is less expensive to implement.

802.11a

The IEEE 802.11a adopted the OFDM modulation technique and uses the 5 GHz band.

802.11a devices operating in the 5 GHz band are less likely to experience interference than devices that operate in the 2.4 GHz band because there are fewer consumer devices that use the 5 GHz band. Also, higher frequencies allow for the use of smaller antennas.

There are some important disadvantages to using the 5 GHz band. The first is that higher frequency radio waves are more easily absorbed by obstacles such as walls, making 802.11a susceptible to poor performance due to obstructions. The second is that this higher frequency band has slightly poorer range than either 802.11b or g. Also, some countries, including Russia, do not permit the use of the 5 GHz band, which may continue to curtail its deployment.

802.11b and 802.11g

802.11b specified data rates of 1, 2, 5.5, and 11 Mb/s in the 2.4 GHz ISM band using DSSS. 802.11g achieves higher data rates in that band by using the OFDM modulation technique. IEEE 802.11g also specifies the use of DSSS for backward compatibility with IEEE 802.11b systems. DSSS data rates of 1, 2, 5.5, and 11 Mb/s are supported, as are OFDM data rates of 6, 9, 12, 18, 24, 48, and 54 Mb/s.

There are advantages to using the 2.4 GHz band. Devices in the 2.4 GHz band will have better range than those in the 5GHz band. Also, transmissions in this band are not as easily obstructed as 802.11a.

There is one important disadvantage to using the 2.4 GHz band. Many consumer devices also use the 2.4 GHz band and cause 802.11b and g devices to be prone to interference.

802.11n

The IEEE 802.11n draft standard is intended to improve WLAN data rates and range without requiring additional power or RF band allocation. 802.11n uses multiple radios and antennae at endpoints, each broadcasting on the same frequency to establish multiple streams. The multiple input/multiple output (MIMO) technology splits a high data-rate stream into multiple lower rate streams and broadcasts them simultaneously over the available radios and antennae. This allows for a theoretical maximum data rate of 248 Mb/s using two streams.

The standard is expected to be ratified by September 2008.

Important: RF bands are allocated by the International Telecommunications Union-Radio communication sector (ITU-R). The ITU-R designates the 900 MHz, 2.4 GHz, and 5 GHz frequency bands as unlicensed for ISM communities. Although the ISM bands are globally unlicensed, they are still subject to local regulations. The use of these bands is administered by the FCC in the United States and by the ETSI in Europe. These issues will impact your selection of wireless components in a wireless implementation.

Cisco CCNA Exploration 4.0 Lan Switching and Wireless chapter 7.1.2.1