A quick overview of the IEEE 802.11 standard and the ammendments that improved data rates. The first standard was accepted in 1997. It was often referred to as Prime as it was the original standard for WLAN. Since 1997 there have been several revisions to the standard. With the revisions have come better technologies as well as increased data rates. Beginning at 1 Mbps to now being capable of supporting Gbps rates the technology has greatly increased.
802.11 1997
Capable radios can broadcast in the 2.4 GHz ISM (industrial, scientific, and medical) band. The radios can use either FHSS (Frequency Hopping Spread Spectrum) or DSSS (Direct Sequence Spread Spectrum). The FCC has different regulations for the two radios. DSSS can use the entire 2.4 spectrum while FHSS is restricted to only 2.40-2.48 GHz. However a DSSS radio cannot communicate with a FHSS radio so it is key to have a single environment. Originally Prime, could transmit at 1 or 2 Mbps whether FHSS or DSSS was used.
802.11a – 1999
802.11a was also published in 1999 along with 802.11b. This was an important draft because it specified the use of the 5GHz UNII (Unlicensed National Information Infrastructure) band. With the 2.4 GHz band being a busy place with cordless phones and Bluetooth as well as other devices, the move to the 5GHz band was necessary. With 802.11a came increased data rates, rates jumped from 11 to 54 Mbps thanks to the use of OFDM (Orthogonal Frequency Division Multiplexing). The rates supported are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. 802.11a radios cannot communicate with 802.11b or 802.11 radios due in part to being in a different band and also because of the different technology used.
802.11b - 1999
802.11b implemented HR-DSSS (High Rate-Direct Sequence Spread Spectrum). The spectrum use is 2.4-2.48 GHz of the ISM band. 802.11 b increased data rates by using CCK (Complementary Code Keying). By using CCK the rates were increased from 1 or 2 Mbps to 11 Mbps. 802.11b offered rates of 1, 2, 5.5, and 11 Mbps. 802.11b is backward compatible with 802.11.
802.11g – 2003
This amendment was published in 2003 and defined 802.11g radios as clause 19 devices allowing use in the 2.4 Ghz spectrum as well as ERP (Extended Rate Physical). To achieve higher data rates ERP-OFDM was used, this allowed rates up to 54Mbps. 802.11g Supported rates of 6, 9, 12, 18, 24, 36, 48, and 54 and retained the ability to be backward compatible with 802.11b devices.
802.11n – 2009
This amendment brought several major breakthroughs in the form of speed and the capability to function in both the 2.4 and 5 Ghz frequencies. A new technology known as HT (High Throughput) and the use of wide channels made enhancements that supported up to 600 Mbps. HT capable radios use MIMO (multiple-input, multiple-output) which greatly reduce the effects of interference. 802.11n is also backward compatible with 802.11 a/b/g radios.
802.11ac – 2013
This amendment is currently the fastest offering rates in Gbps. 802.11ac uses several interesting technologies. VHT (Very High Throughput) was introduced and operates in the 5 Ghz frequency. 802.11ac also uses up to 160 Mhz channels and up to 8 spacial streams. It also uses MU-MIMO (Multi User – Multiple in Multiple Out) supporting several clients at once, 256 QAM ( Quadrature Amplitude Modulation) is also used. A good write up on QAM is located here: http://community.arubanetworks.com/t5/Technology-Blog/What-is-QAM/ba-p/114747
* 2.4 is not part of .ac certification but .ac devices are backwards compatible
