Friday, July 1, 2011
The very first 802.11 wireless networking standard was ratified in 1997. These first wireless networks were very slow, and barely usable. Early 802.11 used FHSS modulation and could only achieve speeds of 1 and 2Mb. It wasn't until 1999 when 802.11b was ratified that wireless networking began to really catch on and speed up. Around the same time, 802.11a access points were available and could support wireless speeds of up to 54Mbps, but 802.11a didn't catch on with enterprise customers or home users since it was more expensive, and there weren't nearly as many client devices that supported the 802.11a (5GHz) frequencies. This pattern of wireless adoption leaning towards 2.4GHz continued on for many years.
In 1999 you could only hope for 2.4GHz wireless speeds of a theoretical 11Mb, but more like 5.5 actual throughput due to the half-duplex nature of wireless technology. The DSSS data rates supported speeds of 1Mb, 2Mb, 5.5Mb and 11Mb. When OFDM for 2.4GHz was released in 2003 the additional data rates of 6, 9, 12, 18, 24, 36, 48, 54 became available in the 2.4GHz frequency. Four years earlier 802.11a had been able to support the same speeds, but there were simply more 802.11b/g client devices available.
With the ratification of 802.11n finally happening in 2009, the 2.4GH frequencies are now capable of the additional speeds when using 20Mhz wide channels of 7.2, 14.4, 21.7, 28.9, 43.3, 57.8, 65 and 72.2. The real speed increases of 802.11n can be realized when two channels are bonded together into a 40Mhz wide channel to double the theoretical throughput to speeds such as 15, 30, 45, 60, 90, 120, 135 and 150. Of course, there are still only three non-overlapping 2.4Ghz channels (1, 6, and 11) so bonding channels together in the 2.4GHz spectrum quickly leaves you with little room for a non-overlapping channel plan. Utilizing the 5GHz spectrum for 40Mhz channel bonding is the obvious choice. The 5GHz spectrum allows for at least 12 non-overlapping channels (depending on the country codes in use).
Early 1 & 2Mb wireless networks usually did not incorporate antenna diversity into the design, but even as early as 1999 access points were designed with antenna diversity capabilities. Antenna diversity is used to increase the odds that you receive a better signal on either of the antennas. This only becomes more important as you can see in 802.11n access points. MIMO (Multiple Input, Multiple Output) antennas are integral to achieving 802.11n wireless speeds.
Higher throughput via 802.11n is possible with mutiple antennas as well as access points that are capapble of sending multiple data streams. The number of spatial streams an access point is capable of supporting is represented by a X b : c. (a) represents the number of transmit antennas (b) is the number of receive antennas, and (c) is the maximum number of spatial streams the access point/radio can support. An access point identified as 3x3:2 has three antennas for transmitting, three for receiving and is capable of sending two concurrent spatial streams. It is possible to achieve data rates up to 600 Mbit/s with four spatial streams using a 40 MHz-wide channel. Of course this also now means you need to use a gigabit switch to connect your access points to the LAN or you're creating a potential network bottleneck at the switch port.