Introduction
Ratified in 1999 (shortly after
802.11b), the 802.11a standard added a new coding technique and operated on the
5GHz band. This allowed it to run at much higher speeds (54mbps) and provided a much larger range of channels to work with. This standard was originally intended to get
WiFi out of the increasingly crowded
2.4GHz band and form the foundation of future revisions.
Unfortunately,
5GHz radio equipment was limited at the time of its ratification so it took until 2001 before mature products appeared on the market. By this time,
802.11b had become firmly entrenched in the mass market so the fact that the two standards were not compatible meant that migration to 802.11a was limited.
When
802.11g was ratified in 2003, it provided the same throughput as 802.11a but maintained compatibility with
802.11b. That, combined with the lower cost of
2.4GHz equipment relegated 802.11a to niche markets. While it is seeing some resergence at this stage, it still holds a much smaller market share than the
2.4GHz 802.11g standard.
Note that due to similarities between the various
WiFi standards, this article discusses factors specific to the 802.11a variant. Please see the
WiFi article for detailed information on those common characteristics.
[
edit]
Modes
[
edit]
Infrastructure
The most common 802.11a mode, infrastructure networks are based around devices called
access points. When running in this mode, the
access point acts as a central control station for the network and provides access to a wired network. This provides more stable communications and makes it easy for wireless devices to connect to the Internet.
[
edit]
Ad-Hoc
The
WiFi standards offer an additional mode called
ad-hoc that allows stations to communicate directly with one another. This allows devices to form networks to be set up in the field without the requirement for additional equipment.
[
edit]
Performance
802.11a networks provide a maximum
theoretical throughput of 54mbps. As range increases and signal strength becomes weaker, the network will automatically step back to slower speeds (48/36/24/18/11/5.5/2/1mbps) to maintain the connection. This provides an adaptive system that provides the maximum throughput possible for the current conditions.
It is important to note that the rates mentioned above are
theoretical transfer rates and real-world networks will never be able to reach that level. The
802.11 standards have significant overheads that limit practical transfer rates to a maximum of approximately 25mbps. Additionally, if multiple devices are active on the network that bandwidth must be shared between them.
[
edit]
Range
The shorter wavelengths used in
5GHz transmission unfortunately mean that 802.11a networks are more sensitive to obstructions. This means that the pratical range of an 802.11a network is generally smaller than that of a similar
802.11g network. Fortunately, the shorter wavelength also means that stock antenae generally have more gain. As such, range varies dramatically from installation to installation so it's not easy to give absolute numbers.
As the
5GHz band is much less crowded than the
2.4GHz band, one of the major advantages to 802.11a is that interference is much less common. The wider selection of non-overlapping channels also means that many 802.11a networks can coexist with one another in confined spaces. This, in turn, means that it is easier to set up a network with many 802.11a
access points to provide full performance over a wide area.
If more range is required, larger antenae and/or more powerful radios can be used to expand the size of the covered area. Most mainstream
WiFi equipment is nowhere near the legal limits, so there is often a lot of room for improvement. It is important to note, however, that some jurisdictions do not allow removable antenae on equipment running in this band so users should verify this before purchasing these devices. Please check local regulations prior to using any form of range expanding equipment.
[
edit]
Channels
Unlike the
2.4GHz WiFi standards, the 300MHz of bandwidth provided by the
5GHz U-NII band allows 802.11a to provide up to 12
non-overlapping channels. Not all of these frequencies will be available in all countries, however the lowest eight channels are permitted in most. Users should check local regulations for more information on the limitations in their jurisdiction.
As all of the available channels are spaced in 20MHz increments (rather than the 5MHz steps used on
2.4GHz networks), adjacent channels can be freely used without interference. This allows a much higher density of 802.11a networks to be used in a given area without degrading performance. Please see the following diagram for an illustration of the three
U-NII bands:
Unlike the
2.4GHz band, this
spectrum is limited to a more focused group of technology. While there are other devices using this band, many of the common systems cluttering the
2.4GHz band aren't present here. Combined with the sheer size of the
5GHz band, this means that there is a lot more room for devices to coexist so interference is much less of an issue.
[
edit]
Dual-Band Radios
Most modern 802.11a equipment is integrated into devices with support for
802.11g as well. To provide this compatibility, these devices use separate
2.4GHz and
5GHz radios for each standard. As such,
access points and wireless
routers using this technology actually host two discrete networks - each with 54Mbps of its own bandwidth. This allows networks using a mixture of the two protocols access to a larger pool of bandwidth.
With the integration of
WiFi support in many new consumer electronics devices, this has lead to a bit of a resurgence in the 802.11a market. Using dual-band configurations, normal network traffic can be isolated from the high-bandwidth tasks these devices need to perform (eg transferring large video files) to maximize throughput.
[
edit]
See Also - 802.11 - Ratified in 1997, 802.11 was the basis of all other WiFi standards.
- 802.11b - Ratified in 1999, was the first standard in the family to become popularized.
- 802.11g - A variant of the standard that provides 802.11a speeds but uses the 2.4GHz band and maintains backward compatibility with 802.11b.
- 802.11n - Still in the ratification process, this next generation protocol incorporates new technology to significantly boost throughput and range.
- 5GHz U-NII Band - The microwave frequencies allocated for use by unlicenced radio equipment.
Linear Mode
