802.11g
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Introduction
Ratified in 2003, 802.11g added a significant increase in throughput to the older 802.11b standard. Unlike the earlier 802.11a revision, 802.11g maintains backward compatibility with 802.11b as it uses the same 2.4GHz band. It has since replaced 802.11b as the dominant standard and is used in the majority of modern WiFi equipment.
Note that due to similarities between the various WiFi standards, this article discusses factors specific to the 802.11g variant. Please see the WiFi article for detailed information on those common characteristics.
Modes
Infrastructure
The most common 802.11g 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.
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.
Unfortunately, most 802.11g devices will revert to the slower 802.11b data rates when used in this mode. This means that ad-hoc networks typically run at 1/4 the speed of infrastructure networks, strongly limiting their utility.
Performance
802.11g 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.
Range
Microwave communication is a complex topic so the range that can be obtained with these networks depends on many different variables. In common circumstances, mainstream 802.11g hardware will generally operate within a 90 meter (300 ft) range with best performance in the inner 30 meters (100 ft) of that radius. That range can be dramatically increased or decreased by the environment where the network is being used.
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. Additionally, WiFi provides provisions for roaming between several access points, so large areas can be covered by a more complex network.
Channels
While the official 802.11 specifications provide for up to 14 possible channels, most countries to not allow the use of them all. In North America, only 11 of these channels are available to users of WiFi equipment. In most other countries (Europe and most of Asia), the first 13 channels are available. The only country currently allowing all 14 channels is Japan.
Unfortunately, when used for WiFi these channels are aranged in a manner where they overlap one another. See the following diagram for an illustration of this arangement:
The 802.11g specifications require approximately 22MHz of bandwidth for each network that is in use. The channels, however, are configured in 5MHz increments so networks should be at least five channels away from others in the area. As such, approximately three 802.11b/g networks (operating on channels 1, 6 and 11) can operate without overlapping one another1. Placing channels closer together will result in interference and negatively effect the throughput and range of both networks.
If more networks are required, four channels (1, 4, 8 and 11) can be used with only minimal negative effects. While these channels will still overlap, they do so at the periphery of their range so the effects are minimized.
It is important to note that the 2.4GHz ISM band is also used by bluetooth, cordless phones, video cameras and many other wireless devices. Further, some Microwave ovens may also emit significant noise in this band when they are operating. As such, the presence of any of these items within range of a wireless network can cause interference and degrade performance. The 5MHz channels are provided for this reason, as they allow WiFi networks to work around these types of interference.
1WiFi signals do not have firm boundaries, instead they slowly decay as you get further from the centre of the channel. The 802.11 standards define the boundaries of the channel as the points where the signal is reduced to -30dB (1/1000th) of their maximum strength.
Extensions
Super G (Unofficial)
A proprietary extension provided by WiFi chipset vendor Atheros extends throughput to 108mbps. In addition to a number of extensions to the standard, this system uses a channel-bonding technique that uses two WiFi channels in parallel. In order to benefit from this technology, both the access point and network adapter must support this technology.
Unfortunately, with WiFi already limited to only three non-overlapping channels the use of two of them for a single network can be an issue. While it works well in facilities where only one network is active, it is not recommended in areas where other networks may be in range of the system.
Airgo True MIMO (Unofficial)
Using some technologies planned for use in the future 802.11n standard, this proprietary extension of the 802.11g standard improves both range and performance of wireless networks. Using a technique called spatial multiplexing, this system uses an array of independant radios coupled with a DSP to send several discrete signals down different paths in order to increase bandwidth.
The range and performance improvements depend on the environment where the devices will be used, however in ideal circumstances the latest iterations of this technology can provide throughput as high as 120mbps. Transfer rates also decay at a much slower rate, so performance at longer ranges is often significantly faster than traditional equipment.
As this technology is significantly more expensive than conventional 802.11g equipment (often 3-4x more), it is often only useful to users who need extremely high transfer rates. It is also important to note that although this system uses technology similar to 802.11n, devices using this system will not be compatible with the upcomming standard.
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.
- 802.11a - Also ratified in 1999, this 5GHz variant of the standard provided higher throughput but wasn't backwards compatible with it's predecessors.
- 802.11n - Still in the ratification process, this next generation protocol incorporates new technology to significantly boost throughput and range.
