Wireless LAN

Wireless Local Area Networks according to IEEE 802.11 standards has become extremely widespread in recent years, in campus networks, for home networking, for convenient network access at conferences, and to a certain point for commercial Internet access provision in hotels, public places, and even planes.

While wireless LANs are usually built more for convenience (or profit) than for performance, there are some interesting performance issues specific to WLANs. As an example, it is still a big challenge to build WLANs using multiple access points so that they can scale to large numbers of simultaneous users, e.g. for large events.

Common problems with 802.11 wireless LANs

Interference

In the 2.4 GHz band, the number of usable channels (frequency) is low. Adjacent channels use overlapping frequencies, so there are typically only three truly non-overlapping channels in this band - channels 1, 6, and 11 are frequently used. In campus networks requiring many access points, care must be taken to avoid interference between same-channel access points. The problem is even more severe in areas where access points are deployed without coordination (such as in residential areas). Some modern access points can sense the radio environment during initialization, and try to use a channel that doesn't suffer from much interference. The 2.4 GHz is also used by other technologies such as Bluetooth or microwave ovens.

Capacity loss due to backwards compatibility or slow stations

The radio link in 802.11 can work at many different data rates below the nominal rate. For example, the 802.11g (54 Mb/s) access point to which I am presently connected supports operation at 1, 2, 5.5, 6, 9, 11, 12, 48, 18, 24, 36, or 54 Mb/s. Using the lower speeds can be useful in terms of adverse radio transmission conditions. In addition, it allows backwards compatibility - for example, 802.11g equipment interoperates with older 802.11b equipment, albeit at most at the lower 11 Mb/s supported by 802.11b.

When lower-rate and higher-rate stations coexist on the same access point, it should be noticed that the lower-rate station will occupy disproportionally more of the medium's capacity, because of increased serialization times at lower rates. So a single station operating at 1 Mb/s and transferring data at 500 kb/s will consume an equal part of the access point's capacity as 54 stations also transferring 500 kb/s each, but at a 54 Mb/s wireless rate.

Multicast/broadcast

Wireless is a "natural" broadcast medium, so broadcast and multicast should be relatively efficient. But the access point normally sends multicast and broadcast frames at a low rate, to increase the probability that all stations can actually receive them. Thus, multicast traffic streams can quickly consume a large fraction of an access point's capacity as per the considerations in the preceding section.

This is a reason why wireless networks often aren't multicast-enabled even in environments that typically have multicast connectivity (such as campus networks). Note that broadcast and multicast cannot easily be disabled completely, because they are required for lower-layer protocols such as ARP (broadcast) or IPv6 Neighbor Discovery (multicast) for work.

802.11n Performance Features

IEEE 802.11n is an recent addition to the standards for wireless LAN offering higher performance in terms of both capacity ("bandwidth") and reach. The standard supports both bands, although "consumer" 802.11n products often work with 2.4 GHz only unless marked "dual-band". Within each band, 802.11n equipment is normally backwards compatible with the respective prior standards, i.e. 802.11b/g for 2.4 GHz and 802.11a for 5 GHz. 802.11n achieves performance increases by

  • physical diversity using multiple antennas in a "MIMO" multiple in/multiple out scheme
  • the option to use wider channels (spaced 40 MHz rather than 20 MHz)
  • frame aggregation options at the MAC levels, allowing larger packets or bundling of multiple frames to a single radio frame, in order to better amortize the (relatively high) link access overhead.

References

-- SimonLeinen - 15 Dec 2005 - 21 Oct 2008

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Topic revision: r2 - 2008-10-21 - SimonLeinen
 
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