August 03, 2010, 2:01 PM — Over the last 10 years 802.11 technology has made remarkable strides -- getting faster, stronger and more scalable. But one problem still haunts Wi-Fi: reliability.
Nothing is more frustrating for network managers than users complaining about flaky Wi-Fi performance, spotty coverage and dropped connections. Dealing with a Wi-Fi environment that you can't see and that is constantly changing is the problem. And radio frequency interference is the culprit.
RF interference can be generated by almost any device that emits an electro-magnetic signal -- from cordless phones to Bluetooth headsets, microwave ovens and even smart meters. But what most companies don't realize is that the single biggest source of Wi-Fi interference is their own Wi-Fi network.
Unlike licensed spectrum that devotes a swath of bandwidth to the highest bidder, Wi-Fi is a shared medium that operates in the unlicensed RFs within the 2.4GHz and 5GHz range.
When an 802.11 client device hears another signal, whether it is a Wi-Fi signal or not, it will defer transmission until the signal ceases. Interference that occurs during transmission also causes packet loss, which forces Wi-Fi retransmissions. These retransmissions slow throughput and result in wildly fluctuating performance for all users sharing a given access point (AP).
While spectrum analysis tools are now being integrated into APs to help IT staff visualize and identify Wi-Fi interference, they are useless if they don't actually fix the problem.
The problem of RF interference is only exacerbated by the new 802.11n standard. 802.11n typically uses multiple radios within an AP to simultaneously transmit several Wi-Fi streams in different directions to achieve faster connectivity.
But now, twice as much can go wrong. If just one of these signals runs into interference, the ability to spatially multiplex or bond channels, two fundamental 802.11n techniques that help to yield dramatically higher data rates, is effectively eliminated.
Common approaches to dealing with interference
Three popular approaches to addressing RF interference include lowering the physical (PHY) data rate, reducing the transmit power of the affected AP or changing the AP's channel assignment. While each of these can be useful in some respect, none of them addresses the fundamental problem of dealing directly with RF interference.
The vast majority of APs on the market today use omni-directional, dipole antennas. These antennas send and receive transmissions equally in all directions. Since these antennas always transmit and receive exactly the same in all situations, when interference crops up, these systems only have one option to combat interference. They must lower their physical data rate until an acceptable level of packet loss is achieved.