Some Practical 802.11n Thoughts

A number of different sources have brought some interesting details to the light of day for me concerning 802.11n products. In the recent edition of the German computer magazine C’t (23/2007) Dusan Zivadinovic published an interesting article on the performance of 802.11n access points. Here are some interesting thoughts:

  • Some access points have a channel auto configuration option. While in general this is a good feature it will always be suboptimal because the access point can just select a channel with low noise from his point of view but can not take the situation of a client into account. For a client the selection of the access point could be less than optimal since it could receive interference from a network which is not visible at the access point.
  • 5GHz band radar avoidance: To be done once every 24 hours for 60 seconds. The access point then stops transmitting and monitors the selected frequency band for spurious signal peaks.
  • Some WLAN routers like those from LANCOM (quite expensive APs) check for general noise (e.g. from other networks) and change the channel automatically.
  • In Europe, the maximum transmission power in the 2.4 GHz band is 100mW. In the 5 GHz band up to 200 mW is allowed which can compensate for the higher absorption rate of signals in this higher frequency band. Dusan notes that in practice he didn’t observe much of a range difference between usage of the 2.4 GHz and 5 GHz band.
  • For distant devices it’s better when the access point falls back to a 20 MHz channel. Makes sense from my point of view because the signal energy can then be concentrated in a narrower band which should increase range.
  • Devices that can be operated in the 2.4 GHz band and 5 GHz band simultaneously must use different sets of antennas for each band. Interesting, I thought, so how does the simultaneous dual band capable Linksys WRT600N do this with 3 external antennas? A bit of research revealed that the WRT600N has 4 external antennas and 2 internal antennas, so 6 in total. For each band 3 antennas are used. Here’s a link to a picture that shows the cables leading to all antennas.

For more on 802.11n on this blog click on the ‘802.11n’ tag below.