Multiple Access Points

The generally accepted practice in running a single Wi-Fi access point is to perform a site survey to determine which is the least used frequency and then place an access point on channel 1, 6 or 11 depending on the results.  If you need an additional access point, then generally using channel 1 for the first and channel 11 for the second is the recommended method, ensuring the least possible interference between your two access points.  This may need to change depending on neighbouring access point activity over time.

If you need more than 2 access points, the generally accepted method of allowing for this is a mesh of access points using channels 1, 6 and 11 placed in appropriate positions around the site, the positions being determined after a comprehensive RF site survey.  A diagram showing how to install a mesh of Wifi access points to cover the entire floor space of a large warehouse is shown below:

 As you can see, there’s some overlap between channels (allowing for devices to roam around the site) and a bit of excess spill into neighbouring premises.  Now, as this is a 2 dimensional representation of the warehouse, this looks all nice.  Were we to look at this in 3 dimensions, it would look a lot less pretty.  If multiple-storey buildings need saturated Wi-Fi coverage, this is where things get, well, hairy.

Also, this is where people start looking at the Four Channel System and come unstuck.  As can be seen on the Cisco site, their research into the use of a 3 Channel System versus a 4 Channel System showed that using a 3 channel system resulted in higher overall throughput.  This is where experience of Wi-Fi, RF and general networking comes in handy and where a lot of misunderstanding by a lot of people will result in a poor performing WiFI network.

3Com, Cisco, DLink and a number of other manufacturers have controllers that will manage wireless roaming across a Wi-Fi mesh.  These systems are not cheap, but they do allow a single device to be recognized by and to authenticate to all Wi-Fi access points in your organization’s mesh infrastructure.

Further discussion on Wi-Fi mesh architecture is beyond the scope of this article, considering it has already gotten technical enough to scare many people off!

802.11b, 802.11g – which to choose?

When configuring an 802.11g Wi-Fi AP you can normally choose between 802.11g only, 802.11b only, or 802.11b+802.11g.  The issue here is that if a single 802.11b device connects to an 802.11b+g AP, the AP will run in reduced speed mode until that device dissociates from the AP, when it will then go straight back to full 802.11g speeds.  That’s fine if you have very few wireless devices and they don’t transfer a lot of data, but is far from acceptable if you are heave WiFi users.  In that case, we’d strongly suggest running an 802.11g-only access point on channel 1 (if possible) and a separate 802.11b AP on channel 11 (if possible) to ensure that “g” users keep running at full speed whilst “b” users can connect as well.  Please note that this "b+g" issue does not mean that "g" traffic runs at "b" speed, but just that it is reduced to a lower speed that were only "g" traffic present on that channel.