Channel Planning isn’t Easy for Algorithms

If you’ve ever had to create a manual channel plan where spectrum is scarce, you know how hard it is to get it right. You run out of virgin spectrum, then the difficult choice of channel reuse is encountered. Often, what looks acceptable on an architectural plan, doesn’t hold up to post-deployment validation. Two AP’s 6 classrooms away on the same channel can hear each other at a loud and clear -65 dBm RSSI. Reuse the same channel in the classroom directly above, and the signal disappears below the noise floor. An extra inch of concrete make all the difference. To get it right, you have test, change, check, test, change, check, etc.

24channelplan
A 2.4Ghz channel plan

Given the challenge, it’s no surprise that I’ve never encountered an automatic channel selection algorithm that produced better results. At least, not in high density designs where spectrum is scarce, which is more and more just about everything I design. AP’s directly adjacent to one and other end up on the same channel, blasting away at max transmit power.

Speaking of power, I’ve also never encountered an algorithm that satisfactorily handled transmit power control in a high density network. They always turn things up WAY too high. As in, I’m manually taking AP’s that were auto-set from 12-20 dBm down to 4-6 dBm to shrink their cells away from the AP’s that share their channel. That can mean a 10x reduction in power! And even when power levels are auto-set to an acceptable level, I’ve yet to meet an algorithm that proportionally adjusts an AP’s receive sensitivity to accommodate the smaller cell.

Another thing algorithms don’t do well is handle DFS channels. I use DFS channels in many high density designs, but there are always some clients that don’t support them. The best thing to do in that case is to evenly distribute DFS channels throughout the WLAN, and only use them where AP density would otherwise cause non-DFS channel overlap. In those environments I like to alternate non-DFS channels with DFS channels so that clients without DFS support are still within range of a 5 GHz radio they can use. My experience with channel selection algorithms has been that a group of adjacent of AP’s may all be set to a DFS channel, creating a 5 Ghz dead-zone for clients that don’t support DFS channels.

What gives?

This is all too bad because auto channel/power features would be ideal as it dynamically adjusts to changes in the RF environment. A neighbor puts up a new AP on one of your channels and, without intervention, the algorithm moves your AP to clean spectrum elsewhere. In urban environments, this is a highly desirable feature because there is so much RF in your environment that is out of your control.

Every WLAN vendor offers automatic channel/power selection. They all ticked that box a long time ago. But who’s got an algorithm that actually works?

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