Static Channel Assignment vs RRM/ARM
Every time I scan a network that uses Radio Resource Management (RRM) or Adaptive Radio Management (ARM) and I see access points on the same channels near each other I cringe. I see this even with networks that are using all 26 5GHz channels. Why does RRM/ARM repeat channels so frequently? Every time I see this and I hear people saying RRM works every time without issue or configuration I have to chuckle a bit. Well, I think you will see as far as Channel planning it does not work well.
I have always been a fan of static channel assignments and static power. I know I am a dinosaur, but this is the only way you can be assured you will not see the same channel repeated very often.
I still recommend using an 8 channel plan, especially when using voice (as I said I am a dinosaur). I believe this is not a problem since back in the day we had 2.4GHz and only had 3 non over lapping channels to work with. Now if we recommend anyone use only 8 channels some network administrators cry like little schoolgirls saying “We want to use all the channels because we want the higher data rates that 40MHz and 80MHz channels give us”.
First of all, if you are using 80MHz in an enterprise environment (on 5GHz) you should be tarred and feathered. We all know how inefficient 80MHz channels are, but some Network Admins have bought into the marketing hype that bonded channels give you higher data rates and a faster more efficient network. This argument has plenty of holes in it.
The two I like to bring up are;
I was once at a site that was using Extreme Access Point running Radio Resource Management (or whatever their version is). They had only 30 Access Points and were using 26 5GHz channels so, in theory, I should have seen only 4 channels repeated once and most channels never repeated. This of course was not the case. There were multiple times when I scanned the network and saw the same channels within range of each other. I can hear the skeptics saying that only happens with brands like Extreme but if you look at the screenshots below you will see this happens with Cisco and Aruba as well.
Each AP vendor has a crazy way of coming up with a list of preferred channels and they use these channels more often than others. Cisco used to heavily use Channel 36; now they’ve seemed to fix the channel 36 issues, but they still repeat channels in an RRM configuration far more than you would if you manually planned the channels.
When we design a wireless network, we always use a static channel and static power; this works extremely well. The problem comes in when we install the system and for whatever reason we let RRM take over after the design is done. Why is this? I have never heard of a design or customer requirement that states let RRM choose the channel and power. When you are planning your design the channel plan may be chosen by the design software like Ekahau or AirMagnet but this is still far better than RRM.
There are some arguments to be made for RRM over static channels design. Two of the arguments are the DFS channel issue and the coverage issues (if an AP goes offline). The first argument is the DFS channel issue. If you assigned your channel plan statically and used DFS channels, when the AP hears a radar event by the regulation, the AP needs to stay off that channel for 30 min. If you used static channels the AP would have to stop transmitting for 30 minutes. The AP does this because it lacks a mechanism to shift from the affected channel. The second argument for RRM is the coverage issue. If an AP goes offline then RRM can manually adjust the power of existing APs to fix the coverage gaps.
There are two fixes for this. The first is to keep to the 8-channel plan and you would never use a DFS channel. The second fix would be to design your network for voice coverage (two APs at -65 or better) if one AP went offline due to a radar event then there would be another AP to make sure the coverage was still good.
There are three screen shots below two from a Cisco network and one from an Aruba Network. These screenshots show multiple examples of RRM/ARM using the same channel in close proximity of each other.
This screenshot shows 2 APs on Channel 36 , 3 APs on Channel 100 . There is separation on some of these APs but my point is that, when left to RRM, Cisco uses channels more than they should on the same floor. This screenshot was filtered to show only the APs that my device was connecting to.
This screenshot is from a Cisco site running RRM. You can see multiple APs have the same channel. There are 5 APs on Channel 36 , 6 APs on Channel 44 , and 2 APs on Channel 149 . This screenshot was filtered to show only the APs that my device was connecting to.
Just in case you are thinking I am just picking on Cisco here is an Aruba site that has 2 APs using channel 44 , 2 APs using Channel 149 , and 2 APs using Channel 100 . This screenshot was filtered to show only the APs my device was connecting to.
In conclusion static channel power is the only way you can be certain that channels will not get used over and over again. RRM and ARM have there place but nothing can implement your channel plan better than you can.
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