The Three Best Ways to Control AP Cell Size (and Two You Shouldn’t Use)

I was watching a video recently where a Wi-Fi engineer was recommending changing the minimum data rate to shrink cells. The scenario very typical, clients were hanging on to APs instead of roaming to an AP in the same room. A few minutes later, this all made sense when I made the connection that he worked with Ruckus gear.

I’ll be transparent in that I don’t like the typical Ruckus design methodology. The idea of leveraging maximum transmit power results in sticky clients, and because clients often measure signal based on the beacon, they think the signal is ok. So when I hear folks trying to use data rates to adjust cell sizing because they won’t use the proper tool for the job, I cringe.


There are three primary methods to alter the size of an APs effective cell: Antenna, Attenuation, and Transmit Power. Sure, there are other options like data rates, RX-SOP, but I classify these as airtime conservation techniques. Can you alter the cell size with these setting? Sure, but you probably shouldn’t.

Antennas:

Antennas are the first mechanism to control cell sizing. They are also the most expensive and impactful way to adjust the size of a cell because you have to purchase and install different antennas. This scenario is likely only an option during installation or refresh. But in the words of Keith Parsons, the goal is to put RF where you want it, and not where you don’t. 

Attenuation:

Attenuation is often not something we have control over. The architects and builders specify the building materials. However, we can control where we place access points to maximize the use of the attenuation that is present. Keeping that cell small by putting it APs where attenuation will absorb more can help you keep cell size smaller.

Transmit Power:

Transmit Power is the last option where you should be controlling cell size, and it’s the one configuration knob built to do it. The more Transmit Power you use, the further your signal will go. If the goal is to make clients roam, you need the signal to drop off to the point where clients decides to roam.


Now, on to how you shouldn’t control your cell sizing.

Data Rates:

Clients base their roaming on RSSI and SNR. Changing the minimum data rate doesn’t change how loudly clients hear the beacon. Therefore it doesn’t help them make roaming decisions. Ultimately the roaming decision gets made when they can no longer hear the beacon which happens suddenly and can have a negative impact on connectivity. Data rates should be used to decrease airtime utilization by reducing the airtime used by management frames. Changing these also affect retries, so this is about the conservation of airtime, not about controlling cell sizing.

RX-SOP:

RX-SOP adjusts the threshold at which an AP will demodulate a frame. I kind of view RX-SOP as the reverse Data Rate. Because like data rates, it cuts off at a particular RSSI. Unlike data rates, which are negotiated at the time of association, the client cannot tell if the RX-SOP threshold is set or if it is negatively impacted. If you adjust RX-SOP far enough to affect the cell size, you likely are going to experience side effects that make its use undesirable.

I tend to classify features based on their primary use. I’m not saying you can’t manipulate cell sizing with these features, but keep in mind these features are related to airtime conservation and utilization. While they can affect cell size, often they come with some significant trade-offs that you are giving up. The further up the list you can adjust your cell size, the less you risk hitting impactful trade-offs.


So there you have it, three great ways to control your cell sizing and two ways not to.

2 comments

  1. To be fair as far as I’ve been talking with Ruckus in the past, they would agree with your post. They said leave the power at max and place the APs in a way that limits the cell size accordingly, then play with power to get the correct coverage.

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