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Blog Post

Mar 02

Access Point Power Caps


Access Point Power Caps: Why restrict AP Transmit Power?

 

 

One of the more common questions I get asked, is: "Why would you want to use power caps to restrict the maximum power an access point can transmit at?"

Surely if the AP can transmit at a higher power, that’s better right? It will give better coverage at a higher power and so why pay for more APs? Makes sense doesn’t it? From a superficial level it does – but actually when you’re supporting lower power devices it doesn't.

Let’s take a trip back in time: Five years ago, most people only ever connected laptops. Nice big batteries, nice big antennas, don’t roam very much in operation.

Coverage tended to be pretty sparse in terms of AP density and although everyone noticed the coverage wasn’t that great, it was good enough.

 

 

Today's WiFi Devices can be weak Transmitters

Symbolisation of a High Power AP
Try and connect a WiFi phone to that old fashioned network though, and you’ll experience some pretty serious dropout and it will just not roam properly. 

The APs have also become more powerful in the last few years too – restrictions have changed and the APs are now allowed to transmit more power on some channels
 

On some channels? Yes – its strange, but not all channels are equal (see this earlier blog, describing this).

So, what are you left with? If you spark up your network and don’t have enough AP density, the APs will run at maximum power. On 5GHz (channel dependant as I say), some will be 25mW and some a whopping 200mW.

Let’s look at how this would look if those APs were a group of people in a room: you’d have some people transmitting whispering right next to people with a megaphone shouting.

Even right next to the guy whispering, the guy with the megaphone is going to drown him out. This is just the Transmission (TX) side of things.

 

Poor WiFi Voice Roaming

How does it look from a client point of view? Well, a Cisco phone has only 40 mW maximum transmission power. There are other weaker devices, such as location tags (check the Ekahau A4 tag for example).

Do you want a much higher power device, such as a phone transmitting at higher power right next to your head? The phones have lower transmit power and a smaller antenna than the AP.

 

Walk around with that phone on 40mW and even when you walk past an AP running at 25mW, you’ll end up connecting to the 200mW AP nearby. The phone is listening to the RSSI of the AP to work out which is closer. It doesn’t know you’re right under an AP, only that the 200mW AP appears like a better choice.

What if that AP is not a good choice – well you’re still connected to it and you’re not going to roam properly and drop out. Sound familiar on your site?

 



Let’s look at what happens if you apply power caps to your APs. Yes – you are indeed hobbling them. You are restricting what the AP can do, but it’s for a specific purpose. You’re balancing the AP transmit power to the client transmit power. You’ll rebalance that conversation to have two people talking at an even volume to each other. When this happens devices start to roam properly.

Restricting AP Transmit Power: Q&A;


Will you appear to open up coverage holes?
   
Yes, you will. You might do a survey at max power and it all looks good. Apply a power cap and resurvey, you now have coverage holes. What you really have revealed is all the areas a phone might not roam properly. 
You’ll need more APs – sorry, that’s the rules – you can’t bend physics.
     
 

Is there another way?
   
Maybe. If you can match the receive sensitivity of your AP and balance this with the transmit power to roughly evenly match them, you’ll likely achieve the same effect. 
It would take some playing with though, and could be considered a theoretical solution to help. Check Cisco’s RX SOP feature for an example of how this might be done.
     
 

Do I really need so many APs?
   
It depends. If you want spare capacity in your network and you want lower powered devices to roam properly, then yes you do. 
Skimping on the number of APs will mean that your IoT devices, phones, tags and so on just don’t work properly. The properly designed network will have more APs and a lower, smoother set of TX powers. Get that right and it will almost be plain sailing.
     
 

Almost plain sailing?
   
Unfortunately, as ever with WiFi it’s not that simple. Specific AP placement is important too – not just the number of APs. That’s another blog though.
     
 

If you need some help working out roaming issues, or want some help to add more APs to your network to bring it up to voice level, email us as sales@iptel.com.au

 

 
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