Technology Blog

Wi-Fi Design: APs in Hallways

Guest Blogger

A common design problem in wireless networks is the tendency to place access points conveniently down a hallway. This creates problems because all the APs in the hallway can hear each other. Much like the idea of one AP per classroom, this seems like a good convenient way to design and install an access point deployment. We’re going to look at some predictive models for an example residence hall so you can visually see some the impact of these design choices. In all these examples, the cutoff for the grey areas is -70dBm. Most clients won’t actually see the signal that well, but this works to illustrate the point.

 

Here we have our APs installed in the hallways with initial power set to 25mw. Coverage looks pretty decent, though room 309 looks like it could use some help. So far, so good.Pasted Graphic 3.pngInitial coverage heat mapHowever, because the access points can hear each other pretty loudly, they will constantly interfere with each other in 2.4GHz, since we don’t have enough channels to prevent overlap. Let’s see how far the signals travel down these hallways:

Pasted Graphic 3.pngHow the radio signal travels down a hallway

As you can see, the AP provides a little room coverage, but is able to reach significant distances down the hall. This is what makes hallway APs less than ideal in most circumstances. Now look at the channel overlap (aka, co-channel interference) and think about what that means for capacity:Pasted Graphic 5.pngChannel overlapGreen means one AP can be heard on the same channel, orange is two APs, and red is 3 APs. It’s mostly green, so that’s good, right? Not so fast. Only one device can transmit on a given channel at a time. If two APs can hear each other on the same channel, that means they must take turns talking. This means that you only get one channel and one AP worth of capacity because only one can transmit at a time. This is not good for your 2.4GHz performance! You might think you can work around this by moving as many clients to 5GHz as possible. I’m sure that would help, but there may be bad news in the 5GHz band, as well.

 

If we use Adaptive Radio Management, the APs will hear all the neighboring radios and will start to turn their power down. Now that the power is turned down, the signal in the rooms will suffer. For example, if you left ARM configured for defaults it may drop power down to 9dBm and this is what your coverage would look like:Pasted Graphic.pngPotential impact of ARMThat’s no good.

 

You could tweak the ARM settings, but that doesn’t solve the 2.4GHz issues. A better solution is to move the APs out of the hall and into the rooms and solve both issues. Here’s the channel overlap if the APs are installed in the rooms:Pasted Graphic 7.pngChannel overlap with APs installed in roomsLooks a lot better, but why is that? Check out the coverage pattern for an AP in a room:

Pasted Graphic 4.pngSingle AP in room heat mapThe RF is much more controlled. This is what we want to see. How does the rest of the design look?Pasted Graphic 8.pngHeat map with APs installed in roomsNow our coverage is much more even than the original design and we’ve greatly reduced the co-channel interface/channel overlap while still providing coverage to the hallways. This means enhanced performance for both 2.4GHz and 5GHz clients. There’s certainly more that could be done here:

  • We could use 40MHz channels on 5GHz to improve throughput and capacity.
  • We could add additional APs to provide more redundancy in case of an AP failure.
  • We could experiment with disabling 2.4GHz radios and playing with power levels.
  • We could experiment with different models of AP or antennas.

 

There are many options you can choose when designing a Wi-Fi network. AP placement is just one of them, but if you choose to place APs in a hallway, you better be very sure that is the best choice for your design and your requirements.

Tags (1)
Comments
New Contributor

I find the implementation of hallway designs can be a mix of either bad design or customer contraints.

 

I have had similar experience deploying WiFi in a University student accommodation block but were restrained by the client's concern for security by having access points in the students rooms - this too could be a concern for hotel/public accommodation owners.

 

This could be counter-measured by installing the AP above ceiling and having ceiling mounted omni-directional panel antennas. However from memory this was an old concrete building with no wall/roof cavity - another constraint.

 

I think as Wireless Engineers we have the responsibility to help customers understand that the improvement to the WiFi environment (by placing APs in rooms) will far out-weigh the cost of possible theft/damage by tennants/guests.  This article would be perfect to show the customer as part of our argument!

 

 

Guest Blogger
Andrew von Nagy also pointed out that hallway designs create poor roaming situations. As a client moves around a corner, it gains and AP and loses one suddenly. There's no time for a clean roam because signal is poor, it just disappears. There's also something to be said for having some APs in the halls to avoid excessive roaming as the client moves past all the rooms. Don't take this blog posting as being a hard and fast "never ever put APs in hallways." It's about thinking through your designs and the ramifications of the design decisions.
Frequent Contributor II

"There's also something to be said for having some APs in the halls to avoid excessive roaming as the client moves past all the rooms."

 

So, to agree with that, if one were to add 1 or 2 hallway APs, where would you add them on the example residence hall?

Search Airheads
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements
Read all about it! If it’s happening now, it’s in the community.

Check out the latest blogs from your community team, the community experts and other industry sources.
Labels