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Designing WLANS: What If We Could Double Our Airtime At 5GHz?

By David Coleman in · Experts · April 5, 2016

WLAN design has evolved drastically over the years. In the early years of 802.11 technology, WLAN design was strictly for coverage. Designing for the number of Wi-Fi clients was usually an afterthought. As we all know, the time haves changed due to the vast increase in the WLAN client population of mobile devices, IoT devices and BYOD devices.

Over the years, this cosmic explosion of Wi-Fi client devices combined with enhanced 802.11 technologies has forced us to rethink about how we design WLANs. Designing for client device capacity has now become the norm. I prefer to say that we should always design for “airtime consumption” which is directly related to capacity design.

If you assemble 200 Wi-Fi experts in one room, such as the WLAN Professionals conference, most likely you will get 200 different opinions as to proper WLAN design for coverage, capacity and airtime consumption. For over sixteen years, I have been asked the question: “How many devices can an access point support?” And for sixteen years, I have responded with the same answer that nobody ever likes: “It depends.”  

There simply is not one single correct answer to that question because there are simply two many variables. Below are just a few variables:

  • What applications are being used and how much bandwidth do they consume?
  • How many devices in certain areas of the building?
  • What type of devices? 802.11b legacy? 1x1:1 MIMO clients? 3X3:3 MIMO clients?
  • What are the walls made of? How big is the room? Where will the device and users be located?

As I mentioned, there are often different approaches to designing for airtime consumption and high capacity WLANs. There is always more than one way to skin a cat.

We all know that proper channel reuse design, lower power settings, disabling lower data rates, short guard interval, etc. are just some of the considerations when planning for a high-density or very-high-density WLAN.

What if we could double our airtime at 5GHz? Aerohive’s AP250 is another tool in your arsenal for WLAN design in high-density environments. The AP250 has a 3x3:3 802.11ac 5GHz fixed radio as well as a software-definable 3x3:3 radio that can transmit on either the 2.4GHz or 5GHz band.

We all know that the 2.4GHz band is overcrowded, does not have enough frequency space, and is very noisy. The 5GHz band offers much more frequency space and a cleaner RF environment. Having the ability to provide client access to two 5GHz radios from the same AP opens a lots of possibilities. However, we will have to rethink our discussions about designing for capacity and airtime consumption.

Here are two design guidelines to immediately consider when deploying dual 5GHz radios in a single access point:

  • 80 MHz or more separation between the two 5GHz channels in the same AP.
  • Only use 20 MHz channels and do not use 40 MHz channels.

One dual 5GHz AP250 could be deployed in one area of client density and still mixed into a design of APs that are dual-frequency.

Now consider the type of 5GHz channel planning  that might be needed if you have multiple dual 5GHz APs deployed throughout an entire building:

  • DFS channels will need to be enabled.
  • Pair non-DFS channels with DFS channels (This ensures connectively for clients that do not support DFS).

Some examples:  

AP250

Channel Pairing

AP250

Channel Pairing

AP #1

36/100

AP #5

149/116

AP #2

40/104

AP #6

153/132

AP #3

44/108

AP #7

157/136

AP #4

48/112

AP #8

161/140

 

This will start to getting complicated as to where the above APs are physically deployed and effectively you will have to plan for two overlapping 5GHz channel reuse patterns to try and best avoid co-channel contention interface. 

Now consider the often-debated “1 AP per classroom” design using AP250s and dual 5GHz radios. Instead of disabling two of every three 2.4GHz radios, two out of every three classrooms could have dual 5GHz radios deployed.  

Dual 5GHz APs are now another tool in your capacity design arsenal. Other Physical layer and MAC layer considerations such as power, data rates, etc. will still have to be considered.

As I stated earlier, there is more than one way to skin a cat and I fully expect multiple blogs and debates in the near future regarding proper capacity designs using dual 5GHz access points. Let’s start the conversation and start skinning some cats.

Want to discuss? You can find me on Twitter @mistermultipath 

For More Information:

1) Designing WLANS: What If We Could Double Our Airtime At 5GHz?

2) Suffering From Wi-Fi Congestion? Dual 5GHz Radios Can Help

3) Is 2.4GHz Dying? Or Is It Already Dead?

David Coleman (@mistermultipath )

David Coleman is a wireless mobility consultant and trainer. For the last seventeen years, David has instructed IT professionals from around the globe in enterprise WLAN design, WLAN security, WLAN administration and WLAN troubleshooting. In his spare time, David writes white papers, blogs and books about enterprise Wi-Fi networking. David is the co-author of Sybex Publishing’s CWNA Study Guide, CWSP Study Guide and CWAP Study Guide. David is the Senior Mobility Leader for Aerohive Networks and is CWNE #4. Aerohive invites you to download a free ebook co-authored by David Coleman: WLAN Troubleshooting and Design here: http://bit.ly/1I5miUu