High Density Wi-Fi SSID Considerations – Part 1

Typically within very high density WiFi deployments we recommend a having a low number of SSIDs particularly in the 2.4 GHz spectrum. We may want to have fewer SSIDs in the 2.4 GHz spectrum than in the 5 GHz spectrum. This is due to the available airtime slots within each specific spectrum. Within this blog  try to explain over a few parts in detail the cause and effect of the performance impact of multiple SSIDs within a high density WiFi environment. It will also help to illustrate the importance of a WLAN Policy within the high density WLANs to manage rogue devices and interferers.

In any WLAN environment we have management traffic and control traffic as well as our client data. This management traffic creates an overhead that has an impact on the amount of airtime that we actually have available to send data which is what we built the WLAN for in the first place. The lower our management and control overhead the more airtime we actually have to transmit data. The trade-off is that without the RF management and control traffic we would not have a WLAN to transmit data in the first place but we can take steps to limit this and claw back some of that valuable airtime. If we do not control it we can see very high channel utilisation and saturation with just management traffic and severely degraded WLAN performance. 

Generally speaking the most prevalent management and control traffic will be beacons, probes and probe responses which is where we focus this discussion for having a low number of SSIDs particularly in 2.4GHz and tools which we can use to mitigate the issues within the Cisco portfolio

Equally the reason we focus on 2.4GHz is that it is by far the most difficult use case with the prevalence of interferers, the lower number of channels and its higher signal propagation.

In the 2.4 GHz spectrum we have only 3 channels as compared to typically 16 to 20 in 5GHz (depending on your regulatory domain etc), the reason we discuss 12 channels in 5GHz particularly. Therefore we have significantly less available airtime if we were to have an equal number of devices in 2.4GHz as we would in 5GHz. 

Typically we are seeing a shift of devices to the 5 GHz spectrum which alleviates a lot of the pain as we have approximately only a fifth of the capacity in 2.4 GHz as we do in 5 GHz or less now with the improved efficiency of 802.11ac.

Defining High Density Wireless

HD Wi-Fi has become de riguer over the last two years or more and no doubt there will be another flavour of the month soon. The reality of HD Wi-Fi as a feature is different in that it is intangible without an easily definable outcome. Usually a solution will either work or not work, however when you are looking to deploy HD Wi-Fi not all vendors are equal, be that due to the chipset, the features or product portfolio.

For example if you deploy EAP-TLS, bonjour guest access or multicast you have either standards based protocols to work with or a clearly defined goal. The issue is one mans high density is another mans normal density.

People often say they need "high density" and ask if the proposed solution can do "high density". My immediate response is “lets define high density” rather than “of course it can everyone's WLAN solution can do high density”.

High density WLANs, like everything else, can only be tested once installed and tested againstthe design brief. If there is no design brief as to how many clients, what type of clients and what type of applications you are simply unable to measure the success or lack thereof of your implementation. So you must absolutely define how many clients etc you will support.

Lets take a look at what two different people deem as high density Wi-Fi

Option 1 - A WLAN deployment in a busy office with lots of people for voice, video and data where you might get 40 clients per access point and each access point is using omni directional antennas may be described as high density. It certainly is higher than perhaps a normal enterprise WLAN density nad as such may be defined as high density.

On the other hand

Option 2 – A stadium where you have 90000 people and each access point covers 400 seats to provide data, video and location.

Option 1 there is a moderate expectation that you will be successful with many vendors, some more successful than others. I respect that this is not that far away from a pretty standard deployment and most enterprise class WLANs should be able to cope with this. 

Lets define the high density environment as 20 seats per cell and 3 devices per seat so we have a maximum design of 60 devices on an access point. 

Simply by defining the expected outcome if you have 61 devices and the WLAN fails you have exceeded the design. OK we need to define in more granularity the applications per cell etc so we do not end up with 60 voice clients but our expectation would be a sensible split of clients and applications. 

This would provide a reasonable user experience for typical office based applications.

Option 2 where we have a stadium typically we would define the total number of clients we could reasonably expect to have on the WLAN simultaneously at any one time. For this example lets assume 25%. We need to use directional antennas to segregate the RF and we want location and video as well. 

Next step is to define the video capabilities and start to manage the customer expectations that it may not work well in all areas. Additionally with location we will not see the granularity that we could expect in an environment that lends itself to a location based deployment.

The requirements in the eyes of the customer are both high density Wi-Fi however the approach, expectation and usage are both very very different and need to be managed differently. Indeed not all vendors would be able to provide a solution for option 2 as very high density WLANs pose many different challenges that you will only see in that type of environment. More vendors perhaps have the capability to solve the challenges of option 1.

Are they both high density or is one simply an extention of the success of Wi-Fi and the growth of devices in the market place and new standards like 802.11ac. All of this coupled with client and user expectation that Wi-Fi is very much commoditised these days means we will always have challenging situations where our Wi-Fi skills and ability to communicate designs successfully will be tested.

What will the next flavour of the month be?