Considering stability in designing for real time applications over WiFi


Let’s face it. WiFi and wireless applications in general are very dynamic. While we like to picture a two dimensional flat heatmap to look at coverage, the reality is that it’s a 3 dimensional fluid creature that is constantly growing and retracting. While you are deploying real time applications, anyway that you can find to tame this beast, and create a more stable WLAN, you will certainly achieve better performance.

Real Time Application Grade Design

No AP vendor will tell you this, but all of the new enhancements that come with each successive WiFi generation pale in comparison to having a quality WiFi design purposefully built, and validated for your particular facility. Creating proper channel separation and seamless roaming with your particular floor plans and wall obstructions is key to achieving the best overall throughput.

Also, part of a well-designed WLAN is figuring out how to add that stability in the design. If someone turns the corner in a hallway of a facility with a VoWiFi phone, is there going to be an available AP for it to associate to in time? Just having a tech walk through and create a “canned” survey report is definitively not close to enough. In fact looking at beautiful heatmaps that look like they provide proper coverage can lead to a false sense of security. Did you have an experienced WiFi engineer take into account TX power consistency, hallway vs. room placement, offset for the client’s particular receive sensitivity? These are just some of the factors that make a huge performance difference for a real time application such as voice.

We know that RF is measured in logarithmic terms. Having the design off 3dB means that the actual client is seeing half or double what you think it is seeing. That is a huge difference. Personally I have seen networks that were off by 10dB from where they thought they were. Think about that. Their clients were walking through a design that was off by 10 times. Needless to say they thought their WLAN was spot on and they were blaming it on the WiFi voice client vendors. Pushing one vendor out and receiving another only to discover that they have the same performance issues. Countless months and dollars spent putting a WLAN design in place that was destined to fail over and over. Getting the WiFi built right the first time will make your investment so much more efficient and cost effective.

TX Power Variance

In having experienced design we know that one way to achieve stability is have less variance overall in the transmit powers. In a real time WiFi engineer’s mind some would most likely wish you a network that is configured statically, carefully designed at one uniform TX power. In today’s enterprise WiFi world statically configured networks may be considered on the endangered species list. Most customer’s are sold on the benefits of using a dynamic configuration approach such as RRM. The administrative burden is certainly reduced and most administrators have been sold on the benefits of the approach. That being said you have to make some threshold adjustments to the default configuration of RRM in order to add some stability.

You can see that we should look at RRM as a possible source of instability if using this feature, rather than just the benefits that it offers. We need to make a designed strategic approach to this feature. Dialing in thresholds to RRM is a key task in order to raise your real time application performance. For example if you had most of your access points set to 14 dBm, and you had a few at 17 dBm, that would be more consistency than having some at max power, and some at min power, with a range of TX powers in between. A good exercise is to picture yourself in the eyes of a WiFi client. If your client is walking through areas that are predictable in its TX power and cell structure, it seems a much easier road to travel than if it turns a corner to find an AP blasting at 100 mW or it must turn the corner from an AP that was set to 1 mW. Make the roaming path easier on your client.

Update Clients

Creating more channel airtime availability will ultimately lead to less delay in accessing the WLAN which increases performance. However, from the viewpoint of stability, you want to get transmissions on and off the medium faster and more efficiently. Spikes in channel utilization lead towards un-stabilizing the WLAN. These temporary accessibility “meltdowns” can confound the average WLAN administrator looking to troubleshoot issues. In 2.4 GHz range this can be a very frequent occurrence which is one reason why we want to get our real time app clients onto 5 GHz if at all possible.

Updating the clients in the environment or at least removing the most legacy will work towards this end. It has been well known for a long time how much legacy clients will reduce performance on the network. Legacy clients will engage protection mechanisms so they can transmit in the same environment as the latest WiFi clients. This creates much overhead on the WLAN as we have seen.

As 802.11ax rolls forward the promise would only be made real by introducing more of the corresponding clients into the environment. We can only achieve these efficiencies if the latest clients make up a larger percentage of the overall client base.

Control In-Network Interference

Reducing co-channel and adjacent channel interference is another key to providing more stability in the WLAN. Without having control over your channel contention, or the size of the intended cells, you essentially lose your ability to assure high performance on the WLAN. There is no replacement for a carefully engineered high performance design to achieve these ends. Paying careful attention to the layout of each cell and creating a quality channel reuse pattern or strategy is key to maintaining stability in this areas. If you have no control over your channel layout then how would you expect to achieve high performance with your WLAN. Even if you are using RRM you need to add some strategy and threshold or you will lose the control and thus the stability of your WLAN. Having a quality design in place is the proper foundation to achieve more stability to an RRM implementation.

Mitigating the Unknown Problem Sources

It is key to have some walkthrough and monitoring of your outside interference sources and rogue AP detection. By having an experience WiFi engineer walk through and perform a spectrum analysis of the facility you can see what forces may be in play to cause performance degradation in the network. Getting control of the unknowns will allow you to achieve greater stability in your WLAN. As an example a customer contacted me once regarding losing their voice calls when entering a cafeteria area. Upon further examination with a spectrum analyzer it was discovered that a kiosk with WiFi located in the cafeteria was having issues and constantly transmitting in error. By rebooting the kiosk the issue was solved and the voice call issue disappeared. It could also be a video camera or some other type of non -WiFi device in the environment that will lead to unplanned performance issues.

Also, rogue Aps can also lend to instability of the network. You took the time to create a great WLAN design and now the neighbor’s Aps or a rogue internal AP are wreaking havoc on your channel plan and channel utilization. This reduces the stability and also the capacity on the network. Getting a handle on what out of network Aps or non-WiFi interference sources are affecting your WLAN is key to achieving more control and also more stability on your WLAN

Stability is Key

You can see how looking at the stability aspect of your WLAN is key to achieving high performance. Looking at how your client see’s the WLAN and how it roams, looking at stabilizing your channel and TX power settings, letting go of legacy clients, and monitoring and mitigating interference sources are key to achieving this stability and thus a path to a high performance WLAN.