By Jake Snyder
One of the guiding principals of the my wireless career has been to engineer wireless networks that meet my customer's goals. In order to achieve this, I had to break down some of the complexity of RF design into rules to help myself design wireless networks. This is not an exhaustive breakdown of RF engineering processes, but it is some very basic techniques to improve RF design.
But over time these rules either hold true or fall out. Often rules get absorbed into a more generic rule or divide into more specific rules. I'll attempt to describe the rules and how they function as part of a wireless design.
Rules of Wi-Fi design:
- Capacity first
- Put RF where clients are
- Use Attenuation to your advantage
- Use antennas to your advantage
- Coverage > Capacity or as i like to call it "I'd rather have co-channel than no-channel"
- Design in 3 dimensions
- Take RRM algorithms into account
- Take device capabilities into account.
- Predictive models are only as good as the data you use to build them.
- Design up and down the stack
I know some of these will sound odd. Let me give some brief descriptions on what I means by some of these.
Rule 1: Capcity First
Knowing how much capacity you need in a given space is paramount. This enables you to plan for the number of cells you need. Only then can you know where you need to put the APs in order to meet your goal. Also, this helps you realize when your goal is unrealistic, improbable or impractical.
Rule 2: Put RF where clients are
This rule is pretty straightforward. If you want people to use wireless devices in a room, why would you install APs in places that aren't that room? This can also be the "Put APs close to where the bulk of clients are" rule. Reducing this distance, increases signal strength for clients, increasing data rates. If you get more clients closer to the AP (or vice versa), you can leverage faster data rates, and therefore create more available capacity.
Rule 3: Use attenuation to your advantage
This rule is all about building capacity. While most people get that attenuation helps shrink cell sizing, it also shrinks contention domains which can be dramatically bigger than the coverage cell. Reducing the contention domain allows for greater channel reuse and ultimately will result in higher performing networks. Remember, capacity gains are only realized when you are not sharing the spectrum with another AP.
Rule 4: Use antennas to your advantage.
A great way to keep a cell size small is to reduce the amount of RF you are putting into a place that you don't want. For example, putting omni antennas in a high ceiling manufacturing area. There's a significant amount of RF going up that you probably don't want. Instead, a patch (even a wide patch) antenna pointed down can help reduce the amount of RF energy going up into the ceiling. Also, directional antennas can help seperate clients into cells. Load-balancing mechanisms aren't needed if clients can easily see which AP they should be connecting to.
Rule 5: Coverage is greater than capacity (aka I'd rather have co-channel than no-channel)
I know this rule is going to work some WLAN pros up. Notice that I didn't say "design for coverage." The intent here is that you can easily measure the performance of no coverage (it's zero). In an ideal world there would be no co-channel interference, but in reality you cannot entirely eliminate it. This rule embodies that you have to have coverage before you can have capacity and helps in your design methodology to make sure you start with getting coverage to an area and then iterate through your design process building capacity. And there are times when you must make a choice, either not covering an area or introducing some co-channel interference.
Rule 6: Design in 3 dimensions
A lot of people talk about measuring wall attenuation to aid in their predictive designs. Just don't forget that in some buildings you will have less attenuation between floors than you do across walls. Measure the floors and take it into account. If you can't account for the inter-floor attenuation, you could see substantially higher co-channel interference that you didn't account for in your model.
Rule 7: Take RRM/ARM into account
If you plan to use RRM/ARM, make sure you understand how it works. My belief is that these technologies reduce ongoing operational costs by reducing the impact of environmental changes. Every vendor does different things with their auto channel and power settings. Understand how these work and how you can help these systems make good decisions. Fail to do so may result in poor decisions.
Rule 8: Take device capabilities and behaviors into account:
Our client devices vary wildly in capabilities. Rarely do you ever have the pure 802.11ac client mix. Make sure you take client capabilities into account with regard to your capacity planning. Spatial streams, supported channel widths, modulation rates, required rates can all have an impact on how much capacity your cells are going to have. Failing to design for device capabilities and capacity is designing to fail.
Rule 9: Predictive Models are only as good as the data you use
All models are wrong, some are useful. The easiest way to getting useful models is to make sure that your models reflect what you see in the real world. Measure wall attenuation, cell sizes, floor attenuation, etc. Use these as inputs into your predictive models. Then sanity check your model to make sure what you predict aligns with what you saw. If you blindly trust that the wall is "Dry wall with metal studs," you will find that sometimes it's accurate and sometimes it's not.
Rule 10: Design up and down the stack
While the goal of this article is Wi-FI Design, Wi-Fi needs to be design up and down the stack. Layer 2 design, routing, DNS, DHCP, firewalls can all be contributing factors to how a network performs. Make sure you are taking these other components into account as well as designing the RF. WiFi is a system, it should be designed as such.
As with most things in Wi-Fi, everyone will have opinions about RF design. These some of mine.
