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  • Writer's pictureLandon Foster

Wireless surveys are alive and well- Out of necessity

Surveys can be expensive; this is a fact. There’s an old joke about an engineer billing a company for “knowing where to hit with a hammer”, and this is exactly that.

Here’s the thing- you have to know what you’re after, especially in the WLAN space. With the advancement of any particular field comes cost. I often see this reflected in reluctance for capital expenditure across almost every sector of IT. Companies just aren’t throwing money at technology ‘on spec’ like they used to. But let me tell you a secret… We wireless engineers don’t want you spending any more of your precious Cap-Ex than you do. Yes, this even applies to consultants- the good ones at least. Here we run into the best argument for wireless surveys, besides just making it “work”. (More on that later)

That sound is the gentle lapping of interference in your ears

Let’s address one of the most common stones thrown at the wireless survey first- “Just install more APs!” Let me explain a bit about why installing more APs is just not a solution. First, and most obviously, it will cost you more money. To elaborate on the second point, I need to explain CCI/CCC (Co-channel-interference/Co-Channel-Contention, CCC henceforth). Radios at the present can only ‘talk’ in one direction at a time, to one client device at a time. Every time that a radio ‘talks’, it has come out on top, so to say, in a contention algorithm. Then it talks, and the process repeats. Every other device that can hear the one talking has to back off, and wait until it wins the spot. In properly designed networks QoS weights it so that the more latency-sensitive devices get the chance to send before other, less sensitive devices. The problem is this: the number of APs you have will scale as large as you can wire and power, (and provide licenses for, looking at you Cisco/Meraki) but the available channels do not likewise scale. In 2.4 GHz (Yes, a lot of clients still lean hard on the lower band) you only have effectively three channels to use in the US. You COULD use channels 2-5 and 7-10, but that actually makes you even more prone to interference, as anything outside of channels 1,6 and 11 physically overlap more channels. (A great explanation is here from the wonderful people at MetaGeek) So then, with more clients, it takes longer to get your data across. This effect is magnified if your clients are older and using legacy data rates (transmitting slower, for all purposes). The problem unfortunately gets worse when you have multiple networks close to each other that aren’t part of the same system. These can actually end up positioned so they can’t hear each other properly and try to talk at the same time. This is interference as we understand it classically, which keeps the AP (or client) from properly decoding the message, and results in retries of the transmission, consuming even more of your precious airtime. There are multiple other reasons to not just add more APs, but for brevity, let’s move on.

This is your 2.4, like, all the time

How does this effect the ROI with survey? Building on our earlier point, one way to cut down on CCC/CCI is to make sure the cells don’t overlap, with techniques like reducing your transmit power, or using directional APs. This reduces the size of the area RF is intelligible and means you can reuse those channels a bit more aggressively. This also means that by making the cell sizes smaller, you can actually have more capacity available. Capacity is the current name of the game, with coverage being a given. Here’s where the survey comes in- Tools like Ekahau are invaluable in this domain. Ekahau (and similar tools) allow you to literally map out a number of functions in different kinds of survey. These include latency, channel overlap, configurable signal strength and more. It’s not exact, but it’s far more than serviceable. If you simply add APs, you can end up with dead areas, or areas with reduced capacity. Things can be tricky that way- RF does in fact do funny things, and can’t be fully predicted. It CAN however, be measured. A survey can be as precise as you are willing to be. Is that AP in the hallway of your hotel covering all six rooms the way you intended? (Spoiler alert; no.) Let’s say that you need a certain response time, and want to find out exactly where you aren’t getting that- Active survey is your game. If you’re actively remediating an environment and you want more control, you can do an APoS survey (Access Point on a Stick) where you place an AP where you project the need to be, and re-survey with it in place. If you’re designing for coverage (Because that’s a prerequisite for capacity) you can do a passive survey and find out how each area will support your desired data rates. You can also simulate your ideal client in this situation with RSSI offsets. The point is, simply throwing up APs and expecting Machine Learning or AI (Both anemic at best in this writer’s opinion, especially without the right seed data- which still requires knowing what in the wide world of RF is going on in your environment to make it work) is simply a waste of your money. Do it right the first time- data is your key. You wouldn’t go into a business transaction knowing nothing about your partners!

Because I can, That's why

Finally, lets talk about the labor portion. If you have the philosophy that you can always just add more APs, not only are you going to spend man hours (and likely consulting hours) fixing your issues, but you are going to be pouring your even more valuable and less flexible OpEx budget into getting the right stuff installed… Again. It reinforces the idea that your IT department is a cost center, instead of a investment… But that’s another article.

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