As mentioned earlier (CWNA Re-certified for 2018), I finally got around to re-certifying the CWNA certification that I originally obtained in 2007.
While I consider the CWNA to be the one essential certification for anyone who’s interested in getting into wireless network engineering, the certification is generally viewed as anything but a beginner cert … primarily because of the range of topics covered in this exam. Unlike the more specialized certifications like CWAP, CWSP, and CWDP, the CWNA covers topics from all three of these specializations … albeit at a higher level.
I originally sat and passed the CWNA exam back in 2007 with the original PW0-100 exam. I started down the path of re-certifying in 2013 after letting my initial CWNA status lapse (the certification is valid for 3 years from the date of the successful exam completion), this time with the PW0-106 exam. Last week, I finally say my second CWNA exam, this time the PW0-107, and passed.
One response I received on LinkedIn after posting about the completion, was asking what study methods I use when going for technical certifications. This question brought up some specific methodologies, but also shed some light for me personally on the topic I usually have the most resistance to, but this time finally had a better grasp on … RF Math.
You see, for most Associate and Professional level certification, my typical learning method usually has me sitting down with an official study guide … working my way through the text and focusing on any practice questions available to make sure I comprehend the content well enough. In the case of the CWNA, my go-to book is the study guide published by Sybex … expertly written by David Coleman and David Westcott. Most anyone who’s read through that book knows (or at least has heard of) the dreaded chapter 3 … “Radio Frequency Components, Measurements, and Mathmatics”. Kind of a mouthful right there, but this is where the beginner wireless engineer gets their first exposure to dB math, and breaking the mindset that big linear deltas in measurements are good … while smaller changes are insignificant. As Matthew Gast said at the first WLPC conference in Austin … ask your manager for a 2dB pay raise and see how significant logarithmic math can be.
So, where am I going with this? Sometime between 2007 when I originally got the CWNA certification and now … I picked up photography as a hobby. Wait, what the heck does photography have to do with wireless, you ask? Simply that both get into physics … one dealing with RF energy, and the other dealing with light energy, and the implications of the Inverse Square Law.
With wireless, we often talk about the rule of 10s and 3s … basically, that every time a signal changes (increases or decreases) by 3 decibels, the signal level has actually doubled or halved in strength. What’s more, a change in 10 dB is like an order of magnitude change: take the original signal level and multiple or divide by 10 for the new level. So remember that 2 dB pay raise … yeah, it’s a lot more than just the standard 2% some people got.
Another quick calculation, and the one that really brought my understanding together (and the reason for this post) is the 6 dB rule: a change of 6 dB effectively doubles or halves the distance of the useable signal.
In photography, controlling light is crucial in capturing great images in camera. (I’ll ignore all the possibilities that come through photo editing, since my interest in photography is to get me out from behind the monitor and out in the world … not to spend hours behind the monitor editing images to make a great image out of a mediocre capture. So if I have this battery powered light, how do I make the light brighter when it’s already turned up as bright as the unit can go? Simply … by moving it halve as close to the subject. If the light was 2 ft from the subject, I can move it 1 ft from the subject and have more light light available at my disposal without using more battery power than I already was. Similarly … if I’m too lazy to adjust settings on my light, I can move that light from 2ft away from the subject to 4 ft away from the subject, and I’ll have 1/4th the light available with the same settings.
And this is where it dawned on me. The typical question when the Inverse Square Law is discussed in photography is “why don’t I have half/double the light when I move the light half/double the distance to the subject? Remember that rule of 3? Every three dB is a half/double in power. But if doubling the distance means I put the power (either light or RF) by 6 dB … that’s not a change of a half … that’s a change of a half and then another half, or quarter power.
So what does this all really mean? For me, the rule of 6 dB is good for understanding coverage as it relates to distance. If you add 6 dB to the signal level, then the effective distance is doubled, while reducing power by 6 dB decreases the distance by half. If those distance changes are relative to a client position and I move the AP twice the distance that it was to where it is (aka, I move it from 2 ft away to 4 ft away), I’m now receiving 1/4th the signal level that I was before.
The rule of 3 dBs explains why that signal level change is what it is. If changing the lowering the signal by 6 dB is the result of doubling the distance from my client to the AP, that’s two different 3 dB changes.
Hopefully that example with light helps. If all I’ve done is further confused the subject (hopefully not), leave me a note in the comments and we’ll discuss it further to either make sure my understanding is right or to get me corrected.