Are you Near, Far or just something else? (multi-part series on a 54Mbps and roaming)

Back in 2017,  I did a little post on Ekahau — basically I had never really played around with the software and had some free time. I spent around four hours just messing around. The point is, you can see the placement of our Access Points. Which is in what I will can an “L” shape. That is mostly our walking pattern in the office. On the first post in the series I talked about the AP towards the front of the building, more specially the one by our elevator. This AP has a single purpose, doing the initial connect of our client devices. Which all happen to be made by Apple, so our results should be pretty much the same and Apple has a really good support doc called About wireless roaming for enterprise.

Now, I talked about setting the minimum supported rate to 54Mbps. Would this cause a Near/Far issue? Hold on, what’s a Near/Far issue? Good question, if you just so happen to have the Sybex CWNA-106 book, you can flip to page 413. But if not, here’s a little of what is Near/Far:

Disproportionate transmit power settings between multiple clients may also cause communication problems within a Basic Service Set (BSS). A low powered client station that is at a great distance from the Access Point could become an unheard client if other high-powered stations are very close to that Access Point.

OK, that sounds cool, BUT, you’re talking about just changing the minimum supported rate of an ESS(Extended Service Set, Sybex CWNA-106 page 250) or BSS to 54Mbps. And, the Near/Far issue is talking about disproportionate power on an STA(Station), i.e. Client.

Ahhhh, good point. We just might be learning something here. I hope so. Let me explain what I’m thinking. So you have this AP, that is the same power as the other APs, in theory you should be able to hear it. And, since all of our devices are Apple, then we should be seeing the exact same thing across the same type of device(all running the latest non-beta iOS, same hardware etc…). Now, I know the real, real world is not 100% static like I’m saying, but just hear me out. Hmm, ok, I’m with you so far, keep going.

So basically if all APs are the same power level, and we’re faithful to the Apple wireless roaming doc, and if you looked at how the APs are placed(talked about on a previous blog post). Then, I should hear at least three, if not all four APs, right?

But, I only see two, sometimes — maybe three APs and that AP at the very front of the building, by the elevator is not heard. So here’s what could be coming into play regarding this: The physical distance to the AP, the line-of-sight propagation from the STA to the AP. And, one could even say you have some FSPL(Free Space Path Loss), hmm, nahh. Also, physical items, i.e. walls–the placement of the APs is in the open region of the office, all open with walls that go maybe six feet high, i.e. dividers, so all the APs are can see each other.

Here’s a couple screen shots showing something interesting. You see two APs, then I moved 10 feet towards the front of the building, same location just 10 feet(on that previous blog post showing the floor-plate, I moved closer to the bottom).

nearfar
four

I now see four Access Points instead of two. OK, so what are you getting at? By changing your minimum supported rate, you in essence could be creating a wall that stops the RF. Huh? That’s not true, RF goes a long, long way. That is correct, but the perceived demodulation of the RF is what counts. Our client devices have tiny little antennas in them. And, by changing the minimum supported rate, you have changed the cell sizing of your access point.

But why would you want to change your cell size? Don’t you need a so called 10-15% overlap from AP to AP for better roaming? Yeah, that’s always a goal, but how do you measure overlap(maybe we can talk about that idea in another post)?

Now, this works for us. But why? The first post in this series, I talked about how we really do not have roaming scenarios. And, what *roaming* we do have, we are 100% fine with the slight 2-5 ms time it takes to rebuild the TCP/IP sessions for that STA.

Blah! I still don’t buy it, sounds like crap! Why not just set the minimum supported rate to 24 or 18 and be a smooth roamer? What about the beacons, they travel at the lowest possible rate anyways or do they?

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