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).


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?

I’m at 54, how about you? (multi-part series on a 54Mbps and roaming)

A couple years back, maybe more, I did this–changed a setting to 54Mbps. My office wireless network is set at 54Mbps minimum supported rate. I said heck why not do 54 and see what happens. OK, for the details of what my network is running and why. First off, we run Cisco Meraki wireless, it does exactly what I want and expect. Our network consists of simple L2/L3 designs, pretty cookie-cutter, darn near everything is Cloud based. With being Cloud based we just need WAN access. We have roughly 18 VLANs, even for a few simple things like printers, those go on a dedicated VLAN. Also, I take the approach of “if it has a network port, make it wired”. Along with isolating devices with VLANs, we also run an entire Apple environment, i.e. all iPhone, iPads and MacBooks(generally within 24 months of the latest physical device released–for the most part all latest-gen). With this approach, I don’t have to go around guessing about what wireless card driver versions are installed, did Windows 7/10 overwrite a newer driver etc… (yes, I did mention Windows 7, we have some legal software that is great at the legal process-but sucks otherwise-and it works well on Windows 7).

Now for our physical Access Point placement, I did not have access to Ekahau or any predictive mapping software when our office was planned. But, I did know the walking patterns and how our lawyers operate(I call this part TACO(I’ll blog about that later) or basically Chapter 2 in the Certitrek CWDP-302 book). They generally *do not roam*. What?? What do you mean?

Let me explain a little on that. First off, people enter the first floor, access the elevator for the second floor(our office is the entire second floor of a three story building). I have an access point roughly 10 feet from the elevator, that is pretty much meant to get the device connected. Hardly any usage on that AP is done, maybe a quick email or two if the elevator is slow that day. That AP is also on the opposite side of the building that the most used offices are. Also, I know that the mobile devices are usually tossed in a pocket or backpack during this time. Sometimes, those devices are not even touched until sitting on the desk in the person’s office. And, in that case they will connect to the AP that is right outside their office.

Knowing how the devices are used, I placed the Access Points in relation to the office usage walking patterns. huh? Basically, I knew how people will walk around in the office, how they will be using a mobile device and what would be used on that mobile device. Lucky for me, I know that our mobiles devices are used for consumption. Lots of PDFs(mostly looking at one or two that are 100’s of pages), along with some Words Docs, hardly any VoIP and/or video used in a “walking around” sense. Very little Facetime/Video, however lots of cell calling(but that’s not my problem).

Now that you have a little background of our network. You can see why I’m forcing a minimum supported rate(see pages 300-301 of Sybex CWNA-106 book, also page 218 for OFDM) of 54Mbps. And, I also know that all of our devices are 802.11ac.

Now comes the roaming part, which we really do not do. Since we run at 54, and know that the our devices will be very close, roughly 20-23 feet lines-of-sight propagation to the Access Point(if not closer). They *should* not have too much of an issue with decoding the higher modulation rate(see pages 640-643 Sybex CWNA-106).

Now, this is not a perfect theory of why this works for us, our office is all concrete floors and ceilings. with lots of lines-of-sight propagation to other Access Points.

However, I’m pretty sure we have a Near/Far issue due to our AP layout, think of it as as big “L” shape, with lots of metal and concrete walls, sitting in the “arm” of the “L”.

Or……are we just hitting the point of demodulation issues on that far away AP, since we’re at 54? Stay tuned for a little more details on that in the next blog post.