Just thought I would share - this is a great piece and is very insightful.  Anyone interested here is the link !  [Link: adtechglobal.com]

A few nice points it discusses:

That addresses two important issues:

1) You have to be clever about your channel separation on your APs if you're setting it up manually.

2) Blast it and forget it doesn't really work. Even placement of multiple lower power APs (such as the UniFi 3 pack, just the regular ones for most properties under 500sqft) yields much better performance.

And well, it adds extra cost but everyone knows there's way less interference in the 5Ghz band.

I sent this on to my sons, and the one who's in charge of enterprise software for a local college replied:

That isn't what is being said at ALL.  The article doesn't say to stop supporting anything - it simply says that instead of blasting an area with the strongest WiFi possible you should use MORE APs and lower their transmit rates (Aruba does this for you so I have been doing this for awhile now). 

When they say to take out the lower broadcast rates that is because you won't be supporting as far-away distances... the network result would still be BYOD. 

This focused on 802.11n but it still has full relevance to 2.4 Ghz.

Bottom line I wanted to point out is that CAPACITY and not STRENGTH is what matters with both enterprise AND our higher-end residential products.  

I have been using Aruba simply because it has Airtime fairness which actually mitigates the "low-quality client" effect on the network by intelligently enforcing airtime based on a devices connection.  It also has HARDWARE-based handover for roaming which increases roaming quality by large percentages over other options.

We use Ubiquiti now in smaller jobs and are also looking into Meraki more and more.  

I missed how "take out the lower broadcast rates" is not the logical thing to do about "low rate clients screw things up for everyone." The quote in my post says that since you have to be BYOD and some people have slow devices, you cannot totally control this. Right, or did I miss something else?

You are missing something. It's not necessarily a matter of locking out client DEVICES that are only capable of low rates (a la .11B, which we are very unlikely to see on our WLANs anymore). The white paper is talking about shutting down lower rate support at the AP to limit edge client devices that may be fully .11N capable but, due to SNR or other issues, are stuck on lower rates, killing other's throughput. Some APs will let you limit rates to, say >(12, 18, 24, 36, 48, 54, you pick where)Mbps. The article suggests the use of another, well paced AP to cover those devices that would otherwise be locked out.

Also, while it pains me to say it, I totally disagree with your boys' concept of "the open and unbordered nature of wireless." In the context I infer from his statement, he seems to be saying that all devices should have access and, if that was the design, he is somewhat right. However, the idea of a well designed WLAN isn't just a connection but, a damn good one. We could give your boy his access by adding well placed APs that don't push out edge case clients based on throughput instead of simply allowing them on an existing AP at the cost of dragging everyone else down. Lastly, and this is largely semantic (you, of all people will appreciate that), a WLAN should very rarely be open and, by their very nature, are always bordered.

The one thing I wish the article talked more on is cochannel vs. adjacent channel interference. Specifically, I train my boys on this every chance I get, is there is NEVER, EVER, EVER, NEVER, EVER, EVER a reason to EVER use any channel on 2.4GHz other than 1, 6 or 11. Ever. This isn't a matter of opinion, it's an absolute statement of fact. Nor is there ever an excuse for setting 2.4GHz channels to an HT40 (40MHz wide channels). Ever. Stick to HT20 always. At 5GHz it is a much different story, designed at the spec level to safely use any channel and benefit from a wider HT.

Very true.

Channels 1, 6, 11 ONLY

NO 40 MHZ

(Unless 5 Ghz).

Too True!  

why channels 1,6,11 only?

Because with 2.4GHz in North America, 1,6,11 are the only non-overlapping channels. 2,3,4,5 will all overlap with both channels 1 and 6 at the same time. 7,8,9,10 will all overlap with both channels 6 and 11 at the same time. 6 is default in many devices that neighbors could be on if they don't set up them properly. It's better to have conflict with one channel than two in a crowded 2.4GHz space...with anything other than 1,6,11 you're likely to conflict with two channels at the same time.

Thank you, Hoogy. Anything that dserves that many nevers or evers in capital letters demands at least a cursory explanation.

Anthony, my son works at a college. There are professors there who must have access to the network. While this is still considreed "enterprise," whatever that exactly means, there are factors here that run counter to your asumptions.

In the past, he's run into profs who have all of their thirty year old research on floppies. This includes data from earlier software, including DOS programs. In these cases, the software was not really commercial, so there was never any money to make it work directly in Windows! Neither he nor the university can simply declare that these guy can't use their computers on the network.

While that in and of itself doesn't mean they have slow devices, it indicates that the college cannot do what a business can do: forbid a user from using certain devices on the network. Sure, the college issues and maintains computers, but if a computer is too old for the college to support, that is, maintain, the profs can still do their own support on older devices. I think that's the nature of the idea that the network must be open to all devices brought to it.

And yes, that's a semantic issue, but even I didn't see the possible dual meaning, and I actually have training in seeing these things.


Diificult to do across a college campus!


Yeah, me too. What's that?


You're saying that the standard is so crappy, or the equipment is so crappy, that fully FOUR channels should be left unused between used channels. What's wrong with things, that this is necessary?

This is a pretty general explanation but, here goes. WLANs are basically hubs, not switches. When you use the ONLY acceptable channels of 1,6 or 11, there is a good chance that you will share radio space with your neighbors. That's called cochannel interference. When data needs to be transmitted, like on a hub, it can only pass one transaction at a time. If your AP "sees" a conflicting AP transmitting, it will wait for it's turn and vise versa. Basically, transactions are queued and you network is adversely affected.

However, when you use overlapping channels, transactions aren't queued BUT, they can shit all over one another. Bits get mangled, bits get dropped, bits get missed. Both for you and your neighbor. This is adjacent channel interference. When these packets get mangled, due to the three way handshake built into TCP/IP, they retransmit. These retransmits can also be mangled. Hence, even greater overhead. This can be far worse than cochannel interference. Far, far worse.

Only in .11N running at 5GHz can you safely use any available channel without fear of overlap.

Simple but, gets the point across...

thank you.

thanks for the great explanations, I was totally unaware thats how it worked

How about a bit deeper dive? :)

The channel spacing in 802.11 is on 5 Mhz basis. So channel 2 is the same as channel 1+5 Mhz and so on. There are 11 such channels in north america.

The frequency you pick in your AP is the "center" frequency (Fc). The way the modulation works, the occupied channel is much larger than one signal 5 Mhz channel. Here is an actual spectrum measurement of 802.11g:



Look at the transmitter on channel 6 in teal color. You see that it has "skirts" extending well to the sides and yes, even into channels 1 and 11! That is just the nature of RF modulation.

The standard has a set of recommendations for output power to put some measure of control over this as shown with the solid color here:



The center frequency is 2.437 Gigahertz. Each grid there is 10 Mhz or two channels in 802.11 parlance. The power limit is -20 dbm at +-11 Mhz which is actually more than four 5 Mhz channels. This is a lot of power when you consider that the transmitter may start with 20 dbm. Subtraccting -20 dbm will only get you to 0 dbm. Consider that a receiver can be sensitive to below -70 dbm and you realize that the overlap represent lots of signal to confuse a receiver.

Even when you go 4 channels below *and* 4 channels above (for 40 Mhz bandwidth), there is still lots of power (down only 40 dbm). No wonder the first display above show so much overlap even when you pick channels 1, 6 and 11.

So for sure you don't want to pack more than three stations in there and if you can go down to two, that would be even better.

I'm not a professional in this area, but found value in this thread nonethless. I'm struggling with getting a reliable WiFi signal at my new Samsung SmartTV. Among the challenges are (a) having only 2.6 Mbps DSL service from AT&T, (b) my Linksys EA4500 router is at the other end of the house, and (c) my neighbor has U-Verse and apparently has a 2Wire420 router across the property line but less than 10 feet from my TV. The signal from her router is MUCH stronger than from my own router near my TV ! The Samsung won't connect to my router's 5 Ghz network at all, probably because Samsung didn't waste an $ on the built-in WiFi adapter. I assume the difficulty the TV has connecting to the 2.4 GHz network is due to interference with the neighbor's. I've now switched to from channel 1 to 11, hoping that will help. Any other advice anyonehere can throw my way would be appreciated.