All Things 802.11ac - Question 3: How Will Wider Channels be Utilized in Production Environments?

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I’ve recently had a number of interesting conversations with end-users and technology suppliers alike about 802.11ac channel plans. Granted, 802.11ac runs in the relatively-underutilized 5 GHz. bands, but 80 MHz. channels may still be difficult to manage in some cases (to say nothing of the complexities inherent in 160-MHz. channels). So, what are the practical issues and best practices with respect to applying these larger channels in production environments? And we’ve had a couple of questions about why .11ac is 5-GHz. only, so could you comment on that as well?
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Craig Mathias

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Posted 5 years ago

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Matthew Gast

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The easiest part of that question to answer is why 802.11ac is 5 GHz only. That's been a common question, and I first answered it as a blog response to Tom Carpenter at last fall's Wireless Field Day: http://blogs.aerohive.com/blog/the-wi.... Briefly, you need lots of spectrum to get high speeds, and the 2.4 GHz band is so crowded and has such a high noise floor that it can't take full advantage of the features of 11ac.
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EvaldasOu

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Good answer Matthew, but what about 160 MHz wide channels? In a real enterprise deployment when you can see a massive numbers of SSIDs from the neighbor companies, will it work at all? :)
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Matthew Gast

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I don't see 160 MHz channels as viable within areas with many networks. Beacons are transmitted at 20 MHz for backwards compatibility, but all it takes is one Beacon transmitter to stop you from using a wide channel. You have to sneak your wide transmissions in between the transmissions from neighbors -- which, granted, may be kind of like "duck between the drops" in a monsoon...
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Tom Carpenter

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Oh, but I really want to use those 80 MHz channels in 2.4 GHz to get back at my neighbors. How about an 80 MHz Intolerant Bit? :-) That would be helpful in 5 GHz actually. It seems we should at least have a 160 MHz Intolerant Bit.
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Matthew Gast

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An important note about channel width in 11ac is that you don't have to pick a single channel width for all clients. 11ac lets you switch the channel width on a per-frame basis, so you can transmit at 20 MHz bandwidth to client 1, 40 MHz bandwidth to client 2, and 80 MHz to client 3 (provided the whole channel is free at the width that you want to use). Devices can even negotiate the channel width. One of the ways that this helps is that if you space out the channels, you can "burst" to the highest capacity available at the time. Here's a part of a figure from my 11ac book. When the entire 80 MHz in the figure is idle, a transmitter can use the whole 80 MHz. However, if part of the channel is busy, you can fall back to a narrower bandwidth.

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Craig Mathias

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Someone asked earlier if 2.4 couldn't take advantage of at least some of the technologies and benefits inherent in .11ac. After all, 2.4 isn't necessarily crowded everywhere nor all the time. Is the IEEE essentially abandoning 2.4 for the future? And what happens when 5 becomes crowded?
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Matthew Gast

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My take on 11ac being 5 GHz only is that it is a tacit admission by the 802.11 working group that 11n is the capstone technology for the 2.4 GHz band, and there won't be much more development.

Fortunately, when 5 GHz becomes crowded, we have 60 GHz. (That was said somewhat in jest -- the 802.11ad MAC has some differences from the 802.11 MAC most of us know really well.)
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Matthew Gast

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It's also worth dropping in a cross-reference to another thread here. There's a reply in this thread that shows the basic idea behind 802.11ac channel planning to make best use of the available spectrum: http://community.aerohive.com/aerohiv...
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Matthew Gast

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We currently have five 80 MHz channels available. If the rules proposed by the FCC at the end of Julius Genachowski's chairmanship go through, we'll get four more. More importantly, we'll go from one 160 MHz channel to four. (I should point out that the 802.11ac spec allows you to build a "160 MHz" channel either by taking 160 MHz of continuous spectrum or taking two 80 MHz channels and treating them together.)
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Micha Van Ravenswaay

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Matthew,
You're saying the AP and client negotiate about the channel width. Is this also already the case with current AP's from Aerohive?
So if you put, for example, the AP121 in 40Mhz it will automatically switch down to 20Mhz if it gets crowded at certain times?
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Matthew Gast

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Channel bandwidth negotiation is an 802.11ac feature, so 11n APs do not support it.