Friday, February 27, 2015

The case "against" 802.11ac

Still a hot topic for debate:  Do you invest in more expensive 802.11ac access points or do you deploy cheaper 802.11n, given that 802.11n is likely more than adequate for today's needs in the small to medium enterprise environment.

The answer:  As always, it depends.  :)

There are only two main improvements in 802.11ac wave 1:

  • 80 MHz channels:  Gives you 2x improvement in speed, but higher noise floor and more subject to external interference
  • 256 QAM:   33% improvement in speed, but you need a > 29 dBm SNR to take advantage of it, meaning that in practical terms you must be within 10’ – 15’ of the AP in open air to get MCS 8 or 9.

Additionally, newer chipsets and APs generally have better performance, though impact is probably at most in the 10% - 15% range.   Given the same environment and the same 2x2:2 5 GHz 802.11n client device, an 802.11ac is likely to perform somewhat better than its 802.11n counterpart.

802.11ac wave 2 will have two other “improvements”:
  • 160 MHz channels:   Gives another 2x improvement in speed, but even higher noise floor and, at least with current FCC allocated spectrum, only two usable channels, making it impractical for multi-AP deployments.
  • MU-MIMO:   I finally understand how this is supposed to work (from WLPC 2015), and will have to go into it in a separate blog post.  Suffice it to say that it will likely only be useful in high density environments, and it requires transmit beamforming feedback from the client devices, meaning that the devices will likely need to be 802.11ac wave 2 compatible.  The feedback mechanism is actually part of 802.11n spec, but most client device vendors didn’t implement it.  It is not clear whether the functionality even was built into the chipsets of most 802.11n or existing 802.11ac wave 1 devices, or even if it was, whether firmware upgrades will ever be made available on current generation devices to make it useful.   Note that the feedback mechanism will also serve improve range, even if the "multi-user" part is not applied.
Add to this the bifurcation of client devices we are likely to see on the networks evolving in the next few years, which will ultimately lead to different classes of service based on FREQUENCY BAND:

  • Smartphone / Tablet Devices (i.e. the iPhone 10):   Don’t know their capabilities, but their being more of them, sucking down more bandwidth more frequently is a pretty safe assumption.   Will use the latest and greatest Wi-Fi chipsets for 5 GHz / dual band.  They will still only be 1x1:1 or 2x2:2 devices because of the need to keep both size and power consumption down.   Even now there are very few high end laptops with 3x3:3 802.11n or 802.11ac capability, and that is unlikely to change going forward.  Hence the desire for MU-MIMO.
  • Internet of Things:  Lots of devices that need to be connected all the time but each one passing very little data.  Consumer / price driven, so likely to have the cheapest Wi-Fi chipsets and antennas that can be found.  Hence, most of these devices will likely be 2.4 GHz only and maybe still 802.11b or 802.11g (i.e. not even 802.11n).
It is true that the bottleneck of the network is the internet bandwidth connection from the carrier.  That is where you want the bottleneck to be.   You never want the internal network (APs, switches, wireless bridges, or routers) to be the bottleneck.

So, still begs the question – do you deploy 802.11n or 802.11ac?   I still think the answer ultimately comes down to the expected lifetime of the network.  Most small / medium enterprise customers are not going to want to upgrade anytime soon.  The minimum lifespan of a network deployed today is at least 5 years, with 7-8 years being more likely.   Hence, do you deploy for today, or do you deploy for tomorrow?   The expectations of performance out of a network deployed today will only increase over time, both in terms of number of devices and in total bandwidth consumption.  I therefore believe today’s networks should have the latest and greatest technology deployed, if only to be 2-3 generations behind in 5 years vs. 4-5 generations behind.

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