Wireless wanderings

The impact of 802.11ac vs 802.11n in high density environments

At the beginning of 2014 the 802.11ac specification had just been ratified and home routers supporting  the draft version were already hitting the shelves in the local computer shops. It is interesting to see the difference between how fast 802.11n was adopted back when it was finally ratified in 2009 and how fast it looks like 802.11ac will be adopted now with the change in device types accessing the wireless medium altering dramatically in such a short space of time. I believe there will be a large difference between what we see in homes versus at offices or in schools as device density becomes increasingly important. Among other statistics, Cisco predicted the number of mobile-connected devices will exceed the world’s population by the end of 2014 and this is key when it comes to the difference between adoption of 802.11ac versus 802.11n. Throughout this article I mention ‘Wave 1’ and ‘Wave 2’ because 802.11ac is arriving like that. Wave 1 is out already with Wave 2 looking like 2016 and there are significant differences in speed between the two with Wave 1 maxing out at a still-very-high 1.3Gbit and Wave 2 achieving dizzy heights of an estimated 7-10Gbits p/second.

Devices and spectrum space

The 802.11n specification brought high throughput (HT) networks to not only the crowded 2.4GHz band but also the very empty 5GHz band, so it would have been a safe bet to predict most devices to be running on 5GHz now, right? Not quite. In several of the homes that I have run inSSIDer at recently I haven’t found one neighboring network running 5GHz because the demand is completely different at home where range is desired much more over speed. Who cares if your router has auto-adjusted to run on channels 4 and 8 and you’ve screwed the network for just about anyone – including yourself – within 40m of you? As long as it reaches the other side of the house it doesn’t matter. Most people reading these sorts of articles would be aware there are only 3 (4 in Japan) non-overlapping channels in the 2.4GHz range but a whopping 23 (country dependent) in the 5GHz spectrum so why aren’t we all on 5? The range is much lower when using the 5GHz network but another factor has a major influence on spectrum adoption and that is device type.

When 802.11n was ratified the iPhone 3GS had only just been released but it wouldn’t be until the the iPhone 5 three years later that mobile phone support for 5GHz 802.11n would be seen with Apple. Other smartphones such as the Samsung Galaxy S III supported them slightly earlier than this. What’s the difference now? 802.11ac has already been incorporated into most popular smartphones and tablets (Apple, Samsung, HTC, Nokia etc) within the same year of ratification accompanied by more marketing to make people aware of the new standard. The marketing alone shows the difference in attitude towards wireless networks now.

With 802.11ac only running in 5GHz network will we now see dramatic shifts in spectrum use?  I’m sure some users can’t wait to access those ‘1.3 Gbps’ Wave 1 speeds that are being advertised on their smartphone but they may want to check the hardware restrictions before buying a $300 home router. Maybe the same reason people aren’t using 5GHz 802.11n at home now will still leave the 2.4GHz crowded in the residential vertical. Meanwhile in schools and offices the adoption of 802.11ac looks set to be driven by two major factors; device density and demand for BYOD support. During the 802.11n golden years (I’ve clearly already condemned it) BYOD has smashed it’s way into the nightmares of every CTO as many businesses have struggled to keep up with demand. Combined with 4G networks providing an always-on connection to the Internet, end users expect to be able to bring their Playstation in and hook up to the BYOD SSID for a quick game of COD. Perhaps not so dramatic but it doesn’t feel far off.

Although there are technically 24 available channels in the 5GHz band there is not actually enough space for full adoption of 802.11ac right now. Many networks these days make use of an excellent technology called channel bonding which allows you to combine two available channels into one for faster speeds. With 802.11n the most you can bond is two channels which should be avoided in the 2.4GHz band but is widely used for 5GHz. As always there are some caveats. Through channel bonding you will halve the number of networks available to 12. Still doesn’t sound too bad though?  Unfortunately part of the 5GHz spectrum space known as the UNII-2 Extended channel set is not fully supported on all devices, particularly ones more than 2-3 years old. At 12 channels this space is quite large and, should you enable them on your network, you may receive some strange service calls from end users standing next to each with one on the network and another not being able to see it at all. This leaves you with 13 available channels (in Australia) or a total of 6 bonded networks. In a high density environment such as a school with one AP per classroom across multiple floors there is already interference in the 5GHz band just by using 802.11n in bonded mode. 802.11ac Wave 1 allows for up to 4 bonded channels and Wave 2 is going to allow for up to 8! Crikey. Fortunately America is already planning to free up more space in the 5GHz spectrum pretty soon which will likely be adopted by the rest of the world after that. Newer devices also support the UNII-2 Extended range which has current network administrators’ fingers hovering over the ‘apply’ button.

In order to access the awesome 1.3 Gbit speeds of 802.11ac Wave 1 you’re going to need a device that supports 80MHz width channels, 3 spatial streams and the network will have to be configured for a 400ns guard interval (GI). Simply put most devices won’t be able to achieve this but it doesn’t matter. 802.11ac comes with a few cool features to help with device density that will really make it worth adopting regardless of top speed.


Without going too deep on the technical front I want to mention a couple of features of 802.11ac that are, well, cool. When I first heard of the channel widths that could be used with 802.11ac I thought there was absolutely no chance people could deploy this in their environment but I’ve changed my tune after researching it more.

Automatic Channel Width – As mentioned earlier, 802.11ac supports either 4 bonded channels (Wave1) or 8 (Wave 2) and this is a concern because although the 5GHz spectrum has a lot more space there certainly isn’t much room left if you decided to bond 8 channels. Something amazing about the new standard is it can alter it’s channel width per frame depending on interference around it. What that means is if a nearby network is utilising one or more of the channels designated for an Access Point’s communications it will detect this and temporarily shift to a lower channel width in order to maximise communications efficiency for everything in that area. Whilst this isn’t an answer to everything, it is a huge boost to how wireless networks operate and will help 802.11ac fit in alongside 802.11n networks without causing too many ripples.


Auto channel width 802.11ac
Auto channel width adjusting with 802.11ac


Modulation – Modulation is the way that the data is encoded to be passed between the physical radios for demodulating at the each end. In the table below you can see the modulation encoding scheme (MCS) numbers going from 0 – 9. If you are on OSX right now you find this number out quite easily by holding your option key down whilst left clicking on the WiFi icon. This will display some statistics including Received Signal Strength Indicator (RSSI), Noise level and the MCS being used. The higher the MCS number the better your speeds. With 802.11n the MCS index goes from 0 – 31 and the best modulation you can achieve is 64-QAM. This enables the highest theoretical speed of 600mbit p/second with four streams which nobody ever realistically gets in an 802.11n network.


802.11ac Modulation
802.11ac Modulation Table. Source: Wikipedia


802.11ac provides a highest modulation of 256-QAM and comes with a top theoretical speed of 433mbit p/second with a single stream device. Multiply this by the number of streams to get your top speed. An iPhone 6 supports only single stream and therefore has a top theoretical speed of 433.3mbit p/second when bonding 4 channels with 802.11ac. Compare this to 150mbit p/second with a single stream using 2 bonded channels with 802.11n and you can see why better modulation = improved performance.

MU-MIMO – Sadly this fantastic improvement won’t be available until Wave 2 comes out. Without bigging it up too much I have seen it labelled as the holy grail of wireless networking so no pressure at all. One of the biggest issues with wireless is that it is a shared medium and therefore everyone has to wait their turn to use a channel. With Multi-User Multiple-Input Multiple-Output (MU-MIMO), multiple users are able to receive data on the same channel “simultaneously”. Meru networks did a nice blog (link below) that outlines the potential problems with MU-MIMO and based off this it may be that we have to wait until the next standard for the real benefits to be seen from it due to slow adoption from devices.


Power Considerations

As I am studying for my CCNP Wireless I feel no guilt in pointing out a few Cisco specific things now!

I recently had to look into hardware replacements for the Cisco 1142 and 3502 APs which gave me an opportunity to delve into the hardware requirements when comparing the 3602 and 3702 models. It is fairly straightforward but easy to miscalculate as I found out when looking at Cisco’s documentation. From my original glance, both APs were compatible with the 802.3af standard but if you want to run 802.11ac you basically need a 802.3at (PoE+) compatible power source for all radios to be operational. Even more confusing is that Cisco implies the PWRINJ4 as 803.at compatible device but am not convinced it is. However, I have seen the Cisco Meraki PWRINJ4 power injector that is 803.at compatible so perhaps that is what they are referring to. If there are two different injectors that have different power ratings I don’t know why they would not call them different things but I’m sure there is a reason! So if you are running them on a PWRINJ4 or a 803.af power device and see power messages in your debug then it might be that the power source is not high enough to power all your radios.

Cisco 3600 AP Datasheet

Cisco 3700 AP Datasheet

Cisco Aironet PoE Application Note

WLC Support

See below for one of the most useful Cisco links ever, the compatibility matrix. Basically take note that if you want to deploy 3700 APs in your environment you will need to upgrade to at least code 7.6.x. Do not forget to upgrade your firmware whilst doing this or you may be come across a few problems!

Cisco Wireless software compatibility matrix


In a nutshell, 802.11ac is huge. Although 802.11n was and is a really great standard, it was adopted slowly due to low trust in wireless and a lack of demand across the verticals. Once smartphones and tablets stormed the market technology demands changed drastically and an explosion of wireless devices occurred. At home we are seeing a demand for range and in some cases speed when it comes to media streaming. In offices and schools users are expecting BYOD support and often internal resources access for their personal devices which means each employee is bringing in 2 or more devices to work everyday and that can be a challenge for any corporate network to handle. Schools can have the same number of devices for different reasons with mobile phones being banned in many places so the demand shifts to 20 or more users accessing resources like Youtube or logging in simultaneously as opposed to the usual ebb and flow of users coming in and out of an office building.

The raw speed of 802.11ac is exciting of course but I believe it is the behind-the-scene features that really make the difference. Improved modulation, faster management frame handling and overall efficiency improvements make it a technology worth adopting in the enterprise environment soon but planning is key. The 3602 AP comes with the capability to add an 802.11ac Wave 1 module into it but the 3702 AP comes with it in built and the capability to install a Wave 2 module when that comes out. If you get quoted a similar price for the two then I think it is a no brainer to buy the 3702 AP and cover yourself for the future at the same time. The real issue at that point will be wired throughput when those theoretical speeds of 7Gbit+ start becoming real. Ask yourself now how you would address providing support for those speeds to a single AP servicing 20 clients and see what you come up with.


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