When you are evaluating wireless chipsets and wireless hardware, there are a lot of things to look at. One thing that seems to be largely overlooked by some vendors is the available guard intervals with 802.11n. If you are in an environment that is prone to bad multipath situations, than you probably don't need to think too much about this, as you would want the longer guard interval of 800ns anyhow. However for office environments, having a Short Guard Interval (SGI), which lowers the interval to 400ns, can give you an increased data rate of 11%.
The guard interval is at a very basic level, time spacing between symbols to prevent inter-symbol interference (ISI). Don't confuse symbols with packets; packets are separated by inter-frame space (IFS). If you really want to know a little bit more about Symbols, read this Wikipedia page and break out your oscilloscope. I don't think I've seen an enterprise grade 802.11n access point that doesn't support SGI when using 40-Mhz channels, which is great if you've got the ample 5ghz coverage in your facility and everyone is using it. However with today’s clients, your mobile devices will often prefer 2.4ghz to 5ghz because the signal is typically stronger, even if the data rate is lower. Short of using a vendor that supports Band Shifting/Steering (ignoring probes/clients seen on 2.4 if they are also seen on 5ghz), or forcing client configurations to 11a, or finding the few clients that do more than look at SNR to choose a band, you may be losing out on a little bit of throughput on the 2.4ghz side.
Since you can't really use a 40mhz channel on 2.4ghz unless you're installing only a single access point in the middle of field and don't have to worry about any neighboring RF, it's unfortunate that some vendors don't support SGI on 20mhz channels.
How can you tell if your AP supports it? If you don't want to rip the cover open and look at the chipset to read its datasheet, you can really just walk close to your installed AP (not directly under it if its ceiling mounted) look at your connected data rate (Windows task tray wireless icon or Mac OS X Network Utility). If you're on 2.4ghz and it says 144Mbps, your AP supports SGI. If it says 130Mbps, you're using the regular Guard Interval of 800ns. (Note that if you aren't close to your AP, MCS14 with SGI is 130Mb, so make sure you're using MCS15/highest rate.
Of course, if you are supporting legacy clients still, it's sort of pointless, as the SGI won't be used, even if the data rate says 144Mbps. You will see a lot of APs that lack SGI support on 20mhz, as one of main chipsets on the market that is in use by enterprise wireless vendors doesn't have support for it. (I'm looking at you Atheros). I know Cisco and Trapeze support short guard on 20mhz channels. If you know of more examples of who has support for it, let me know.
Is an 11% datarate increase on 2.4ghz really a big deal? Not really, especially because it's unlikely in a lot of situations to be running in a Greenfield mode with no legacy clients. It could be just another great reason to move your 802.11n clients to 5ghz and use 40-mhz channels. Would I not buy a vendors hardware because they don't support it? It doesn't bother me that much personally, though a lot of times I look at things like this and wonder what the thought process was. I bet there is a logical reason, whether technical, time to market or financial, but I don't know.
(Note that this blog post will date itself as APs with more than 2 spatial streams come on the market and MCS15 isn't the highest you can go anymore, but the theory will remain, just the method of checking will require you to look up the higher MCS data rates for your spatial stream configuration.)
Erik Parker is a wireless network engineer for a Fortune 500 e-commerce company based in the United States. Erik was previously a wireless engineer at Toyota and consulting network engineer for International Network Services (Now BT-INS) prior to that. He has experience with Routing, Switching, Wireless, Security, and Linux systems engineering. His primary focus is on wireless infrastructure, 802.11 protocol analysis, RF, and mobility. Erik's hobbies include arm-chair electronics using Arduino, Parallax, and nearly anything else you can hook random sensors into. Erik has maintained his CISSP designation since 2002, has spoken at multiple Gartner mobility summits, and continues to be active in the wireless community.
This blog represents the personal views of the author and does not necessarily represent the views of his employer.