Sunday, 23 September 2012

Which 5GHz Channels Does My Device Support?

I've been on a bit of a 5GHz quest recently, trying to get to grips with all of the nuances of supporting WiFi devices on this rather (in my mind) troubling band.

Until fairly recently, it seems that the default band of choice for many WiFi devices has been 2.4GHz (802.11g/n). But as the whole 'bring your own device' area has exploded, networks require more high-density deployments, 802.11ac is on the horizon and consumer grade devices are starting to support 5GHz in increasing numbers, it looks like 5GHz is going to transition to being the band of choice over the next year or two.

However, there seem to be a number of considerations that need to be taken in to account when delving in to the 5GHz 'wonderland'. There are far more non-overlapping channels available (19 in the UK) compared to 2.4GHz (generally 3 channels), which is going to potentially deliver much better performance gains (with the mitigation of co-channel interference, lower noise floor etc.). However,  terms such as 'DFS' and' TPC' start to crop-up and a myriad of channel usage restrictions across the band need to be understood (e.g. indoor/outdoor use, varying power levels, varying DFS support etc.).

As if this isn't enough, I was recently warned of the dangers of ensuring that client devices support all of the 5GHz channels that you intend to use. I must admit that my assumption (until recently) was that if a device supports 802.11a/n, then it will support all of the channels that are dictated by the local  regulatory authority (e,g, FCC, ETSI etc.). But, apparently not...

I'm not sure how extensive the lack of support for all available 5GHz channels is among 802.11a/n devices, but it is certainly a consideration when designing WiFi networks that use the 5GHz band (I am reliably informed). I'd be interested to know if this issue is only applicable to older devices, or is due to device assumptions about their regulatory domain - drop me a comment with your experiences/knowledge!

I fired up a couple of devices to check their support for 5GHz channels as a bit of an experiment. I have an iPad 2 and a Galaxy S3 phone, which both support 5GHz.

My initial thought was to fire up my test AP on channel 36 and test both devices to verify they can associate to a test SSID. Then I would move on to channel 40 and check again. Checking each of the channels supported in my particular part of the world, I could verify if all channels are in fact supported by both devices.

However, I had a WiFi analyzer to hand and was capturing a few frames to see what was going on during my testing. Luckily, I spotted some capability fields in the association request of each device which really helped me out. In the association frame there are 'supported channels' fields that show which 5GHz channels the client device supports. This is exactly the information I needed to know!

I've posted a couple of screen shots below to show what I saw using a WiFi analyzer:

Fig. 1 - iPad 2 Association Frame

Fig. 2 - S3 Association Frame
In each association request, you can see the supported channels for the client device. It's a bit tricky to read on first glance, but if you look carefully, you can see the start of a channel range (called 'First Channel') followed by the number of channels in that range.

Looking at the iPad frame, we have channels 36, 40, 44, 48 (first channel - 36, num of channels = 4), followed by channels 52, 56, 60, 64 (first channel - 52, num of channels = 4), followed by 15 other channels (see frame detail). This gives the full 23 channels you might expect in the USA, but is actually more than we can support here in the UK!

The S3 supports just the 19 channels we have available here in the UK. I'm guessing this is because it is manufactured specifically for Europe.

Looking at these frame captures, we shouldn't have any issues using these here in the UK, using all 19 channels we have over here on 5GHz, if we wanted to use them.

It's certainly worth checking your devices when planning to use them on a 5GHz WLAN, just to be sure you don't have clients that  only work on a subset of the channels you intend to use on your WLAN (which could obviously cause connectivity issues to some clients).

If you don't have a wireless analyzer to do this investigation, it might be worth checking out running Wireshark on Linux. You can sometimes find a wireless NIC card that will run with a Linux distribution in a promiscuous mode, so that it can capture frames. I recently downloaded the latest copy of the Backtrack distribution (the penetration toolkit) and found that the built-in wireless NIC in my Dell laptop is now supported (a nice surprise!). I can run the Backtrack distribution by booting from a USB stick and use it as a great wireless analyzer (Wireshark is part of the distribution) - I don't have to disturb the main OS on my laptop at all.

Anyhow, as I said, I'd be interested to hear of other folks experiences with client support on 5GHz. Drop me a comment if you can.

Nigel.

Related Post: Which iPads Are on the Network?