Wi-Fi Riddles: Invisible 40MHz

A while ago I stayed in a small hotel that, had virtually all the problems of ‘small hotel wi-fi’ (including unresponsive one-device-per-room hotspot and lack of support outside of business hours. How many people stay at the hotel inside the business hours?). This is very typical of a WLAN installation made by a small local jack-of-all-trades shop. Unfortunately, I see this issue way too often, especially in SMB deployments. I hope this article will help some of the SMB integrators and their clients.

Before we move on – what’s wrong with this picture? (Nexus7 1st gen)

BW-InRoomOf course, with such SSID naming there will be no roaming, but that’s not the main problem (especially in the hotel). Not even that the ARBOIS7 AP was closer to me (about 10m away on the same floor), and yet, the ARBOIS9 AP (located right under ARBOIS7 on the other floor) offered stronger signal. The connection was bad even w/o it, so I decided to check with my laptop.

BW- Laptop 40MHzI think the picture speaks for itself (clickable). It’s not the first time I see planning for ‘channels 1/6/11’ without taking 40MHz into consideration.

Allowing 40Mhz channels in 2.4GHz on consumer devices (this installation uses EnGenious, according to the MAC OUI) was one decision that, I believe, IEEE made wrong when drafting 802.11n (I know there was a lot debate around this). And now, we may as well be facing the same issue with 80/160MHz channels in 5GHz range, albeit with lesser consequences. Here’s a quote from EnGenious AP manual that explains a lot, in my opinion. Who would want to restrict their channels to 20MHz after reading that?

EnGenious40MHzGood thing, though, is that in 2.4GHz one cannot force 40MHz channels, and APs will ‘fold’ back to their primary 20MHz channel once they see a 20MHz transmission around them (such as a 20MHz beacon). The problem is, that this does not apply to a situation when all APs around are at 40MHz. So, in the diagram above, when ARBOIS7 transmits, the ARBOIS9 has to sit idle, because its channel is busy (should be noted that in 802.11ac setup ARBOIS7 would have been able to use the free 20MHz of it’s channel). Plus, this generates unnecessary overhead for the 20MHz transmissions (both b/g and n) due to protection modes. Given that the Internet pipe is such hotels is rarely >75Mbps of effective throughput, a fixed 20Mhz plan would have probably been better for them.

Here’s another example (albeit dirty). Note how the wide channel 4+8 overlaps with every single channel in the 2.4GHz band.

WideChannel6I had similar situation in a conference we held (the offending AP was used by the stage management guys). The owner denied having any problems and showed his survey tool with channel visualization as a proof – his AP was on 20MHz channel 4 (“…Because we want to minimize interference by going between ch1 and ch6…”) without any overlap with anyone else. Of course he was running it on a tablet (in fact, same Nexus7 as mine). After showing him how my laptop interprets his design decision, we have reached consensus quickly 🙂

Conclusion: not seeing 40MHz channels, does not mean they aren’t there. Proper survey tools must be used, including capable hardware. Same, BTW, happens in WLAN security: not seeing a rogue doesn’t guarantee it does not exist, except that the results of the negligence are much more serious.

Plus, one should remember, that tablets and phones have much weaker receivers. The difference of 10dB and more is not uncommon – just look how the laptop in the picture sees ARBOIS7 at -50dBm, while the tablet sees it only at ~-67dBm! Surveying with the weakest client possible is a good idea to ensure the bottom line. …And please don’t do the ‘one-device-per-customer‘ hotspots in 2014!

BTW, why the AP that was closer had weaker signal than the AP on the other floor? My other hotel all-time favourite: installing APs in maintenance shafts to preserve the ‘decor‘. The best one I’ve seen so far was installed right inside of an aircon duct!

Arbois-40MHz-APmountBottom line:

I understand that ‘I-must-do-it-all-from-pulling-cable-to-configuring-asterisk’ people simply do not have time to become gurus in everything they do, but hope this post will help them have happier customers! 🙂


One thought on “Wi-Fi Riddles: Invisible 40MHz

Add yours

  1. Absolutely spot on. It makes me sad to see so many bits of consumer kit not configured correctly. The industry could very easily fix it to take account of best practice and just tweak the default configs.

    Trouble is – 40MHz = double the bandwidth you can advertise.

    You won’t sell any of your slow 150Mbps kit if I advertise mine as capable of 300Mbps


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