wifi SSID channels

Banzai_Joe · 7 Oct 2015 at 15:56

So,
got multiple Sophos AP's around our building controlled by a Sophos UTM. (not that it matters).
Each AP has 2 x SSIDs; 1 for employees, the other is the default guest network, but both wifi's are broadcasting on the same channel.
I would have though it better (less congestion) if each SSID was on a different channel.
Employees SSID: ch 9
Guest SSID: ch 6 (or whatever)

Previously all the AP's had auto channel assignment and it caused loads of drops out for connecting clients, so our IT support (who support me) put everything on the same channel, and i've only just noticed.

What's your thoughts?

bremen1874 · 7 Oct 2015 at 16:19

Are they actually capable of supporting different channels for each SSID? That would imply they have two completely separate radios.

holdin09 · 7 Oct 2015 at 20:20

Each channel needs to be at least 5 apart so 1, 6, 11. Otherwise it will cause drop outs.

Caged · 7 Oct 2015 at 21:08

Each radio (2.4GHz or 5GHz) broadcasts/listens on a channel. If you're using anything other than 1, 6, or 11 in the 2.4GHz band then you're stupid, broadly speaking.

Having 16 SSIDs on an AP on the same channel isn't going to cause interference issues, since it's the same radio. If you're seeing issues where radio airtime is being so heavily utilized that it's impacting on network performance then your only options are to add more radios to create a more dense deployment (difficult with only three non-overlapping channels), boot off weak (therefore airtime hogging) clients (reducing the range of your network unless you go back and fill in the holes after turning the power down), or throw all your pre-AC gear away and replace all the APs and clients.

Each radio should definitely be on a different channel to an adjacent one, but I don't think that's the question you're asking.

bremen1874 · 7 Oct 2015 at 21:15

I've never really understood why all of the channels are still selectable. Life would be simpler if the firmware hid the inappropriate options.

Trifid · 7 Oct 2015 at 21:36

Caged said:
Each radio (2.4GHz or 5GHz) broadcasts/listens on a channel. If you're using anything other than 1, 6, or 11 in the 2.4GHz band then you're stupid, broadly speaking..

There is an argument for using 1, 5, 9 and 13 in the UK though. (Beware of American devices not being able to connect to channel 13 though.)

Caged said:
Having 16 SSIDs on an AP on the same channel isn't going to cause interference issues, since it's the same radio.

It is almost as bad having that many SSIDs as it is having multiple access points on the same channel in close proximity. Each SSID takes up a good amount of air time. http://www.revolutionwifi.net/revolutionwifi/p/ssid-overhead-calculator.html

Caged · 7 Oct 2015 at 21:44

Depends if you go mental with the beacon rate. That's not interference though, which is what the OP alluded to.

If you try and run 1, 5, 9, 13 you will have a lot of packet loss when things get busy, I've never seen it done successfully. Adding more APs and actually set the radio power up properly will go a long way to resolving issues you thought you'd solve by using 4 overlapping channels.

Trifid · 7 Oct 2015 at 21:50

I certainly wouldn't change it from the default of 100. You could remove slower data rates to help improve the performance if desired (beacons are always sent at the slowest supported speed) but this depends on your environment.

The channels are only slightly overlapping at the edges of spectrum, this will cause a very slight reduction in throughput, but this is more then made up by having more total bandwidth available. As said, there is an argument for using it. I personally don't, but can see the benefit and wouldn't want the option of having it taken away from me.

Caged · 7 Oct 2015 at 22:11

It's at the edge of the spectrum in theory, different radios will crap different amounts of noise outside of the neat semicircles that get drawn on spectrum plots.

I certainly would never design a network to use four channels. It could potentially be a short-term fix but if Wi-Fi is that important then presumably you have some control over AP placement and the clients in use.

Steveocee · 8 Oct 2015 at 07:03

Can you actually separate them? On my Mikrotik I've got 3 SSID's on the same wireless interface but the wireless itself only broadcasts on 1 channel, the actual separation is done I the router. It could be the same?

Trifid · 8 Oct 2015 at 08:20

Caged said:
It's at the edge of the spectrum in theory, different radios will crap different amounts of noise outside of the neat semicircles that get drawn on spectrum plots.

Yes, there is some noise, but it is significantly weaker then the actual signal, but as said the extra channel and extra bandwidth made available outweighs this. The AP and client works around it, reducing the bandwidth available on that channel slightly.

Caged said:
presumably you have some control over AP placement and the clients in use.

That is the argument for using 4 channels. It is procuring devices from North America as well as visitors that cause problems, because channel 13 isn't allowed over there.

Caged said:
I certainly would never design a network to use four channels.

The 3 channel model is American thinking and a lot of the training material is based around this. If they did allow channel 13, then the 4 channel model would be much more widely used, due to the benefits of the extra channel and total amount of bandwidth available.

Caged · 8 Oct 2015 at 22:13

Steveocee said:
Can you actually separate them? On my Mikrotik I've got 3 SSID's on the same wireless interface but the wireless itself only broadcasts on 1 channel, the actual separation is done I the router. It could be the same?

No, you can't. Each radio can be set to a channel.