Dual band

[Matthias May]

On 02/01/18 16:25, Fejes József wrote:
> On Tue, Jan 2, 2018 at 4:14 PM, Ben Greear <greearb at candelatech.com> wrote:
>>
>>
>>
>> On 01/02/2018 04:28 AM, Fejes József wrote:
>>>
>>> Hi,
>>>
>>> This question was asked a couple of years ago (eg. here
>>> http://lists.infradead.org/pipermail/hostap/2012-January/025117.html)
>>> with a negative answer.
>>>
>>> However, it seems like things have changed in Linux at least. Recent
>>> wifi cards support multiple virtual interfaces with multiple channels.
>>> Eg. with the command: iw dev wlan0 interface add wlan1 type __ap .
>>> Thus even if I have one card with one chip, I can have multiple wlan
>>> devices and the card will handle communication with different
>>> bands/channels accordingly.
>>
>>
>> Which cards do you think support this option?
>>
> 
> For example iw reports this for a BCM4366 (it also reports both 2.4GHz
> and 5GHz bands in the channel list):
> 
> valid interface combinations:
> * #{ managed } <= 1, #{ P2P-device } <= 1, #{ P2P-client, P2P-GO } <= 1,
>   total <= 3, #channels <= 1
> * #{ managed } <= 1, #{ AP } <= 1, #{ P2P-client } <= 1, #{ P2P-device } <= 1,
>   total <= 4, #channels <= 1
> * #{ AP } <= 4,
>   total <= 4, #channels <= 1, STA/AP BI must match
> 
> Last combination is the key, you can create 4 AP-type interfaces at
> the same time. Although it says channels<=1 which is weird for
> dual-band but whatever. I've seen pastes where channels was 2.
> 
> 

Yes this allows you to create 4 APs, but on the same frequency.
The key here is not that you can create 4 APs, but the "channels <= 1", meaning the card can only operate on a single
frequency.
Dual Band usually means the card can operate on 2.4Ghz and 5Ghz, but not at the same time.

I'm not aware of "any" card which supports multiple frequencies at the same time, not only from software side, but
mainly from hardware side.
How would this work? How would you isolate multiple transceivers transmitting at the same time?

Scenario:
Put 2 different cards into a single device, and put terminators on one, and a power meter on the other:
When the card with terminators transmits something, you will still see the signal with ~20dB attenuation on the other card.

BR
Matthias