Virtual WiFi on Linux?

Jim Thompson jim
Wed Oct 19 13:33:03 PDT 2005

On Oct 19, 2005, at 10:18 AM, Jean Tourrilhes wrote:

> On Wed, Oct 19, 2005 at 09:50:17AM -1000, Jim Thompson wrote:
>> On Oct 19, 2005, at 9:00 AM, Jean Tourrilhes wrote:
>>>    Performance is not everything. Sometime, you want more
>>> flexibility. I can see scenario where I would be willing to trade
>>> performance for the flexibility given by this scheme. I can see
>>> scenario where I would even be willing to use the AP scheduling
>>> described above, which in term of performance is really a drag.
>> I think you don't understand the issues at the PHY layer.
>     Most likely. I though it was impossible to do 802.11 and BT
> through a single radio and was proven wrong.

Well, they're entirely different PHYs, so it is impossible.

Now, if you're talking about being able to have both functions  
running on the same chipset
(though with different front-ends (PHYs), using the same T/R chain,  
then it just requires some...


You really don't want 'B' to transmit while 'A' is attempting to  
receive a frame.  Doing so changes the channel (an by 'channel' here  
I mean "the characteristics of the medium", not "the center  
frequency"), which means that any channel correlation is suddenly  
quite obsolete, and demodulation of the incoming frame is now  
impossible (with high probability).   Put simply, "A" can't ACK that  
frame (because it turned to goo.)

802.11 has this idea of "Clear Channel Assessment".    The PHY will  
sit and listen to the media.  If the media is 'busy' (successfull  
PLCP header received and/or signal strength above a threshold), then  
CCA goes low, and the transmitter is barred from sending a frame.

This is why, sans co-ordination (or a lot of filtering), two radios  
(even on ch1 and ch11) in the same box don't work.

Turns out that some chipsets expose a pin (hardware) or register  
(software) with this CCA signal (or its analog) on it, and this can  
be used to co-ordinate multiple MACs (802.11 or not) from stomping on  
an incoming frame.   Vivato's 'multiple radio' based products work  
this way.  You could even inject enough RF signal in and adjacent  
transceiver while receiving a frame to ensure that CCA gets set,  
precluding any transmits by co-located transceivers.

Doing similar things internal to a chipset that implements BT and  
802.11 (as Mobilian implemented) is really straight-forward.


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