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Question about Promiscuous Mode

Posted: Mon Mar 12, 2012 12:44 pm
by alef
Hello,
I'm reading a "802.11 WLAN Packets and Protocols" from Network Glossary on this site. There is explanation about Promiscuous Mode:
"Device C acknowledges the receipt and processes the packet normally, passing it to the next layer of the protocol software. Device D also captures the transmission and passes the packet to OmniPeek. The machine on which OmniPeek is running will not send any acknowledgement, because Promiscuous Mode is strictly a listening mode. Radio transceivers cannot listen and send at the same time on the same frequencies because their own transmission would drown out any incoming traffic."

My question is - how come device C is able to send an acknowledgement if it also listens on the same frequency and acknowledgement transmission may drop incoming traffic in the same way?

RE: Question about Promiscuous Mode

Posted: Mon Mar 12, 2012 12:51 pm
by DJWP
Because Device C is not in promiscuous mode, it can send and receive. Device D is the capture adapter.

Re: RE: Question about Promiscuous Mode

Posted: Mon Mar 12, 2012 1:24 pm
by alef
[quote="DJWP"]Because Device C is not in promiscuous mode, it can send and receive. Device D is the capture adapter.[/quote]

Ok, thanks for the answer but as for next sentence: " Radio transceivers cannot listen and send at the same time on the same frequencies because their own transmission would drown out any incoming traffic." is it correct? Does it mean that Device C (which is not in promiscuous mode) can send and receive on the same frequency? Or it uses different frequency to send acknowledgements?

RE: Re: RE: Question about Promiscuous Mode

Posted: Mon Mar 12, 2012 2:43 pm
by DJWP
Any single 802.11 radio must transmit and receive on the same frequency.

RE: RE: Re: RE: Question about Promiscuous Mode

Posted: Mon Mar 12, 2012 5:02 pm
by alef
So device D does not send acknowledgments not because of "Radio transceivers cannot listen and send at the same time on the same frequencies because their own transmission would drown out any incoming traffic." but because it is in listening mode, which does not allow to send something. Right?

RE: RE: RE: Re: RE: Question about Promiscuous Mode

Posted: Tue Mar 13, 2012 7:59 am
by DJWP
Yes. Device D is listening only, it is the capture adapter

RE: RE: RE: RE: Re: RE: Question about Promiscuous Mode

Posted: Tue Mar 27, 2012 2:24 pm
by zonker
To answer the root question, the 802.11 radio conversation must be half-duplex. When the adapter wants to transmit, it cannot simultaneously receive. So, when it receives a packet, it will stop listgening, transmit the ACK, and go back to listening.

Best regards,

-Z-

RE: RE: RE: RE: RE: Re: RE: Question about Promiscuous Mode

Posted: Wed Mar 28, 2012 8:03 am
by DJWP
This is not correct. When the adapter using the WildPackets driver is used for capture, it cannot be used for transmission at all. It is always in receive mode only.

When OmniPeek is not using the adapter for capture it goes back to normal send/receive operation.

RE: RE: RE: RE: RE: RE: Re: RE: Question about Promiscuous M

Posted: Wed Mar 28, 2012 10:07 am
by zonker
To clarify; The node that you are sniffing is the radio that stops listening when it transmits... The radio in the sniffer is going to be listening all the time.

RE: Question about Promiscuous M

Posted: Wed Mar 28, 2012 10:56 am
by DJWP
Thank you for the clarification-