Why can I sniff 802.11g with my 802.11b card?

[Irongeek]

Penguin and I would both like to know this. Why can I sniff 802.11g with my 802.11b card? I need to look again, maybe it’s only seeing the headers of the packets. I already know they do use the same frequency, but the encoding is different. Since they are a little different in how they encode things you would think that a B card could not sniff G, but I know for a fact that my B card in my laptop can sniff traffic from my G card in my desktop to my G router (I just tested). Maybe someone else could enlighten us to why this is so.

Edit: By sniff I mean see data packet (emule in this case), not just find WAPs.

[djscribble]

it depends on your settings -- if your router is in "mixed" mode then it is most likely that while you have a g card in your desktop it is performing at the b standard. if this is not the case then, while g and b are different as far as encoding goes because they operate in the same frequency range and g is technically backward compatible b is able to snif it because the sniffer that you are using is not looking at standards, but rather just seeing that there is something that it thinks is a packet and then it captures it

my money is that you are running in fixed mode (i have only gotten the second case to work successfully once)

[jinxy]

802.11g is backwardly compatible with 802.11b, so i do not see a reason not to be able to sniff it.

[Irongeek]

I understand that, but the G card in my desktop is talking to the G chipset in my router. Why should it need to send data in a format the B card in my laptop can read? I always assumed that a G device encoded differently if it was sending to another G device (so as to get the bandwidth increase).

[jinxy]

The benifits of G over B Are speed of transmission and reception, leaving out the added security possabilities. So a carrier wave is produced with the infomation attached to it, that can be recieved by any reciever working on the same frequency. It is the operating system that decodes the information.

I have to say this is just me taking a quess.

[Irongeek]

You are correct, but the encoding must be different to get those extra bits over the same frequency. I would not think the B card would know how to decode a G to G packet. I’ll do some more looking into it.

[moxnix]

Here is a fairly good artical on the coding used in all 3 standards used today (a,b,and g).
http://www.fact-index.com/i/ie/ieee_802_11.html
That should tell you what you are questioning.

[jinxy]

So G transmits all three transfer methods at the same time, or am i missing something.

DSSS, HR-DSSS, OFDM

[3rr0r]

All I know is that it clearly isn't just a software issue, because otherwise we could just upgrade the drivers of our old 802.11b cards to run at 802.11g speeds - instead you need new hardware.
My guess is that when you associate your B card with the G device, it immediately switches to a compatible mode of transmission, which is B. Check the G device while you're sniffing to see what data rate it claims to be operating at.

[Irongeek]

While my B card is on the network my G card says it's working at 54Mbps. After trying to sniff my web connections it looks like Ethereal can see only some of the G packets. As best I can tell, the B card can sniff some of the G data some of the time (but very little, only the occasional packet). The G can see all of the G and B traffic. But why can the B see some of the G date even some of the time? Something must force the G card to send out it’s data encoded in the old format from time to time, maybe just to let the B devices know someone else out there is talking on the channel?