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

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.

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)

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

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).

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.

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.

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.

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

DSSS, HR-DSSS, OFDM

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.

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?

Analyzing:

Both B and G use the same frequency, which has been noted. It is 2.45 Gighertz located in the LSM band. ALL packets must be included in this band.

B uses a media access similar to Ethernet CSMA/CA. That like Ethernet is in the firmware not in the OS or driver software. Lower level stuff. B divides the radio spectrum into 14 to 22 overlapping channels; depending on what country you are in. They overlap because they are not all used at once. (Anything over 14 is NON-standard)

G operates in the same band uses the same transfer methods (DSSS and HR-DSSS) same media access and adds one transfer method for A (OFDM). It uses the same security measures as B (WEP) then adds some depending on hardware (AES or Wi-Fi protected access). It uses all the same frequencies, so in essence there is not much of a difference so far…. And no reason why you can’t sniff packets, in essence it is exactly the same….

So I am digging through tech. Documents my guess is there is not much of a difference at all and they merely bond more channels together and switch the data rate faster on the link. But that is a wild ass guess.

B can see G because it's the SAME lacking certain channels at random. (another SWAG until confimed)

\\EDIT It's acctually the way the radio is Keyed. Keying a radio is like hiting the talk button on a walkie-talkie. Only in hispeed data this happens VERY fast and is related to the data transmitted. Meaning it's part of the intelligence mechanism and it too trasmits part of the puzzle, not just the modulation schema along with channel hopping. So as these key sequences come in alliance with the B mechanism you will see random bits of data but not all data. Hope that helps muddle things up more. ;)

most implimentations of G transmit using multiple channels, so you are most likely able to sniff *some if not all of the transmission due to this fact. If your channel hopping and your delay is short enough you can feasibly pick up quite a bit of G traffic.

Whether the vendor implementation encodes the data is another story, but this is most likely why your picking up the traffic