How does a 802.11 client work?

Unanswered Question

Hi,I'm just wondering how a pc with a 802.11 interface send and receive traffice from the net. Documents say that it uses its 802.11 interface mac as its source and the ap's mac as its destination. This is what puzzles me. If I type arp -a from the client PC, I can only see the gateway's mac (that's what I think is right), no ap's mac. So why and how does the client PC send traffic to ap's mac?

Anyone help? thanks in advance.

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (3 ratings)
Loading.
Robert.N.Barrett_2 Sun, 04/12/2009 - 08:34

That's a pretty broad question - I would recommend reading Matthew Gast's "802.11 Wireless Networks - The Definitive Guide". The short answer is that there are 4 address fields usable in an 802.11 data packet.

From the book:

Frames destined for an access point take their source/transmitter address from the network interface in the wireless station. Access points do not perform filtering, but instead use the third address to forward data to the appropriate location in the distribution system.

jeff.kish Mon, 04/13/2009 - 10:27

Think of an AP like a switch. If you have a client connected to a switch, it uses its own MAC as the source, but it doesn't use the switch as the destination. The destination is the target client's MAC is it's a local transmission, or the default gateway if it's sending traffic to a different subnet.

Wireless works the same way. The destination MAC address is either the receiving client or default gateway. It does not use the AP MAC as the destination.

I hope that helps, and please let me know if you have more questions.

Jeff

EDIT: Haha, overlapped with Rob again. We seem to do that a lot in these forums, Rob! +5 for the additional info.

rob.huffman Mon, 04/13/2009 - 11:09

Hi Jeff,

Good stuff my friend! Your post contained great info and mine was just a simple link to the excellent guide referenced by Robert. You deserve the +5 points here buddy.

Keep up the good work!

Cheers!

Rob

Thank you guys.I have read the book refered by you guys. This is what I concluded: Right the pc use its mac as source and the client's or the default gate's mac as its destination. However before frames reach ethernet, they are 802.11 frames so their frame formats are different from that of wired ethernet. There are commonly used three types of mac addresses. The client uses the address 1, which is the mac address (or BSSID) of the AP, as its receiver address; address 2, as its source address;address 3, which is the mac address of the gateway, as destination address. And , after the AP reveives the frame, it use the address 2 as source, address 3 as destination, transforms 802.11 frame into ethernet frame. And then the frame goes into wired net.

Correct?

jeff.kish Tue, 04/14/2009 - 10:39

That sounds right, though I don't have the book in front of me to reference. The 802.11 frame is what's key, as there are parameters there that don't appear in a normal Ethernet frame.

Johannes Luther Tue, 04/14/2009 - 10:53

It's kinda simple:

Client to Client (Ad-hoc):

Mac1 = Destination address

Mac2 = Source address

Mac3 = BSSID

From AP to Client

Mac1 = Destination address

Mac2 = BSSID

Mac3 = Source address

From Client to AP

Mac1 = BSSID

Mac2 = Source address

Mac3 = Destination address

From AP to AP (f.e. in bridge mode)

Mac1 = Receiver address

Mac2 = Transmitter address

Mac3 = Destination address

Mac4 = Source address

Hope that helps

Actions

This Discussion