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In an ESS, if a frame is being sent from a station in one BSS to a station in another BSS then that frame will cross the DS from one AP to another.

When the frame leaves the sending station the To DS bit will be set and the address 1 field will be the MAC of that station's AP. The address 2 field will be the MAC of the send station and the address 3 field will be the MAC of the destination station in the other BSS.

The frame will then cross the wired network from one AP to the other.

Now when the frame leaves the AP in the destination BSS the From DS bit will be set and the address 1 field will be the MAC of destination station. The address 2 field will be the MAC of this AP and the address 3 field will be the MAC of the sending station in the other BSS.

If any of this is incorrect please set me straight.

My question is when the frame is sent across the DS from the sender's AP to the receiver's AP what MAC addresses are used in the Etherent II frame? Since it's not an 802.11 frame, we have only a source and destination address not four address fields.

My follow-up question is when the sender's AP receives the frame from the sending station how does it know to send it to the receiver's AP? In other words, how does it know that it's a station in a BSS and not a wired station (assuming the wired station was in the same broadcast domain)?

TIA,
Brad

Brad,

These are good questions about a topic that isn't well understood. Instead of trying to correct your statements, I'll make some of my own. Here is how the To DS / From DS bits change the MAC addresses:

To DS: 1
From DS: 0
Address 1: Receiver (This is the immediate wireless device, in this case the AP)
Address 2: Transmitter (In this case, the client STA)
Address 3: Ethernet Destination

To DS: 0
From DS: 1
Address 1: Receiver (This is the immediate wireless device, in this case the client STA)
Address 2: Transmitter (In this case, the AP)
Address 3: Ethernet Source

So, how does the AP handle the "conversion" from wireless addressing to wired? As you can tell, depending on the direction of traffic, the third address changes roles.

In situation one, address 3 is the Ethernet Destination. So, what is the source? The AP knows that the device that transmitted the frame (The client STA) MUST be the originator of the frame, so it just uses the transmitter address as the source.

As a frame comes into the AP such as situation two, the AP reads the Ethernet headers and looks at the MAC addresses. Here is what it knows:

Ethernet Source: I need to tell my wireless device the source, so that goes in address number three.

Ethernet Destination: I know that I need to get it to the destination, so that has to be the device that is receiving my data, so, destination is "translated" into receiver.

This takes me an hour on a whiteboard to teach in class because it gets confusing.

Remember this: An AP is a layer two device and does not EVER use it's own MAC address in any packets that are going THROUGH the AP. Just like a wired switch.

I've written a lot here and if I have confused you, I apologize. Just post back and I'll try to clear anything up.

We'll talk about how to address a WDS later. :)

GTHill

GT,

Thanks for the explanation. That makes perfect sense now that I hear it. In regards to my follow-up question I think I can answer it myself now. Please let me know if I'm off track here.

An AP maintains a list of MACs of wireless stations that are associated with it. Any time the AP sees one of those MACs as a destination address in a frame on the wired DS then it takes it in and forwards it to that wireless station.

I'm looking forward to hearing your comments on a WDS.

Brad

Your statement above is correct. Just to make sure, lets take it one step further. On the wired port, how does the AP receive only (well, mostly) data for its STA's and not all wireless STA's on the network? Since the AP passes all traffic through it, the switch maintains a MAC table of all wireless STA's.

As far as a WDS goes, want to give it a shot?

GT

GT,

OK, I'm thinking that the "well mostly" is for any unknown unicasts (or multicasts) the switch (connecting the two APs) receives.

Here's my shot at the WDS. I'm assuming that there are two BSSes connected via a WDS so each AP is acting as both an AP and a bridge. (Not sure if vendors actually do this). However, the two BSSes could just as well be wired networks.

Frame from wireless station A to it's AP
To DS: 1
From DS: 0
Address 1: Receiver (A's AP)
Address 2: Transmitter (Station A)
Address 3: Station B

Frame from wireless station A's AP to station B's AP
To DS: 1
From DS: 1
Address 1: Receiver (Station B's AP)
Address 2: Transmitter (Station A's AP)
Address 3: Destination (Station B)
Address 4: Source (Station A)

Frame from wireless station B's AP to station B
To DS: 0
From DS: 1
Address 1: Receiver (Station B)
Address 2: Transmitter (Station B's AP)
Address 3: Source (Station A)

Thanks for all your help,
Brad

Excellent work! You don't need me... you have it down for sure. Have you taken your CWNA yet?

GT

Thanks. You helped me a lot.

I posted my question in a couple wireless forums and the only answer I got was someone saying it was too technical and I should post it here.

No, I'm not really interested in certs just knowledge.

Thanks again. Have a great weekend.

Brad

GTHill Escribi?3:

Remember this: An AP is a layer two device and does not EVER use it's own MAC address in any packets that are going THROUGH the AP. Just like a wired switch.

True.

But if you have access points all over the place and want to "flatten" them together, a common thing to do in cable environments is to have them L2TP tunnel back to a common source so that you can authenticate the users (e.g. RADIUS) and hand out addresses on the same DHCP server.

This way, there is some continuity between access points that might be in completely separate networks.

Rog,

To keep things simple(r) I usually discuss things like MAC addressing in reference to stand-alone AP's. You are talking about lightweight controller based AP's right?

GT

GTHill Escribi?3:

To keep things simple(r) I usually discuss things like MAC addressing in reference to stand-alone AP's. You are talking about lightweight controller based AP's right?

That's understandable.

I don't typically deal with lightweight APs that require controllers, hence the reason I can tunnel from them and stuff like that.