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Context: I understand the first two MAC addresses in a WiFi setting are for source (UE) and destination (AP) with the third address used to signify the layer-3 next-hop. The fourth address may be involved in the answer to this question (if so, the Wikipedia entry on 802.11 should be updated with any information on the fourth address!)

Setting:

[UE] ~~ [Booster] ~~ [AP] -- [Switch] -- [Router] -- [The Internets]

  • UE: User Equipment, such as a laptop
  • Booster: Signal boosting device
  • AP: Access point out of range of UE without the aid of the booster
  • Switch: Simple Ethernet switch
  • Router: First-hop connection to the remainder of internet
  • ~~ Wireless (WiFi)
  • -- Ethernet cable

Suppose the booster isn't needed or used for a moment:

  • MAC Address 1 is the receiver, the AP.
  • MAC Address 2 is the transmitter, the UE.
  • MAC Address 3 is the layer-3 first-hop, the Router.
  • MAC Address 4 is probably unused.

Now with the booster in the situation what happens? How are the addresses used differently after the frame reaches the booster (if at all)?

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You are a bit confused about the nature of addresses in 802.11 frames. Since your question isn't actually about the address fields, let me address that first and then go into the details on the address fields later in my answer if you are interested.

Most devices to which you are referring (whether they are call boosters, repeaters, extenders, etc) don't actually play a part in 802.11 as either a station or part of the distribution system (DS, also most commonly the access point or AP). They are simply devices that listen for and then repeat a signal.

For example, let's say Bob and Bill need to pass a message by radio. Unfortunately, they are too far apart (or maybe there is a mountain range in between) so they cannot reach each other directly. Ben happens to be in the middle (or on the top of the mountain) and since Bob and Bill can't directly talk to each other, Ben offers to repeat their messages so they can communicate.

So when Bob says "Hey, Bill, how are you?" on his radio, Ben hears it and then repeats it back into the air and Bill then gets the message. Any replies from Bill are handled the same way. Ben is not the source, the destination, or the intended receiver of the message. He doesn't really directly play a part in the conversation at all. This is the same process a "booster" will typically use on a 802.11 network.


The way the address fields are used in an 802.11 header is determined by the status of the "To DS" and "From DS" flags.

Address 1 will always be the receiver address (RA) and address 2 will always be the transmitter address (TA). This provides a consistent way for radios to easily determine if they need to receive the frame.

If neither of the "To DS" or "From DS" flags are set, this is a management or "station to station" frame. In these cases, address 1 is is the destination address (DA) in addition to the RA. Address 2 is the source address (SA) in addition to the TA. Address 3 would contain the BSSID (so radios not in the BSS can filter it) or be set to broadcast (all F's). Address 4 is unused.

If only the "To DS" flag is set, this is a frame bound from a station to a DS. In this case, address 1 is both the RA and the BSSID, address 2 will be both the TA and the SA, and address 3 will be the DA. This makes sense as while the station needs to send the frame to the DS, it is actually destined for some other host on the network and not for the DS.

If only the "From DS" flag is set, this would be a frame from the DS (or AP) to a station. Address 1 will be both the RA and the DA, address 2 will be both the TA and BSSID, and address 3 will be the SA. Again this makes sense as while the DS is transmitting the frame, it was sourced from another device on the network.

If both the "To DS" and "From DS" flags are set, this would be frames sent from one DS to another DS, such as you would find when using wireless access points to bridge traffic for other devices. Address 1 will only be the RA, address 2 will only be the TA, address 3 will only be the DA and address 4 will only be the SA.

  • Thank you. I only recently "learned" about these MAC addresses in WiFi; I obviously misunderstood a lot at that point. – BlackVegetable Mar 6 '15 at 16:56

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