• Hi guys ,

    Iam a bit confused about

    Source Address,Destination Address ,Receiver Address
    and Transmiter Address.

    Can some one educate me on the same.

    Best regards,


  • Pages 147-148 in the CWAP study guide.



  • Hi SS of Chennai:

    Source Address identifies the original transmitter of the frame payload.
    Destination Address identifies the ultimate receiver of the frame payload.
    Transmitter Address identifies the immediate transmitter of the frame.
    Receiver Address identifies the immediate receiver of the frame.
    BSSID identifies a set of stations sharing a coordination function.

    I hope this helps. Thanks. /criss

  • Devin /criss

    Thanks a lot for the reply.

    Please validate the following scenario.

    Let me assume that there are 2 wireless APs onnected in the LAN and 2 wireless clients in the LAN.

    a) One AP is wired to the Swicth.
    b) Second AP is completely wireless [Repeater/WDS/wireless backhaul].

    a) WLAN Client No 1 is asociated to AP one
    b) WLAN client no 2 is associated to [WDS] AP two

    In this scenario if Wlan client no 1 trasmits a frame to Wlan client no 2.

    WLAN client no 1 --- Source Address
    WLAN Client no 2 ----Destination Address
    Transmitter Address --- AP one
    reciver Address ------AP Two

    Best regards,
    S.Senthilraj CWNA ,CWSP

  • Hi SS:

    In your scenario there are three transmissions of three frames. Each frame carries the same payload and has the same SA and DA addresses. But each frame has its own set of RA and TA addresses.

    I hope this helps. Thanks. /criss

  • S.Senthilraj,
    The question is a bit more complicated. In the situation you described, keep in mind that the originating sender doesn't know that there is a wireless repeater down the line. So, it wouldn't know its address. This is how that situation would work.

    I am putting these in the same order that they are in a frame. The will be listed in order of Address1 to Address4 (when applicable)

    STA1 to AP1
    Rec - AP1
    Trans - STA1
    Dest - STA2

    AP1 to AP2
    Rec - AP2
    Trans - AP1
    Dest - STA2
    Source - STA1

    AP2 to STA2
    Rec - STA2
    Trans - AP2
    Source - STA1

    To expand on this a bit (kind of a pun), how does a receiver know what the third address is supposed to represent? Notice that the third address can be either the source or destination. This is where the ToDS FromDS bits come in. ToDS means going to the Distribution System, which means going to the AP. FromDS means coming from the AP. This is how the receiver knows how to interpret the addresses.

    The first address is always the receiver and the second address is always the transmitter. So, if the ToDS bit is set to 1, and the FromDS bit is set to 0, what would the third address represent?

    Look forward to hearing from you.


    Thanks for educating me.

  • STA1 ----AP1 AP2----STA2


    Scenario 1:
    AP1 is wired to the switch ,AP2 and AP3 wireless backhaul to AP1 .

    Sta 1 wants to transmit a frame to STA2 .The frame
    gets forwared to AP1 how will the AP1 know whether
    STA2 is associated to AP2 and AP1.

    What if Sta 2 roams between AP2 and AP3 often.

    Scenario 2:

    AP1 ,AP2 ,AP3 are all wired to the swicth.
    Sta1 associated to AP1 wants to tranmit a frame to
    Sta2 assoiciated to AP2.

    How the frame gets forwared to STA2.[Assuming Roaming is in place]

    I will be more than happy if some one educates me on this .

    I'm also planning to take CWAP soon........

    Best regards,
    S.Senthilraj CWNA,CWSP

  • Common some one have to reply my question.


  • Ok, scenario number one. I had to draw this one out on the board to see exactly what was going on. What you have described is one AP in "root" mode (AP1) and AP2 and AP3 in repeater mode.

    When a frame enters AP1 from STA1, it will have 3 addresses. Then AP1 will have determined on which AP the destination STA is located. This is exactly like a wired switch does. When a frame comes into a switch, it must either know or find out which physical port the destination STA is located. Instead of AP1 sending it to a specific physical port (which it can?¡é?€??t do since it is wireless) it will direct the frame to the destination AP with the ?¡é?€??receiver?¡é?€?? address. When AP1 sends the frame on to the next AP, it will have four addresses. See my previous post for exact addressing. The receiver address will be the next AP in line. If the destination STA roams between the repeaters, (AP2 and AP3) then AP1 will have to continually update which AP is handling that STA. Now, let me say this. This is highly irregular, although possible, not used in any enterprise scenario I have ever seen.

    Scenario 2 is just like any normal infrastructure wireless setup. The only slight anomaly is that you have client to client communication which is not all that prevalent. When a STA associated to AP1 transmits a frame to STA2, AP1 is sending the frame out its Ethernet port, so end of wireless discussion. AP1 has no idea that it is destined for an AP. When AP2 receives it, it doesn't know that it came from an AP. On the Ethernet side, it will only have Source and Destination, as Ethernet only has two MAC addresses in its header.

    This may sound slightly confusing, due in part to the complexity of the question and the fact that I explain things much better while speaking in front of a whiteboard. Ok, end of excuses. Let me know if you need any more help, but if you understand this entire thread, you will do fine on the addressing portion of the test. Good luck to you.

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