• I am lacking a little clarity on the probe and beacon issue.

    1. Beacons are always transmitted using long preambles @ 1Mbps. Is this true for 802.11b/g?

    I understand that is STAs support short preambles, and the beacon identifies support as well, then the reply can implement short preambles.

    2. Probe requests are always sent at 1 Mbps, but the response from the AP will be sent at the highest basic rate of both devices. Any truth to that? This concerns the echo rule which confuses me a bit as well.

    Any feedback is greatly appreciated!

  • Hi RunDawg:

    Beacons must be transmitted at one of the basic rates for the BSS. In practice vendors choose the least demanding modulation supported by the BSS. For the 2.4 GHz band this is typically long preambles @ 1 Mbps.

    Probe requests must be transmitted at one of the supported rates for the STA doing the probing. In practice vendors choose the least demanding modulation supported by the STA in order to discover BSSs at the greatest distance. For the 2.4 GHz band this is typically long preambles @ 1 Mbps.

    Probe responses must be transmitted at one of the rates supported by both the transmitter and the receiver. Since the probe request includes the supported rates information element, the probe responder can choose a common rate even if different from the rate used by the probe request. I do not know what vendor practice is, but I would respond with the same rate as used by the probe requester since a higher rate may not have an adequate reach.

    Notice that probe requests and responses are exchanged not within a BSS but between a BSS and a STA that is outside the BSS. An associated station momentarily un-synchronizes itself from its BSS while it probes, usually across many channels besides the one used by its BSS, and promptly resynchronizes when it choses not to reassociate to a different BSS. Consequently these frame transmissions are not limited to the basic rates of a BSS.

    I hope this helps. Can you add your location to your forum profile? Thanks. /criss

  • Criss,

    What do you mean that a STA "un-synchronizes" itself from the BSS before performing probe requests on other channels? It just goes into PSM before switching channels so its AP will buffer any frames while its off channel. Is there something I'm missing?

  • Hi GT:

    When a WLAN station scans for beacons from other BSSs than the one it has previously joined, it momentarily suspends its ability to maintain synchronization with its BSS. In effect it has tuned out, is on its own, and is not limited by the synchronization parameters of any BSS such as basic rates or supported rates.

    IEEE 802.11: "5.2.2 STA membership in a BSS is dynamic. A STA?¡é?€??s membership in a BSS is dynamic (STAs turn on, turn off, come within range, and go out of range). To become a member of a BSS, a STA joins the BSS using the synchronization procedure described in To access all the services of an infrastructure BSS, a station shall become ?¡é?€??associated.?¡é?€??

    "11.1 Synchronization. All STAs within a single BSS shall be synchronized to a common clock using the mechanisms defined herein.

    "11.1.1 Basic approach. A timing synchronization function (TSF) keeps the timers for all STAs in the same BSS synchronized. All STAs shall maintain a local TSF timer.

    " Synchronizing with a BSS. Upon receipt of an MLME-Join.request, a STA shall adopt the BSSID, channel synchronization information, and TSF timer value of the parameters in the request.

    Note in Annex C: "Scan leaves station in Idle state because synchronization with a previous BSS is lost. Implementations may save and restore TSF and association info to automatically rejoin a previous BSS."

    I hope this helps. Thanks. /criss

  • Thanks for the reply Criss. So, how does the AP know that the STA is on a different channel? It seems like the AP could attempt to send a frame, but the STA could be off looking for another BSS on another channel. Thoughts?

  • Hi GT:

    AP will not know and could retransmit and/or drop data frames for lack of ACKs from the client.

    A vendor could creatively code his client stations to toggle power save modes just before and after scanning other channels.

    And what of FHSS? A scanning client may simply remain tuned to a single channel and wait for all FHSS BSSs to hop by.

    I hope this helps. Thanks. /criss

  • Criss,
    So by the standards a STA will not toggle PSM when searching for other AP's? That actually makes sense because you would see a whole bunch of Null Data frames if that were true. Right? Thanks!

  • Hi GT:

    IEEE 802.11 does not require stations to take any precautions against data loss while scanning.

    In relative terms active scanning with probe request frames doesn't take much time away from being well synchronized with one's BSS. (FHSS may be a different story but who cares.)

    My point is that what a client does while scanning is not constrained by modulation limits set by its BSS. In this sense scanning activities are outside the BSS of the client doing the scanning.

    For example a BSS may set the lowest basic rate at 11 Mbps. This should not prevent a client associated to that BSS from transmitting probe requests at 1 Mbps.

    Too little attention has been given to what it means for a station to "join" and be synchronized to a BSS. We have been satisfied with discussing authentication and association frame exchanges because we can see their frames in a protocol analyzer and vendors display these words in client utilities. Joining is spoken much of in the standard but is invisible in the field.

    I hope this helps. Thanks. /criss

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