• Hi,
    I have read the all the discussions regarding preamble and short slot time. Presently iam working on 11g.I have a few doubts regarding these.

    1) Suppose if you configure 802.11b/g AP to use Long only preambles.
    In the capability field is set to 0 , but the barker bit is not set as mentioned
    in the standard. (I have observed this in Cisco Fat AP).

    2) AP- 802.11b/g uses short preamble, A 802.11b client gets associated with the AP.
    Now entire BSS uses long preamble. But in capability field of beacon,
    short preamble is still set.
    Now if another short preamble capable 802.11b station wants to associate,
    how does it know that the BSS is using short preamble. (It cannot decode ERP - Barker preamble which is set)

    3) AP - 802.11b/g is configured to use long-only preamble.
    If the short preamble capable station wants to associate and sets short preamble capability, how does AP react.
    As mentioned earlier in your posts that it is implementation dependent,
    what would be better to do?

    Please look into my questions with paitience....

    With regards,

  • Murali,

    Can you clarify your scenario in question #2?

    Are you saying that:

    a B/G AP is using short preambles

    and then a B STA that is incapable of short preambles joins the BSS and forces the BSS into using long preambles

    after which another B STA capable of long preambles wishes to associate?

    Is your question "How does the last station know to use long preambles?"



  • Hi,
    Thanks for responding,

    Let me make my question clear.

    AP - Cisco device (11b/g) - short preamble enabled
    (Short preamble option is enabled in AP )

    A Station 11b incapable of short preambles (long only) joins 11b/g AP.
    Now the AP/BSS starts using long preambles and the barker bit.
    in the Beacon will also be set.

    But in beacon the capability of short premable is still set even when the
    network or BSS is using long preambles.

    Now if a 11b station which is capable of short preambles wants to join
    the network.How does it recognise that the network/BSS is using long preambles since it cannot decode the ERP element (which is 11g), and hence the barker bit.

    In Association request it will set the short preamble capability since it supports it.And in response also the short preamble will be set since AP also supports short preamble.

    So, How does a 11b station recognise that the network is using long preambles and begins to use it.

  • Murali,

    See if the following helps: (italics added by me)

    from 802.11b-1999 clause 18.2.6

    The receive procedures for receivers configured to receive the mandatory and optional CPs, rates, and modulations are described in this subclause. A receiver that supports this High Rate extension of the standard is capable of receiving 5.5 Mbit/s and 11 Mbit/s, in addition to 1 Mbit/s and 2 Mbit/s. If the PHY implements the Short Preamble option, it shall detect both short and Long Preamble formats and indicate which type of preamble was received in the RXVECTOR.

    from 802.11g-2003 clause 19.1.2

    The ERP has the capability to detect ERP and Clause 18 preambles whenever a clear channel assessment (CCA) is requested. Because protection mechanisms are not required in all cases, the ERP CCA mechanisms for all preamble types shall be active at all times.

    Since long preambles are being used would the STA detect them in the beacons and/or probe response?



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