• Hello everybody,

    Here is a question I got on the Boson practice tests:

    SIFS are used for short durations and are used when transmitting
    A. ACK
    B. RTS
    C. PIFS
    D. CTS

    Select the 3 best answers.

    From what I read in the official guide (version 3.0), the answers should be A and D only, but Boson says that B is correct as well.

    Can anyone shed some light please?

  • From 802.11 doc:

    " Short IFS (SIFS)
    The SIFS shall be used for an ACK frame, a CTS frame, the second or subsequent MPDU of a fragment
    burst, and by a STA responding to any polling by the PCF. It may also be used by a PC for any types of
    frames during the CFP (see 9.3)."

    It makes sense that a station should use DIFS before RTS since RTS is announcing impending use of the medium.


  • All:

    I concur with Moe of Birmingham who went to the source to validate the answer.

    All of us should have the IEEE 802.11 PDF's and Adobe Reader for ready searching.

    Thanks. /criss

  • OK I'll get a copy of the 802.11 standard.
    However, I didn't quite get what you meant in your reply. Are you referring to the phrase "It may also be used by a PC for any types of
    frames during the CFP"?

    Note that the official guide also says that "DIFS is used to transmit most data and management frames, including the RTS frame that begins an RTS/CTS/Data/ACK excahnge and the first fragment of a fragment burst".

    So, I don't understand how SIFS is used when transmitting an RTS frame.

  • Hi BlueIce:

    Sorry for allowing a confusion.

    Moe's answer is that SIFS is >not< used ahead of an RTS frame. RTS is not mentioned in IEEE 802.11 as one of the frames preceded by a SIFS.

    The sentence you asked about has nothing to do with RTS. The acronyms used are:

    PCF = Point Coordination Function
    PC = Point Coordinator (an access point)
    CFP = Contention Free Period

    Thanks. /criss

  • What I am saying is that SIFS is not used prior to RTS- DIFS is used prior to RTS.

    Where I quoted the 802.11 doc:

    "The SIFS...It may also be used by a PC for any types of frames during the CFP."

    I was doing so for completeness. I did not intend for you to consider CFP relative to your question about RTS and SIFS.

    I'm sorry for the confusion.


  • I apologise if I did not explain myself clearly.
    I'm still studying...

    What I understand from your reply, is that SIFS is NOT used prior to an RTS frame (which I agree).

    But then, Boson's answer is not correct.

    What do you think?

  • I agree with you.. I believe Bosun is wrong.

  • Boson is wrong.

    People always get confused with SIFS and DIFS. For people that understand arbitration, there is a very simple way to determine which will be used.

    SIFS: Arbitration has been decided.

    DIFS: Arbitration has not been decided.

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