• What is the difference between the protection mechanism described on page 387 (CTS-to-Self) and the one described in the sub-section "802.11g Operating Modes" under section "OFDM in 802.11g" on page 201-202?
    Are they the same protection mechanism? They don't seem to be. It seems to me that the second one sends one frame with headers DSSS-OFDM modulated and the payload OFDM modulated. Is this possible in the same frame or is it achieved in 2 different frames?

  • blueice Escribi?3:

    Are they the same protection mechanism?
    No they are not the same protection mechanism.

    From IEEE 802.11g-2003 9.10 Protection Mechanism:

    "ERP STAs shall use protection mechanisms (such as RTS/CTS or CTS-to-self) for ERP-OFDM MPDUs of type Data or an MMPDU when the Use_Protection field of the
    ERP Information element is set to 1."

    From IEEE 802.11g-2003 19.7 DSSS-OFDM operation specifications:

    "This optional mode provides systems the ability to use OFDM in a mode that is fully compatible with Clause 15 and Clause 18 BSSs without requiring additional coordination. That is, it does not need a protection mechanism." Italics added.

    blueice Escribi?3:

    Is this possible in the same frame or is it achieved in 2 different frames?

    It is possible in the same frame. See 802.11g-2003 19.7.2 Single carrier to multicarrier transition requirements.



  • 802.11g standard:

    9.2.11 NAV distribution
    When a node needs to distribute NAV information, for instance, to reserve the medium for a transmission of
    a non-basic rate frame (that may not be heard by other nodes in the BSS), the node may first transmit a CTS frame with the RA field equal to its own MAC address (CTS-to-self) and with a duration value that protects the pending transmission, plus possibly an ACK frame.

    The CTS-to-self NAV distribution mechanism is lower in network overhead cost than is the RTS/CTS NAV distribution mechanism, but CTS-to-self is less robust against hidden nodes and collisions than RTS/CTS. STAs employing a NAV distribution mechanism should choose a mechanism such as CTS-to-self or RTS/
    CTS that is appropriate for the given network conditions. If errors occur when employing the CTS-to-self mechanism, STAs should switch to a more robust mechanism.

  • I thought it was impossible to transmit one frame with the headers and data modulated using different techniques.

    Thanks moe and Devinator.

  • blueice,

    I'm not sure if your statement was a question, and if it was, can you please restate it more clearly? I'm not really sure what you're saying. thanks!


  • OK. I'm probably still confused about this.
    My re-phrased question is: when the protection mechanism is used in a mixed 802.11b and 802.11g environment, do all frames transmitted use 2 different modulation techniques for the header and for the payload, i.e., part of each frame is modulated using DSSS-OFDM and the rest OFDM?

    Thanks for helping in clearing the confusion.

  • When DSSS-OFDM modulation is used, no protection mechanisms are used. In this case, the headers are modulated one way (DSSS) and the frame bodies are modulated another way (OFDM).

    When protection mechanisms such as RTS/CTS and CTS-to-Self are in use, they are used because DSSS transmitters cannot recognize OFDM modulated frames. OFDM transmitters send RTS and CTS frames using DSSS modulation so that the receivers can demodulate the frames and read the duration value in the header. This tells them to set their NAV to a given value so that they will not speak during the transmission of an OFDM-modulated frame.


  • Excellent, that's much better now.
    But is there a performance degradation in terms of data rate, when DSSS-OFDM is used?

  • Headers can only be sent at 802.11b data rates. Additionally, PHY headers are always sent at 1 or 2 Mbps depending on their preamble length. Only the payloads are transmitted at 6 - 54 Mbps. So, yes, there's a performance hit when using DSSS-OFDM. However, when you compare this to the extreme overhead of using protection mechanisms, it's about a wash in my humble opinion.

  • I agree. Thanks Devinator.

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