• All,

    802.11g defines the protection mechanism CTS-to-self for use by 802.11g stations to notify 802.11b stations when they are going to transmit.

    802.11g also specifies an optional PPDU type consisting of a DSSS preamble and header, but acceptiong an ERP-OFDM PPDU as its PSDU. This results in not requiring protection mechanisms.

    So, is DSSS-OFDM implemented in APs and/or cards by any vendors? If so, what are the pros and cons of using protection vs. DSSS-OFDM?

  • I will respond to myself on this one. I Googled DSSS-OFDM and not a single hit on the first page was marketing material (testing tools excepted). So I will assume that DSSS-OFDM is not widely implemented (if at all).

    I also got a hit back to this site and this forum dated Mar 04, 2005..the author was kf8rd and Criss_Hyde responded. See for the topic.

    A residual thanks to Criss :)

  • Hi moe,

    You are correct in saying that DSSS-OFDM is not implemented. It's an optional PHY, and I personally think it's a better implementation of "backward compatibility" than using protection mechanisms. Apparently equipment vendors disagree with me on that. :)


  • By (Deleted User)

    Devinator would this help in the site survey process and help avoid the 802.11g only APs from rate shifting or the Ripple effect when they come in contact with an 802.11b AP?

    Wouldn't it (DSSS-OFDM) be better for wLANs that currently support 802.11b Wifi phones? Is this a way they market or sell more access points? :)

  • Using DSSS-OFDM wouldn't affect a site survey. A site survey is primarily a Layer-1 function (RF coverage, capacity, and interference).

    802.11g APs don't rate shift in the presence of 802.11b, but rather they enable protection mechanisms which cause a throughput decrease due to the extra frame overhead.

    I do agree that DSSS-OFDM would be better for a VoWLAN environment.


  • By (Deleted User)

    Thanks for the clarification. I am mixing up DSSS-OFDM and ERP-OFDM. I see the light. This is a great forum.

    Are vendors really using protection mechanisms on their APs? I have read what the IEEE asks them to do but is this really taking place?

    The reason I ask is this, would during a site survey, me utilizing a 802.11b access point as the surveying AP, because I am planning a Voice over WLAN network in addition to the data network, affect the 802.11g access points already on the air. Would just software selecting 802.11g only for those data specific APs be sufficient and help avoid the ripple? Should we utilize separate APs to avoid the amount of negotiations going on with the voice traffic?

    I understand the QoS and vendor specific protocol priority schemes with Voice over Wireless as work arounds for this decrease in throughput, but I want to make sure APs for data are not going to be challenged by 802.11b handsets.

    The QoS isssues are huge and as I understand for VoWLAN you need a lot of APs to handle the roaming (coverage) issues and data networking traffic.

    Thanks much

  • Yes, every vendor I have seen (which is most) implements RTS/CTS and CTS-to-Self according to the standard.

    Yes, if you were using an 802.11b AP for a survey, the nearby 802.11g APs would be affected dramatically. Please refer to my Ripple whitepaper on the Learning Center for further clarification. Almost nobody configures APs for 802.11g-only mode because that can cause even worse problems than protection mechanisms and ripple. It would cause major RF interference, corrupted frames, and retransmissions.

    My suggestion is to keep mission-critical applications such as VoWLAN (and similar applications) on 802.11a and keep data on 802.11b/g.

    Even 802.11e draft compliant and proprietary QoS mechanisms can't come close to making up for a 50%+ throughput loss due to protection mechanisms. QoS can give you traffic prioritization, but doesn't address overall capacity issues.


  • By (Deleted User)


    Thanks again abouting reading the Ripple Whitepaper
    I have several times and it explains the cause and effect very well!


  • The only exception I would see (I may be wrong) is when using an AP that can disable 11b data rates, which is different than some vendors "g only" mode. "g only" mode still "hears" 11b STA's, but won't communicate with them, but they will still put the AP into protection mode. For example, in a Cisco 1200 series AP you can disable 1,2,5.5,and 11Mbps data rates. This is the only way I have seen to make sure an AP won't go into protection mode.
    This is not to be in contrast to Devin's suggestions for the VoWLAN and Data separation. Just a thoughts as to how to prevent protection, although may not work for your environment.

    GT Hill

  • I would be looking at 802.11a as a very viable alternative for VoIP phones. 802.11a has more non-overlapping channels, no protection mechanisms to worry about, and they should be available in the very near future.

    GTHill is correct about 802.11g-only is different than disabling 802.11b rates. When you disable 1, 2, 5.5, & 11 Mbps on a Cisco AP, it disables protection altogether.


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