RTS/CTS question

20 posts by 5 authors in: Forums > CWNA - Enterprise Wi-Fi Admin
Last Post: June 9, 2005:
  • Hi Criss

    So, if I understand it correctly, a node -in this case the AP- does not have to have RTS/CTS enabled in order to return a CTS in response to an RTS?


  • exactly

  • Hi to Criss,Phil and Denis.
    May I ask a question?
    Then what is the RTS/CTS in AP for?
    Thanks a lot!

  • Hi Raul:

    Given one BSS with no other BSS within range, the AP would never benefit from RTS/CTS. Every client station is within range of frames transmitted by its AP.

    Given two BSS's overlapping in channel and area, each AP may benefit from RTS/CTS since some client stations of each BSS may be out of range of the other BSS's AP.

    I hope this helps. Thanks. /criss

  • Yes it helps. Thanks Criss!

  • Criss,

    I can understand that in one AP BSS it is no use for the AP to use RTS/CTS to relay the packet to the intended recipient but i still can't figure out the advantage in multiple BSS AP's configuration.

    Can you explain a bit more on how each AP is advantaged by using RTS/CTS?

  • Hi CGO:

    The classic RTS/CTS explanation involves a lineup of STA1, AP1, STA2. STA1 and STA2 are within range of AP1 but out of range of each other. STA1 begins transmitting a data frame but before finishing, STA2 begins transmitting. AP1 receives neither frame uncorrupted. Since AP1 is by definition within range of all its client stations, AP1 would incur an overhead cost but receive no benefit from using RTS/CTS to protect its transmissions.

    But the IEEE standard is more general than this. Imagine a lineup of AP1, STA1, STA2. AP1 and STA1 are members of one BSS. STA2 is a member of another BSS using the same or overlapping radio channel. AP1 and STA2 are within range of STA1 but out of range of each other. AP1 begins transmitting a data frame but before finishing, STA2 begins transmitting. STA1 receives neither frame uncorrupted. In this case AP1 might benefit from using RTS/CTS to protect its transmissions.

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

  • thanks criss. First of all i must say that setting up a different BSS using the same channel adjacent to each other is a poor design in the first place.

    However, i am still a bit puzzled by some of the detail. In this case, for example, STA 2 starts transmitting to an AP in BSS2 while STA1 is receiving a data from AP1. And you are suggesting that since the STA's are in range within each other then STA2's data will corrupt STA1's data. but doesnt STA2 do CCA and hear that STA1 is receiving? does CCA depends on BSS? My understanding is that regardless of the BSS when you do CCA then you scan the medium around you and sense if it is busy. Doesnt it mean that STA2 (though in a different BSS) can hear STA 1 receiving?

  • Hi CGO of Sydney:

    The key sentence in my example is: "AP1 and STA2 are within range of STA1 but out of range of each other." Thus STA2 can not "hear" that STA1 is "receiving". AP1's signal makes it to STA1 but attenuates into the noise by the time it reaches STA2.

    Regarding the poor design of two BSS's using the same (or overlapping) channels in the same space consider this. The standard does not assume that overlapping BSS's are administered by the same people; the standard protects you from your neighbor. Also, an IEEE standard Wireless Distribution System (WDS) requires that two or more BSS's use the same channel in the same space. WDS compromises channel efficiency to avoid wires between AP's.

    I hope this helps. Thanks. /criss

  • if that is the case then i can understand the advantage. I see where i am having problem understanding. It is to do with how the STA actually listens to the medium. Anyway it is cleared up now.

    Thanks a lot.

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