• Howdy Group,

    Another friendly question which is I could use some insight...

    I feel comfortable with the "general" knowledge about the rates of frames for preambles (long/short) and the headers, etc.

    What is getting me is specifics like the frame rate for probe requests, probe responses. In reading the study guide, I just want to presume the answer is that they (probe requests/responses) are always sent at the same speed. In looking at the diagrams in the book, it seems like that is the correct answer...but.....

    I guess my question is:

    when are frames transmitted at the lowest supported rate versus the lowest basic rate, because I think they are two separate rates (or could be)

  • A station must be able to modulate and demodulate for each of the mandatory rates for its PHY. A station may be able to modulate and/or demodulate at optional rates as well. The set of rates a station can actually modulate and demodulate is said to be supported by that station but is not given any special name.

    Every station may limit itself to transmit with only a subset of the rates it actually supports. This is the operational rate set of the station. The operational rate set when published in certain management frames is called the supported rate set of that station. Please notice that "support" has two meanings here, one that each station keeps to itself and the other that each station publishes to stations with which it communicates.

    The supported rate set is a promise by a station to other stations that transmissions sent by those other stations at any of the modulations in the set will be demodulated if within range, and a request that no transmissions be sent at any other modulation even if the station could actually demodulate it. Each other station records for each of its communicating partners that partner’s published supported rate set.

    Each access point advertises its supported rate set, rates it guarantees it can demodulate if within range, and also a basic rate set for the BSS, a subset of its supported rate set that must be supported by each client that associates with that BSS. In effect each of the BSS's basic rates must appear in the supported rate set published by each station in the BSS. A client that does not support any one of the basic rates of the BSS shall not be associated. The access point's other supported rates, if any, are NonBasic rates for the BSS.

    Frame exchanges other than communications within a BSS use the lower or lowest of the available transmission rates since they are more likely to be both compatible and of sufficient range for the intended receiving station or stations. Alternatively the transmitter may use the rate observed to have been used recently by the receiver.

    Although every station theoretically could have its supported rates list configured by an administrator, typically only the supported rates list of the access point is configured. Including a rate means "please feel free to talk to me with this rate". Omitting a rate means "please don't talk to me with this rate". Frames received at other rates actually supported by the station may still be demodulated and acted upon. An example of this is when an ERP access point receives HR/DSSS probe request frames at 1 Mbps, even though that rate is disabled at the access point, and the access point responds with probe response frames but at a different rate.

    The basic rates list for the BSS is only configured at the access point. Including a rate means "you must be able to demodulate this rate in order to associate with this BSS". Omitting a rate means "the ability to demodulate this rate is not a requirement to associate with this BSS".

    The interplay of these rate lists is seen in the web configuration page of a Cisco access point as an array of buttons. Each of the access point's actually supported modulations is represented as a row of three radio buttons labeled "require", "enable", and "disable". These terms are vendor specific. According to Cisco a “required” modulation is in both the basic rates list and the supported rates list. A merely “enabled” modulation is only in the supported rates list. A “disabled” modulation is in neither list. It is important to note that "disabling" a rate prevents the access point from transmitting at that rate but does not prevent it from receiving and demodulating at that rate.

    I hope this helps. Thanks. /criss

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