• Has anyone found in a possible mutli-path environment, that fixing the transmit rate to a slower (1,2M) speed can help with this problem?


  • Data rate change is not a solution to the multi path problem. To mitigate multi path you can use antenna diversity. To remove the problem you need to move the antennae so as to avoid the reflective surface which is causing the problem or remove the obstruction that is causing the reflection. Changing the data rate would just slow down your network. The Fresnel zone would still be the same size unless you changed frequency or the distance between the antennae.

  • Thanks Bryan.

    What has better odds of making it to the destination-packets going out at 2M or packets going out at 11M?

    Is'nt that why management packets are sent out at slower speeds?


  • Since both the lower data rate and higher data rate frames are using the same frequency they have the same wave size. Neither speed has a better chance of making it through the air waves than the other. The reason management frames are sent at the lowest supported data rate is to allow STAs at great distance or older clients supporting only the lower rate to hear and use the frames. They are not transmitted at lower rates to avoid propagation issues. Remember, data rates and modulation method go hand in hand but have no effect upon propagation. Frequency, distance and environmental issues effect propagation.

  • Understood, Why then if the RSSI is below certain thresholds on AP's
    do they scale there data rate down?

    To me that means the propagation is bad, so it tries to get through at a slower rate?

    Thank Bryan

  • Higher data rates require greater power. The STAs are causing that by using Dynamic Rate Selection. They will lower their data rate to stay connected. The AP is configured to allow that rate and broadcasts that in the beacon. The STA will detect a lower RSSI value and change data rate to avoid connection loss. The AP allows data rates and the STAs select the best rate they can use with the given signal strength. If the AP were set to allow only the higher rates then the STAs would drop connection as the moved away because they would not have a strong enough signal to support the better modulation and coding techniques.

  • Hi Heck:

    As the data rates go up, the complexity of the RF symbols used by the corresponding modulation technique increases in order to carry the additional bits/second. As the complexity goes up, the necessary difference between the received signal strength and the local noise floor goes up.

    Assuming the local noise floor is constant, as the distance to the receiver increases the received signal strength decreases. When the difference is insufficient such that the receiver can't convert the fuzzy itsy bitsy RF symbols to bits, the transmitter and receiver agree to shift to a lower data rate. The lower data rate has a lower complexity of RF symbols such that the required difference between noise floor and received signal strength is lowered and communication continues.

    I hope this helps. Thanks. /criss

  • Criss,

    Excellent answer. As I was already composing a similar answer in my head, I'll go ahead and post too. I don't think I'll say anything that you didn't, but maybe hearing it a different way will help some readers to make sense of it.

    Different 802.11 data rates are created by varying the complexity of the signal. More complex signals can carry more 1's and 0's in the same amount of time, but they are also more susceptible to corruption, and vice versa for less complex signals. Specifically, more complex signals require a greater signal-to-noise ratio in order to receive them correctly. Assuming a constant amount of ambient noise, that statement is equivalent to saying that higher data rates require more signal strength.

    Multipath cancellation causes a decrease in signal strength in a specific location. Depending on the amount of the decrease in signal strength, lowering the data rate that the station uses might be sufficient to address the problem, since it will allow the station to receive the packets even though they are weaker.

    The real question that I would pose is, "What makes you think that multipath is the issue, and not something else, like interference or a shadow from an attenuating obstruction?" In my experience, it's very easy to say, "Signal strength is low here," but very hard to say, "And that's because of multipath." This matters because if you are specifically tackling multipath as the issue, you might not be addressing the real problem, which could be something else.

    Whatever the issue, decreasing the distance and obstructions between the client and the AP by relocating the AP or adding another AP will probably solve it. That's not always the preferable solution, but as a last-ditch effort, it usually works, at least up to the point where you have so many APs that you run out of channels and get interference.

  • Thanks guys very informative.

    I guess I do not know for sure if it is multi-path or not. All is I know is I am not going very far there is metal involved and
    And I loose signal.

    The app is in a small cement cannel that a rail car goes back an forth on. The max distance is about 300?¡é?€?? and there is line of site
    With at least one of the two antennas at all times. It work fine for the first 100?¡é?€?? feet and then I start to loose data. I have plenty of signal
    And antenna for this distance.

    Why I was thinking muti-path was because the omni antenna placement has to be near (3?¡é?€??) metal pipes.

    Thanks Heck.

  • Hi Heck:

    Along your 300' line of sight between client station and access point, are there obstructions near the line of sight (above, below, right, left)? If so how close are these obstructions to the line of sight and how far along the line of sight are they?

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

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