• Criss,

    Good idea to start a new thread. :) First, to answer the question about the AP/Client Channel. You are correct in your assumption. It doesn't matter to me, I am just talking about 2 or more 802.11 devices on the same channel within range of each other. From an interference and performance perspective, it doesn't matter if they are in the same BSS or not. Probably more to come about that.

    Note that some of the data included is not for your (Criss's) benefit as I know your expertise. I just want to make sure that we provide enough information for all to make their own determinations.

    Ok, which characteristics of an 802.11 signal causes a receiver to treat a transmission as a frame or as interference? I want to note that there is a third state that is worth mentioning. A device that understands a signal as being 802.11, but having a bad FCS, thereby not setting the its NAV and returning to an idle receive mode and waiting an EIFS.

    Keep in mind that an 802.11 device, whether STA or AP has to arbitrate in the same way to get network access. One does not have priority over the other.

    1. Channel of the transmitter - Yes - If two or more devices are on the same channel and in range, it will either cause interference or arbritrate with each other. Whether it is interference or understandable communication depends on many factors, many of which are listed below.

    2. Signal strength at the receiver. - Yes - If the signal strength is too low, then the receiver will either interpret it as a bad 802.11 frame (then go into an EIFS), or as RF noise. If it receives an intelligible 802.11 frame (no matter what BSS it was from) it will set its NAV appropriately and abritrate with that device.

    3. CRC check at the receiver. See note earlier. If the FCS is good, then it will set it's NAV and arbitrate accordingly. If FCS is bad, wait EIFS and begin again.

    4. Distance the signal has traveled - Yes, but directly related to number 2.

    5. Power Setting of the transmitter - Yes, but directly related to number 2.

    6. Value of the Channel field of the physical header. - Not sure on this one - It is possible and common for a device to be able to receive a good frame from a device on an overlapping channel. For example, a device on Ch 1 may receive a good frame from a device on Ch 2. What I don't know is will it ignore the frame because it is a different channel? My gut says "no", it won't ignore it.

    7. Value of Address 3 in the MAC header - No - Devices will arbitrate no matter what MAC addresses are in the frame.

    8. Values of the FromDS/ToDS fields. - No - Devices will arbitrate regardless of where it came from or where it is going. If it can read the frame, it will arbitrate with that device.

    9. Value of the Address 1 field. - No - See #7.

    To restate my opinion. It is never correct (in the US) to use anything besides channels 1, 6, and 11. Here is my reason, by siting an example. If you have a situation where there are devices within RF range using all three channels, some will say that you should put your devices on channel 3 (for example) to avoid interference from the other devices.

    Channels 1 and 3 and 3 and 6 severely overlap each other. This isn't like a CB radio where you can change from channel 1 to channel 3 and not hear the conversations on channel 1. This is spread spectrum, which means it uses more than just the center frequency to transmit, as opposed to narrowband for a CB radio. For those of you that have the CWNA v3 guide, see page 189 and 190. With 802.11, when you have a device on channel 3, you are receiving RF signals from all devices on channel 1 up to about channel 9. The main problem with this is you are receiving RF signals from devices on all of these other channels, but you can't understand what they are saying. From the channel 3 device's perspective, there is just a lot of noise on the network, which will in turn cause a lot of collisions among other things. Now, lets put our device on Ch 1. Sure, we now have more devices on Ch 1, but now they can understand each other. In 802.11, when a device receives a complete frame from another device, it goes through some processes to help prevent collisions with that other device. Again, note that this has nothing to do with which BSS or SSID this device is associated with. Now, instead of causing and receiving bad packets by being on Ch 3, it will arbitrate with other devices on Ch 1, thereby improving throughput and transmission quality over the Ch 3 alternative. Here are some lab results of testing this exact setup. I have performed this lab in class many times. Here are the results from memory, but you guys get the idea.

    There are two AP's, each with a different SSID. Each AP has one STA associated. We start with one AP on Ch 1, and one on Ch 11. These are figures in 802.11g without protection enabled. The main thing to note is not the speeds themselves, but the differences in speeds depending on channel.

    Ch 1 Ch 11
    15 Mbps 14Mbps

    Ch 1 Ch 6
    13 Mbps 13Mbps

    Ch 1 Ch 1
    9 Mbps 9Mbps

    Ch 1 Ch 3
    6 Mbps 5Mbps

    Again, in a testing environment you will get different results, but no matter where I have done the test one result always stays the same. The scenario with Ch 1 and Ch 3 is always the lowest cummulative throughput. Also to note, Ch 1 and Ch 6 are slightly overlapping, which is why in a clean RF enviornment you will see a lower speed than with Ch 1 and Ch 11.

    So, I think that explains my thoughts. Sorry for the long post, but I like to try and be throurough.

    Having all of the AP's on the same channel is a bad idea. So, your answer should have been to put the top floor and Ch 1, next floor down Ch 6, next Ch 11 and the last floor on Ch 1. With the great distance between the two Ch 1 AP's, they probably didn't even interfere (also meaning arbitrate) with each other, which is good.

    The purpose of our discussion is to say why you shouldn't use an other channel besides 1, 6, and 11 when devices are within RF range of each other. If you have your AP's on any other channels, you will cause yourself and any other 802.11 devices around you problems.

    Sorry to cause you confusion. If this isn't clear now, let us know and we'll try and clear it up!

    Take care to both of you and I look forward to continuing the discussion!

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