• By (Deleted User)

    hi all, am afirst time poster inhere as u can see..
    well, i cant really figure out how would the hidden
    node prob. occur in a scenario as when am
    covering a whole large area with many access points
    ,so its really normal to find a node for ex. in the
    far east side which its coverage area cant reach the
    far west side, and thats really normal unless
    i wouldnt have the need for many APs from the
    i know am missing something in here,is that a problem
    which deals with just one AP coverage area? or?

    thanks in advance.

  • I don't completely understand your situation, but hidden node is pretty straightforward. An access point of course serves multiple client stations. If two of the stations are close enough to the AP to communicate with it, but not close enough to each other to "hear" each other, then that is hidden node. An example is if the AP is in the middle and each station is on opposite ends of a room, building etc where they can't hear each other.
    802.11 communication relies heavily on the ability for each station to be able to hear each other (Carrier Sense, Clear Channel Assessment) so they know when they can talk (transmit).
    Analogy: Imagine you are standing in the hallway with a door open to your left and one open to your right. You represent the AP. You have two people talking to you (stations), one in the back of each room. You can hear both of them just fine, but due to obstructions and distance, they can't hear each other. The problem is that very easily they could both start talking to you at the same time, thereby confusing you and not understanding either conversation. This is hidden node.
    Hopefully this helps, and thanks for bearing with my endless analogies. I love'em, but I tend to go overboard. :)

    GT Hill

  • That's actually a good analogy for those who have trouble understanding it.

  • By (Deleted User)

    thanks so much for ur great analogy and making it easier to understand, but what am asking for is how this problem would occur in multiple APs environment, so node A associated with AP "X" located in the far right side would hear node B associated with another AP "Y" located in the far left?
    or the hidden node prob. is "per access point" ?

    and what if the 2 APs is just connected together with ethernet and not in wifi coverage range of each other?

    thanks again..

  • Hi Still:

    IEEE 802.3 Ethernet assumes all nodes in a LAN can hear each other, and only each other. IEEE 802.11 WLAN assumes all nodes in a WLAN may not hear each other, and worse may hear nodes in other WLANs sharing the same space and channel.

    The classic example of the 802.11 model is two clients in one infrastructure Basic Service Set (BSS) too far from each other to hear the other. As one transmits to the access point (AP) the other client may transmit as well. In this case both frames are corrupted, neither is acknowledged by the AP, and both frames are retransmitted.

    With this in mind the IEEE included the optional RTS/CTS feature. Later the IEEE Handbook coined the phrase "the hidden node problem" to help explain the situation. While only one node was ever referred to as hidden, in fact each of the two (or more) nodes is hidden from the other(s).

    As you suspect RTS/CTS helps in more situations than two distant clients sharing one access point. They could be two clients in range of each other (same space and channel) belonging to two BSSs whose APs are out of range from each other.

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

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