Importance of RTS Threshold on IEEE 802.11 MAC Protocol
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Thread: Importance of RTS Threshold on IEEE 802.11 MAC Protocol

  1. #1
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    May 2006
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    Importance of RTS Threshold on IEEE 802.11 MAC Protocol

    Importance of RTS Threshold on IEEE 802.11 MAC Protocol


    The RT is a switch for the RTS/CTS mechanism. In 802.11 we use RTS/CTS clearing procedure to help with large frames. Actually this RTS/CTS procedure is developed to combat the hidden node problem. The standard specifies that the RTS threshold should be set to 2347 bytes. If the frame size is greater than this threshold value then we should go for the 4 way hand shake instead of the 2 way handshake procedure.

    Actually we are more concerned with the bandwidth. The RTS/CTS mechanism consumes extra bandwidth which has a negative effect on the performance of the network, packets with collision probability slightly
    Greater than the probability of a RTS packet should also be sent directly.
    If the frame size is lesser than the RTS Threshold value then we can avoid RTS/CTS method and make the medium less congested.

    Another important point to use RTS/CTS mechanism is that if the collision happening in the medium is more then we should enable the RTS/CTS mechanism.

    So we can say that RTS Threshold is a switch for RTS/CTS mechanism and should be used wisely. If network throughput is slow or there are large no of retransmissions we should enable RTS/CTS by decreasing the RTS Threshold.


    Hidden node problem

    In wired networks stations can easily find whether some other station is transmitting using the CSMA/CD method. But in wireless world networks have fuzzier boundaries and even in one BSS typical problems like Hidden node problem can come.

    Hidden node problem can explain as follows. Consider 3 stations in one BSS. Imagine that station 1 can reach station 2 and station 3 can reach station 2. But due to some problem station 1 and station 3 are unaware of each other, the cause might be distance. So if both station 1 and station 3 start transmitting data at the same time then station 2 cant distinguish the data. And since the problem is local to the station 2 station 1 and station 3 wont know about this problem.

    In order to avoid this problem We are introducing the RTS/CTS mechanism. The RTS is the Request to send frame and CTS is Clear to send frame. RTS is a request to send the frame and reserves the medium for the transmission and CTS will give the clear to send flag as well as silences the stations near to it. So even it RTS frame is not reachable more than the receiver , the receivers CTS frame silences the neighboring stations and thus avoids problems.

    Vimal raj


    Vimalraj is a software engineer in WITEL, wireless division of the cranes software’s.

  2. #2
    Join Date
    Nov 2003
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    The RTS/CTS concept was part of the IEEE 802.11 protocol from the beginning. You make it sound like you or whom you work for have developed something new?

    I am not sure what you are trying to say.

  3. #3
    Join Date
    Jul 2003
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    Georgia USA
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    Rather than lowering the threshold, thus increasing the overhead, the preferred method of dealing with this problem is installing enough AP's in the first place.

    Coverage isn't throughput....

  4. #4
    Join Date
    Nov 2005
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    29
    I have a few questions I want to ask here...Question #1 Why are you implimenting a mechanism that has existed in the 802.11 protocol sinc it was invented in 1996 ??...#2 how will rts\cts help a signal saturated with noise from a saturated amplifier ??..#3 How will rts\cts help in an enviroment where a microwave oven is used...#4 why are you posting writings from a white paper on hidden nodes published by Lucent 7 or 8 years ago ??.....I know the answers but I just want to see if you know what you are talking about...

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