Monday, February 23, 2009

The CSMA/CA access protocol does not detect collision as CSMA/CD does. How does this limitation affect link-layer and transport-layer performance?

In wired network, when two transmitters start simultaneously collision occurs and that can be detected by transmitters. Then they abort their transmission and retry later. However, in wireless networks transmission and reception is done in the same band that makes collision detection difficult. Hence, the WLAN use CSMA/CA to avoid collision. CSMA/CA, is listen before talk mechanism, where transmitters wait for a random interval after the medium becomes idle before starting transmission. The transmitter that starts first seizes the medium and all others back off, there by reducing the change of collisions. However, there may be collision that is not detected by CSMA/CA and corrupted frames not retransmitted by CSMA/CA are considered by the link-layer and transport-layer. Link layer is responsible for flow control, acknowledgement and error recovery. It also species which mechanism are to be used for addressing stations over the transmission medium and for controlling the data exchanged between the sender and receiver. To recover from wireless losses, including undetected collisions, transmitters may need to optionally ask for acknowledgements to retransmit transparently unacknowledged frames at the link layer. Further, in some cases, wireless transmission errors are not related to network congestion but due to corrupted packets that not handed over to TCP. TCP assumes those packets are lost and reduces its congestion window size that leads to throughput degradation.

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