Cater: An Opportunistic Medium Access Control Protocol for Wireless Local Area Networks


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Virginia Tech


An adaptive MAC protocol is developed and analyzed that offers a "best case" scenario by allowing the MAC to control medium parameters thereby fully exploiting the channel of an ad hoc wireless LAN. This new, opportunistic medium access control protocol is called CATER (Code Adapts To Enhance Reliability) and is based on the proposed MAC standard for wireless local area networks (WLAN)-IEEE 802.11 [IEE96]. As currently proposed, IEEE 802.11 uses a fixed pseudo-noise (PN) code for spreading the information signal, implying a fixed process gain at the receiver. When the channel degrades, IEEE 802.11 offers only retransmissions at the MAC layer to combat a corrupt medium. However, CATER allows communicating stations to reconfigure their transceivers to use a longer PN code after a prescribed number of failed retransmissions. This longer code increases the process gain of the receiver and reduces the error rate. After the two stations are reconfigured, the source station sends the frame in question. Immediately after that frame is acknowledged, the source station may send additional frames during the reconfigured period.

Simulation and emulation are used to demonstrate and validate the adaptive protocol's capabilities. Results show that this new protocol offers substantial improvement in system throughput when the channel degrades to a point that reliable transmission of frames is not feasible in a standard IEEE 802.11 WLAN. Specifically, CATER continues to function, permitting up to 14 percent normalized aggregate throughput at times when IEEE 802.11 permits no frames to pass through the WLAN. In addition, throughput experiences only a small decrease due to protocol overhead during periods when stations experience a good channel with few bit errors. Moreover, CATER does not adversely affect the predominate transport layer protocol (i.e., TCP), and provides equitable service to all stations within the network.



Adaptive MAC Protocol, IEEE 802.11, Spread Spectrum, Wireless