A comparison of Aloha and CSMA in Wireless Ad-Hoc Networks under Different Channel Conditions
Résumé
In this paper we compare Aloha and Carrier Sense Multiple Access (CSMA) in Wireless Ad-Hoc Networks. We use a Signal-to-Interference-and-Noise Ratio (SINR) model where a transmission is assumed to be successful when the SINR is larger than a given threshold. Regarding channel conditions we consider both negligible and standard Rayleigh fading. For slotted and non- slotted Aloha we use analytical models as well as simulations to study the density of successful transmissions in the network. As it is difficult to build precise models for CSMA, we use only simulations to compute the performance of this protocol. We compare the two Aloha versions and CSMA on a fair basis, i.e.; when they are optimized to maximize the density of successful transmissions. For slotted Aloha, the key optimization parameter is the medium access probability, for non-slotted Aloha we tune the mean back-off time, whereas for CSMA it is the carrier sense threshold that is adjusted. Our study shows that CSMA always outperforms slotted Aloha, which in turn outperforms its non-slotted version. The gain in the density of successful transmissions depends however on the model's parameters: the path-loss exponent, SINR threshold, the existence or not of the fading, as well as the transmission range. Our conclusions differ from these of the seminal paper by Nelson an Kleinrock (1983), see [15], where the performance of Aloha is found comparable to this of CSMA in a simple geometric model, the reason being a non-optimal choice of the sensing range in the latter model.
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