Collisions are a main cause of throughput degradation in wireless local area networks. The current contention mechanism used in the IEEE 802.11 networks is called carrier sense multiple access with collision avoidance (CSMA CA). It uses a binary exponential backoff technique to randomize each contender attempt of transmitting, effectively reducing the collision probability. Nevertheless, CSMA CA relies on a random backoff that while effective and fully decentralized, in principle is unable to completely eliminate collisions, therefore degrading the network throughput as more contenders attempt to share the channel. To overcome these situations, carrier sense multiple access with enhanced collision avoidance (CSMA ECA) is able to create a collision-free schedule in a fully decentralized manner using a deterministic backoff after successful transmissions. Hysteresis and fair share are two extensions of CSMA ECA to support a large number of contenders in a collision-free schedule. CSMA ECA offers better throughput than CSMA CA and short-term throughput fairness. This paper describes CSMA ECA and its extensions. In addition, it provides the first evaluation results of CSMA ECA with non-saturated traffic, channel errors, and its performance when coexisting with CSMA CA nodes. Furthermore, it describes the effects of imperfect clocks over CSMA ECA and presents a mechanism to leverage the impact of channel errors and the addition withdrawal of nodes over collision-free schedules. Finally, the experimental results on throughput and lost frames from a CSMA ECA implementation using commercial hardware and open-source firmware are presented.
L-MAC is able to achieve higher throughput than CSMA CA with a very fast convergence speed. Nevertheless, the choice of @math suppose a previous knowledge of the number of empty slots ( @math , where @math is the number of contenders), which is not easily available to CSMA CA or may require a centralised entity @cite_33 .
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Carrier sense multiple access with enhanced collision avoidance (CSMA ECA) is a distributed MAC protocol that allows collision-free access to the medium in WLANs. The only difference between CSMA ECA and the well-known CSMA CA is that the former uses a deterministic backoff after successful transmissions. Collision-free operation is reached after a transient state during which some collisions may occur. This paper shows that the duration of the transient state can be shortened by appropriately setting the contention parameters. Standard absorbing Markov chain theory is used to describe the behaviour of the system in the transient state and to predict the expected number of slots to reach the collision-free operation. The paper also introduces CSMA E2CA, in which a deterministic backoff is used two consecutive times after a successful transmission. CSMA E2CA converges quicker to collision-free operation and delivers higher performance than CSMA ECA, specially in harsh wireless scenarios with high frame-error rates. The last part of the paper addresses scenarios with a large number of contenders. We suggest dynamic parameter adjustment techniques to accommodate a varying (and potentially high) number of contenders. The effectiveness of these adjustments in preventing collisions is validated by means of simulation.