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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Recent studies on MAC scheduling have shown that carrier sense multiple access (CSMA) algorithms can be throughput optimal for arbitrary wireless network topology. However, these results are highly sensitive to the underlying assumption on static' or fixed' system conditions. For example, if channel conditions are time-varying, it is unclear how each node can adjust its CSMA parameters, so-called backoff and channel holding times, using its local channel information for the desired high performance. In this paper, we study channel-aware' CSMA (A-CSMA) algorithms in time-varying channels, where they adjust their parameters as some function of the current channel capacity. First, we show that the achievable rate region of A-CSMA equals to the maximum rate region if and only if the function is exponential. Furthermore, given an exponential function in A-CSMA, we design updating rules for their parameters, which achieve throughput optimality for an arbitrary wireless network topology. They are the first CSMA algorithms in the literature which are proved to be throughput optimal under time-varying channels. Moreover, we also consider the case when back-off rates of A-CSMA are highly restricted compared to the speed of channel variations, and characterize the throughput performance of A-CSMA in terms of the underlying wireless network topology. Our results not only guide a high-performance design on MAC scheduling under highly time-varying scenarios, but also provide new insights on the performance of CSMA algorithms in relation to their backoff rates and underlying network topologies. A novel media access control (MAC) protocol named carrier sense multiple access with idle detection (CSMA ID) is proposed to handle variable-length packets over an all-optical ring network. To evaluate optimal utilization of channel bandwidth, we study packet scheduling based on three transmitting queue discipline (TQD) architectures and four idle space allocation (ISA) algorithms with regard to their impact on performance. For numerical evaluation of performance, an analytical model is developed by a preclassification queue with weighted round-robin (PCQWRR) architecture and a random algorithm. Moreover, three related MAC protocols are examined and compared, namely, multitoken, carrier sense multiple access collision avoidance (CSMA CA) and carrier sense multiple access collision preemption (CSMA CP). Simulation results indicate that, of the TQDs, better performance is obtained by PCQWRR compared with first-in-first-out and preclassification queues. The first fit space (FFS) algorithm has the best performance of the ISAs. The 12 combinations of TQDs-ISAs are then considered. It is found that the combination of PCQWRR with FFS provides the greatest efficiency and has the lowest packet latency, providing better throughput than three different MAC protocols under either symmetric or asymmetric traffic load on all-optical ring networks. This paper proposes and analyzes the performance of a simple frequency-agile CSMA MAC protocol. In this MAC, a node carrier-senses multiple frequency channels simultaneously, and it takes the first opportunity to transmit on any one of the channels when allowed by the CSMA backoff mechanism. We show that the frequency-agile MAC can effectively 1) boost throughput and 2) remove temporal starvation. Furthermore, the MAC can be implemented on the existing multiple-frequency setup in Wi-Fi using multi-radio technology, and it can co-exist with the legacy MAC using single radio. This paper provides exact stationary throughput analysis for regular 1D and thin-strip 2D CSMA networks using a "transfer-matrix" approach. In addition, accurate approximations are given for 2D grid networks. Our closed-form formulas accurately quantify the throughput gain of frequency-agile CSMA. To characterize temporal starvation, we use the metric of "mean residual access time" (MRAT). Our simulations and closed-form approximations indicate that the frequency-agile MAC can totally eliminate temporal starvation in 2D grid networks, reducing its MRAT by orders of magnitude. Finally, this paper presents a "coloring theorem" to justify the use of the frequency-agile MAC in general network topologies. Our analysis and theorem suggest that with enough frequency channels, the frequency-agile MAC can effectively decouple the detrimental interactions between neighboring links responsible for low throughput and starvation. In this paper, we analyse the impact of the Contiki Operating System (OS), and its Carrier Sense Multiple Access and Collision Avoidance (CSMA-CA) implementation on an IEEE 802.15.4 node's throughput and wireless channel utilization. The analysis is based on Contiki's Rime networking protocol stack, and its target is to determine an upper bound for the stated metrics. We explain that in Contiki with CSMA-CA as a MAC layer protocol, a node's throughput is limited to 8.1 kbps, at maximum, even without power saving features. In order to maximize a node's transmission capability, we modified Contiki's CSMA-CA implementation. A number of simulations are performed, and it is observed that with our modifications node throughput reaches 45 kbps, at maximum. Simulation results for estimating the channel capacity with our modified CSMA-CA MAC layer protocol show that the average per-node delay is low when the offered data load remains below 100 kbps. For an offered load of 100 kbps, the channel drops almost 20 of packets. Going beyond 100 kbps results in large latencies and significant packet loss. Results presented in this paper can serve as basis for the available bandwidth estimation in Wireless Sensor Networks (WSNs), QoS-based routing, and design of congestion control algorithm.