Modeling, Analysis and Impact of a Long Transitory Phase in Random Access Protocols

In random access protocols, the service rate depends on the number of stations with a packet buffered for transmission. We demonstrate via numerical analysis that this state-dependent rate along with the consideration of Poisson traffic and infinite (or large enough to be considered infinite) buffer size may cause a high-throughput and extremely long (in the order of hours) transitory phase when traffic arrivals are right above the stability limit. We also perform an experimental evaluation to provide further insight into the characterisation of this transitory phase of the network by analysing statistical properties of its duration. The identification of the presence as well as the characterisation of this behaviour is crucial to avoid misprediction, which has a significant potential impact on network performance and optimisation. Furthermore, we discuss practical implications of this finding and propose a distributed and low-complexity mechanism to keep the network operating in the high-throughput phase.Comment: 13 pages, 10 figures, Submitted to IEEE/ACM Transactions on Networkin

Decentralised Learning MACs for Collision-free Access in WLANs

By combining the features of CSMA and TDMA, fully decentralised WLAN MAC schemes have recently been proposed that converge to collision-free schedules. In this paper we describe a MAC with optimal long-run throughput that is almost decentralised. We then design two \changed{schemes} that are practically realisable, decentralised approximations of this optimal scheme and operate with different amounts of sensing information. We achieve this by (1) introducing learning algorithms that can substantially speed up convergence to collision free operation; (2) developing a decentralised schedule length adaptation scheme that provides long-run fair (uniform) access to the medium while maintaining collision-free access for arbitrary numbers of stations

Providing service guarantees in 802.11e EDCA WLANs with legacy stations

Although the EDCA access mechanism of the 802.11e standard supports legacy DCF stations, the presence of DCF stations in the WLAN jeopardizes the provisioning of the service guarantees committed to the EDCA stations. The reason is that DCF stations compete with Contention Windows (CWs) that are predefined and cannot be modified, and as a result, the impact of the DCF stations on the service received by the EDCA stations cannot be controlled. In this paper, we address the problem of providing throughput guarantees to EDCA stations in a WLAN in which EDCA and DCF stations coexist. To this aim, we propose a technique that, implemented at the Access Point (AP), mitigates the impact of DCF stations on EDCA by skipping with a certain probability the Ack reply to a frame from a DCF station. When missing the Ack, the DCF station increases its CW, and thus, our technique allows us to have some control over the CWs of the legacy DCF stations. In our approach, the probability of skipping an Ack frame is dynamically adjusted by means of an adaptive algorithm. This algorithm is based on a widely used controller from classical control theory, namely a Proportional Controller. In order to find an adequate configuration of the controller, we conduct a control-theoretic analysis of the system. Simulation results show that the proposed approach is effective in providing throughput guarantees to EDCA stations in presence of DCF stations.European Community's Seventh Framework ProgramPartly funded by the Ministry of Science and Innovation of Spain, under the QUARTET project (TIN2009-13992-C02-01)Publicad

Towards a Collision-Free WLAN: Dynamic Parameter Adjustment in CSMA/E2CA

Carrier Sense Multiple Access with Enhanced Collision Avoidance (CSMA/ECA) is a distributed MAC protocol that allows collision-free access to the medium in WLAN. 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 article shows that the duration of the transient state can be shortened by appropriately setting the contention parameters. Standard absorbing Markov Chain theory can be 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 article 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/CA in harsh wireless scenarios with high frame error rates. To achieve collision-free operations when the number of contenders is large, it may be necessary to dynamically adjust the contention parameter. The last part of the article suggests an approach for such parameter adjustment which is validated by simulation results

MAC design for WiFi infrastructure networks: a game-theoretic approach

In WiFi networks, mobile nodes compete for accessing a shared channel by means of a random access protocol called Distributed Coordination Function (DCF). Although this protocol is in principle fair, since all the stations have the same probability to transmit on the channel, it has been shown that unfair behaviors may emerge in actual networking scenarios because of non-standard configurations of the nodes. Due to the proliferation of open source drivers and programmable cards, enabling an easy customization of the channel access policies, we propose a game-theoretic analysis of random access schemes. Assuming that each node is rational and implements a best response strategy, we show that efficient equilibria conditions can be reached when stations are interested in both uploading and downloading traffic. More interesting, these equilibria are reached when all the stations play the same strategy, thus guaranteeing a fair resource sharing. When stations are interested in upload traffic only, we also propose a mechanism design, based on an artificial dropping of layer-2 acknowledgments, to force desired equilibria. Finally, we propose and evaluate some simple DCF extensions for practically implementing our theoretical findings.Comment: under review on IEEE Transaction on wireless communication

RCFD: A Novel Channel Access Scheme for Full-Duplex Wireless Networks Based on Contention in Time and Frequency Domains

In the last years, the advancements in signal processing and integrated circuits technology allowed several research groups to develop working prototypes of in-band full-duplex wireless systems. The introduction of such a revolutionary concept is promising in terms of increasing network performance, but at the same time poses several new challenges, especially at the MAC layer. Consequently, innovative channel access strategies are needed to exploit the opportunities provided by full-duplex while dealing with the increased complexity derived from its adoption. In this direction, this paper proposes RTS/CTS in the Frequency Domain (RCFD), a MAC layer scheme for full-duplex ad hoc wireless networks, based on the idea of time-frequency channel contention. According to this approach, different OFDM subcarriers are used to coordinate how nodes access the shared medium. The proposed scheme leads to efficient transmission scheduling with the result of avoiding collisions and exploiting full-duplex opportunities. The considerable performance improvements with respect to standard and state-of-the-art MAC protocols for wireless networks are highlighted through both theoretical analysis and network simulations.Comment: Submitted at IEEE Transactions on Mobile Computing. arXiv admin note: text overlap with arXiv:1605.0971

Improving the Performance of Wireless LANs

This book quantifies the key factors of WLAN performance and describes methods for improvement. It provides theoretical background and empirical results for the optimum planning and deployment of indoor WLAN systems, explaining the fundamentals while supplying guidelines for design, modeling, and performance evaluation. It discusses environmental effects on WLAN systems, protocol redesign for routing and MAC, and traffic distribution; examines emerging and future network technologies; and includes radio propagation and site measurements, simulations for various network design scenarios, numerous illustrations, practical examples, and learning aids

Service differentiation in multihop wireless packet networks

This work explores the potential of link layer scheduling combined with MAC layer prioritization for providing service differentiation in multihop wireless packet networks. As a result of limited power, multihop characteristic and mobility, packet loss ratio in wireless ad hoc networks tends to be high compared to wireline and one-hop mobile data networks. Therefore, for wireless ad hoc networks, DiffServ-like distributed service differentiation schemes are more viable than hard QoS solutions, which are mainly designed for wireline networks. The choice and implementation of proper queuing and scheduling methods, which determine how packets will use the channel when bandwidth becomes available, contributes significantly to this differentiation. Due to the broadcast nature of wireless communication, media access is one of the main resources that needs to be shared among different flows. Thus, one can design and implement algorithms also at MAC level for service differentiation. In this study, in addition to the scheduling discipline, IEEE 802.11 Distributed Coordination Function is used to increase the media access probability of a specific class of traffic. It is shown that the service requirements of a class can be better met using this two level approach compared to the cases where either of these schemes used alone