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

A cross-layer mechanism for QoS improvements in VoIP over multi-rate WLAN networks

In IEEE 802.11 WLANs, Link Adaptation mechanisms, which choose the transmission rate of each node, provoke unexpected and random variations on the effective channel capacity. When these changes are towards lower bitrates, inelastic flows, such as VoIP, can suffer from sudden congestion, which results on higher packet delays and losses. In this thesis, a VoIP codec adaptation algorithm is proposed as a solution, based on a cross-layer feedback from RTCP packets and the MAC layer, which can adapt the codecs of active calls to adjust them to the multirate scenario. A combination of this algorithm with a call admission control mechanism is also studied. The results show an important improvement in terms of the QoS of the already active flows as also in the total hotspot's capacity. Additionally, by defining a new Grade of Service related parameter, the Q-Factor, which captures the trade-off between dropping and blocking ratio and perceived speech quality, the codec adaptation algorithm can be tuned to achieve maximum capacity without severely penalizing any of those variables, and hence satisfying both technical and user quality requirements. Finally, a new QoS-enabled AP, which implements these enhancements is designed.En las redes inalámbricas del estándar IEEE 802.11, los mecanismos de adaptación de enlace que eligen la tasa de transmisión de cada nodo, pueden provocar variaciones aleatorias e inesperadas en la capacidad efectiva del canal. Cuando estos cambios son hacia tasas de transmisión mas bajas, los flujos inelásticos, tales como los de VoIP, pueden de repente sufrir congestión, lo que se traduce en aumento de retrasos y pérdidas de paquetes. En esa tesis, se propone un algoritmo de adaptación de codificadores de voz como solución, basado en técnicas multinivel (cross-layer) que combinan el uso de información de diferentes capas, como los paquetes RTCP y la capa MAC, y que puede adaptar los codecs de las llamadas activas para ajustarlos al escenario "multi-rate". Adicionalmente, la combinación de este algoritmo con un mecanismo de control de admisión de llamadas (CAC) se ha estudiado. Los resultados muestran una importante mejora en términos de QoS de los flujos activos como también en la capacidad total del hotspot. Además, mediante la definición de un nuevo factor, el Q-Factor, que puede captar la compensación entre la tasa de corte y de bloqueo de llamadas y de la calidad percibida por esas, el algoritmo de adaptación de codecs se puede ajustar para lograr la máxima capacidad sin penalizar severamente ninguna de esas variables y así satisfacer los requisitos técnicos de calidad y los usuarios. Por último, un nuevo punto de acceso (AP) habilitado para ofrecer calidad de servicio, ha sido diseñado que lleva a cabo estas mejoras

Dynamic measurement-based codec selection for VoIP in multirate IEEE 802.11 WLANs

In wireless multirate environment users can suffer rate changes due to various factors while maintaining active voice over IP (VoIP) sessions with other users connected to the wired network. This creates a very hostile environment for VoIP, provoking serious quality of service degradation in all active sessions. Building upon our previous results, we propose a centralized version of our codec adaptation algorithm, which allows a cell-wide optimization of network resources and voice quality, based on the combined feedback from Real-Time Control Protocol (RTCP) packets and the MAC layer. A comparison of both the centralized and distributed versions of the algorithm is provided for a wired-wireless scenario, showing an important capacity and quality increase over the standard case. Keywords: RTP/RTCP, QoS, VoIP, multi-rate 802.1

A comparative survey of adaptive codec solutions for VoIP over multirate WLANs: a capacity versus quality performance trade-off

In multi-rate WLANs, users can suffer transmission rate changes due to the link adaptation mechanism. This results in a variable capacity channel, which is very hostile for VoIP and can cause serious quality of service (QoS) degradation in all active calls. Various codec adaptation mechanisms have been proposed as a solution to this, as well as to solve congestion problems on WLAN environments. Here, these solutions are presented, categorized according to the adaptation policy and scenario they implement, and evaluated at call-level in terms of the resulting blocking and dropping probabilities, as well as the perceived voice quality. To define a common performance metric, a new index named VGoS-factor is presented, which, by combining these capacity and quality indicators, can provide an overall view of the capacity versus quality trade-off of the proposed mechanisms and consequently help in choosing the adequate policy for each scenari

A comparative survey of adaptive codec solutions for VoIP over multirate WLANs: a capacity versus quality performance trade-off

In multi-rate WLANs, users can suffer transmission rate changes due to the link adaptation mechanism. This results in a variable capacity channel, which is very hostile for VoIP and can cause serious quality of service (QoS) degradation in all active calls. Various codec adaptation mechanisms have been proposed as a solution to this, as well as to solve congestion problems on WLAN environments. Here, these solutions are presented, categorized according to the adaptation policy and scenario they implement, and evaluated at call-level in terms of the resulting blocking and dropping probabilities, as well as the perceived voice quality. To define a common performance metric, a new index named VGoS-factor is presented, which, by combining these capacity and quality indicators, can provide an overall view of the capacity versus quality trade-off of the proposed mechanisms and consequently help in choosing the adequate policy for each scenari

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

<p/> <p>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.</p

Taking advantage of overhearing in low power listening WSNs: a performance analysis of the LWT-MAC protocol

LWT-MAC is a new Low Power Listening MAC protocol for WSNs designed to rapidly react to instantaneous increases of the network load. It takes advantage of overhearing by waking up all nodes at the end of a transmission to send or receive packets without needing to transmit the long preamble before. In this work, detailed analytical models of the LWT-MAC and B-MAC protocols, for both saturated and unsaturated conditions, are presented. Moreover, the key LWT-MAC parameters are optimized in order to minimize the energy consumption, constrained to obtain the same throughput as the IEEE 802.11 (CSMA/CA) MAC protocol. From the behavior of the optimal LWT-MAC parameters, a heuristic configuration is proposed. Finally, the LWT-MAC is compared to B-MAC, in both single and multi-hop scenarios, showing improvements in energy consumption, throughput and delay.This work has been partially supported by the Spanish Government under the projects TEC2008-0655/TEC (GEPETO, Plan Nacional I+D) and CSD2008-00010 (COMONSENS, Consolider-Ingenio Program) and by the Catalan Government (SGR2009#00617

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 collisionfree/naccess to the medium in WLANs. The only difference between CSMA/ECA and the well-known CSMA/CA is that the former/nuses a deterministic backoff after successful transmissions. Collision-free operation is reached after a transient state during which/nsome collisions may occur. This paper shows that the duration of the transient state can be shortened by appropriately setting the/ncontention parameters. Standard absorbing Markov chain theory is used to describe the behaviour of the system in the transient/nstate and to predict the expected number of slots to reach the collision-free operation. The paper also introduces CSMA/E2CA,/nin which a deterministic backoff is used two consecutive times after a successful transmission. CSMA/E2CA converges quicker to/ncollision-free operation and delivers higher performance than CSMA/ECA, specially in harsh wireless scenarios with high frameerror/nrates. The last part of the paper addresses scenarios with a large number of contenders. We suggest dynamic parameter/nadjustment techniques to accommodate a varying (and potentially high) number of contenders. The effectiveness of these/nadjustments in preventing collisions is validated by means of simulationThe present paper is the extended version of a MACOM/nworkshop paper. The authors benefited from the feedback/nand discussion in the workshop. The authors are specially/nindebted with N. Abramson for his clarifications regarding/nAloha and with D. Malone for providing recent and/nvaluable references. This work has been partially supported/nby the Spanish Government under Projects nos. TEC2008-/n0655/TEC (GEPETO, Plan Nacional I+D) and CSD2008-/n00010 (COMONSENS, Consolider-Ingenio Program)