Anna Sfairopoulou

Low energy operation in WSNs: A survey of preamble sampling MAC protocols

The limited energy resources of sensor nodes are among the most important constraints in Wireless Sensor Networks (WSNs). Consequently, the Medium Access Control (MAC) layer design is crucial, due to its influence on the transceiver, which is the most energy-consuming component of a sensor node. Among the different MAC protocols designed for WSNs, preamble sampling techniques provide extremely low energy consumption at low loads and have a notably simple operation and a lack of synchronisation requirements, which are characteristics that are especially appealing to WSNs. In this work, a survey of the different types of MAC protocols designed for WSNs is presented with a special focus on preamble sampling MAC protocols. The aim of this work is to give a detailed overview and classification of the most relevant preamble sampling MAC protocols, being motivated by the extremely large number of MAC protocols designed for WSNs in recent years. Moreover, a simple set of guidelines for matching the most suitable MAC protocol category to a given application is provided in this work.

CSMA with Enhanced Collision Avoidance: A Performance Assessment

CSMA with Enhanced Collision Avoidance (CSMA/ECA) uses a deterministic backoff after successful transmissions to significantly reduce the number of collisions. This paper assesses by means of simulations the throughput and conditional collision probability obtained from a single-hop ad-hoc network using CSMA/ECA. A comparison with the legacy CSMA/CA reveals that the proposed protocol outperforms the legacy one in all considered scenarios. Specifically, it is shown that CSMA/ECA presents advantages for both rigid and elastic flows.

A low power listening MAC with scheduled wake up after transmissions for WSNs

This work presents a new MAC protocol for WSNs that combines an unscheduled channel access, based on lowpower listening, with an opportunistic scheduled wake up after transmissions mechanism. The proposed MAC provides a good tradeoff between energy consumption, complexity and performance in terms of throughput and delay. It achieves the benefits of scheduled MAC protocols without the cost of maintaining and sharing the schedule, as well as the low consumption and simple operation of unscheduled MAC protocols.

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 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.

How to tune VoIP codec selection in WLANs

In IEEE 802.11e-based 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 bit-rates, inelastic flows, such as VoIP, can suffer from sudden congestion, which results on higher packet delays and losses. A VoIP codec selection algorithm has been proposed as a solution to this issue, which is triggered both by channel rate changes as well as in combination with a call admission control mechanism. The results show that an important improvement in terms of hotspot capacity for VoIP calls can be achieved by choosing the VoIP codec adaptively in a multi-rate scenario. By defining a new grade of service-related parameter, Qmacr, which captures the tradeoff between dropping and blocking probabilities and perceived speech quality, the codec selection algorithm can be tuned to achieve maximum capacity without severely penalizing any of those variables, and hence satisfying both technical and user quality requirements.

Wireless open metropolitan area networks

IEEE 802.11 technologies allow the deployment of metropolitan access networks. This paper details the reasons for making these networks open to several service providers. The sharing of the wireless access network fosters competition and ultimately benefits the users. The second part of the paper describes the available technologies that allow such infrastructure sharing and compare them in terms of security, scalability, maturity and convenience.

Carrier sense multiple access with enhanced collision avoidance: a performance analysis

Carrier Sense Multiple Access with Enhanced Collision Avoidance (CSMA/ECA) is a recently proposed modification to the well-known CSMA/CA protocol. By using a deterministic backoff after successful transmissions, the number of collisions decreases. This article presents a model that captures the behaviour of CSMA/ECA in both saturated and non-saturated scenarios. The results, which are validated by simulations, show that CSMA/ECA effectively prevents collisions and, therefore, it can deliver a higher throughput than CSMA/CA.

Tuning the EDCA parameters in WLANs with heterogeneous traffic: A flow-level analysis

The need to support multimedia services in WLANs has motivated the research on traffic differentiation mechanisms at the MAC layer. The most common approach is the tuning of the different MAC parameters of the heterogeneous traffic profiles in order to provide different channel access probabilities. The benefits of these mechanisms in terms of throughput and transmission delay, as well as their traffic differentiation and QoS capabilities, have been thoroughly studied. However, there are very few results on how the tuning of the MAC parameters impacts on the flow-level metrics, such as blocking probability or average flow duration. In this article, several EDCA-based tuning algorithms have been evaluated by comparing their flow-level response in presence of rigid (e.g. VoIP) and elastic (e.g. P2P) flows. Results show that those algorithms which adapt better to the changing WLAN state (number and type of active flows), and that are designed under multiple objectives, provide significantly higher performance and QoS than static and single-objective configurations.

QoS Adaptation in SIP-based VoIP calls in Multi-rate IEEE 802.11 Environments

In this paper we study the impact of rate changes at the physical layer on VoIP flows in multi-rate IEEE 802.11-based environments. We determine how the networks rate changes affect the QoS of the active flows, and how these effects can be quantified and corrected using a mixture of RTCP and MAC layer information. We propose a decision mechanism that, upon hints of QoS degradation, triggers the transmission of RTCP packets to collect QoS feedback as perceived by the application, together with MAC layer information on the networks state. Subsequently, the VoIP flow parameters are changed, by adapting their codecs and choosing a more appropriate codec combination for the new conditions. Our results show a significant increase in network usage, without a significant QoS loss for the active flows in the cell.

A Novel MAC Protocol for Event-Based Wireless Sensor Networks: Improving the Collective QoS

WSNs usually combine periodic readings with messages generated by unexpected events. When an event is detected by a group of sensors, several notification messages are sent simultaneously to the sink, resulting in sporadic increases of the network load. Additionally, these messages sometimes require a lower latency and higher reliability as they can be associated to emergency situations. Current MAC protocols for WSNs are not able to react rapidly to these sporadic changes on the traffic load, mainly due to the duty cycle operation, adopted to save energy in the sensor nodes, resulting in message losses or high delays that compromise the event detection at sink. In this work, two main contributions are provided: first, the collective QoS definitions are applied to measure event detection capabilities and second, a novel traffic-aware Low Power Listening MAC to improve the network response to sporadic changes in the traffic load is presented. Results show that the collective QoS in terms of collective throughput, latency and reliability are improved maintaining a low energy consumption at each individual sensor node.