Mário Serafim Nunes

Performance evaluation of IEEE 802.11e

This paper presents an evaluation of the QoS enhancements to the IEEE 802.11 standard, named IEEE 802.11e, currently under specification. Both the enhanced distributed coordination function (EDCF) and hybrid coordination function (HCF) modes of medium access control (MAC) operation are analysed and compared with legacy distributed coordination function (DCF) and point coordination function (PCF). Performance evaluation is attained through computer simulation of a scenario of 802.11 b/e access to an IP core network through an access point (AP) in an infrastructure WLAN.

A scheduling algorithm for QoS support in IEEE802.11 networks

This article presents a scheduling algorithm for the IEEE 802.11e hybrid coordination function under definition by the IEEE 802.11e task group. HCF can be used to provide IP quality of service guarantees in IEEE802.11e infrastructure WLANs. The enhanced distributed coordination function is mainly used for data transmission without QoS guarantees, but can also be used to decrease the transmission delay of QoS-sensitive traffic. Scheduling of queued packets follows a delay-earliest-due-date algorithm. The proposed algorithm is compatible with the link adaptation mechanisms implemented in commercial WLANs, as it limits the amount of time during which the stations control the wireless medium. The performance of the algorithm is evaluated through computer simulation and compared with the reference scheduler presented by the IEEE 802.11e task group.

DTSN: Distributed Transport for Sensor Networks

This paper presents the distributed transport for sensor networks (DTSN), a novel reliable transport protocol for convergecast and unicast communications in wireless sensor networks (WSNs). In DTSN, the source completely controls the loss recovery process in order to minimize the overhead associated with control and data packets. The basic loss recovery algorithm is based on selective repeat ARQ, employing both positive and negative acknowledgements. DTSN is able to detect when all packets of a session are lost, besides scattered gaps in the packet sequence. Caching at intermediate nodes is used to avoid the inefficiency of the strictly end-to-end transport reliability TCP-like model, commonly employed in broadband networks. Reliability differentiation is achieved by means of the smart integration of partial buffering at the source, integrated with erasure coding and caching at intermediate nodes. The simulation results attest the effectiveness of both the full reliability and the reliability differentiation mechanisms in DTSN.

Terminal independent mobility for IP (TIMIP)

This article presents Terminal Independent Mobility for IP (TIMIP), which is a new architecture for IP mobility in wireless access networks. TIMIP is based on principles similar to those in the CIP and HAWAII architectures proposed at IETF and is equally suited for micromobility scenarios. With TIMIP, terminals with legacy IP stacks have the same degree of mobility as terminals with mobility-aware IP stacks. Nevertheless, it still uses MIP for macromobility scenarios. In order to support seamless handoff, TIMIP uses context-transfer mechanisms compatible with those currently in discussion at the IETF SeaMoby group.

Distributed Latency-Energy Minimization and interference avoidance in TDMA Wireless Sensor Networks

This paper presents Latency-Energy Minimization Medium Access (LEMMA), a new TDMA-based MAC protocol for Wireless Sensor Networks (WSNs), specially suited to extend the lifetime of networks supporting alarm-driven, delay-sensitive applications characterized by convergecast traffic patterns and sporadic traffic generation. Its cascading time-slot assignment scheme conciliates low end-to-end latency with a low duty-cycle, while supporting multi-sink WSN topologies. Unlike most of the current solutions, LEMMAs time-slot allocation protocol makes decisions based on the interference actually experienced by the nodes, instead of following the simple but potentially ineffective n-hop approach. Simulation results are presented to demonstrate the ineffectiveness of the n-hop time-slot allocation in comparison with LEMMA, as well as to evaluate the performance of LEMMA against L-MAC, T-MAC and Low Power Listening. The results show that under the target scenario conditions, LEMMA presents lower interference between assigned time-slots and lower end-to-end latency, while matching its best contender in terms of energy-efficiency.

Evaluation of the H.264 Scalable Video Coding in Error Prone IP Networks

The Joint Video Team, composed by the ISO/IEC Moving Picture Experts Group (MPEG) and the ITU-T Video Coding Experts Group (VCEG), has standardized a scalable extension of the H.264/AVC video coding standard called scalable video coding (SVC). H.264/SVC provides scalable video streams which are composed by a base layer and one or more enhancement layers. Enhancement layers may improve the temporal, the spatial or the signal-to-noise ratio resolutions of the content represented by the lower layers. One of the applications of this standard is related to video transmission in both wired and wireless communication systems, and it is therefore important to analyze in which way packet losses contribute to the degradation of quality, and which mechanisms could be used to improve that quality. This paper provides an analysis and evaluation of H.264/SVC in error prone environments, quantifying the degradation caused by packet losses in the decoded video. It also proposes and analyzes the consequences of QoS-based discarding of packets through different marking solutions.

Link-adaptation and transmit power control for unicast and multicast in IEEE 802.11 a/h/e WLANs

This paper proposes a mechanism to combine link-adaptation and transmit power control (TPC) in IEEE 802.11a WLANs. The link-adaptation and TPC algorithms try to maximize the goodput of the WLAN, while minimizing transmit power. The algorithm runs at the sender, but it relies on feedback from the receiver. The latter notifies the sender about the signal-to-interference-plus-noise-ratio experienced in previous transmissions, allowing a prediction of the channel status. An extension of the algorithm for multicast is also presented. The implementation of the proposed mechanism in IEEE 802.11a/h/e WLANs is discussed and its performance evaluated.

A Subjective Quality Estimation Tool for the Evaluation of Video Communication Systems

This paper presents a quantification tool for the evaluation of scalable and non-scalable video communication systems. The proposed mechanism estimates the subjective quality of experience (QoE) of a human viewer according to the temporal resolution, the spatial resolution and the Root Mean Square of the Error (RMSE) between the original image and the encoded one. According to these three dimensions of quality it enables an encoder and server to search for the best combination of each of these scalability factors in order to deliver the best quality. The proposed quantification tool was obtained through subjective tests using a panel of evaluators and a new methodology which have shown good correlation factors between measurement data and estimating functions.

Performance Evaluation of DTSN in Wireless Sensor Networks

The guaranteed delivery of critical data is an essential requirement in most Wireless Sensor Network (WSN) applications. The paucity of energy, communication, processing and storage resources in each WSN node causes the TCP transport model (widely used in broadband networks) to be inefficient in WSNs, a reason why new WSN-specific reliable transport protocols have been proposed in the past few years. This paper presents one of these protocols, the Distributed Transport for Sensor Networks (DTSN). DTSN is able to efficiently support unicast communications in WSNs due to its capabilities to tightly control the amount of signaling and retransmission overhead. The basic loss recovery algorithm is based on Selective Repeat ARQ, employing both positive and negative acknowledgements. Caching at intermediate nodes is used to avoid the inefficiency typical of the strictly end-to-end transport reliability commonly assumed in broadband networks. DTSN is currently implemented in TinyOS. Preliminary simulation results using this code show that DTSN is quite efficient providing block oriented reliability, while the caching mechanism employed in DTSN decreases packet delay for more than one hop.

Personalized Coverage of Large Athletic Events

This article presents a platform that lets users direct their own coverage of large athletic events, letting them set up their own virtual director and orchestrate event viewing according to their preferences.