António Grilo

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.

An Integrated WSAN and SCADA System for Monitoring a Critical Infrastructure

Wireless sensor and actuator networks (WSAN) constitute an emerging technology with multiple applications in many different fields. Due to the features of WSAN (dynamism, redundancy, fault tolerance, and self-organization), this technology can be used as a supporting technology for the monitoring of critical infrastructures (CIs). For decades, the monitoring of CIs has centered on supervisory control and data acquisition (SCADA) systems, where operators can monitor and control the behavior of the system. The reach of the SCADA system has been hampered by the lack of deployment flexibility of the sensors that feed it with monitoring data. The integration of a multihop WSAN with SCADA for CI monitoring constitutes a novel approach to extend the SCADA reach in a cost-effective way, eliminating this handicap. However, the integration of WSAN and SCADA presents some challenges which have to be addressed in order to comprehensively take advantage of the WSAN features. This paper presents a solution for this joint integration. The solution uses a gateway and a Web services approach together with a Web-based SCADA, which provides an integrated platform accessible from the Internet. A real scenario where this solution has been successfully applied to monitor an electrical power grid is presented.

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.

Immune System Simulation through a Complex Adaptive System Model

Evolutionary algorithms and cellular automata are two computational approaches to model complex adaptive systems. Here is described an immune system simulator that uses a cellular automaton to model the physical environment along with an evolutionary genetic algorithm to attain adaptation and selection. Agent genetic coding comprised within the genotype is a set of rules which expresses behavior. Moreover, an agent includes a collection of operators which use the genetic code in order to interact with other agents and physical sites. We also depict the system’s methodology as well as some of the obtained results.

Efficient multimedia transmission in wireless sensor networks

Real-time multimedia data such as video are usually loss-tolerant but require timely delivery in order to be useful to the application. Loss recovery through the retransmission of lost data may introduce unacceptable delays, which is the reason why these data types are usually delivered with no transport layer reliability, using erasure coding and similar techniques to maximize data recovery at the receiver. However, in Wireless Multimedia Sensor Networks (WMSNs), these mechanisms are not enough to provide an acceptable image quality and, thus, reliable transport protocols adapted to these requirements are needed. This paper presents some mechanisms to improve multimedia transmissions in WMSNs when reliable transport layer protocols are used. They consist of assigning a budget of time for the sending of certain amount of information and estimating if the channel conditions allow to complete the transmission or not. If it is not likely to complete it, then the transmission is stopped, thus saving important energy resources in the sensors. We evaluate this approach by modifying the behavior of a previously proposed reliable transport protocol (DTSN). Our proposal, M-DTSN, improves DTSN flexibility by managing the trade-off between media quality and timely delivery for real-time multimedia data with some degree of loss-tolerance. The simulation results demonstrate that the advantages of M-DTSN for the transmission of multimedia data are quite significant when compared with the original DTSN protocol.

An analytical model for transport layer caching in wireless sensor networks

Reliable transport protocols have traditionally been designed to perform end-to-end error control transparently to the intermediate nodes (e.g., TCP). However, the resource constraints featured by Wireless Sensor Network (WSN) require a different paradigm where intermediate nodes are able to cache packets, retransmitting them on-demand in order to avoid incurring on costly end-to-end retransmissions. This paper presents an analytical model of end-to-end delivery cost for WSN reliable transport with intermediate caching. The model calculates the cost as the total number of physical layer transmissions using a probabilistic formulation that has been validated through network simulation. Although the model is based on a specific transport protocol (DTSN), the addressed mechanisms are more generic, allowing it to be easily adapted to other WSN transport protocols that also feature intermediate caching. Numerical results confirm the improved efficiency introduced by a transport layer with intermediate caching in comparison with end-to-end approaches that are based exclusively on MAC layer reliability. Different cache partitioning policies were tested, and it is shown that cache partitioning policies should take into account the network conditions experienced by concurrent flows, namely the status of the radio links and the flow lengths.