M. V. Bueno-Delgado

Analysis of DFSA anti-collision protocols in passive RFID environments

Frame slotted Aloha (FSA) protocols are promising anti-collision protocols for passive RFID systems. They aim at decreasing the time to detect all the tags in range (identification delay). In FSA, the maximum identification rate (average number of tags identified per slot) is achieved when the number of contending tags matches the cycle length (number of slots in a frame). Therefore, the reader should ideally know the actual number of competing tags. However, in RFID scenarios this figure varies randomly, and the reader has to guess the number of contenders somehow. This paper analyzes the most relevant anti-collision algorithms; taking into account the limitations imposed by the world-wide de-facto standard EPCglobal Class-1 Gen-2 for passive RFID systems.

A comparative study of RFID schedulers in dense reader environments

In realistic deployments, several readers may be placed in the same area, forming a, so-called, dense reader environment. These scenarios are susceptible to suffer Reader Collision Problems, characterized by Reader-to-Tag and Reader-to-Reader interferences. Both affect network throughput, decreasing the overall number of tags identified per reader. This paper reviews the mechanisms proposed to mitigate the Reader Collision Problems. Besides, the constraints of these techniques are pointed out. The mechanisms have been evaluated to study the efficiency of the resources allocation.

On the optimal frame-length configuration on real passive RFID systems

The majority of the anti-collision protocols proposed for passive RFID systems are based on frame slotted aloha (FSA). They assume a classical result in FSA-based protocols which states that the theoretical identification throughput is optimized when the number of competing tags in coverage equals the number of slots in the frame. However, this is not exact in real RFID systems, as the so-called capture effect is neglected. The capture effect occurs when a tag identification signal is successfully decoded from a collision slot. This paper analyzes the identification performance of real RFID systems, taking into account not only the capture effect, but also the requirements imposed by the de facto standard EPCglobal Class-1 Gen-2. The analysis is addressed by discrete time Markov chains. From the analysis, a set of relevant results is extracted: the frame-length values that, configured into the readers studied, guarantee the best identification performance (maximum throughput). The analytical results have been confirmed by means of simulations and by a set of measurements performed on a real passive RFID system. Results closely match the analysis predictions, which demonstrate a notable impact of the configuration on the performance.

Dimensioning the Add/Drop Contention Factor of Directionless ROADMs

Reconfigurable Optical Add/Drop Multiplexers (ROADM) are the optical switching equipment of transparent optical networks. Directionless ROADMs permit the network carriers to change the direction of an added and/or dropped lightpath without the need of a technician on-site intervention in the lightpath end nodes. Colorless ROADMs provide the same versatility for changing the lightpath transmission wavelength. Cost-effective directionless ROADM architectures (colorless or colored) can be built if the maximum number of lightpaths that can be added/dropped using the same wavelength is limited. We name this limit as the node add/drop contention factor, and denote it as C. In this paper we investigate the network lightpath blocking performance as a function of this add/drop contention factor of the nodes. The scenarios considered are the static planning of a network (i) with unprotected traffic, (ii) with traffic 1+1 protected for single-link failures, and (iii) with traffic 1+1 protected for single-link or single-node failures. Since for these scenarios, the wavelength of an existing lightpath does not have to be dynamically reconfigured, the work in this paper applies to both colorless and colored nodes. An ILP model and an effective heuristic are presented to solve the so-called Add/Drop Contention Aware RWA (ADCA-RWA) planning of the network. Extensive results are reported. In all the cases, an add/drop factor C = 2 is sufficient to provide the same performance as contentionless nodes (C = ∞). Furthermore, in all the tests a factor C = 1 was also sufficient, or produced a minor lightpath blocking performance degradation (below 0.5% in the unprotected cases, and below 2.5% in the 1 + 1 protected cases).

Add/drop contention-aware RWA with directionless ROADMs: The offline lightpath restoration case

Directionless reconfigurable add/drop multiplexers (ROADMs) permit changing the direction of an added/dropped lightpath without manual intervention. If the ROADM is also colorless, its wavelength can be automatically reconfigured. In broadcast-and-select directionless ROADM architectures, the number of lightpaths that can be added/dropped using the same wavelength is limited by the so-called add/drop contention factor C. This is a source of light-path blocking that reduces the network capacity. A previous work investigated this aspect for the case of unprotected and 1+1 protected lightpaths. In this paper, we address the case in which lightpath restoration is the fault recovery technique in the network. We hypothesize that, using appropriate network planning, the add/drop contention effects can be mitigated. For this, we propose and investigate the add/drop contention-aware routing and wavelength assignment with lightpath restoration offline planning problem, considering colored ROADMs. To solve the problem, an effective heuristic is presented. Extensive results are reported for four reference topologies. In our tests, the reduction in the network capacity caused by add/drop contention is eliminated in practice for factors C = 2, or even C = 1 in some topologies. These results contribute to the dimensioning of directionless ROADMs.

Base technologies for vehicular networking applications: review and case studies

In this paper, we review the state of the art of two key hardware technologies that support vehicular applications: on-board embedded systems and wireless sensor networks (WSN). We focus on pre-competitive or state-of-the-art hardware, and illustrate its use with two case studies: on-line navigation assistance and data collection in a mobile WSN. In the first case (based on a joint collaboration within project FUNCMOV PGIDIT05TIC00501CT, Xunta de Galicia, Spain), we describe our development experience with automotive embedded systems. In the second case, we analyze the feasibility of wake-up schema to gather data from highly dispersed sensor nodes. The goal of the paper is to offer a perspective on the current possibilities of these hardware systems.

On the Optimal Identification of Tag Sets in Time-Constrained RFID Configurations

In Radio Frequency Identification facilities the identification delay of a set of tags is mainly caused by the random access nature of the reading protocol, yielding a random identification time of the set of tags. In this paper, the cumulative distribution function of the identification time is evaluated using a discrete time Markov chain for single-set time-constrained passive RFID systems, namely those ones where a single group of tags is assumed to be in the reading area and only for a bounded time (sojourn time) before leaving. In these scenarios some tags in a set may leave the reader coverage area unidentified. The probability of this event is obtained from the cumulative distribution function of the identification time as a function of the sojourn time. This result provides a suitable criterion to minimize the probability of losing tags. Besides, an identification strategy based on splitting the set of tags in smaller subsets is also considered. Results demonstrate that there are optimal splitting configurations that reduce the overall identification time while keeping the same probability of losing tags.

An Analytical Approach to the Optimal Deployment of Wireless Sensor Networks

In this work we propose and investigate a novel research topic in Wireless Sensor Networks (WSNs): sensor deployment in order to maximize the interest of the gathered information. The target areas are characterized by zones with different interest levels. We model the interest variable as an “importance function” that assigns a quantitative reference to each point. Due to the nature of WSNs, the sensor deployment must guarantee that the information of all nodes can reach certain control nodes or sinks. Otherwise, the events captured by the nodes are lost. This condition is equivalent to ensuring the existence of a communication path from every node to at least one of the sinks. We propose a global optimization model that fulfills all the conditions and we solve it with a simulated annealing algorithm that determines optimal node placement. Moreover, we also characterize the effects of an aerial deployment, that is, the effect of node placement inaccuracy in network behavior and performance. We have found that small placement variations may lead to strong fluctuations in the quality of the connectivity properties of the network topology, and thus to a significant performance degradation in terms of captured interest.

Distributed online RWA considering add/drop contention in the nodes for directionless and colorless ROADMs

This paper presents an online distributed RWA algorithm that considers the add/drop ports occupation in directionless-colorless ROADMs, to reduce the lightpath blocking. Results support our approach to reduce the number of ROADM transponder banks.

Optimal p-Persistent MAC Algorithm for Event-Driven Wireless Sensor Networks

In event-driven wireless sensor networks nodes transmit information only if the monitored physical magnitude levels have triggered an alarm. In these networks, traffic exhibits a high spatial correlation, since it is likely that neighbor nodes detect and try to notify the same events. Thus, the probability of packet collision raises up, as well as notification delay, just as opposite as required. In this work we propose to use a p- persistent mechanism in the access control layer. The aim is reducing collisions and saving energy. We compute the optimal p for a coherent network deployment and describe the experimental implementation of our proposal. Theoretical computations predict a notable improvement, specially in terms of energy, and experiments reveal that our proposal achieves up to 67% of energy saving compared against a perfect (collision-free) mechanisms.