Rodrigo M. Santos

Modeling IoT-Based Solutions Using Human-Centric Wireless Sensor Networks

The Internet of Things (IoT) has inspired solutions that are already available for addressing problems in various application scenarios, such as healthcare, security, emergency support and tourism. However, there is no clear approach to modeling these systems and envisioning their capabilities at the design time. Therefore, the process of designing these systems is ad hoc and its real impact is evaluated once the solution is already implemented, which is risky and expensive. This paper proposes a modeling approach that uses human-centric wireless sensor networks to specify and evaluate models of IoT-based systems at the time of design, avoiding the need to spend time and effort on early implementations of immature designs. It allows designers to focus on the system design, leaving the implementation decisions for a next phase. The article illustrates the usefulness of this proposal through a running example, showing the design of an IoT-based solution to support the first responses during medium-sized or large urban incidents. The case study used in the proposal evaluation is based on a real train crash. The proposed modeling approach can be used to design IoT-based systems for other application scenarios, e.g., to support security operatives or monitor chronic patients in their homes.

Improving the schedulability of soft real-time open dynamic systems: The inheritor is actually a debtor

This paper presents the Clearing Fund Protocol, a three layered protocol designed to schedule soft real-time sets of precedence related tasks with shared resources. These sets are processed in an open dynamic environment. Open because new applications may enter the system at any time and dynamic because the schedulability is tested on-line as tasks request admission. Top-down, the three layers are the Clearing Fund, the Bandwidth Inheritance and two versions of the Constant Bandwidth Server algorithms. Bandwidth Inheritance applies a priority inheritance mechanism to the Constant Bandwidth Server. However, a serious drawback is its unfairness. In fact, a task executing in a server can potentially steal the bandwidth of another server without paying any penalty. The main idea of the Clearing Fund Algorithm is to keep track of processor-time debts contracted by lower priority tasks that block higher priority ones and are executed in the higher priority servers by having inherited the higher priority. The proposed algorithm reduces the undesirable effects of those priority inversions because the blocked task can finish its execution in its own server or in the server of the blocking task, whichever has the nearest deadline. If demanded, debts are paid back in that way. Inheritors are therefore debtors. Moreover, at certain instants in time, all existing debts may be waived and the servers are reset making a clear restart of the system. The Clearing Fund Protocol showed definite better performances when evaluated by simulations against Bandwidth Inheritance, the protocol it tries to improve.

A least upper bound on the fault tolerance of real-time systems

This paper presents a method to deal with the reexecution of tasks in a hard real-time system subject to temporary faults. The set of tasks follows the Liu and Layland model: they are periodic, independent and preemptible. Time is considered to be slotted. The system is said to be k-schedulable if it is schedulable in spite of the fact that in the interval between its release and its deadline, every task admits that k slots are devoted to uses other than its first execution. In this case, the k slots are used to reexecute tasks subject to temporary faults. Since the value of k can be easily determined, a least upper bound on all the possible combinations of faults that the system can tolerate while meeting the hard time-constraints, follows immediately. The method is bandwidth preserving and the expression of the bound is a diophantic inequality relating k, the execution time and the period of each task. The method is compared to methods proposed by other authors to solve the same problem and it is evaluated through extensive simulations performed on random generated sets of tasks.

A real-time analysis approach in opportunistic networks

There are several mobile work scenarios requiring real-time messages. Examples of these scenarios are disaster relief or mobile work in isolated areas. Although opportunistic networks are not intended for real-time messages, under certain conditions the communication could be feasible. This paper presents the schedulability analysis of an opportunistic network for real-time traffic. It includes a stochastic and deterministic analysis of the network performance. Moreover, a scheduling policy is also proposed.

Scheduling heterogeneous multimedia servers: different QoS for hard, soft and non real-time clients

This paper presents a probabilistic model of a client/server multimedia system based on the theory of Markov processes. The server provides different qualities of service (hard real-time, soft real-time and best-effort) and is operated using a new scheduling concept based on a hybrid deterministic/probabilistic analysis. The model also provides answers to some fundamental engineering design questions related to disk price/performance tradeoffs.

Disseminating shared information in disaster relief efforts: A communication computable model

VHF radio systems commonly used to support search and rescue activities after disasters limit the flow of information among response teams deployed in the field. It generates islands of information that jeopardizes the coordination and effectiveness of the response activities. This article proposes a communication model that uses opportunistic networks and real-time messages delivery to help address such limitation. Since the communication model is computable, it is possible to diagnose the flow of information expected for a particular work scenario. The diagnose results allow identifying elements that could help improve the information flow in such scenario. This proposal allows first responders to address the stated problem during the phases of preparedness, response and recovery from a disaster.

Rate monotonic scheduling of real-time control systems with the minimum number of priority levels

When applying the Rate Monotonic discipline to schedule a set of periodic preemptible real-time tasks, the scheduler may be able to distinguish only a limited number of priority levels. This is common in control applications using low cost embedded controllers. If the number of tasks to be scheduled is larger than the number of distinguishable levels, the set of tasks must be partitioned in a set of priority classes. RM can be used only to arbitrate conflicts between tasks of different classes. In this paper a method to determine the minimum number of priority levels necessary to schedule the set of tasks is formally proved and its complexity analysed. Finally, a systematic method to obtain all the possible partitions with the minimum number of classes, resembling the Quines method to minimize Boolean functions, is also given.

On-line schedulability tests for adaptive reservations in fixed priority scheduling

Adaptive reservation is a real-time scheduling technique in which each application is associated a fraction of the computational resource (a reservation) that can be dynamically adapted to the varying requirements of the application by using appropriate feedback control algorithms. An adaptive reservation is typically implemented by using an aperiodic server (e.g. sporadic server) algorithm with fixed period and variable budget. When the feedback law demands an increase of the reservation budget, the system must run a schedulability test to check if there is enough spare bandwidth to accommodate such increase. The schedulability test must be very fast, as it may be performed at each budget update, i.e. potentially at each instance of a task; yet, it must be as efficient as possible, to maximize resource usage.In this paper, we tackle the problem of performing an efficient on-line schedulability test for adaptive resource reservations in fixed priority schedulers. In the literature, a number of algorithms have been proposed for on-line admission control in fixed priority systems. We describe four of these tests, with increasing complexity and performance. In addition, we propose a novel on-line test, called Spare-Pot algorithm, which has been specifically designed for the problem at hand, and which shows a good cost/performance ratio compared to the other tests.

Reducing energy consumption in human-centric wireless sensor networks

Energy consumption is a main research issue in wireless sensor networks; and particularly in those where nodes collaborate to reach a goal. This article explores the energy consumption in mobile devices participating in a human-based wireless sensor network. Specifically, the paper proposes the use of a message predictor to help detect and reduce the number of unnecessary control packets delivered by the nodes as a way to keep updated the network topology. In order to evaluate this proposal, the Optimized Link State Routing protocol was modified to add a message predictor between the routing and the network layers. Eleven simulations were performed using a particular setting. The preliminary results indicate the use of the message predictor can help reduce considerably the nodes energy consumption without affecting the routing capability of the protocol. Although these results are still preliminary, they are highly encouraging.

Modeling interactions in human-centric wireless sensor networks

The recent growth of social networking and sensing applications has enabled people to perform crowd computing. In turn, this activity has opened several opportunities to address peoples needs in various application areas, such as tourism, security, entertainment and emergency response. However, the design of these applications has also brought several challenges to software designers. Since users of these systems interact among them in heterogeneous physical scenarios, their interactions should be formally considered in the design of these applications in order to determine whether or not the services embedded in the system are suitable to support those interactions. The lack of guidelines to address this modeling issue not only jeopardizes the suitability of these applications, but also implies the services provided by the system can be evaluated only after implementation, which is clearly risky and expensive. This article proposes an initial modeling language that allows software designers to address this challenge through the representation of the interaction among users of a system. The interaction model represented using the proposed language allows designers to evaluate, at the design time, the information flow and the availability of interaction supporting services in the system. The usability and usefulness of the proposal are shown using a running example.