Collisions are a main cause of throughput degradation in wireless local area networks. The current contention mechanism used in the IEEE 802.11 networks is called carrier sense multiple access with collision avoidance (CSMA/CA). It uses a binary exponential backoff technique to randomize each contender attempt of transmitting, effectively reducing the collision probability. Nevertheless, CSMA/CA relies on a random backoff that while effective and fully decentralized, in principle is unable to completely eliminate collisions, therefore degrading the network throughput as more contenders attempt to share the channel. To overcome these situations, carrier sense multiple access with enhanced collision avoidance (CSMA/ECA) is able to create a collision-free schedule in a fully decentralized manner using a deterministic backoff after successful transmissions. Hysteresis and fair share are two extensions of CSMA/ECA to support a large number of contenders in a collision-free schedule. CSMA/ECA offers better throughput than CSMA/CA and short-term throughput fairness. This paper describes CSMA/ECA and its extensions. In addition, it provides the first evaluation results of CSMA/ECA with non-saturated traffic, channel errors, and its performance when coexisting with CSMA/CA nodes. Furthermore, it describes the effects of imperfect clocks over CSMA/ECA and presents a mechanism to leverage the impact of channel errors and the addition/withdrawal of nodes over collision-free schedules. Finally, the experimental results on throughput and lost frames from a CSMA/ECA implementation using commercial hardware and open-source firmware are presented.

Collisions are a main cause of throughput degradation in Wireless LANs. The current contention mechanism for these networks is based on a random backoff strategy to avoid collisions with other transmitters. Even though it can reduce the probability of collisions, the random backoff prevents users from achieving Collision-Free schedules, where the channel would be used more efficiently. Modifying the contention mechanism by waiting for a deterministic timer after successful transmissions, users would be able to construct a Collision-Free schedule among successful contenders. This work shows the experimental results of a Collision-Free MAC (CF-MAC) protocol for WLANs using commercial hardware and open firmware for wireless network cards which is able to support many users. Testbed results show that the proposed CF-MAC protocol leads to a better distribution of the available bandwidth among users, higher throughput and lower losses than the unmodified WLANs clients using a legacy firmware.

The two main drawbacks of receiver-initiated Medium Access Control (MAC) protocols for Wireless Sensor Networks (WSNs) are that i) they require all nodes to send a beacon each time they wake up, and that ii) broadcast traffic is not efficiently supported. In this work, we propose addressing these limitations by extending receiver-initiated MAC protocols with scheduling, i.e, coordinating sensor nodes to wake up at nearly the same instant. Following this approach, only one sensor node in the neighborhood sends a beacon per wake-up period and, as all nodes are awake at the same time, broadcast transmissions are naturally supported. A distributed learning technique is used to establish the order of beacon transmissions. We present the protocol description and the time to convergence when a fully connected network is considered.

This paper proposes an agile model-based systems engineering (SE) methodology (LITHE methodology) to engineer the contemporary large, complex, and interdisciplinary systems of systems. The methodology relates the processes, the methods, and the tools in order to support an effective model-based development context. The LITHE uses a universal and intuitive SE base process, reducing the complexity and intricacy of the base methods, emphasizing the agile principles such as continuous communication, feedback and stakeholders' involvement, short iterations, and rapid response, and rousing the utilization of a coherent system model developed through the benchmark systems graphical modeling languages. Aiming to support the development of successful systems, which satisfy the stakeholders' expectations, the methodology is particularly concerned with human systems integration, so the related fundamental aspects are considered throughout the engineering process. The LITHE methodology also includes a supporting graphical tool that aims to be an agile instrument to be used by systems engineers in a model-based development environment. To illustrate the effectiveness of the proposed methodology and to provide some validation, an empirical case study related with the development of a real large and complex system (the Guiding Urban Intelligent Traffic and Environment system) is also described.

To engineer the modern large, complex, interdisciplinary systems-of-systems (SoS), the collaborative world teams must “speak” the same language and must work on the same “matter.” The “matter” is the system model and the communication mechanisms must be supported by standard, flexible, and friendly modeling languages. The evolving model-based systems engineering (MBSE) approach is leading the way and is expected to become a standard practice in the field of systems engineering (SE) in the next decade. As an emerging paradigm for the systems of the 21st century, it seems useful to overview its current state of the art concerning the developing standards, the embryonic formalisms, the available modeling languages, the methodologies, and the major applications.

The local authorities have a fundamental role in improving the traffic & environment system of their urban jurisdictions. Locally developed tailor-made solutions, based on active public participation processes, are essential to make the city more attractive, cleaner and healthier thus, a better place to live, work, and play. This challenge, of sustainable urban transportation, demands a comprehensive holistic approach able to cope with the complexity, multidisciplinarity, and socio-technical dimension of the system-of-interest. The environmental aspect of sustainability is critical being at the agenda of the modern world. Modelling the traffic and the related environmental impacts at the micro level is decisive to describe adequately the urban dynamics, to provide personalized real-time information, to test Intelligent Transportation Systems strategies and evaluate their impacts, and to support accurate decision-making processes. This work highlights the relevance of this micro modelling approach, particularly for traffic & air quality, providing a guide through the existing modelling approaches and suggesting the appropriated models and tools to undertake the green mission.

This paper presents CSMA/ECA, which combines the efficiency of reservation-based protocols and the simplicity of random access mechanisms. The maximum efficiency of CSMA/CA with optimal parameter adjustment is easily exceeded by CSMA/ECA, even when fixed parameters are used by the latter. CSMA/ECA stations fairly coexist with legacy CSMA/CA and increase the portion of time that is devoted to successful transmissions while decreasing the number of collisions and empty slots. The proposed mechanism initially behaves as a CSMA/CA network, but it progressively converges to a collision-free deterministic operation. The convergence process can be modelled as a Markov Chain to assess the duration of the transitory phase.

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.

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.