Luciano Bononi

Runtime optimization of IEEE 802.11 wireless LANs performance

IEEE 802.11 is the standard for wireless local area networks (WLANs) promoted by the Institute of Electrical and Electronics Engineers. Wireless technologies in the LAN environment are becoming increasingly important and the IEEE 802.11 is the most mature technology to date. Previous works have pointed out that the standard protocol can be very inefficient and that an appropriate tuning of its congestion control mechanism (i.e., the backoff algorithm) can drive the IEEE 802.11 protocol close to its optimal behavior. To perform this tuning, a station must have exact knowledge of the network contention level; unfortunately, in a real case, a station cannot have exact knowledge of the network contention level (i.e., number of active stations and length of the message transmitted on the channel), but it, at most, can estimate it. We present and evaluate a distributed mechanism for contention control in IEEE 802.11 wireless LANs. Our mechanism, named asymptotically optimal backoff (AOB), dynamically adapts the backoff window size to the current network contention level and guarantees that an IEEE 802.11 WLAN asymptotically achieves its optimal channel utilization. The AOB mechanism measures the network contention level by using two simple estimates: the slot utilization and the average size of transmitted frames. These estimates are simple and can be obtained by exploiting information that is already available in the standard protocol. AOB can be used to extend the standard 802.11 access mechanism without requiring any additional hardware. The performance of the IEEE 802.11 protocol, with and without the AOB mechanism, is investigated through simulation. Simulation results indicate that our mechanism is very effective, robust, and has traffic differentiation potentialities.

A Cross Layered MAC and Clustering Scheme for Efficient Broadcast in VANETs

In this paper, we illustrate the design of a cross-layered MAC and clustering solution for supporting the fast propagation of broadcast messages in a vehicular ad hoc network (VANET). A distributed dynamic clustering algorithm is proposed to create a dynamic virtual backbone in the vehicular network. The vehicle-members of the backbone are responsible for implementing an efficient messages propagation. The backbone creation and maintenance are proactively performed aiming to balance the stability of backbone connections as well as the cost/efficiency trade-off and the hops-reduction when forwarding broadcast messages. A fast multi-hop MAC forwarding mechanism is defined to exploit the role of backbone vehicles, under a cross-layered approach. Simulation results show the effectiveness of the mutual support of proactive clustering and MAC protocols for efficient dissemination of broadcast messages in VANETs.

Simulation and analysis of network on chip architectures: ring, spidergon and 2D mesh

NoC architectures can be adopted to support general communications among multiple IPs over multi-processor systems on chip (SoCs). In this work we illustrate the modeling and simulation-based analysis of some recent architectures for network on chip (NoC). Specifically, the ring, spidergon and 2D mesh NoC topologies have been compared, both under uniform load and under more realistic load assumptions in the SoC domain. The main performance indexes considered are NoC throughput and latency, as a function of variable data-injection rates, source and destination distributions, and variable number of nodes. Results show that the spidergon topology is a good trade-off between performance, scalability of the most efficient architectures inherited from the parallel computing systems design, constraints about simple management, and small energy and area requirements for SoCs

Smart Radios for Smart Vehicles: Cognitive Vehicular Networks

This article discusses the current state-of-the-art research on CRV networks. While it is envisaged that this technology will help to realize high-bandwidth multimedia applications, the research on CRV networks is still at a preliminary stage. The spectrum management functions proposed for general-purpose CR networks will need to be revisited by taking into account the characteristics of the vehicular environment, such as the role of the mobility, and the cooperation possibilities. The lack of realistic test beds and of simulation tools is a serious limitation, and effort needs to be invested in building such evaluation platforms that can provide realistic insights on the performance of CRV networks before the potential for this technology can be fully realized.

End-to-end protocols for Cognitive Radio Ad Hoc Networks: An evaluation study

Cognitive radio ad hoc networks (CRAHNs) constitute a viable solution to solve the current problems of inefficiency in the spectrum allocation, and to deploy highly reconfigurable and self-organizing wireless networks. Cognitive radio (CR) devices are envisaged to utilize the spectrum in an opportunistic way by dynamically accessing different licensed portions of the spectrum. To this aim, most of the recent research has mainly focused on devising spectrum sensing and sharing algorithms at the link layer, so that CR devices can operate without interfering with the transmissions of other licensed users, also called primary users (PUs). However, it is also important to consider the impact of such schemes on the higher layers of the protocol stack, in order to provide efficient end-to-end data delivery. At present, routing and transport layer protocols constitute an important yet not deeply investigated area of research over CRAHNs. This paper provides three main contributions on the modeling and performance evaluation of end-to-end protocols (e.g. routing and transport layer protocols) for CRAHNs. First, we describe NS2-CRAHN, an extension of the NS-2 simulator, which is designed to support realistic simulation of CRAHNs. NS2-CRAHN contains an accurate yet flexible modeling of the activities of PUs and of the cognitive cycle implemented by each CR user. Second, we analyze the impact of CRAHNs characteristics over the route formation process, by considering different routing metrics and route discovery algorithms. Finally, we study TCP performance over CRAHNs, by considering the impact of three factors on different TCP variants: (i) spectrum sensing cycle, (ii) interference from PUs and (iii) channel heterogeneity. Simulation results highlight the differences of CRAHNs with traditional ad hoc networks and provide useful directions for the design of novel end-to-end protocols for CRAHNs.

Analyzing the potential of cooperative Cognitive Radio technology on inter-vehicle communication

Recent studies demonstrate that Cognitive Radio (CR) technology can increase the spectrum efficiency of wireless systems, provided that the activity of primary users (PUs) must be carefully protected. For this reason, several sensing schemes leverage the cooperation among nodes to increase the accuracy of PU detection. In this paper, we propose to employ the CR principles in the vehicular environment in order to increase the spectrum opportunities for inter-vehicle communication (IVC). We propose a cooperative sensing and spectrum allocation scheme through which vehicles can share information about spectrum availability of TV channels on their path, and dynamically decide the channels to use on each road segment. Moreover, we investigate the role of vehicular mobility in the cooperation process, which might allow a vehicle to know in advance the spectrum availability on future locations along its path. Simulation results confirm the ability of our scheme in providing robust PU detection under fading conditions, and analyze the impact of some vehicular networks characteristics into the operations of CR systems.

Cooperative spectrum management in cognitive Vehicular Ad Hoc Networks

In recent years, Cognitive Radio (CR) technology has received significant attention from the research community as it enables on-demand spectrum utilization, based on the requests of the end-users. An interesting application area of CR technology is Vehicular Ad Hoc Networks (VANETs). In such networks, several innovative services and applications based on inter-vehicular communication have strict requirements in terms of bandwidth and delay, which might not be guaranteed by a fixed spectrum allocation paradigm. In this paper, we propose two key contributions pertaining to CR-VANETs: (i) an experimental study of the spectrum availability and sensing accuracy in a moving vehicle and (ii) a collaborative spectrum management framework (called Cog-V2V), which allows the vehicles to share spectrum information, and to detect spectrum opportunities in the licensed band. As part of this framework, we design a collaborative sensing and decision algorithm, enabling the vehicles to share spectrum information and to know in advance the spectrum availability at future locations along their motion paths. The simulation results, produced through a novel integrated simulation platform for CR networks, reveal significant improvements of Cog-V2V in sensing accuracy and pair-wise communication performance compared to classical fixed spectrum approaches.

A distributed mechanism for power saving in IEEE 802.11 wireless LANs

The finite battery power of mobile computers represents one of the greatest limitations to the utility of portable computers. Furthermore, portable computers often need to perform power consuming activities, such as transmitting and receiving data by means of a random-access, wireless channel. The amount of power consumed to transfer the data on the wireless channel is negatively affected by the channel congestion level, and significantly depends on the MAC protocol adopted. This paper illustrates the design and the performance evaluation of a new mechanism that, by controlling the accesses to the shared transmission channel of a wireless LAN, leads each station to an optimal Power Consumption level. Specifically, we considered the Standard IEEE 802.11 Distributed Coordination Function (DCF) access scheme for WLANs. For this protocol we analytically derived the optimal average Power Consumption levels required for a frame transmission. By exploiting these analytical results, we define a Power Save, Distributed Contention Control (PS-DCC) mechanism that can be adopted to enhance the performance of the Standard IEEE 802.11 DCF protocol from a power saving standpoint. The performance of an IEEE 802.11 network enhanced with the PS-DCC mechanism has been investigated by simulation. Results show that the enhanced protocol closely approximates the optimal power consumption level, and provides a channel utilization close to the theoretical upper bound for the IEEE 802.11 protocol capacity. In addition, even in low load situations, the enhanced protocol does not introduce additional overheads with respect to the standard protocol.

Design and Performance Evaluation of a Distributed Contention Control (DCC) Mechanism for IEEE 802.11 Wireless Local Area Networks

This paper focuses on the design and performance evaluation of a new mechanism, named Distributed Contention Control (DCC), for the adaptive contention reduction in LAN networks that utilize random access MAC protocols. The proposed mechanism could be executed on the top of a preexistent access protocol, with no additional overhead introduced. Specifically, we consider the IEEE 802.11 wireless LAN (WLAN). The DCC mechanism requires a simple and rough estimate of the contention level, and this can be achieved by estimating any parameter, directly connected with the amount of contention on the shared channel. The main characteristics of the proposed mechanism are represented by its simplicity, integration with the Standard, complete distribution, absence of overheads, and prompt reaction to changes in the network congestion. The protocol automatically adapts to the network congestion by monitoring the channel contention level through the estimation of the contention parameter. In this paper we show that the information needed for the contention estimation is already available to a 802.11 station, with no additional costs. Simulation experiments to evaluate the performance of an IEEE 802.11 WLAN, with and without the DCC mechanism, have been carried out. Results confirmed the effectiveness of the DCC mechanism in improving the performance, stability, and congestion reaction of the IEEE 802.11 access scheme. The DCC mechanism also provides a simple way to implement a distributed priority mechanism.

Modeling and performance evaluation of transmission control protocol over cognitive radio ad hoc networks

Cognitive Radio (CR) technology constitutes a new paradigm to provide additional spectrum utilization opportunities in wireless ad hoc networks. Recent research in this field has mainly focused on devising spectrum sensing and sharing algorithms, to allow an opportunistic usage of licensed portions of the spectrum by Cognitive Radio Users (CRUs). However, it is also important to consider the impact of such schemes on the higher layers of the protocol stack, in order to provide efficient end-to-end data delivery. Since TCP is the de facto transport protocol standard on Internet, it is crucial to estimate its ability in providing stable end-to-end communication over Cognitive Radio Ad Hoc Networks (CRAHNs). The contributions of this paper are twofold. First, we propose an extension of the NS-2 simulator to support realistic simulation of CRAHNs. Our extension allows to model the activities of Primary Users (PUs), and the opportunistic spectrum management by CRUs in the licensed band. Second, we provide an accurate simulation analysis of the TCP performance over CRAHNs, by considering the impact of three factors: (i) spectrum sensing cycle, (ii) interference from PUs and (iii) channel heterogeneity. The simulation results show that the sensing interval and the PU activity play a critical role in deciding the optimal end-to-end performance, and reveals the inadequacy of classical TCP to adapt to variable spectrum conditions.