Luca Tavanti

A survey on multi-constrained optimal path computation: Exact and approximate algorithms

The paper presents a survey on the techniques to solve the multi-constrained optimal path (MCOP) problem. Computing the MCOP is a task shared by many research areas, from transportation systems to telecommunication networks. In the latter, the MCOP is often related to the issue of Quality of Service (QoS) routing, which consists in finding a route between a couple of nodes that meets a series of QoS requirements such as bounded delay, packet loss, and other parameters. The MCOP problem has been faced by several authors and a plethora of solving methods is now available. In the present work, we draw the state of the art of exact and approximate MCOP computation algorithms, with particular attention to the networking area. We describe and analyse the most representative methods, and for each of them we derive the worst case computational complexity. In addition, we provide the reader with a uniform notation and with the detailed pseudo-code of various algorithms, so that the paper can indeed serve as a workable starting point for further studies on the MCOP problem.

Experimental assessment of the coexistence of Wi-Fi, ZigBee, and Bluetooth devices

Many wireless technologies used to build local or personal area networks (WLANs, WPANs) operate into the 2.4 GHz ISM band. Due to the mutual interference, the coexistence of such devices working at the same time in the same area can be troublesome. This paper reports the result of an extensive experimental study on the performance of three popular technologies (ZigBee, Wi-Fi, and Bluetooth), under their mutual disturbance. In addition to assessing the effect of the interference between pairs of technologies, we also analyzed the simultaneous interference among the three systems, a scenario that has never been studied before. Our work partly confirms previous findings, but also reveals some unexpected behaviors, imposing to reconsider some assumptions about the supposed interference-free ZigBee and Bluetooth channels.

Experimental evaluation of two open source solutions for wireless mesh routing at layer two

The paper reports the outcome of an experimental evaluation of two open source solutions for realising a Wireless Mesh Network (WMN). Both works at layer two of the ISO/OSI stack and are transparent to the IP layer, i.e. they allow keeping the existing TCP/IP stack unchanged and avoid dealing with the complex IP configuration and management tasks. The first solution is the upcoming IEEE 802.11s standard, as implemented by the open 80211s project, and the other is the B.A.T.M.A.N. routing protocol, in its layer-2 version. We compared them in a small experimental testbed, with main focus on their behaviour in typical mesh situations. We found that both have strengths and shortcomings, but none can be claimed to be completely mature.

Implementation frameworks for IEEE 802.11s systems

The standardisation process of IEEE 802.11s requires some effort to be devoted to assess the effectiveness of the proposed solutions. Specifically, the path selection and forwarding facilities, embodied by the Hybrid Wireless Mesh Protocol (HWMP), are the major and most delicate components. A working prototype can be profitably used to experimentally evaluate such features, thus returning immediate and significant feedback. In this scenario, the paper offers a threefold contribution. First, we review the current 802.11s implementation efforts, such as the Open 80211s and One Laptop Per Child projects, highlighting the pros and cons of each solution. Then, we describe a prototype IEEE 802.11s Mesh Access Point (MAP) we developed starting from common off-the-shelf hardware and software. The main innovation of this prototype is the support of both modes of operation of HWMP. Conversely to the other projects, which implement only the reactive mode, our device also supports the proactive mode, and runs it in the same way as suggested by the draft standard, thus becoming a more complete evaluation platform. As a final contribution, we provide some simulation results to assess the behaviour of the mixed proactive-reactive mode. From this evaluation, we can confirm that employing the proactive algorithm can bring some benefits to the operation of the 802.11s network, even though a careful approach must be taken to avoid some drawbacks.

Providing air-time usage fairness in IEEE 802.11 networks with the deficit transmission time (DTT) scheduler

Wireless systems based on the IEEE 802.11 standard are known to suffer a performance degradation when just a single station in the network experiences bad channel conditions toward the Access Point (AP). This phenomenon, known as the “performance anomaly”, is mainly due to the max-min throughput fairness of the CSMA/CA algorithm of the 802.11 MAC. The simple FIFO scheduling policy usually implemented in the AP also contributes to this problem. In order to overcome the performance anomaly, we propose the Deficit Transmission Time (DTT) scheduler. The aim of DTT is guaranteeing each station a fair medium usage in terms of transmission time. This feature, directly related to the proportional fairness concept, allows to ideally achieve exact isolation among the traffic flows addressed to different stations. DTT achieves this goal taking advantage of measurements of actual frame transmission times. Experiments carried out using a prototype implementation of DTT are compared with analogous tests performed with a classic FIFO queue of a commercial AP and a recently proposed traffic shaping scheme aimed at solving the same 802.11 performance anomaly.

A joint experimental and simulation study of the IEEE 802.11s HWMP protocol and airtime link metric

One of the main features of the upcoming IEEE 802.11s standard is the path selection procedure, which is performed by the Hybrid Wireless Mesh Protocol by means of thetextitairtime link metric (ALM). Through a small experimental testbed, and using the open80211s code, which provides one of the most compliant draft implementations, we analysed the impact that the implementation choices of the developers have on the system performance. The analysis revealed that even small deviations from the specified routines (or different choices in the estimation of the parameters used in the online calculation of the ALM) can lead to noteworthy performance degradations. We thus realized some fixes to improve the performance of the system and verified the effectiveness of such amendments in a realistic simulation environment. We then compared the performance of the modified version to the original one. The outcome of the test highlighted the remarkable advantage achieved by the proposed fixes. Copyright

Does Traffic Consolidation Always Lead to Network Energy Savings

In this paper we study the behavior of a general optimization model for reducing the power consumption of core networks employing energy-aware network equipment. Specifically, we assess how the energy profiles of the devices affect the outcome of the optimization model and hence determine the general power saving policy. The computational analysis performed on several real topologies shows that the widespread traffic consolidation strategy does not always provide the best results. In fact, for devices presenting a cubic (convex in general) energy profile, the highest energy savings are achieved by spreading the traffic on the network.

Notes on Implementing a IEEE 802.11s Mesh Point

Wireless Mesh Networks (WMNs) are gaining wide popularity as a flexible and cost-effective access technology. Many vendors and network operators have already deployed their own proprietary solutions, and, in the meantime, the IEEE has set off Task Group 802.11s (TGs) to develop a common standard for WMNs. So far, however, TGs has not yet produced a final document and is still working on the draft. In this context, we have built a prototype mesh node as much compliant as possible with the (still unofficial) 802.11s draft. A software framework was developed using common off-the-shelf technology and deployed on top of the legacy 802.11 interface card. We believe that the availability of such a prototype will permit to readily test the features and the amendments to the draft as soon as they are proposed, thus returning immediate and significant feedbacks on their effectiveness. This is an important milestone, as simulation trials, though very useful, often do not give answers on the actual feasibility of the tested feature. And, in fact, the prototype already allowed us to experimentally evaluate the basic 802.11s characteristics, pointing out some shortcomings and suggesting possible improvements.

A branch-and-Benders-cut method for nonlinear power design in green wireless local area networks

We consider a problem arising in the design of green wireless local area networks. Decisions on powering-on a set of access points (APs), via the assignment of one power level (PL) to each opened AP, and decisions on the assignment of the user terminals (UTs) to the opened APs, have to be taken simultaneously. The PL assigned to an AP affects, in a nonlinear way, the capacity of the connections between the AP and the UTs that are assigned to it. The objective is to minimize the overall power consumption of the APs, which has two components: location/capacity dimensioning costs of the APs; assignment costs that depend on the total demands assigned to the APs. We develop a branch-and-Benders-cut (BBC) method where, in a non-standard fashion, the master problem includes the variables of the Benders subproblem, but relaxes their integrality. The BBC method has been tested on a large set of instances, and compared to a Benders decomposition algorithm on a subset of instances without assignment costs, where the two approaches can be compared. The computational results show the superiority of BBC in terms of solution quality, scalability and robustness.

Network-Based Micro-Mobility in Wireless Mesh Networks: Is MPLS Convenient?

The paper reports an analysis about the convenience of employing Multi-Protocol Label Switching (MPLS) to provide mobility in Wireless Mesh Networks (WMNs). In such networks, which are expected to serve a considerable number of mobile users, the efficient support of mobility is a non-trivial issue. The starting point of our work is Proxy Mobile IPv6 (PMIPv6), a standardised network-based protocol for localised mobility management that has already shown good performance. We analyse and compare PMIPv6 with a recently proposed extension that aims at employing MPLS tunnels in place of IPv6 tunnels. Our contribution is novel in two aspects. First, the analysis focuses on a recent application scenario (the WMN) and accounts for its typical features. Second, we investigate a non-standard use of MPLS (i.e. for micro-mobility management) and provide the first evaluation work of the MPLS tunnel for PMIPv6 technique.