No abstract

There is an increasing demand for middleware for nomadic computing applications. Owing to the inherent characteristics of such environments, these platforms have to address two fundamental issues: (i) device disconnections and the limitations of wireless networks may force users to experience short periods of service unavailability; and (ii) the complexity to design and develop next-generation mobile computing applications. This paper proposes the Esperanto Broker (EB), a communication platform that addresses mobility issues via an integrated approach, i.e. at data-link, network, and middleware levels. Decoupling interactions are achieved via a tuple-space underlying infrastructure. To support developers with advanced services, the EB enhances the distributed objects computing model providing the abstraction for the communication paradigms standardized by the W3C. Esperanto applications can be modeled as sets of objects that are distributed over mobile devices, which communicate via remote method invocations (RMIs). RMIs natively implement pull and push models, in both one-to-one and one-to-many multiplicity. The paper focuses on the EB design issues, essential aspects of the implementation, and performance evaluations of the implemented prototype. INTRODUCTIONRecent advantages achieved in wireless and in low-power consumer electronics have lead to new computing paradigms, generally described as mobile computing. Nomadic computing (NC) is a form of mobile computing where the communication may take place during users' movements between different service locations such as their office, home, hotel, airport, car, and so on [1][2][3]. Nowadays, the interest in middleware for NC environments is still growing as such infrastructures are becoming increasingly widespread: suffice to say, telecom operators are competing to quickly interconnect different wireless networks (such as GSM, UMTS, and Wi-Fi) to make the Wireless Internet the cuttingedge market where new services can be provided at a huge profit.Focusing on distributed communication, NC middleware must deal with new challenging issues that are mainly the result of the inherent characteristics of wireless networks and mobile devices [4,5]. Weak connectivity or battery power constraints may lead users to experience short periods of service unavailability. Moreover, users may initiate a communication just before traveling from office to home. Even if the network connection is available both outdoor and indoor (e.g. via GSM and Wi-Fi) switching from one technology to another may lead to a disconnection. Therefore, the temporary unavailability of counterparts is the rule rather than the exception. If traditional communication mechanisms required both counterparts to be available during the interaction, mobile computing requires mechanisms to let users communicate in a loosely coupled fashion.From the software development perspective, the development of next-generation mobile computing applications needs to rely on high-level abstractions and advanced services. Mobile ...

End-to-end delay estimation is a crucial issue in the design of network monitoring systems for wireless best-effort infrastructures. This work demonstrates that estimations based on one-way delay are the most suitable for evaluating the delay on wireless networks such as Wi-Fi LANs andBluetooth piconets. To prevent delay measurement from being affected by clock synchronization issues, raw measures need to be corrected by estimating clock synchronism attributes such as clock skew and offset. However, due to network noise and clock adjustments, this estimation process may affect the reliability of network monitoring systems. In this paper we propose a trustworthy network performance monitor designed to support adaptive and/or soft real-time applications in wireless environments. The monitor corrects one-way delay measurements through an on-line algorithm for evaluating clock synchronism attributes. Simulation experiments show the effectiveness of our approach. RationaleWe are witnessing an increasing interest on the so-called next generation of distributed mobile applications, such as tele-conference, tele-presence, and tele-control. These applications are characterized by soft real-time requirements, and they are typically deployed over hybrid infrastructures, encompassing wired as well as wireless technologies (e.g. * This work has been partially supported by the Italian Ministry for Education, University and Research (MIUR), within the framework of the FIRB Project "Middleware for advanced services over large-scale, wiredwireless distributed systems (WEB-MINDS), and by the Regione Campania, within the framework of the "Centro di Competenza Regionale ICT". Wi-Fi [3], Bluetooth [1], UMTS [2]). Due to the best effort policy of current wireless technologies, it is crucial to realize reliable monitoring systems in order to provide applications with feedback for adaptivity to network changes [8,13]. To this aim, network monitoring systems must be accurate, since errors in the network performance estimation could lead the application to erroneous behaviors.A plenty of research studies has been conducted to estimate performance of Internet. The ITU-T [7] recommended network performance parameters (end-to-end delay, delay variation, and information loss) to be used for QoS estimation in IP-based networks. Several algorithms have been proposed for addressing issues in end-to-end delay estimation, the crucial parameter from which the other network performance indicators can be derived. Two are the most common metrics: i) the Round Trip Time (RTT); and ii) the One-way Transit Time (OTT). Asymmetric paths affect delay estimations based on RTT, whereas clocks synchronization affects delay estimations based on OTT. Although RTT is a good estimation of end-to-end delay in traditional LANs (such as IEEE 802.3x), we experience that it does not represent the favored choice for performance estimation of wireless environments, such as WLANs and Bluetooth piconets. As for the OTT, approaches for the correction of raw measures include...

Network infrastructures composed of wireless access points (e.g. IEEE 802.11 APs) connected via a LAN have been enabling a form of mobile computing known as Nomadic Computing (NC). In order to manage migrations of a mobile terminal among different wireless networks, such infrastructures are often decomposed in several domains: a domain can be a building floor, a building room, or a certain zone of a campus. Since the traditional middleware are unsuitable for mobile computing [1], during the past years a great deal of research has been conducted. Research efforts have been progressed along the following directions: i) providing mechanisms, such as context awareness, reconfiguration, spontaneous discovery; ii) dealing with QoS aspects such as security. While we recognize these studies as fundamental milestones for the pursuit of new middleware for mobile computing, most of them do not effectively explore how the distributed interaction (such as RPC or RMI) has to take place

No abstract