Stability and performance analysis of a MAC protocol for a time-code air interface in LEO mobile satellite systems

The paper deals with low Earth orbit mobile satellite systems (LEO-MSSs) belonging to third-generation mobile networks and proposes a novel packet-access scheme suitable for a hybrid time-code wideband air interface. This medium-access control (MAC) protocol is a modification of the packet-reservation multiple-access scheme with hindering states (PRMA-HS) and is called code-division PRMA-HS (CD-PRMA-HS). Each mobile terminal that needs to transmit makes random accesses on available time-code resources until it receives a positive acknowledgment from the satellite. Both real-time voice and Web-traffic sources have been considered in our LEO-MSS scenario. An in-depth analysis of the random-access phase has been carried out, thus identifying the conditions that allow a good CD-PRMA-HS behavior. Finally, a performance analysis has been developed in order to evaluate the quality of service parameters for both voice and Web-traffic sources.     

QoS provisioning in GEO satellite with onboard processing using predictor algorithms

Recently, IP satellite networks have attracted considerable interest as a technology to deliver high-bandwidth IP-based multimedia services to nationwide areas. In particular, IP satellite networks seem to be one of the most promising technologies for connecting users in rural areas, where a wired high-speed network (e.g., xDSL) is not foreseen to be used. However, one of the main problems arising here is to guarantee specific quality of service constraints in order to have good performance for each traffic class. Among various QoS approaches used in the Internet, recently the DiffServ technique has become the most promising solution, mainly for its scalability with respect to the IntServ approach. Moreover, in satellite communication systems, DiffServ computational capabilities are placed at the edge points, reducing the implementation complexity of the satellite onboard equipment. This article deals with the problem of QoS provisioning for packet traffic by considering some resource allocation schemes, including bandwidth allocation techniques and priority-driven onboard switching algorithms. As to the first aim, the proposed technique takes advantage of proper statistical traffic modeling to predict future bandwidth requests. This approach takes into consideration DiffServ-based traffic management to guarantee QoS priority among different users. Moreover, the satellite onboard switching problem has been addressed by considering a suitable implementation of the DiffServ policy based on a cellular neural network.     

Performance Evaluation of Polling Protocols for Data Transmission on Wireless Communication Networks

The growing interest in mobile computing and communication devices leads to the necessity of wireless broadband network. Data transmission over such networks requires suitable error control schemes to guarantee high data reliability as well as efficient bandwidth utilization.In this paper we propose an accurate yet simple analytical approach to evaluate the performance of wireless networks using gated and exhaustive polling protocols combined with the Stop and Wait (SW) or Go Back N (GBN) ARQ schemes [Bertsekas and Gallager, 2]. Moreover, simulation results concerning the performance of polling protocols combined with the Selective Repeat (SR) ARQ scheme are also shown for comparison purposes.Protocol performance is estimated under very general assumptions, such as: AWGN or fading channels, arbitrary value of the round trip delay and arbitrary distribution of the traffic load (i.e., both symmetric or asymmetric system have been considered).     

Performance Evaluation of a Virtual Queuing – Channel Allocation Scheme for Microcellular Networks

This paper deals with the performance evaluation of a novel distributed channel allocation scheme that attains a high resource reuse in cellular networks. A suitable interference model has been envisaged. Resources are nominally assigned to cells with Fixed Channel Allocation (FCA), according to a given reuse distance. Whenever a channel demand from a mobile user does not find a free nominal resource in a cell, a channel is temporarily borrowed violating the reuse distance constraint, provided that the co-channel interference level is acceptable. As soon as a nominal channel becomes available in this cell, the borrowed resource is released (virtual queuing). The performance of the proposed channel allocation scheme has been evaluated considering both uniform and non-uniform traffic patterns. A performance analysis approach has been also developed and validated by simulations. Comparisons with the classical FCA scheme and other dynamic and borrowing channel allocation strategies have permitted to highlight the good performance of the proposed technique.     

Proposal and performance evaluation of an efficient multiple-access protocol for LEO satellite packet networks

This paper deals with a modified version of the packet reservation multiple-access (PRMA) protocol suitable for integration of real-time (voice) and best effort (data) traffic in low Earth orbit (LEO) satellite communication systems. The proposed scheme differs from previous alternatives on the method adopted to handle access requests for voice and data terminals, and to transmit data messages. An analytical approach is proposed and validated in the case of voice and classical (i.e., geometric distributed) data traffic in order to derive system performance in terms of mean data message delay and voice packet dropping probability. However, in order to better highlight the advantages of the proposed approach typical interactive and background traffics types have been also considered. Performance comparisons with previous proposed PRMA protocols for voice and data transmission in LEO satellite communication systems are also shown in order to highlight the better behavior of the proposed scheme. Finally, a brief discussion concerning the extension of the proposed S-PRMA protocol to the case of different satellite communication systems is also provided.     

Communications and networking over satellites: SatNEx experimental activities and testbeds

One of the aims of the SatNEx (Satellite Network of Excellence, http://www.satnex.de ) European project is to provide the scientific community that operates on satellite communications with measurement campaign results and evaluation tools, in order to support research activities that investigate problems in satellite communications at different levels of the OSI stack. In fact, the research activities developed in SatNEx address the most relevant aspects of satellite communications, such as channel modulation and coding schemes, access strategies, advanced networking, end‐to‐end quality of service, transport performance, delay tolerant networks, and new applications. Even if SatNEx is mainly devoted to satellite communications, integration of satellite and terrestrial wireless segments has also been taken into account. This paper presents the results obtained in the specific research activity devoted to trials and testbeds developed during the SatNEx project. The high number of authors of this paper is an indication of the strong collaborative work sustained, the numerous measurement campaigns, and the complexity in developing integrated testbeds. Copyright © 2008 John Wiley & Sons, Ltd.     

Ostensible oncocytoma of accessory lacrimal glands

The selective 5-HT1B agonist CP-94,253 (3- (1,2,5,6-tetrahydro-4-pyridyl)-5-propoxypyrrolo[3, 2-b] pyridine) (5-40 mumol/kg) reduced the intake of both pellets and a 10% solution of sucrose (ID50 = 12.5 and 22.8 mumol/kg, respectively) in mildly deprived rats. Time-sampled observations revealed that CP-94,253 terminated feeding earlier, without disrupting the continuity of feeding. CP-94,253 increased standing but did not promote resting during satiation. Microstructural analysis of licking indicated that CP-94,253 decreased the frequency, but not the size, of bursts and clusters of licks without altering oral motor efficiency. The peripherally acting 5-HT1B agonist, CP-93,129 (3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one) had no effect on food intake. These results imply that CP-94,253 probes a role for central 5-HT1B receptors in the regulation of meal size and duration, but that recruitment of other 5-HT receptor subtypes may be needed for the full expression of satiety.     

Analysis of secure handover for IEEE 802.1x-based wireless ad hoc networks

The handover procedure in secure communication wireless networks is an extremely time-consuming phase, and it represents a critical issue in relation to the time constraints required by certain real-time traffic applications. In particular, in the case of the IEEE 802.1X model, most of the time required for a handover is used for packet exchanges that are required for authentication protocols, such as Extensible Authentication Protocol Transport Layer Security (EAP-TLS), that require an eight-way handshake. Designing secure re-authentication protocols to reduce the number of packets required during a handover is an open issue that is gaining interest with the advent of a pervasive model of networking that requires realtime traffic and mobility. This article presents the 802.1X model and evaluates its application to ad hoc networks based on IEEE 802.11 i or IEEE 802.1 be standards, focusing on the problems that must be evaluated when designing handover procedures, and suggesting guidelines for securing handover procedures. It also presents a novel protocol to perform secure handovers that is respectful of the previous analysis and that has been implemented in a mesh environment.