Thesis (M.S. in Information Systems)--Naval Postgraduate School, September 1990. / Thesis Advisor(s): Tung Xuan Bui. Second Reader: Smith, Henry H. "September 1990." Description based on title screen as viewed on December 16, 2009. DTIC Identifier(s): Local area networks, security, control systems, thesis. Author(s) subject terms: Computer security, local area network (LAN), security and control. Includes bibliographical references (p. 87-90). Also available in print.

In general, rural areas in Africa are seen as unprofitable by operators and hence these areas do not benefit from typical wired Internet access. Due to the cost involved, only a small number of people in Africa benefit from the Internet. On the other hand, the Global System for Mobile communications (GSM) has thoroughly penetrated Africa and in many cases unutilized capacity exists in rural areas. Since the cost of GSM services is still extremely high when compared to the average income of African communities, many organizations are now considering alternative business models for the provision of Internet access to disadvantaged populations. To try to bridge the digital divide, the aim of this contribution is to show that basic but affordable Internet Protocol (IP) connectivity can be provided to rural communities by using spare capacity on GSM networks to carry Wireless Local Area Network (WLAN) traffic. Since the main problem with WLAN in Africa is not the last mile, but rather finding a way to connect the wireless access point to an existing backbone network, a solution to integrate WLAN with GSM is proposed.

Project report (M. Eng.)--Virginia Polytechnic Institute and State University, 1990. / Vita. Abstract. Includes bibliographical references (leaf 97). Also available via the Internet.

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1991. / Vita. Abstract. Includes bibliographical references (leaves 87-88). Also available via the Internet.

Thesis (M.S. in Computer Science)--Naval Postgraduate School, March 2005. / Thesis Advisor(s): Thuy D. Nguyen, Cynthia E. Irvine. Includes bibliographical references (p. 53). Also available online.

Thesis (M.S.C.I.T.)--Regis University, Denver, Colo., 2005. / Title from PDF title page (viewed on Dec. 28, 2005). Includes bibliographical references.

The purpose of this research is twofold - the first objective being to develop Markovian models that can be used to analyze the performance of the various medium access control protocols of slotted ring type local area networks.

This thesis presents the development and implementation of a token bus protocol for a Power Line Local Area Network (PLLAN) which utilizes intra-building power distribution circuit as the physical transmission medium. This medium provides a low cost means for data communications with a high degree of portability. Due to the characteristics of the power line and the prototype modem, the network would be easily saturated with data and would have a high collision probabilities. The IEEE 802.4 token bus standard is modified to fit the PLLAN and to bring its performance up. A comparative performance of the original protocol and the modified version shows that the latter provides an improvement in network throughput of up to 15 percent and a reduction in the network join-ring delay of up to 20 percent for a wide workload range. The performance figures of the modified version in a power line network of three SUN 3/50 workstations¹ transmitting at 9.6 kilo-bit per second is also presented and analyzed. ¹Sun workstation is a trademark of Sun Microsystems. / Science, Faculty of / Computer Science, Department of / Graduate

Dans l'évaluation d'Internet au cours des années, un grand nombre d'applications apparaissent, avec différentes exigences de service en termes de bande passante, délai et ainsi de suite. Pourtant, le trafic Internet présente encore une propriété de haute variabilité. Plusieurs études révèlent que les flux court sont en général liés à des applications interactives-pour ceux-ci, on s'attend à obtenir de bonne performance que l'utilisateur perçoit, le plus souvent en termes de temps de réponse court. Cependant, le schéma classique FIFO/drop-tail déployé des routeurs/commutateurs d'aujourd'hui est bien connu de parti pris contre les flux courts. Pour résoudre ce problème sur un réseau best-effort, nous avons proposé un nouveau et simple algorithme d'ordonnancement appelé EFD (Early Flow Discard). Dans ce manuscrit, nous avons d'abord évaluer la performance d'EFD dans un réseau câblé avec un seul goulot d'étranglement au moyen d'étendu simulations. Nous discutons aussi des variantes possibles de EFD et les adaptations de EFD à 802.11 WLAN - se réfèrent principalement à EFDACK et PEFD, qui enregistre les volumes échangés dans deux directions ou compte simplement les paquets dans une direction, visant à améliorer l'équité à niveau flot et l'interactivité dans les WLANs. Enfin, nous nous consacrons à profiler le trafic de l'entreprise, en plus de elaborer deux modèles de trafic-l'une qui considère la structure topologique de l'entreprise et l'autre qui intègre l'impact des applications au-dessus de TCP - pour aider à évaluer et à comparer les performances des politiques d'ordonnancement dans les réseaux d'entreprise classiques. / As the Internet evolves over the years, a large number of applications emerge with varying service requirements in terms of bandwidth, delay, loss rate and so on. Still, the Internet traffic exhibits a high variability property – the majority of the flows are of small sizes while a small percentage of very long flows contribute to a large portion of the traffic volume. Several studies reveal that small flows are in general related to interactive applications – for which one expects to obtain good user perceived performance, most often in terms of short response time. However, the classical FIFO/drop-tail scheme deployed in today’s routers/switches is well known to bias against short flows over long ones. To tackle this issue over a best-effort network, we have proposed a novel and simple scheduling algorithm named EFD (Early Flow Discard). In this manuscript, we first evaluate the performance of EFD in a single-bottleneck wired network through extensive simulations. We then discuss the possible variants of EFD and EFD’s adaptations to 802.11 WLANs – mainly refer to EFDACK and PEFD. Finally, we devote ourselves to profiling enterprise traffic, and further devise two workload models - one that takes into account the enterprise topological structure and the other that incorporates the impact of the applications on top of TCP - to help to evaluate and compare the performance of scheduling policies in typical enterprise networks.

Wireless local area networks (WLANs) have experienced tremendous growth with the proliferation of IEEE 802.11 devices in past two decades. Wireless operators are embracing WLAN for cellular offloading in every smartphone nowadays [1]. The traffic over WLAN requires significant improvement of both WLAN throughput and efficiency. To increase throughput, multiple-input and multiple-output (MU-MIMO) has been adopted in the new generation of WLAN, such as IEEE 802.11ac. MU-MIMO systems exploit the spatial separation of users to increase the network throughput. In an MU-MIMO system, efficient channel sounding is essential for achieving optimal throughput. We propose a dynamic sounding approach for MU-MIMO systems in WLANs. We analyse and show that the optimal sounding intervals exist for single user transmit beamforming (SU-TxBF) and MU-MIMO for given channel conditions. We design a low-complexity dynamic sounding approach that adjusts the sounding interval adaptively in real-time. Through our collected over-the-air channel measurements, we demonstrate significant throughput improvements using our proposed dynamic sounding algorithm while being compliant with IEEE 802.11ac standard. Subsequently, we investigate the user grouping problem of downlink WLANs with MU-MIMO. Particularly, we focus on the problem of whether SU-TxBF or MU-MIMO should be utilized, and how many and which users should be in a multi-user (MU) group. We formulate this problem for maximizing the system throughput subject to the multi-user air time fairness (MU-ATF) criterion. We show that hypergraphs provide a suitable mathematical model and effective tool for finding the optimal or close to optimal solution. We show that the optimal grouping problem can be solved efficiently for the case where only SU-TxBF and 2-user MU groups are allowed in the system. For the general case, where any number of users can be assigned to groups of different sizes, we develop an efficient graph matching algorithm (GMA) based on graph theory principles with near-optimal performance. We evaluate the proposed algorithm in terms of system throughput using an 802.11ac emulator using collected channel measurements from an indoor environment and simulated channel samples for outdoor scenarios. We show that the approximate solution, GMA, achieves at least 93% of the optimal system throughput in all considered test cases. A complementary technique for MU-MIMO is orthogonal frequency-division multiple access (OFDMA), which will be the key enabler to maximize spectrum utilization in the next generation of WLAN, IEEE 802.11ax. An unsolved problem for 802.11ax is maximizing the number of satisfied users in the OFDMA system while accommodating the different Quality of Service (QoS) levels. We evaluate the possibility of regulating QoS through prioritizing the users in OFDMA-based WLAN. We define a User Priority Scheduling (UPS) criterion that strictly guarantees service requests of the spectrum and time resources for the users with higher priorities before guaranteeing resources to those of lower priority. We develop an optimization framework to maximize the overall number of satisfied users under this strict priority constraint. A mathematical expression for user satisfaction under prioritization constraints (scheduler) is formulated first and then linearized as a mixed integer linear program that can be efficiently solved using known optimization routine. We also propose a low-complexity scheduler having comparable performance to the optimal solution in most scenarios. Simulation results show that the proposed resource allocation strategy guarantees efficient resource allocation with the user priority constraints in a dense wireless environment. In particular, we show by system simulation that the proposed low-complexity scheduler is an efficient solution in terms of (1) total throughput and network satisfaction rate (less than 10% from the upper bound), and (2) algorithm complexity (within the same magnitude order of conventional scheduling strategy. / Ph. D.