Zero-day exploits against servers pose one of the most challenging problems faced by system and security administrators. Current solutions rely mainly on signature databases of known attacks and are not efficient at detecting new attacks not covered by their attack signature database. We propose using a dummy server, i.e. A mirror of the server to be protected but without the real data. Consequently, any incoming network packet is first tested against the dummy server and once it is ensured that the packet is benign, it is delivered to the real server. This would prevent all types of attacks, including those based on zero-day exploits, from reaching the protected server.

In this paper, we present GRMax for WSN (Greedy Routing for Maximum Lifetime in Wireless Sensor Networks) whose goal is to manage the restricted energy so that the Wireless Sensor Network (WSN) stay connected for the maximum time possible. We evaluate the performance of GRMax for WSN by replacing the greedy routing of Greedy Perimeter Stateless Routing (GPSR) protocol with GRMax. Evaluation and comparison of GPSR and GRMax for WSN is done through NS2. Results show that GRMax for WSN performs better than GPSR with respect to packet delivery fraction, number of dropped packet and network lifetime. With the introduction of VIP nodes we avoid early network partition whatever random the traffic is. And with our new metric, we make sure that battery power resources are used efficiently since we suppose that every node can fix his transmission range depending on the distance to the selected next hop. As metric we use the cosine of the deviation angle. We can use our resources efficiently because with our metric, from the source to the destination we use a path close to the straight line.

In this paper, we introduce an extension of Stream Control transmission Protocol (SCTP) aiming for improving the efficiency of delay-sensitive transmission of multimedia data such MPEG-4 traffic. Briefly, our SCTP extension exploits one-way delay between SCTP endpoints, which allows the SCTP extension to retransmit lost packets selectively depending on whether the lost packets would still arrive before the schedule time. By computer simulations, we show that our extension provides the better performance than a simple retransmission method based on lifetime under both wired and wireless access links. Specifically, we confirm that our extension maintains the fair sharing of bandwidth among the legacy TCP flows, irrelevant to wired and wireless access links.

The stream control transmission protocol (SCTP) is a reliable transport layer protocol standardized by IETF as RFC 2960 in 2000. Because of its attractive features such as multi-homing and multi-streaming, SCTP has received much attention from researchers in worldwide with an expectation that would address limitations of TCP performance encountered in wired-cum-wireless networks. In this paper, we focus on the multi-streaming feature of SCTP. Making use of the partial reliability extension in SCTP, we compare performance of MPEG-4 video quality under the policy with/without differentiated retransmission among streams. Differentiated policy allows packets belonging to a specific stream to be retransmitted, while packets are transmitted exactly one time (no retransmissions) in homogeneous policy. We show by simulation, that video object traffic streams encoded by MPEG-4 can be efficiently conveyed under lossy link by SCTP with differentiated policy among streams than with homogeneous policy. We further analyze and show, also by simulation, that SCTP with optimally tuned retransmission policy can minimize the play- out delay time as well as keep high quality video decoding of MPEG-4 traffic stream.

In this paper, we derive spatial correlation functions of linear and circular antenna arrays for three types of angular energy distributions: a Gaussian angle distribution, the angular energy distribution arising from a Gaussian spatial distribution, and uniform angular distribution. The spatial correlation functions are investigated carefully. The spatial correlation is a function of antenna spacing, array geometry and the angular energy distribution. In order to emphasize the research and their applications in diversity reception, as an example, performance of the antenna arrays with MRC in correlated Nakagami fading channels is investigated, in which analytical formulas of average BER for the spatial correlation are obtained.

We investigate the validity of the analytical model of new call channel limiting scheme (NCCL) by means of simulation. NCCL limits new calls to access the total channels with parameter a while handover calls have no restriction to access the channels. We focus on the effects of users' mobility and the parameter a on the blocking probabilities of new and handover calls. We observe that the analytical model presents a good match to the simulation model for low mobility users whereas there exists a small disagreement for high mobility users. Furthermore, we find that the variability of mobile users' velocity affects the applicability of the analytical model, in particular, for small /spl alpha/ and light traffic. We extend the analytical model by including the influence of the distribution of mobile users' velocity. We show that the extended analytical model yields better agreement with the simulation results.

This paper compares the performance of the orthogonal-variable-spreading-factor (OVSF) code assignment among three proposed schemes so far, i.e., the conventional OVSF-code assignment scheme (CONV), the dynamic OVSF-code assignment scheme (DCA), and region division assignment scheme (RDA). In OVSF-CDMA, the maintenance of orthogonality among the assigned OVSF codes causes the code blocking problem which increases the high blocking rate for high-rate requests. DCA is aimed at eliminating the code blocking problem in order to reduce the high blocking rate for high-rate requests, whereas RDA has been proposed to mitigate the code blocking problem in order to efficiently provide services for all supported rates. The computer simulation results show that the blocking probability of RDA is lower than that of DCA under heavy traffic conditions. The simulation results also show that the calculation time of DCA is significantly larger than that of RDA. Furthermore, RDA is shown to be able to simultaneously handle many more voice sources than DCA.

This work focuses on the problem of efficient assignment of the orthogonal-variable-spreading-factor (OVSF) codes for multimedia communications in W-CDMA systems. Due to the problem of code blocking in OVSF-code assignment considered by Minn and Siu, and the different probability of requests for each data rate, the OVSF-code assignment algorithms proposed so far cannot efficiently serve the high-rate requests when traffic load is high. Even though Minn and Siu proposed the algorithm to completely eliminate the code blocking, a system implemented with such an algorithm has a lower number of simultaneous voice conversations than the system with the code blocking problem. To efficiently provide the services for all supported rates, we propose an OVSF-code assignment scheme called the region division assignment (RDA). The performance of the proposed RDA scheme is evaluated by simulation for voice and video sources based on the multiple access protocol in UMTS/IMT-2000.

Along with the constant growing need for mobile communications, frequency utilization becomes a very important issue to be solved. Cellular systems offer ability to reuse frequency so that the system capacity can be improved. Frequency utilization is also affected by the way channels are allocated to users. On the other hand, the ability to support various access rates, namely multi-rate access, is required in today's communication systems. Time-frequency slicing (TFS) system has been studied and was found that it could support multi-rate access effectively. In order to even improve the frequency utilization of TFS system for the use in cellular systems, we study an implementation of TFS with dynamic channel allocation (DCA). Call blocking probability is used as a performance measure of the system. Numerical results show that this system can reduce blocking probability significantly, namely, it has capacity gain due to the increase of trunking efficiency.

The authors examine the structural relationship between load and blocking probability for a fully connected homogeneous nonhierarchical network when blocked messages cause retrial and when different alternate routing strategies are employed to overcome congestion. The model employed for analysis is based on the flow approximation. It is found that retrials push the stable equilibrium state away from the zero blocking state, thereby aggravating congestion. The alternate routing strategies accommodate the stable equilibrium state close to the zero blocking state. However, both the retrials and the alternate routing strategies do not alter the fundamental structure of the system, namely, the fold catastrophe. By solving the degenerate equation, the value of appropriate control parameters can be obtained. According to the present model, it is only under schemes with no alternate routing that the system attains stability, in the mathematical sense of the term.>

An architecture for the copy network that is an integral part of multicast ATM switches is described. The architecture makes use of the property that the broadcast banyan network (BBN) is nonblocking if the active inputs are cyclically concentrated and the outputs are monotone. In the architecture, by employing a token ring reservation scheme, the outputs of the copy network are reserved before a multicast cell is replicated. By the copy principle, the number of copies requested by a multicast call is not limited by the size of the copy network so that very large multicast switches can be configured in a modular fashion. The sequence of cells is preserved in the structure. Though physically separated, buffers within the copy network are completely shared, so that the throughput can reach 100%, and the cell delay and the cell loss probability can be made to be very small. The cell delay is estimated analytically and by computer simulation, and the results of both are found to agree with each other. The relationship between the cell loss probability under various traffic parameters and buffer sizes is studied by computer simulation.>

A global message communication network for low-density traffic, using satellites at low altitudes, is described. This network affords around-the-clock service to any part of the globe, including the polar regions. Such a network can accommodate static and mobile user terminals simultaneously. The oblate globe is modeled as a regular polyhedron with 12 facets for setting up the orbits. Satellites are positioned uniformly in low-altitude symmetrical orbits. The symmetrical orbits are those whose axes are symmetrical in the three-dimensional space. The authors study the coverage aspects of the 6-orbit scheme and the 10-orbit scheme, each with satellites deployed at an altitude of 5000 km. At this altitude, the terrestrial user terminals can access satellites at a grazing angle of 45 degrees . The method of access over the crosslinks is the slotted ALOHA scheme. For low-density traffic, the downlink and uplink throughput rates are estimated. Simulation results agree with these analytical estimates for low values of network offered load.>

Store-and-forward techniques in computer communication networks are addressed. The performance estimation and the stability aspects of an input buffer limiting (IBL) scheme are presented. The model is formulated with respect to the state-dependent effective input rate and effective service rate. The computation of the performance parameters with respect to the effective service rate yields satisfactory results when blocking probabilities are not restricted to very low values (>

A numerical method is suggested for estimating the performance parameters of a node with input buffer limiting, throughout the range of blocking probabilities. Congestion is attributed to the sudden increase in the effective service time when the input rate increases. This sudden increase, resulting in the thrashing of the throughput rate, can be related to changes in the shape of the potential function of the node. The methodology of deriving the potential function of the node from its gradient dynamics and dynamic flow conservation considerations is presented. Using the results from catastrophe theory, it is shown that the abrupt changes in the throughput can be expected due to the existence of a catastrophe point in the parameter space. The behavior of a node in a homogeneous network can be characterized by a fold catastrophe. The simulation results are found to agree with the results obtained by the numerical method.>

Stability and throughput are investigated for a combined random-reservation access scheme employed in a satellite-based very small aperture terminal (VSAT) packet communication network. The analysis is based on the use of catastrophe theory by which the system behavior is characterized by the cusp catastrophe. A stable region is given in terms of system and user parameters such as packet generation rate, retransmission rate, channel division ratio and propagation delays. A numerical example is given to illustrate the analysis developed. Simulation results agree with the steady-state throughput of the model.>

We develop a new tool for performance evaluation of a multiaccess communication system. We give an explicit detailed analytic description of a cusp catastrophe in a computer communication system. An approximate model is formulated for a slotted Aloha system assuming a single buffer, based on the Markovian model studied by Lam [2] and Carleial [3]. Next, we outline the research on quantitative processing of thrashing and its results. Sudden changes, which can be observed in the throughput rate, the average delay, and the average number of backlogged users, are induced by smooth alterations of control parameters in the behavior of a slotted Aloha system. These catastrophic phenomena, such as long lasting periods of vanishing throughput rate and very high delays, are analyzed in line with catastrophe theory. The system behavior in a slotted Aloha system is characterized by the cusp catastrophe. In particular, introducing a new parameter σ in our model, we make it clear why and in what cases these catastrophic phenomena occur. The amount of change is also estimated.

We develop here an approximate method for the design and performance prediction of a multiaccess communication system which employs the ALOHA packet-switching technique. Our model is based on the use of a diffusion process approximation of an ALOHAlike system (with or without time-slotting). A simple closed-form solution for the variableQ(t), a variant of the number of backlog messages at timet, is given in terms of a few system and user parameters. Final results are expressed in terms of ordinary performance measures such as throughput and average delay. Several numerical examples are given to demonstrate the usefulness of the approximation technique developed.