The IP multimedia subsystem (IMS) is being deployed in the third generation (3G) networks since it supports many kinds of multimedia services. However, the security of IMS networks has not been fully examined. This paper presents a novel DoS attack against IMS. By congesting the presence service, a core service of IMS, a malicious attack can cause chained automatic reaction of the system, thus blocking all the services of IMS. Because of the low-volume nature of this attack, an attacker only needs to control several clients to paralyze an IMS network supporting one million users. To address this DoS attack, we propose an online early defense mechanism, which aims to first detect the attack, then identify the malicious clients, and finally block them. We formulate this problem as a change-point detection problem, and solve it based on the non-parametric GRSh test. Through trace-driven experiments, we demonstrate that our defense mechanism can throttle this DoS attack within a short defense time window while generating few false alarms.

Presence is a service that allows a user to be informed about the reachability, availability, and willingness of communication of another user. Presence service has become a key enabler for many popular applications such as instant messaging and push-to-talk. Traffic to a presence server is resulted by user behaviors such as login/logout, online status modification, and automatic status refresh by client application. Mathematical models are proposed in this paper to study the user behaviors associated with a presence server during the time of a day such that traffic characteristics to a presence server can be inferred. The correctness of our estimation and analysis are verified by extensive simulations using Matlab. This study on the relationship between user behaviors and traffic to a presence server can help network operators to plan network capacity, optimize server performance and detect traffic anomaly.

The energy detection is recently studied extensively as a promising candidate for primary signals detection in cognitive radio. However, one fundamental disadvantage of energy detection is that its performance degrades in the presence of inaccurate knowledge of noise power (noise uncertainty) which inevitably occurs in practical implementations. In this paper, we introduce a novel Bayesian estimation based energy detection (BEED) which, confirmed by simulations, can almost completely eliminate detrimental effects of noise uncertainty. In addition, the consistency of BEED is proved which means that it will correctly detect primary users with probability one when the sampling number increases to infinity.

In hard cooperative spectrum sensing (HCSS) for cognitive radio (CR), where local secondary users (sensors) sense whether primary users exist or not and then send decisions to the center where the final decision is made, correlation between sensors has been conventionally believed and experimentally shown to have detrimental effects on performance of HCSS. In this paper, we restudy effects of correlation and demonstrate that, contrary to common belief, the correlation has beneficial effects if the fusion rule at the center is convex and has detrimental effects if the fusion rule is concave. Further, we specially analyze the effects of correlation on AND HCSS and OR HCSS, two existing HCSS rules in the literature.

Presence is a service that allows a user to be informed about the reachability, availability, and willingness to communicate of another user. Presence service has become a key enabler for many popular applications such as instant messaging and push-to-talk. However a study shows that presence service can account for 50% or more of the total signalling traffic the IMS core network handles (C. Urrutia-Valds et al., 2006). This is quite a burden for a real IMS network and need to be tackled. In this paper, a thorough analysis of the traffic load distribution reveals that notify messages account for the largest portion of the traffic load on a presence server. We propose a mathematical model of a queueing system with batch arrival and controlled vacation to describe the processing of notify messages inside a presence server. The model is then used to calculate the optimal value of the timer that controls the notify queue, which is an essential parameter to adjust in network sizing upon deploying a presence server. An approximation of the model is also provided so that the performance of a presence server can be tuned in an online manner based on real traffic statistics.

Handover between various networks or cells of the same network is an important problem faced by mobile nodes. With so many handover algorithms available, how to evaluate and select them become critical. A mathematical model for wrong decision probability and handover probability is given in this paper such that they can be used as criteria to evaluate handover algorithms. Explicit expressions for these two parameters are vigorously studied via a general bandwidth based handover algorithm. A WDP predication based handover algorithm is also proposed to improve the handover performance.

The IEEE 802.16 standard provides a mechanism for creating multi-hop mesh network, which can be deployed as a cost-effective broadband wireless access solution to provide high speed Internet connection to backhaul network. However, the multiple-access interference reduces the concurrent transmission and limits the network capacity and scalability. This paper proposes an efficient approach for centralized scheduling to increase the bandwidth efficiency in WiMAX mesh networks. An odd-even layer alternation algorithm is presented to augment the concurrent transmission rate and reduce latency, which can be implemented in centralized scheduling for interference-aware data transmission. Simulation results show the proposed scheduling algorithm achieves better throughput and latency performance over the basic solution for IEEE 802.16 mesh network.

Flooding is a technique used by link state routing protocols to exchange network status information. In packet switched networks, flooding traffic competes with user traffic for the same physical medium, and a flooding scheme which has a good impact on QoS routing may degrade the performance of best-effort traffic greatly. In this paper, lazy flooding schemes are defined for packet-switched networks. An analytical model is proposed to study how much traffic is added by lazy flooding schemes and periodic flooding scheme in single link case. And the impact of different flooding schemes on transmission delay of best-effort traffic in whole network case is studied by simulation.