-The paper analyzes systems vulnerability targeted by TCP (Transmission Control Protocol) segments when SYN flag is ON, which gives space for a DoS (Denial of Service) attack called SYN flooding attack or more often referred as a SYN flood attack. The effects of this type of attack are analyzed and presented in OPNET simulation environment. Furthermore, the paper presents two anomaly detection algorithms as an effective mechanism against this type of attack. Finally, practical approaches against SYN flood attack for Linux and Windows environment which are followed by are shown.

Wireless sensor networks (WSN) consists of light-weight, lowpower and small size sensor nodes (SNs). They have ability to monitor, calculate and communicate wirelessly.In this paper we present a performance evaluation of ZigBee which is IEEE 802.15.4 standard, including the Physical (PHY) layer and Media Access Control (MAC) sub-layer, which allow a simple interaction between the sensors.We provide an accurate simulation model with respect to the specifications of IEEE 802.15.4 standard. We simulate and analyzed two different scenarios, where we examine the topological features and performance of the IEEE 802.15.4 standard using OPNET simulator. We compared the three possible topologies (Star, Mesh and Tree) to each other.

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Wireless networks are built upon a shared medium which makes it easy for an adversary to launch a Denial of Service (DoS) attack. Jamming, as a part of DoS attacks at physical layer, is one of the main security considerations within the wireless networks. It holds the potential to totally cripple the network and cause a loss of service and revenue. Jamming attacks can severely interfere with the normal operation of wireless networks and, consequently, mechanisms are needed that can cope with jamming attacks. Jamming techniques vary from simple ones based on the continual transmission of interference signals, to more sophisticated attacks that aim at exploiting vulnerabilities of the particular used protocol. Considering the fact that the jamming is one of most common attacks against military communications, this paper is examining the wireless military network behavior in the presence of such attacks. For this purpose we simulate the effects of the various jamming attacks using OPNET Modeler. Several effective counter-measures against low power jamming are proposed. These measures are based on both transmit power and packet reception power threshold optimization, supported by spread spectrum techniques. The results are considerably improved when all of these mechanisms are combined