Selvamuthu Dharmaraja

Analytic modeling of handoffs in wireless cellular networks

In this paper, we report our recent work on closed form solutions to the blocking and dropping probability in wireless cellular networks with handoff. First, we develop a performance model of a cell in a wireless network where the effect of handoff arrival and the use of guard channels are included. Fast recursive formulas for the loss probabilities of new calls and handoff calls are given. Algorithms to determine the optimal number of guard channels and the optimal number of channels are developed. To determine the handoff arrival rate into a cell, a fixed-point iteration scheme is proposed, and the uniqueness of the fixed-point is shown. Then, we allow for failure and repair of channels and hence provide a performability analysis for cellular systems. We present a hierarchical model combining both performance and availability and its closed form solution. Performability measures of interest are explicitly given.

WHOP: Wormhole attack detection protocol using hound packet

The lack of centralised infrastructure in ad hoc network makes it vulnerable to various attacks. MANET routing disrupts if participating node do not perform its intended function and start performing malicious activity. A specific attack called Wormmhole attack enables an attacker to record packets at one location in the network, tunnels them to another location, and retransmits them into the network. In this paper, we present a protocol for detecting wormhole attacks without use of any special harware such as directional antenna and precise synchronised clock and the protocol is also independent of physical medium of wireless network. After the route discovery, source node initiates wormhole detection process in the established path which counts hop difference between the neighbours of the one hop away nodes in the route. The destination node detects the wormhole if the hop difference between neighbours of the nodes exceeds the acceptable level. Our simulation results shows that the WHOP is quite excellent in detecting wormhole of large tunnel lengths

Combined Guard Channel and Mobile-Assisted Handoff for Cellular Networks

For cellular communication systems, mobility and limited radio coverage of a cell require calls to be handed over from one base station system (BSS) to another. Due to the limited bandwidth available in various cells, there is a finite probability that an ongoing call, while being handed off, may get dropped. Minimizing the dropping of ongoing calls during handoff is an important design criterion. Some digital cellular systems, e.g., the global system for mobile communications and the IS-136, use mobile-assisted handoff (MAHO), in which a mobile terminal (MT) assists its BSS, and a mobile switching center in making handoff decisions. MAHO requires an MT to regularly report, back to its serving BSS, its current radio-link state [defined in terms of the received signal strength indicator (RSSI) and the bit error rate (BER)] of transmissions received from neighboring BSSs. Some researchers have suggested that a base station needs to give priority to the handoff calls over the new calls. This requires each cell to reserve a number of guard channels (GCs) to be used exclusively for processing of the handoff calls. Since MAHO makes handoff decisions based solely on RSSI/BER measurements, there is a finite probability that some handoff calls may get dropped due to the nonavailability of free channels in the neighboring cell to which the call is being handed off. Conversely, if a handoff decision is based solely on the availability of a free channel, without regard to the signal quality, it may result in some of the handed-off calls being dropped due to poor signal quality. In this paper, we propose a new handoff technique by combining the MAHO and GC techniques. In the proposed technique, the MT reports back not only the RSSI and the BER but the number of free channels that are available for the handoff traffic as well. This will ensure that a handed-off call has acceptable signal quality as well as a free available channel. The performance of this handoff technique is analyzed using an analytical model whose solution gives the desired performance measures in terms of blocking and dropping probabilities.

Reliability and Survivability Analysis for UMTS Networks: An Analytical Approach

Reliability and survivability are the two important attributes of cellular networks. In the existing literature, these measures were studied through the simulation. In this paper, we construct an analytical model to determine reliability and survivability attributes of third generation and beyond Universal Mobile Telecommunication Systems (UMTS) networks. Hierarchical architecture of UMTS networks is modeled using stochastic models such as Markov chains, semi-Markov process, reliability block diagrams and Markov reward models to obtain these attributes. The model can be tailored to evaluate the reliability and survivability attributes of other beyond third generation cellular networks such as All-IP UMTS networks and CDMA2000. Numerical results illustrate the applicability of the proposed analytical model. It is observed that incorporating fault tolerance increases the network reliability and survivability. The results are useful for reliable topological design of UMTS networks. In addition, it can help the guarantee of network connectivity after any failure, without over dimensioning the networks. Moreover, it might have some impact from the point of view of the design and evaluation of UMTS infrastructures.

Performance modeling of wireless networks with generally distributed handoff interarrival times

Handoff is an important issue in cellular mobile telephone systems. Recently, studies that question the validity of the assumption of handoff arrivals being Poissonian have appeared in the literature. The reasoning behind this claim can be summarized as follows: even if the new call arrival process is assumed to be Poisson, the handoff process due to dependencies with neighboring cells, call blocking and other reasons is not necessarily Poisson. The above-mentioned fact mandates the need to consider more general performance models that allow for arbitrarily distributed interarrival times. In this paper we provide numerical solutions for new and handoff call blocking probabilities with arbitrary handoff interarrival time distribution. For this purpose, we first prove that the underlying stochastic process is a Markov regenerative process and subsequently we use their mathematical theory to develop numerical techniques for important Quality of Service measures. Our results can be seen as a generalization of the recent work by Haring et al. [IEEE Trans. Vehi. Technol. 50 (2001) 664] where handoff traffic was assumed to form a Poisson process. Our work can be used for more accurate dimensioning of cellular systems with realistic traffic.

Performance analysis of IEEE 802.11 DCF with stochastic reward nets

In this paper, we present a performance study to evaluate the mean delay and the average system throughput of IEEE 802.11-based wireless local area networks (WLANs). We consider the distributed co-ordination function (DCF) mode of medium access control (MAC). Stochastic reward nets (SRNs) are used as a modelling formalism as it readily captures the synchronization between events in the DCF mode of access. We present a SRN-based analytical model to evaluate the mean delay and the average system throughput of the IEEE 802.11 DCF by considering an on–off traffic model and taking into account the freezing of the back-off counter due to channel capture by other stations. We also compute the mean delay suffered by a packet in the system using the SRN formulation and by modelling each station as an M/G/1 queue. We validate our analytical model by comparison with simulations. Copyright

Performability modelling of wireless communication systems

The high expectations of performance and availability for wireless mobile systems has presented great challenges in the modelling and design of fault tolerant wireless systems. The proper modelling methodology to study the degradation of such systems is so-called performability modelling. In this paper, we give overview of approaches for the construction and the solution of performability models for wireless cellular networks. First, we start with the Erlang loss model, in which hierarchical and composite Markov chains are constructed to obtain loss formulas for a system with channel failures. Consequently, we develop two level hierarchical models for the wireless cellular system with handoff and channel failures. Then, for a TDMA system consisting of base repeaters and a control channel, we build a hierarchical Markov chain model for automatic protection switching (APS). Finally, we discuss stochastic reward net (SRN) models for performability analysis of wireless systems. Copyright

Analytical modeling of bidirectional multi-channel IEEE 802.11 MAC protocols

This paper presents an analytical approach to model the bi-directional multi-channel IEEE 802.11 MAC protocols (Bi-MCMAC) for ad hoc networks. Extensive simulation work has been done for the performance evaluation of IEEE 802.11 MAC protocols. Since simulation has several limitations, this work is primarily based on the analytical approach. The objective of this paper is to show analytically the performance advantages of Bi-MCMAC protocol over the classical IEEE 802.11 MAC protocol. The distributed coordination function (DCF) mode of medium access control (MAC) is considered in the modeling. Two different channel scheduling strategies, namely, random channel selection and fastest channel first selection strategy are also presented in the presence of multiple channels with different transmission rates. M/G/1 queue is used to model the protocols, and stochastic reward nets (SRNs) are employed as a modeling technique as it readily captures the synchronization between events in the DCF mode of access. The average system throughput, mean delay, and server utilization of each MAC protocol are evaluated using the SRN formalism. We also validate our analytical model by comparison with simulation results. The results obtained through the analytical modeling approach illustrate the performance advantages of Bi-MCMAC protocols with the fastest channel first scheduling strategy over the classical IEEE 802.11 protocol for TCP traffic in wireless ad hoc networks. Copyright

Semi-Markov modeling of dependability of VoIP network in the presence of resource degradation and security attacks

Nowadays VoIP has become an evolutionary technology in telecommunications. Hence it is very important to study and enhance its dependability attributes. In this paper, an analytical dependability model for VoIP is proposed. The study is focused on analyzing the combined effects of resource degradation and security breaches on the Quality of Service (QoS) of VoIP, to enhance its overall dependability. As a preventive maintenance policy to prevent or postpone software failures which cause resource degradation, software rejuvenation is adopted. The dependability model is analyzed using semi-Markov process which captures the effects of non-Markovian nature of the time spent at various states of the system. The steady-state as well as the time-dependent analysis of the dependability model is presented. The steady-state results are obtained analytically, whereas the time-dependent results are obtained from simulation. Also, the analytical model is validated via simulation. The model analysis using a numerical example indicates the feasibility of our approach. Various dependability attributes such as availability, reliability and confidentiality are also obtained. A comparative study is also done between our proposed model and the existing models.

BAAP: Blackhole attack avoidance protocol for wireless network

The lack of centralised infrastructure in adhoc network makes it vulnerable to various attacks. MANET routing disrupts if the participating node start performing malicious activity instead of the intended function. One such specific attack is a blackhole attack in which malicious node falsely claiming itself as having the fresh and shortest path to the destination. In this paper, we propose a protocol for avoiding blackhole attack without the contraint of special hardware and dependency on physical medium of wireless network. BAAP forms link disjoint multi-path during path discovery to provide greater path selection in order to avoid malicious nodes in the path using legitimacy table maintained by each node in the network. Non-malicious nodes gradually isolate the blackhole nodes based on the values collected in their legitimacy table and avoid them while making path between source and destination.