Bivas Mitra

Smart Association Control in Wireless Mobile Environment Using Max-Flow

WiFi clients must associate to a specific Access Point (AP) to communicate over the Internet. Current association methods are based on maximum Received Signal Strength Index (RSSI) implying that a client associates to the strongest AP around it. This is a simple scheme that has performed well in purely distributed settings. Modern wireless networks, however, are increasingly being connected by a wired backbone. The backbone allows for out-of-band communication among APs, opening up opportunities for improved protocol design. This paper takes advantage of this opportunity through a coordinated client association scheme where APs consider a global view of the network, and decide on the optimal client-AP association. We show that such an association outperforms RSSI based schemes in several scenarios, while remaining practical and scalable for wide-scale deployment. We also show that optimal association is a NP-Hard problem and our max-flow based heuristic is a promising solution.

Analyzing the vulnerability of superpeer networks against attack

In this paper, we develop an analytical framework to measure the vulnerability of superpeer networks against attack. Two different kinds of attacks namely deterministic and degree dependent attack have been introduced here. We formally model the superpeer networks with the help of bimodal structure and different attacks with the help of graph dynamics. Our analysis shows that fraction of superpeers and their connectivity have profound impact upon the stability of the network. The results obtained from the theoretical analysis are validated through simulation. The agreement between the simulation results and theoretical predictions is almost perfect.

An algorithm for optimal assignment of a wavelength in a tree topology and its application in WDM networks

In this paper, we present a polynomial time algorithm that gives an optimal solution to the routing and wavelength assignment (RWA) problem in a tree topology. One of the major design issues in wavelength-division multiplexed networks is the assignment of the limited number of wavelengths among network stations so that greater capacity can be achieved. The problem of RWA is known to be NP-hard problem. Many researchers have tackled the problem of RWA with a number of efficient heuristic algorithms. This paper presents an algorithm that optimally assigns a single wavelength to maximize one-hop traffic in a tree topology. The algorithm uses dynamic programming and is shown to be optimal with a time complexity of O(N/sup 4/). We also propose a heuristic scheme to use our optimal algorithm for wavelength assignment in a general graph. The heuristic works on the tree subgraphs of a given graph and the remaining spare wavelengths can be assigned with an existing RWA policy.

How do Superpeer Networks Emerge

In this paper, we develop an analytical framework which explains the emergence of superpeer networks on execution of the commercial peer-to-peer bootstrapping protocols by incoming nodes. Bootstrapping protocols exploit physical properties of the online peers like resource content, processing power, storage space, connectivity etc as well as take the finiteness of bandwidth of each online peer into consideration. With the help of rate equations, we show that execution of these protocols results in the emergence of superpeer nodes in the network - the exact degree distribution is evaluated. We validate the framework developed in this paper through extensive simulation. The analysis of the results shows that the amount of superpeers produced in the network depends on the protocol as well as the properties of the joining nodes. Interestingly, our analysis reveals that increase in the amount of resource and the number of resourceful nodes do not always help to increase the fraction of superpeer nodes in the network. The impact of the frequent leaving of the peers on the topology of the emerging network is also evaluated. As an application study, we show that our framework can explain the topological configuration of commercial Gnutella networks. The developed model can almost perfectly match the degree distribution of Gnutella network.

Correlations in complex networks under attack

For any initially correlated network after any kind of attack where either nodes or edges are removed, we obtain general expressions for the degree-degree probability matrix and degree distribution. We show that the proposed analytical approach predicts the correct topological changes after the attack by comparing the evolution of the assortativity coefficient for different attack strategies and intensities in theory and simulations. We find that it is possible to turn an initially assortative network into a disassortative one, and vice versa, by fine-tuning removal of either nodes or edges. For an initially uncorrelated network, on the other hand, we discover that only a targeted edge-removal attack can induce such correlations.

Measuring robustness of superpeer topologies

In this paper, we propose an analytical framework based on percolation theory to assess the robustness of superpeer topologies in face of user churns and/or attacks targeted towards important nodes. It is observed in practice that in spite of churn of peers, superpeer networks show exceptional robustness and do not disintegrate into disconnected components. With the help of the analytical framework developed, we formally measure its stability against user churn and validate the general observation. The effect of intentional attacks upon the superpeer networks is also investigated. Our analysis shows that fraction of superpeers in the network and their connectivity have profound impact upon the stability of the network. The results obtained from the theoretical analysis are validated through simulation. The simulation results and theoretical predictions match with high degree of precision.

UrbanEye: An outdoor localization system for public transport

Public transport in suburban cities (covers 80% of the urban landscape) of developing regions suffer from the lack of information in Google Transit, unpredictable travel times, chaotic schedules, absence of information board inside the vehicle. Consequently, passengers suffer from lack of information about the exact location where the bus is at present as well as the estimated time to be taken to reach the desired destination. We find that off-the-shelf deployment of existing (non-GPS) localization schemes exhibit high error due to sparsity of stable and structured outdoor landmarks (anchor points). Through rigorous experiments conducted over a month however, we realize that there are a certain class of volatile landmarks which may be useful in developing efficient localization scheme. Consequently, in this paper, we design a novel generalized energy-efficient outdoor localization scheme - UrbanEye, which efficiently combines the volatile and non-volatile landmarks using a specialized data structure, the probabilistic timed automata. UrbanEye uses speed-breakers, turns and stops as landmarks, estimates the travel time with a mean accuracy of ±2.5 mins and produces a mean localization accuracy of 50 m. Results from several runs taken in two cities, Durgapur and Kharagpur, reveal that UrbanEye provides more than 50% better localization accuracy compared to the existing system Dejavu [1], and consumes significantly less energy.

Attacks on correlated peer-to-peer networks: An analytical study

Analysis of attacks on real-world p2p networks and their impact on the topology of the network is difficult as the interconnections among the peers are not random; rather they evolve based on the needs of the connected peers and this brings in degree-degree correlation in the network. We develop an analytical framework to analyze the change in topology of a correlated network and propose a generalized model based on percolation theory to measure the resilience of a correlated network against any arbitrary attack. We present the results and analysis mainly on correlated superpeer networks and correlated bimodal networks. Some of the intricate questions on the stability of real-world superpeer network that we answer analytically are: (a) dependence of percolation threshold of a superpeer network on its peer degree, superpeer degree at different levels of degree-degree correlation (b) minimum peer degree required to make a superpeer topology more resilient. All our theoretical results are validated through simulations and the results are in very good agreement.

Fair bandwidth allocation in wireless network using max-flow

This paper proposes a fair association scheme between clients and APs in WiFi network, exploiting the hybrid nature of the recent WLAN architecture. We show that such an association outperforms RSSI based schemes in several scenarios, while remaining practical and scalable for wide-scale deployment.

Survivable Routing in WDM Weighted Networks

In this paper, we investigate the problem of routing lightpaths on an arbitrary physical topology following a Design Protection approach, such that virtual topology remains connected even after the failure of a single fiber link. This is called survivable routing. It is known to be an NP-complete problem. To address the problem, first, we have proved that embedded Hamiltonian circuit in a mesh network is a must for its survivability. Then using a polynomial time algorithm for generating Hamiltonian circuit i.e. embedded ring virtual topology, we establish lightpaths to the ring. Finally, we design two RWA algorithms to assign lightpaths to other requests in the network, giving priority to wavelength and traffic respectively. We analyze the numerical results obtained for random undirected networks with random normal traffic demands with performance metrics such as maximum one-hop and minimum multi-hop protected traffic, wavelength utilization, number of multi-hops, Buffer size etc.