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Past Research Works


Engineering IEEE 802.11s Wireless Mesh Networks



Current deployment of wireless community and municipal area networks provide ubiquitous connectivity to end users through wireless mesh backbone, that aims at replacing wired infrastructure through wireless multi-hop connectivity. IEEE 802.11s standard is published recently to support the mesh connectivity over well-deployed IEEE 802.11 architecture based on Wireless Fidelity (WiFi) access network. This research work explores a number of research directions to optimize the mesh peering, channel access, scheduling and mesh path selection protocols for IEEE 802.11s mesh network. The standard provides three major protocols to support mesh functionality - \textit{Mesh Peer Management Protocol} (MPM) to establish mesh connectivity and for topology management, \textit{Mesh Coordinated Channel Access} (MCCA) for channel access and scheduling, and \textit{Hybrid Wireless Mesh Protocol} (HWMP) to support mesh path establishment based on link layer characteristics. The objective of this work is to augment the existing protocols for better connectivity and efficient usage of the resources. In a mesh network, the efficiency of the backbone network can be improved through directional communication by exploring spatial reuse capability. However, uses of directional antennas impose several new research challenges that are explored in this work.

The first outcome of this work enhances the functionality of the mesh channel access and path selection protocols to support directional communication over an IEEE 802.11s mesh backbone. Though MCCA provides reservation based channel access, the standard does not implement any specific mechanism for multi-class traffic services to improve the \textit{Quality of Service} (QoS) for the end-users. The next contribution in this direction is to provide QoS support and service differentiation for MCCA based channel access mechanism over the multi-interface communication paradigm. Modern wireless hardwares are capable of providing multiple data rate supports depending on wireless channel dynamics. As a consequence, the MPM protocol has been augmented to support multi-rate adaptation over IEEE 802.11s protocol elements. In the next contribution, a two phase mesh path selection protocol has been proposed over HWMP, by exploring the limitations of the proactive and reactive path selection paradigms. Finally, the performances of the proposed set of protocols have been evaluated through the results obtained from a practical indoor mesh testbed. As a whole, this work improves the performance of the basic IEEE 802.11s mesh protocols in terms of network as well as end user throughput, end-to-end delay and fairness, with the addition of multi-rate and QoS supports as new features over the standard.


IITGMesh: A High Throughput Mesh Networking Platform



IITGMesh is an IEEE 802.11(b/g/n)+s Wireless Indoor Mesh Testbed deployed at IIT Guwahati CSE Research Labs. The testbed consists of 11 mesh points (10 mesh access points, 1 portal point), connected wirelessly to form a multi-hop mesh architecture. The mesh points can operate upto 300 Mbps data rate, with IEEE 802.11n physical layer. The mesh portal point is connected to the institute backbone network through Gigabit Ethernet. The particulars of hardware/software used are as follows.



Performance Analysis of IEEE 802.11 Power Save Mode


This work has been done jointly with Dr. Pravati Swain, Assistant Professor - CSE, NIT Goa, who was the primary contributor.

The IEEE 802.11 standard for wireless local area networks defines a power management algorithm for Independent Basic Service Set (IBSS) allowing it to save critical battery energy in low powered wireless devices. The power management algorithm for IBSS uses beacon intervals (BIs) as the time unit, where every BI consists of an Announcement Traffic Indication Message (ATIM) window and a data window. The stations that have data to send need to go through a handshaking procedure in the ATIM window. If this handshaking is successful, the station remains awake in the data window and participates in the data communication. Otherwise, it goes into the sleep mode. This work presents an analytical model to compute the throughput, expected delay and expected power consumption in an IEEE 802.11 IBSS in Power Save Mode (PSM) for different traffic conditions in the network. The impact of data arrival rate, network size and size of the BI on the performance of the IEEE 802.11 DCF in PSM is also analyzed. This analysis reveals a clear trade-off among throughput, delay and average power consumption. The trade-off analysis is useful for designing efficient power consumption algorithms while maintaining the consistence performance of the network in terms of throughput and delay.



Data Forwarding in Application Specific Wireless Sensor Networks


This work has been done jointly with Dr. Suchetana Chakraborty, Assistant Professor - CS, BITS Pilani Hyderabad Campus, who was the primary contributor.

Designing an efficient scheme for data forwarding in wireless sensor network is a challenging problem due to the limitation in sensor resources and the inherent dynamics in the environment. Distributed nature of a sensor network requires localized solutions, in absence of any centralized control and complete knowledge of the communication network. Additionally, the design strategy demands to satisfy certain performance criteria like bounded delay, low cost and high reliability etc. as specified by different applications. Many applications in sensor network exploit the advantages of tree-based data gathering, where sensory data from all sensor nodes are collected at a sink or base station for statistical analysis. The sensors form a data gathering tree rooted at the sink, such that the data packets from every sensor node is forwarded towards the sink via its parent node in the tree. The existing works in the literature, that have studied the properties of tree based data gathering, lack in providing an efficient scheme that deals with all design parameters like connectivity, sensing coverage, fault tolerance, network lifetime and other application specific performance metrics. Considering all these aspects, the main objective of this work is to design and develop novel schemes for efficient data gathering in sensor network under various constraints.

The first outcome of this work proposes a distributed BFS tree construction scheme rooted at the sink node for crash-tolerant data gathering in a sensor network. Every node computes an alternate parent during the tree construction phase, such that on sudden failure of the parent node, the affected path can be repaired locally using a low-cost proactive approach. Thus application messages are delivered to the sink with minimum loss or redundancy even in presence of an arbitrary node crash. Moreover, multiple simultaneous node failures have also been handled through a reactive repairing technique. To extend the network lifetime by supporting arbitrary node failure, the primary objective is to maintain both the connectivity and the sensing coverage in the network. While the first contribution focuses only on the connectivity aspect, the second contribution of this work considers both the connectivity and the coverage maintenance as the design objectives.

Considering irregular terrain property and optimal positioning of the sink, the energy depletion rate gradually decreases from the sink towards the terrain periphery. So, both the connectivity and the sensing coverage are affected as the nodes near the sink die out of energy sooner than the leaves of the tree rooted at the sink, creating network holes. For an improved network lifetime, a gradient based node deployment strategy has been proposed that also satisfies the initial connectivity and the coverage criteria. It has been observed that the network lifetime will be improved if the density of the deployed nodes follow the gradient, which is estimated as the amount of energy dissipation at any intermediate node to that of the leaf nodes in its rooted subtree. The proposed theory has been justified through the worst case analysis of the sensor network calculus. As unbalanced data gathering tree escalates the problem of uneven energy depletion in the network, a load-balanced distributed BFS tree construction scheme has been proposed. Further, to handle arbitrary node failure a localized and cost-effective tree maintenance scheme has been introduced. Finally, the characteristics and the design objectives of tree based data gathering with failure support have been explored considering two different applications, one for road traffic monitoring, and the other for critical infrastructure monitoring.

The inherent challenges in distribution and management of sensor network along the road require an application-specific protocol support for the network connectivity, sensing coverage, reliable data forwarding and the network lifetime improvement. The third outcome of this work introduces the concept of k-strip length coverage along the road, that ensures a better sensing coverage for the detection of moving vehicles, compared to the conventional barrier coverage and full area coverage, in terms of the availability of sufficient information for statistical processing as well as the number of sensors required to be active. To extend the network lifetime, every sensor follows a sleep-wakeup schedule maintaining the network connectivity and the k-strip length coverage. This scheduling problem is modeled as a graph optimization, the NP-hardness of which motivates to design a centralized heuristic, providing an approximate solution. The properties of the proposed centralized heuristic are then explored to design a per-node solution based on local information.

Sensor network deployed for critical infrastructure monitoring requires high degree of reliability in sensory data gathering, in spite of arbitrary node or sink failures. The final outcome of this work proposes RelBAS, a robust data gathering scheme, specially designed for the sensory applications on critical infrastructure monitoring. Redundancy in sensor network, in terms of both the number of deployed sensors and the sensory data, is explored to design an effective protocol that ensures reliable data delivery. A set of active sensors, that participate in the data forwarding, are selected from all sensors based on the connected-coverage criteria. The rest of the nodes go to the sleep state, and act as a replacement on failure of an active node. The proposed protocol aims to find out multiple node-disjoint paths to multiple sinks, so that the loss of connectivity in one path due to node failure does not disrupt the application services. The forwarding path selection at every node in RelBAS is based on the computation of a metric that is a function of two parameters - the hop-count distance to the sink and the node's residual energy. Moreover, RelBAS is capable of detecting an affected zone due to multiple node failures.



Vertical Handover between UMTS and WiFi Network


This work has been done jointly with Dr. Moushumi Barooah, Professor, Assam Engineering College, Guwahati, who was the primary contributor.

In recent years, Cellular wireless technologies like GPRS, UMTS, CDMA and Wireless Local Area Network (WLAN) technologies like IEEE 802.11 have seen a quantum leap in their growth. Cellular technologies can provide data services over a wide area, but with lower data rates. WLAN technologies offer higher data rates, but over smaller areas, popularly known as `Hot Spots'. The demand for an ubiquitous data service can be fulfilled, if it is possible for the end-user to seamlessly roam between these heterogeneous technologies. In this work, a novel framework is proposed consisting of intra-ISP network called `Intermediate Switching Network'(ISN) fused between UMTS and WLAN networks as well as data (Internet) services for providing seamless mobility without affecting user's activities. The ISN uses MPLS and Multiprotocol-BGP to switch the data traffic between UMTS to IEEE 802.11 networks, as per the movements of the user. The ISN is integrated with the UMTS network at the GGSN-3G and at the Access Point for IEEE 802.11 network respectively. The simulation result shows the improved performance of the ISN based framework over existing schemes. We have also evaluated the performance of ISN for TCP traffics, and desgned QoS aware vertical handover protocols for the ISN framwork.



Context Aware Handover in Wi-Fi and Its Extension to WiMAX


This work has been done jointly with Dr. Abhijit Sarma, Assistant Professor, Gauhati University, Guwahati, who was the primary contributor.

IEEE 802.11 or `Wireless Fidelity' has become a popular wireless technology to offer high speed Internet access at public places called the `Hot-Spots' as well as to support ubiquitous Internet connectivity through institute wide wireless local area networks (WLANs). However, existing researches has shown that due to wide-spread deployments of WiFi based network connectivity zones, more numbers of wireless access points (APs) are deployed than requirements, however, users tend to concentrate at few areas making traffic load imbalance across the network. The design philosophy of IEEE 802.11 connection establishment and handover from one AP to another is based on signal strength which is biased towards the distance between the AP and the client nodes. Severe performance and quality of service (QoS) degradation and capacity underutilization are observed due to this imbalance traffic distribution, which is the main concern of research in this work.

The first outcome of the work explores the inherent problems of IEEE 802.11 handover management policies, and proposes a context-aware handover mechanism to balance traffic load across the network. The proposed mechanism works in coordination of information exchange between the AP and the wireless client that experiences performance degradation due to traffic overload at its present point of attachment. This coordination helps the wireless client to perform a horizontal handover to another AP in the vicinity, that significantly improves the network capacity. The performance of the proposed context aware handover mechanism is analyzed using theoretical analysis as well as from practical testbed results. The second contribution of the work extends the context aware handover to incorporate multiple traffic classes, where different traffic classes require different amount of bandwidth to sustain for acceptable quality of experience (QoE) to the end users. Consequently, a class aware load balancing is designed to reserve traffic resources a priori when an impending handover is observed. The effect of class aware load balancing and context aware handover over the QoS and QoE of multiple traffic classes has been analyzed using practical testbed results. To this end, the class aware load balancing and the context aware handover mechanism are further extended for vertical handover between WiFi and IEEE 802.16 or WiMAX networks, as the third contribution of this work. The performance of the proposed set of context aware vertical handover management schemes is analyzed using simulation results, and compared with other existing mechanisms.