Irfan Sheriff

Routing stability in static wireless mesh networks

Considerable research has focused on the design of routing protocols for wireless mesh networks. Yet, little is understood about the stability of routes in such networks. This understanding is important in the design of wireless routing protocols, and in network planning and management. In this paper, we present results from our measurement-based characterization of routing stability in two network deployments, the UCSB MeshNet and the MIT Roofnet. To conduct these case studies, we use detailed link quality information collected over several days from each of these networks. Using this information, we investigate routing stability in terms of route-level characteristics, such as prevalence, persistence and flapping. Our key findings are the following: wireless routes are weakly dominated by a single route; dominant routes are extremely short-lived due to excessive route flapping; and simple stabilization techniques, such as hysteresis thresholds, can provide a significant improvement in route persistence.

Multipath Selection in Multi-radio Mesh Networks

Research has shown that multi-radio multi-channel mesh networks provide significant capacity gains over single-radio mesh networks. Traditional single path routing can lead to poor utilization of the available channels in these networks. Opportunistic multipath routing can better exploit the available channel diversity in a multi-radio network. The goal of this paper is to select multiple paths that, when used concurrently, provide high end-to-end throughput. To this end, we present a metric for multipath selection in multi-radio networks. We evaluate the metric through simulations in Qualnet and show that intelligent multipath routing significantly outperforms single path routing in multi-radio mesh networks.

TDM MAC protocol design and implementation for wireless mesh networks

We present the design, implementation, and evaluation of a Time Division Multiplex (TDM) MAC protocol for multi-hop wireless mesh networks using a programmable wireless platform. Extensive research has been devoted to optimal scheduling algorithms for multi-hop wireless networks assuming a perfect TDM MAC protocol. However, the problem of designing and implementing such a protocol has not received due attention. We introduce a design framework that addresses the three main challenges that comprise this problem: (i) How to calibrate and optimize the TDM MAC protocol parameters given a wireless platform, (ii) how to achieve network-wide synchronization with high accuracy, minimal overhead, and most importantly, bounded delay, and (iii) how to integrate the synchronization algorithm with the TDM MAC protocol state machine using minimal hardware resources. We apply our design framework to our platform and evaluate the resulting TDM MAC protocol through controlled experiments in a wireless mesh testbed. The results demonstrate the protocols ability to provide fairness and graceful performance degradation under packet losses and multi-hop traffic patterns that arise in mesh network deployments.

A multi-radio 802.11 mesh network architecture

The focus of this paper is to offer a practical multi-radio mesh network architecture that can realize the benefits of multiple radios. Our architecture provides solutions to challenges in three key areas. The first is the construction of a split wireless router that enables modular wireless mesh routers to be constructed from commodity hardware. The second is the design of a centralized channel assignment algorithm that considers the inter-dependence between channel assignment and routing in order to create high-throughput channel-diversified routes. Third is the design and implementation of several communication protocols that are necessary to make our architecture operational. Our system is comprehensively evaluated on a 20-node multi-radio wireless testbed. Results demonstrate that our architecture makes feasible the deployment of large-scale high-capacity multi-radio mesh networks built entirely with commodity hardware. Our implementation is available to the community for research and development purposes.

Effect of payload length variation and retransmissions on multimedia in 802.11a WLANs

Multimedia transmission over wireless local area networks is challenging due to the varying nature of the wireless channel as well as the inherent difference between multimedia and data traffic. In the MAC layer, a single bit error in the packet can lead to the entire packet being discarded. This results in a higher packet error rate for larger payload sizes. Retransmission due to packet errors causes the contention window to double, and this leads to a decrease in throughput if the wireless channel does not improve for the retransmitted packets. Hence, throughput is a function of packet payload length as well as the maximum number of allowable retransmissions. In this paper, we investigate the effect of payload length adaptation and retransmissions on the throughput and capacity of multimedia users. Numerical results and simulations reveal that careful payload adaptation significantly improves the throughput performance at low signal to noise ratios (SNRs). It is also observed that excessive retransmissions reduce the effective throughput, thereby decreasing the capacity of multimedia users in the presence of data users. Since multimedia traffic is more latency constrained and less error constrained, by carefully selecting the payload length and maximum number of allowable retransmissions based on the channel conditions, a greater number of multimedia users can be supported.

Measurement-driven admission control on wireless backhaul networks

IEEE 802.11 wireless networks perform poorly in the presence of large traffic volumes. Measurements have shown that packet collisions and interference can lead to degraded performance to the extent that users experience unacceptably low throughput, which can ultimately lead to complete network breakdown [12]. An admission control framework that limits network flows can prevent network breakdown and improve the performance of throughput and delay-sensitive multimedia applications. In this paper, we present a measurement-driven admission control scheme that leverages wireless characteristics for intelligent flow control in a static wireless network. Experiments on the 25 node UCSB MeshNet show that the proposed admission control scheme can enhance network performance such that the QoS requirements of real time applications, such as VoIP, can be met.

Integrated Data Location in Multihop Wireless Networks

Multihop wireless networks are ideal as infrastructures for location-aware network applications, particularly for disaster recovery operations. However, one missing component is an efficient and scalable distributed data location service. Existing approaches impose significant communication overhead on the underlying wireless layer and generally limit the total number of locatable objects in a network. To address this problem, we present the integrated data location protocol (IDLP), which provides scalable location of a large number of objects by integrating compressed summaries of object signatures into the routing layer. We evaluate our approach using extensive simulations in Qualnet, as well as detailed measurements from a deployed AODV-implementation on the UCSB MeshNet testbed. Results show that IDLP maintains low communication overhead while efficiently locating up to a hundred objects per node.