Saquib Razak

Measurement and Analysis of Link Quality in Wireless Networks: An Application Perspective

Estimating the quality of wireless link is vital to optimize several protocols and applications in wireless networks. In realistic wireless networks, link quality is generally predicted by measuring received signal strength and error rates. Understanding the temporal properties of these parameters is essential for the measured values to be representative, and for accurate prediction of performance of the system. In this paper, we analyze the received signal strength and error rates in an IEEE 802.11 indoor wireless mesh network, with special focus to understand its utility to measurement based protocols. We show that statistical distribution and memory properties vary across different links, but are predictable. Our experimental measurements also show that, due to the effect of fading, the packet error rates do not always monotonically decrease as the transmission rate is reduced. This has serious implications on many measurementbased protocols such as rate-adaptation algorithms. Finally, we describe real-time measurement framework that enables several applications on wireless testbed, and discuss the results from example applications that utilize measurement of signal strength and error rates

Modeling and analysis of two-flow interactions in wireless networks

Interference plays a complex and often defining role in the performance of wireless networks, especially in multi-hop scenarios. In the presence of interference, Carrier sense multiple access MAC protocols are known to suffer from the hidden terminal and exposed terminal problems, which can cause poor performance and unfairness. In this paper, we examine the possible interference modes arising among two interfering one-hop connections under a two-disc model of interference. We classify the large set of resulting configurations into five categories and develop closed form expressions to compute their probability of occurrence. The analysis exposes two new categories, whose occurrence is common, and whose behavior differs significantly from the three known interference categories. Further, the frequency of occurrence of the categories differ significantly from existing results (obtained with a simpler unit disc model of interference). We develop throughput estimation models for the different categories and validate them using simulation.

How do wireless chains behave?: the impact of MAC interactions

In a Multi-hop Wireless Networks (MHWN), packets are routed between source and destination using a chain of intermediate nodes; chains are a fundamental communication structure in MHWNs whose behavior must be understood to enable building effective protocols. The behavior of chains is determined by a number of complex and interdependent processes that arise as the sources of different chain hops compete to transmit their packets on the shared medium. In this paper, we show that MAC level interactions play the primary role in determining the behavior of chains. We evaluate the types of chains that occur based on the MAC interactions between different links using realistic propagation and packet forwarding models. We discover that the presence of destructive interactions, due to different forms of hidden terminals, does not impact the throughput of an isolated chain significantly. However, due to the increased number of retransmissions required, the amount of band-width consumed is significantly higher in chains exhibiting destructive interactions, substantially influencing the over-all network performance. These results are validated by testbed experiments. We finally study how different types of chains interfere with each other and discover that well behaved chains in terms of self-interference are more resilient to interference from other chains.

Analysis of TCP performance on multi-hop wireless networks: A cross layer approach

In Multi-Hop Wireless Networks (MHWNs), wireless nodes cooperate to forward traffic between end points that are not in direct communication range. Specifically, traffic is forwarded from a source towards its destination through intermediate nodes that form a wireless multi-hop chain. Researchers have studied the performance of TCP over chains discovering properties such as how the number of hops reduces chain throughput as neighboring links contend for the shared medium. Moreover, the presence of hidden terminals has also been shown to negatively affect performance of example chains. In this paper, we leverage recent characterization of how competing wireless links interact to develop an in-depth analysis of TCP performance over wireless chains. In particular, there are a number of possible modes of interference between competing links with distinct implications on performance and fairness; to our knowledge, this is the first work that studies the impact of these different modes on TCP chain performance. We classify chains according to interference modes considering both the forward (data) and reverse (acknowledgment) traffic. Chain geometry limits the types of chains that arise most frequently in practice. We evaluate TCP performance over the most frequently occurring chain types and observe significant performance differences between chains that have the same hop count. Different four-hop chains, for example, show a throughput difference of up to 25% and a retransmission overhead difference of over 90%. We discuss the implications of these differences on network performance: specifically, route instability and bandwidth usage generated. We extend this analysis to two single-hop TCP flows and quantify the effect of interference interactions between two flows. This study is a first step towards completely understanding the performance of multiple TCP flows over multiple hops in a MHWN.

Self-interference in Multi-hop Wireless Chains: Geometric Analysis and Performance Study

In the presence of interference, two single hop links can interact in a number of different ways, exhibiting significantly different behavior. In this paper, we consider the impact of these two-flow interactions on multi-hop chains. Specifically, we characterize the different types of interactions that arise in chains between hops that do not share a common node. We develop closed formed expressions to estimate the probability of occurrence of these interaction combinations. We use simulation to characterize the performance of the most common types of chains. We make a number of interesting observations: (1) the most destructive types of two-flow interactions do not arise commonly in chains; (2) the throughput of chains does not vary significantly with the types of arising interactions, because of the self-regulating effect of packets in the chain (later hops can only transmit when they receive packets from earlier ones); however, (3) the chains exhibiting destructive interactions suffer frequent collisions and require many more retransmissions. As such, in general scenarios, such chains reduce the available bandwidth within the network.

Link quality analysis and measurement in wireless mesh networks

Protocols and applications in wireless mesh networks often optimize their performance by measuring the quality of wireless links. However, measuring and characterizing link-quality is a challenging task due to the nature of wireless channel and device-specific properties of radios. The paper proposes two aspects of link-quality measurement and estimation in realistic networks that benefit higher-layer protocols. First, we analyze the statistical properties of link-quality metrics, such as received signal strength and packet error rates, in an indoor IEEE 802.11 mesh network. We show that the statistical distribution and memory properties vary across different links, but are predictable. The next contribution of the paper is a real-time measurement framework that enables higher-level protocols in wireless mesh networks. We discuss the architectural requirements and our implementation experiences of a measurement framework. In addition, we provide three concrete applications that use the measured link-quality and statistical inference to better adapt their behavior.

TCP over Multi-Hop Wireless Networks: The Impact of MAC Level Interactions

In Multi Hop Wireless Networks (MHWNs), nodes act both as end-hosts as well as intermediate routers. When communication occurs, these nodes form chains between different sources and destinations. Researchers have studied how these chains behave, discovering that MAC level interactions play a major role in determining their performance. In this paper, we extend this analysis to study how TCP connections, which involve bidirectional flows, behave over wireless chains. First, we break down and examine the types of chains that occur most frequently in TCP configurations and classify them by the nature of the MAC level interactions that arise in each. We then show that the throughput of TCP over a wireless chain is greatly affected by the type of interactions within the chain. Finally, we show the implications of the MAC level interactions on network performance: specifically, route instability and number of retransmissions.

Interference across Multi-hop Wireless Chains

Chains or multi-hop paths are the fundamental communication structure in Multi-Hop Wireless Networks. Understanding chain behavior is critical in order to build effective higher layer protocols. This paper examines the problem of how MAC level interactions influence chain behavior in a general multi-hop wireless network where multiple chains coexist. We first classify chains based on the MAC interactions observed between its hops when there is no external traffic. Then we identify the interactions across two interfering chains for the most common categories of chains. We study the probability of occurrence, and estimate the effect of MAC interactions on the performance of the chains. We also show that different chains exhibit different transmission patterns; this is an effect that is necessary for accurately estimating chain performance. We observe that destructive interactions arise more frequently among two interfering chains than they do within a single chain. Moreover, chains that have hidden terminals due to self-interference are more prone to have cross-chain hidden terminals. Thus, both intra-chain as well as cross-chain interactions, ultimately provide significant insight into how chains interact.

Modeling and Analysis of Two-Flow Interactions in Wireless Networks

Interference plays a complex and often defining role in the performance of wireless networks, especially in multi-hop scenarios. In the presence of interference, Carrier sense multiple access MAC protocols are known to suffer from the hidden terminal and exposed terminal problems, which can cause poor performance and unfairness. In this paper, we examine the possible interference modes arising among two interfering one-hop connections under a two-disc model of interference. We classify the large set of resulting configurations into five categories and develop closed form expressions to compute their probability of occurrence. The analysis exposes two new categories, whose occurrence is common, and whose behavior differs significantly from the three known interference categories. Further, the frequency of occurrence of the categories differ significantly from existing results (obtained with a simpler unit disc model of interference). We develop throughput estimation models for the different categories and validate them using simulation.

A MAC interaction aware routing metric in wireless network

Carrier Sense Multiple Access (CSMA) based MAC protocols induce several types of harmful interactions, such as hidden and exposed terminals, in wireless networks. Existing routing protocols do not consider the effect of these MAC interactions on route quality, leading to the selection of inefficient routes. We propose a MAC Interaction Aware Routing Metric (MIAR) that explicitly accounts for MAC interactions among the links forming a route. The protocol favors routes with links that have better interactions and avoids ones with detrimental interactions. We compare the performance of the proposed metric with an existing shortest-path routing protocol and show that our metric substantially improves the performance and efficiency of the network.