Sachin Garg

Can I add a VoIP call

In this paper, we study the inherent limitations of the 802.11 (a/b) distributed coordination function (DCF) in supporting VoIP calls over a wireless LAN. Specifically, we evaluate the upper bound on the number of simultaneous VoIP calls that can be placed in a single cell of an 802.11 (a/b) network. Making one additional VoIP call in that cell would degrade the quality of all VoIP call. The upper bound is calculated as a function of the choice of VoIP codec and the length of the audio payload. As an example, when a G711 codec with 20 millisecond audio payload is used, an 802.11b cell can support only 3 to 12 simultaneous VoIP calls. The actual number depends on the effective transmission rate of the wireless station, which for 802.11b can be 1 Mbps, 5.5 Mbps and 11 Mbps. We also study the effect of spatial distribution of the wireless stations on the upper bound which is the dominant factor in determining the effective transmission rate of a station.

An experimental study of throughput for UDP and VoIP traffic in IEEE 802.11b networks

We present experimental studies on the throughput of IEEE 802.11b wireless networks for UDP and VoIP traffic. Our experiments show that the maximum data throughput of a single station sending out UDP traffic is 6.1 Mbps. The maximum number of VoIP calls in a single cell of an IEEE 802.11b network is six if the ITU G711a-Law codec is used with 10 milliseconds of audio per RTP packet. The experiments also show that the effective available bandwidth in the wireless network is reduced by ongoing VoIP connections. Specifically, for the above codec settings, each VoIP connection reduces the bandwidth available for data traffic by 900 kbps.

Network survivability performance evaluation:: a quantitative approach with applications in wireless ad-hoc networks

Network survivability reflects the ability of a network to continue to function during and after failures. Our purpose in this paper is to propose a quantitative approach to evaluate network survivability. We perceive the network survivability as a composite measure consisting of both network failure duration and failure impact on the network. A wireless ad-hoc network is analyzed as an example, and the excess packet loss due to failures (ELF) is taken as the survivability performance measure. To obtain ELF, we adopt a two phase approach consisting of the steady-state availability analysis and transient performance analysis. Assuming Markovian property for the system, this measure is obtained by solving a set of Markov models. By utilizing other analysis paradigms, our approach in this paper may also be applied to study the survivability performance of more complex systems.

Response prediction using collaborative filtering with hierarchies and side-information

In online advertising, response prediction is the problem of estimating the probability that an advertisement is clicked when displayed on a content publishers webpage. In this paper, we show how response prediction can be viewed as a problem of matrix completion, and propose to solve it using matrix factorization techniques from collaborative filtering (CF). We point out the two crucial differences between standard CF problems and response prediction, namely the requirement of predicting probabilities rather than scores, and the issue of confidence in matrix entries. We address these issues using a matrix factorization analogue of logistic regression, and by applying a principled confidence-weighting scheme to its objective. We show how this factorization can be seamlessly combined with explicit features or side-information for pages and ads, which let us combine the benefits of both approaches. Finally, we combat the extreme sparsity of response prediction data by incorporating hierarchical information about the pages and ads into our factorization model. Experiments on three very large real-world datasets show that our model outperforms current state-of-the-art methods for response prediction.

Admission control for VoIP traffic in IEEE 802.11 networks

In this paper, we propose a metric for measuring the utilization of an IEEE 802.11b wireless network and outline how this metric can be accurately estimated using data that is readily available in most access points. We furthermore describe how this metric can be used to perform admission control for VoIP traffic and describe experiences with a prototype implementation. Admission control for VoIP traffic in 802.11 networks is necessary since the number of simultaneous VoIP connections in a single cell of an 802.11 network is very small.

Optimal software rejuvenation for tolerating soft failures

Abstract In recent studies, the phenomenon of software “aging” has come to light which causes performance of a software to degrade with time. Software rejuvenation is a fault tolerance technique which counteracts aging. In this paper, we address the problem of determining the optimal time to rejuvenate a server type software which experiences “soft failures” (witnessed in telecommunication systems) because of aging. The service rate of the software gradually decreases with time and settles to a very low value. Since the performability in this state is unacceptable, it is necessary to “renew” the software to its peak performance level. We develop Markov decision models for such a system for two different queuing policies. For each policy, we define the look-ahead- n cost functions and prove results on the convergence of these functions to the optimal minimal cost function. We also prove simple rules to determine optimal times to rejuvenate for a realistic cost criterion. Finally, the results are illustrated numerically and the effectiveness of the MDP model is compared with that of the simple rules.

Performance and reliability evaluation of passive replication schemes in application level fault tolerance

Process replication is provided as the central mechanism for application level software fault tolerance in SwiFT and DOORS. These technologies, implemented as reusable software modules, support cold and warm schemes of passive replication. The choice of a scheme for a particular application is based on its availability and performance requirements. In this paper we analyze the performability of a server software which may potentially use these technologies. We derive closed form formulae for availability throughput and probability of loss of a job. Six scenarios of loss are modeled and for each, these expressions are derived. The formulae can be used either of time or online to determine the optimal replication scheme.

Dependability enhancement for IEEE 802.11 wireless LAN with redundancy techniques

The presence of physical obstacles and radio interference results in the so called “shadow regions” in wireless networks. When a mobile station roams into a shadow region, it loses its network connectivity. In cellular networks, in order to minimize the connection unreliability, careful cell planning is required to prevent the occurrance of the shadow regions in the first place. In 802.11b/g wireless LANs, however, due to the limited frequency spectrum, it is not always possible to prevent a shadow region by adding another cell at a different frequency. Our contribution in this paper is to propose the alternate approach of tolerating the existence of “shadow regions” as opposed to prevention in order to enhance the connection dependability. A redundant access point (AP) is placed in the shadow region to serve the mobile stations which roam into that region. Since the redundant AP operates on the same frequency as the primary AP, it does not constitute a separate cell. In fact, the primary and the secondary AP communicate to grant medium access to stations within the shadow region. We consider two configurations, which differ in how the two APs communicate with each other. In the first, the secondary AP is connected to the same distribution system as the primary AP. In the second, the secondary AP acts as a wireless forwarding bridge for traffic to/from the mobile stations in the shadow region to the primary AP. The paper outlines the details of how redundancy may be implemented by making enhancements to the basic 802.11 channel access protocol. To evaluate the dependability of the network under study, we present the reliability, availability and survivability analysis of the two configurations and compare them with the scheme with no redundancy. With numerical examples, we show that the redundancy schemes demonstrate significant improvement in connection dependability over Mobile terminal

Learning URL patterns for webpage de-duplication

Presence of duplicate documents in the World Wide Web adversely affects crawling, indexing and relevance, which are the core building blocks of web search. In this paper, we present a set of techniques to mine rules from URLs and utilize these rules for de-duplication using just URL strings without fetching the content explicitly. Our technique is composed of mining the crawl logs and utilizing clusters of similar pages to extract transformation rules, which are used to normalize URLs belonging to each cluster. Preserving each mined rule for de-duplication is not efficient due to the large number of such rules. We present a machine learning technique to generalize the set of rules, which reduces the resource footprint to be usable at web-scale. The rule extraction techniques are robust against web-site specific URL conventions. We compare the precision and scalability of our approach with recent efforts in using URLs for de-duplication. Experimental results demonstrate that our approach achieves 2 times more reduction in duplicates with only half the rules compared to the most recent previous approach. Scalability of the framework is demonstrated by performing a large scale evaluation on a set of 3 Billion URLs, implemented using the MapReduce framework.

Modeling software systems with rejuvenation, restoration and checkpointing through fluid stochastic Petri nets

In this paper, we present a Fluid Stochastic Petri Net (FSPN) based model which captures the behavior of aging software systems with checkpointing, rejuvenation and self-restoration, three well known techniques of software fault tolerance. The proposed FSPN based modeling framework is novel in many aspects. First, the FSPN formalism itself is extended by adding flush-out arcs. Second, the three techniques are simultaneously captured in a single model for the first time. Third, the formalism enables modeling dependencies of the three techniques on various system features such as failure, load and time in the same framework. Further, our base FSPN model can be viewed as a generalization of most previous models in the literature. We show that these FSPNs can not only mimic previously published models but can also extend them. For one FSPN model, we present numerical results to illustrate their usage in deriving measures of interest.