Parameshwaran Krishnan

A system for LEASE: location estimation assisted by stationary emitters for indoor RF wireless networks

We present LEASE, a new system and framework for location estimation assisted by stationary emitters for indoor RF wireless networks. Unlike previous studies, we emphasize the deployment aspect of location estimation engines. Motivated thus, we present an adaptable infrastructure-based system that uses a small number of stationary emitters (SEs) and sniffers employed in a novel way to locate standard wireless clients in an enterprise. We present the components of the system and its architecture, and new non-parametric techniques for location estimation that work with a small number of SEs. Our techniques for location estimation can also be used in a client-based deployment. We present experimental results of using our techniques at two sites demonstrating the ability to perform location estimation with good accuracy in our new adaptable framework.

Supporting soft real-time tasks in the xen hypervisor

Virtualization technology enables server consolidation and has given an impetus to low-cost green data centers. However, current hypervisors do not provide adequate support for real-time applications, and this has limited the adoption of virtualization in some domains. Soft real-time applications, such as media-based ones, are impeded by components of virtualization including low-performance virtualization I/O, increased scheduling latency, and shared-cache contention. The virtual machine scheduler is central to all these issues. The goal in this paper is to adapt the virtual machine scheduler to be more soft-real-time friendly. We improve two aspects of the VMM scheduler -- managing scheduling latency as a first-class resource and managing shared caches. We use enterprise IP telephony as an illustrative soft real-time workload and design a scheduler S that incorporates the knowledge of soft real-time applications in all aspects of the scheduler to support responsiveness. For this we first define a laxity value that can be interpreted as the target scheduling latency that the workload desires. The load balancer is also designed to minimize the latency for real-time tasks. For cache management, we take cache-affinity into account for real time tasks and load-balance accordingly to prevent cache thrashing. We measured cache misses and demonstrated that cache management is essential for soft real time tasks. Although our scheduler S employs a different design philosophy, interestingly enough it can be implemented with simple modifications to the Xen hypervisors credit scheduler. Our experiments demonstrate that the Xen scheduler with our modifications can support soft real-time guests well, without penalizing non-real-time domains.

On the accuracy of signal strength-based estimation techniques

In this paper, we address the problem of finding the inherent uncertainty of signal strength-based location estimation techniques. We propose a mathematical model for mapping uncertainty in signal strength space to uncertainty in physical space. We then analyze this model to compute the minimum value of the uncertainty in location estimation using signal strength measurements. The results of this analysis are used to draw conclusions about the dependence of the minimum uncertainty of various factors such as the signal variance, number of APs, distance between the APs and the propagation constant. We provide an argument linking the minimum uncertainty with a lower limit on the median error in location estimation using classification techniques.

The theory and practice of signal strength-based location estimation

Location estimation enables many innovative applications and is an exciting area of research. With the growing use of wireless technology in enterprise networks, it is an interesting technical challenge to develop techniques for indoor location estimation that leverage the deployed wireless infrastructure. Elegant, cost-effective techniques would present a compelling business proposition. In this paper, we summarize various approaches researchers have studied for the problem of indoor location estimation, concentrating on signal-strength based techniques directed towards 802.11 wireless networks. We also summarize an interesting insight into the best possible accuracy achievable by any technique due to limits imposed by the physical behavior of the radio signal. We identify that the issues driving research work in this area will not only be location accuracy, but other factors like deployment ease, management simplicity, adaptability, and cost of ownership and maintenance. We conclude with some open research problems in this area

Performance implications of hosting enterprise telephony applications on virtualized multi-core platforms

Virtualization technology has gained significant adoption in various domains as a means to lower costs and enable greener solutions. Recently, there has been a significant amount of interest in employing virtualization technology in the telecommunications domain in order to save costs through server consolidation and to provide energy-efficient solutions. The availability of high-end multi-core servers provides powerful platforms for deployment. However, the telecommunications domain poses unique challenges for virtualization technology to be successfully deployed even in these compute-rich multi-core environments. This work discusses these challenges. It provides a detailed analysis of the performance implications of hosting enterprise IP telephony infrastructure in virtualized environments. Unlike signaling applications that are comparatively more tolerant of underlying platform performance, media applications are far more demanding. Our work, therefore, focuses on the performance of media applications (media server, voice-mail, etc.) in virtualized environments. We develop a model for workloads used in enterprise IP telephony. We then evaluate the impact of various hypervisor scheduler and I/O parameters in order to determine good parameter settings for such workloads. Our experiments use the Xen virtualization platform. The results presented in this work will be useful for telecommunication solution providers to understand the capabilities and limitations of virtual environments when deploying their applications.

VoIP network failure detection and user notification

The impact of network failures can be minimized if users are promptly notified by appropriately designed applications. Specifically, for voice-over-IP (VoIP) networks, an RTP/RTCP-based detection method can be used to rapidly distinguish between network congestion and network failures. Users and network managers can exploit this information in various ways, such as rapid network recovery or seeking alternatives at the application layer. In this paper, we present the main ideas behind these proposals, some analytical/simulation results, insights from a Linux-based implementation and experimental results.

Infrastructure-based location estimation in WLAN

This paper focuses on fundamental system deployment aspects of location estimation in 802.11-based wireless networks. We concentrate on adaptable infrastructure-based approaches, where sniffers measure received signal strength from clients to locate them. Our implementation experience and experimental results show that sniffer-based location estimation is feasible and works well provided some important rules are followed. By studying data over a 6-month period, we observe that adaptation of models is necessary for good location estimation, and we show that our techniques enable location estimation with minimal profiling. We also present an intriguing client-assisted approach for location estimation where a client, APs and sniffers collaborate to locale a terminal in enterprise (infrastructure-mode) wireless networks.

Estimating signal strength coverage for a wireless access point

Estimating the signal strength coverage of a wireless access point (AP), given its desired location at a site, is essential in many instances, e.g., to determine optimal AP placement. In this paper, we present a new non-parametric modeling solution, based on sampling, for this problem. We use sample measurements from several fixed APs in a novel way to compute a signal coverage model for an AP at a random location. Our technique automatically takes into account the signal strength propagation characteristics of the site and also allows for an efficient deployment methodology. We present an experimental study that validates the practical usability of the proposed technique.

Measurement Techniques in On-Demand Overlays for Reliable Enterprise IP Telephony

Maintaining good quality of service for real-time applications like IP Telephony requires quick detection and reaction to network impairments. In this paper, we propose and study novel measurement techniques in ORBIT, which is a simple, easily deployable architecture that uses single-hop overlays implemented with intelligent endpoints and independent relays. The measurement techniques provide rapid detection and recovery of IP Telephony during periods of network trouble. We study our techniques via detailed simulations of several multi-site enterprise topologies of varying sizes and three typical fault models. We show that our proposed techniques can detect network impairments rapidly and rescue IP Telephony calls in sub-second intervals. We observed that all impacted calls were rescued with only a few relays in the network and the run-time overhead was low. Furthermore, the relay sites needed to be provisioned with minimal additional bandwidth to support the redirected calls.

Hypervisor-assisted application checkpointing in virtualized environments

There are two broad categories of approaches used for checkpointing: application-transparent and application-assisted. Typically, application-assisted approaches provide a more flexible and light-weight mechanism but require changes to the application. Although most applications run well under virtualization (e.g. Xen which is being adopted widely), the addition of application-assisted checkpointing - used for high availability - causes performance problems. This is due to the overhead of key system calls used by the checkpointing techniques under virtualization. To overcome this, we introduce the notion of hypervisor-assisted application checkpointing with no changes to the guest operating system. We present the design and a Xen-based implementation of our family of application checkpointing techniques. Our experiments show performance improvements of 4× to 13× in the primitives used for supporting high availability compared to purely user-level approaches.