Surendar Chandra

Wireless network interface energy consumption: implications for popular streaming formats

Abstract.With the proliferation of mobile streaming multimedia, available battery capacity constrains the end-user experience. Since streaming applications are expected to be long running, wireless network interface cards (WNIC) energy consumption is particularly an acute problem. In this work, we explore various mechanisms to conserve client WNIC energy consumption for popular streaming formats such as Microsoft Windows media, Real and Apple Quicktime. First, we investigate the WNIC energy consumption characteristics for these popular multimedia streaming formats under varying stream bandwidth and network loss rates. We show that even for a high bandwidth 2000 kbps stream, the WNIC unnecessarily spent over 56% of the time in idle state; illustrating the potential for significant energy savings.Based on these observations, we explore two mechanisms to conserve the client WNIC energy consumption. First we show the limitations of IEEE 802.11 power saving mode for multimedia streams. Without an understanding of the stream requirements, these scheduled rendezvous mechanisms do not offer any energy savings for multimedia streams over 56 kbps. We also develop history-based client-side strategies to reduce the energy consumed by transitioning the WNICs to a lower power consuming sleep state. We show that streams optimized for 28.8 kbps can save over 80% in energy consumption with 2% data loss. A high bandwidth stream (768 kbps) can still save 57% in energy consumption with less than 0.3% data loss. We also show that Real and Quicktime packets are harder to predict at the network level without understanding the packet semantics. As the amount of cross traffic generated by other clients that share the same wireless segment increases, the potential energy savings from our client side policies deteriorate further. Our work enables multimedia proxy and server developers to suitably customize the stream to lower client energy consumption.

A client-side statistical prediction scheme for energy aware multimedia data streaming

The recent proliferation of streaming multimedia on a variety of mobile devices has severely tested their battery lifetime. The long running nature of typical streaming applications results in significant energy consumption by the wireless network interface card (WNIC) in these mobile devices. In this paper we explore linear prediction-based client-side strategies that reduce the WNIC energy consumption to receive multimedia streams by judiciously transitioning the WNIC to a lower power consuming sleep state during the no-data intervals in the multimedia stream, without explicit support from the multimedia servers themselves. Experimental results on popular streaming formats such as Microsoft Media, Real and Apple QuickTime show that a linear prediction-based strategy performs better than history-based strategies that use simple temporal averaging

Trace driven analysis of the long term evolution of Gnutella peer-to-peer traffic

Peer-to-Peer (P2P) applications, such as Gnutella, are evolving to address some of the observed performance issues. In this paper, we analyze Gnutella behavior in 2003, 2005, and 2006. During this time, the protocol evolved from v0.4 to v0.6 to address problems with overhead of overlay maintenance and query traffic bandwidth. The goal of this paper is to understand whether the newer protocols address the prior concerns. We observe that the new architecture alleviated the bandwidth consumption for low capacity peers while increasing the bandwidth consumption at high capacity peers. We measured a decrease in incoming query rate. However, highly connected ultra-peers must maintain many connections to which they forward all queries thereby increasing the outgoing query traffic. We also show that these changes have not significantly improved search performance. The effective success rate experienced at a forwarding peer has only increased from 3.5% to 6.9%. Over 90% of queries forwarded by a peer do not result in any query hits. With an average query size of over 100 bytes and 30 neighbors for an ultra-peer, this results in almost 1 GB of wasted bandwidth in a 24 hour session. We outline solution approaches to solve this problem and make P2P systems viable for a diverse range of applications.

Dynamic, power-aware scheduling for mobile clients using a transparent proxy

Mobile computers consume significant amounts of energy when receiving large files. The wireless network interface card (WNIC) is the primary source of this energy consumption. One way to reduce the energy consumed is to transmit the packets to clients in a predictable fashion. Specifically, the packets can be sent in bursts to clients, who can then switch to a lower power sleep state between bursts. This technique is especially effective when the bandwidth of a stream is small. This work investigates techniques for saving energy in a multiple-client scenario, where clients may be receiving either UDP or TCP data. Energy is saved by using a transparent proxy that is invisible to both clients and servers. The proxy implementation maintains separate connections to the client and server so that a large increase in transmission time is avoided. The proxy also buffers data and dynamically generates a global transmission schedule that includes all active clients. Results show that energy savings within 10-15% of optimal are common, with little packet loss.

Revisiting multimedia streaming in mobile ad hoc networks

Mobile ad hoc networks have been the subject of active research for a number of years. This paper investigates the feasibility of using such networks for transmitting multimedia streams. We observe that wireless network IO operations can be expensive (e.g., programmed IO cost, energy to operate wireless). Moreover, compared to nodes in infrastructure networks that either read or write network traffic, ad hoc traffic requires the intermediate node to perform many expensive network operations twice (read and then resend) and on behalf of other nodes. This observation raises an important question for the ad hoc community, should they a) demand that ad hoc routers support some minimum hardware resources (for example, full DMA support, twice the battery capacity)?, b) force an end-to-end resource management scheme that cooperatively reduces the network flow to half of what can be serviced by the weakest link? This would ensure that no intermediate node would see enough traffic to overwhelm them? or c) require that the local nodes protect themselves from transit traffic? This paper explores the last mechanism in order to provide some control over the resource consumed without a major revamp of existing operating systems or requiring special hardware. We implement our mechanism in the network driver and present encouraging preliminary results.

A statistical prediction-based scheme for energy-aware multimedia data streaming

The proliferation of multimedia-capable mobile devices and ubiquitous high-speed network technologies to deliver multimedia objects has fueled the demand of mobile streaming multimedia. A necessary criterion for the mass acceptance of mobile devices is acceptable battery life of these devices. This paper explores linear prediction-based client-side strategies to reduce the wireless network interface card (WNIC) energy consumption by transitioning the WNIC to a lower power consuming sleep state. The basic idea of this strategy is to selectively choose proper periods of time to suspend communication by switching the WNIC to sleep state. A linear prediction-based time series forecasting technique is used to predict future no-data intervals. Simulation results show that linear prediction-based strategy gives better results than those based on simple averaging [Surendar Chandra et al., (2002)].

Improving Search Using a Fault-Tolerant Overlay in Unstructured P2P Systems

Gnutella overlays have evolved to use a two-tier topology. However, we observed that the new topology had only achieved modest improvements in search success rates. Also, the new two-tier topology had not reduced the message routing overhead and bandwidth consumption. In this work, we used local information at each node to construct an overlay, Makalu, that improved search performance and reduced bandwidth consumption. The overlay maximized the expansion from each nodes neighborhood while minimizing the latency to its neighbors. We show that for a 100,000 node system, wild card searches using flooding successfully resolved most queries within four hops for object replications ratios as lows as 0.05% (50 randomly distributed copies) with less than 3% duplicate messages. Using attenuated bloom filters to route messages for exact identifier searches, we show that Makalu resolved most queries with less than ten messages for networks as large as 100,000 nodes. The performance of this search is comparable to that of structured P2P systems. Finally, using data from traffic traces of Gnutella in 2003 and 2006, we demonstrated search success rates that were up to five times more successful and required 75% less bandwidth on a Makalu overlay than on a modern Gnutella overlay.

Understanding the practical limits of the Gnutella P2P system: an analysis of query terms and object name distributions

A number of prior efforts analyzed the behavior of popular peer-to-peer (P2P) systems and proposed ways for maintaining the overlays as well as methods for searching for contents using these overlays. However, little was known about how successful users could be in locating the shared objects in these system. There might be a mismatch between the way content creators named objects and the way such objects were queried by the consumers. Our aim was to examine the terms used in the queries and shared object names in the Gnutella file-sharing system. We analyzed the object names of over 20 million objects collected from 40,000 peers as well as terms from over 230,000 queries. We observed that almost half (44.4%) of the queries had no matching objects in the system regardless of the overlay or search mechanism used to locate the objects. We also evaluated the query success rates against random peer groups of various sizes (200, 1K, 2K, 3K, 4K, 5K, 10K and 20K peers sampled from the full 40,000 peers). We showed that the success rates increased rapidly from 200 to 5,000 peers, but only exhibited modest improvements when increasing the number of peers beyond 5,000. Finally, we observed Zipf-like distribution for query terms and the object names. However, the relative popularity of a term in the object names did not correlate with the terms popularity in the query workload. This observation affected the ability of hybrid P2P systems to guide searches by creating a synopsis of the peer object names. A synopsis created by using the distribution of terms in the object names need not represent relevant terms for the query. Our results can be used to guide the design of future P2P systems that are optimized for the observed object names and user query behavior.

On the need for query-centric unstructured peer-to-peer overlays

Hybrid P2P systems rely on the assumption that sufficient objects exist nearby in order to make the unstructured search component efficient. This availability depends on the object annotations as well as on the terms in the queries. Earlier work assumed that the object annotations and query terms follow Zipf-like long-tail distribution. We show that the queries in real systems exhibit more complex temporal behavior. To support our position, first we analyzed the names and annotations of objects that were stored in two popular P2P sharing systems; Gnutella and Apple iTunes. We showed that the names and annotations exhibited a Zipf like long tail distribution. The long tail meant that over 98% of the objects were insufficiently replicated (less than 0.1% of the peers). We also analyzed a query trace of the Gnutella network and identified the popularity distribution of the terms used in the queries. We showed that the set of popular query terms remained stable over time and exhibited a similarity of over 90%. We also showed that despite the Zipf popularity distributions of both query terms and file annotation terms, there was little similarity over time (<20%) between popular file annotation terms and popular file terms. Prior P2P search performance analysis did not take this mismatch between the query terms and object annotations into account and thus overestimated the system performance. There is a need to develop unstructured P2P systems that are aware of the temporal mismatch of the object and query popularity distributions.

Automated Storage Reclamation Using Temporal Importance Annotations

This work focuses on scenarios that require the storage of large amounts of data. Such systems require the ability to either continuously increase the storage space or reclaim space by deleting contents. Traditionally, storage systems relegated object reclamation to applications. In this work, content creators explicitly annotate the object using a temporal importance function. The storage system uses this information to evict less important objects. The challenge is to design importance functions that are simple and expressive. We describe a two step temporal importance function. We introduce the notion of storage importance density to quantify the importance levels for which the storage is full. Using extensive simulations and observations of a university wide lecture video capture and storage application, we show that our abstraction allows the users to express the amount of persistence for each individual object.