Jack Y. B. Lee

Parallel video servers: a tutorial

In conventional video-on-demand (VoD) systems, compressed digital video streams are stored in a video server for delivery to receiver stations over a communication network. This article introduces a framework for the design of parallel video server architectures and addresses three central architectural issues: video distribution architectures, server striping policies, and video delivery protocols.

On a unified architecture for video-on-demand services

Current video-on-demand (VoD)) systems can be classified into two categories: 1) true-Voll) (TVoD) and 2) near-VoD (NVod)). TVoD systems allocate a dedicated channel for every user to achieve short response times so that the user can select what video to play, when to play it, and perform interactive VCR-like controls at will. By contrast, NVoD systems transmit videos repeatedly over multiple broadcast or multicast channels to enable multiple users to share a single video channel so that system cost can be substantially reduced. The tradeoffs are limited video selections, fixed playback schedule, and limited or no interactive control. TVoD systems can be considered as one extreme where service quality is maximized, while NVoD systems can be considered as the other extreme where system cost is minimized. This paper proposes a novel architecture called Unified VoD) (UVoD) that can be configured to achieve cost-performance tradeoff anywhere between the two extremes (i.e., TVoD and NVoD). Assuming that a video client can concurrently receive two video channels and has local buffers for caching a portion of the video data, the proposed UVoD architecture can achieve significant performance gains (e.g., 400% more capacity for a 500-channel system) over TVoD under the same latency constraint. This paper presents the UVoD architecture, establishes a performance model, and analyzes UVoDs performance via numerical and simulation results.

Comparative Effectiveness and Safety of Oral Phosphodiesterase Type 5 Inhibitors for Erectile Dysfunction: A Systematic Review and Network Meta-analysis

CONTEXT Phosphodiesterase type 5 inhibitors (PDE5-Is) are currently the first-line therapy for erectile dysfunction (ED), but available studies investigating the comparative effects of different PDE5-Is are limited. OBJECTIVE To compare the efficacy and safety of different classes of oral PDE5-Is for ED. EVIDENCE ACQUISITION A systematic search was performed in PubMed, Cochrane Library, and Embase to identify randomized controlled trials that compared different PDE5-Is or PDE5-Is with a placebo for ED. The methodological quality of included studies was appraised with the Cochrane Collaboration bias appraisal tool, and the quality of evidence was evaluated using the Grading of Recommendations, Assessment, Development, and Evaluation system. EVIDENCE SYNTHESIS A total of 118 trials (31 195 individuals) were included. There was no major difference in the results between the traditional meta-analysis and the network meta-analysis. Network meta-analysis demonstrated that PDE5-Is were superior to placebo to improve erectile function. Compared with tadalafil (relative risk [RR]: 0.61; 95% confidence interval [CI], 0.33-0.90) and vardenafil (RR: 0.63; 95% CI, 0.35-0.92), avanafil was less effective on Global Assessment Questionnaire question 1. Tadalafil was more effective than vardenafil (mean difference [MD]: 1.49; 95% CI, 0.50-2.50) and udenafil (MD: -1.84; 95% CI, -3.31 to -0.33) as measured by the erectile function domain of the International Index of Erectile Function. For all efficacy outcomes, the absolute effects and the rank tests indicated that tadalafil and vardenafil were the most effective agents. After adjusting for dosage, the conclusion remained the same. Safety analysis showed there was no major difference among different agents. CONCLUSIONS In recommended doses, oral PDE5-Is are more effective than placebo for ED, and tadalafil seems to be the most effective agent, followed by vardenafil. PDE5-Is are generally safe and well tolerated, and there is no major difference on the safety profile.

Performance analysis of a pull-based parallel video server

In conventional video-on-demand systems, video data are stored in a video server for delivery to multiple receivers over a communications network. The video servers hardware limits the maximum storage capacity as well as the maximum number of video sessions that can concurrently be delivered. Clearly, these limits will eventually be exceeded by the growing need for better video quality and larger user population. This paper studies a parallel video server architecture that exploits server parallelism to achieve incremental scalability. First, unlike data partition and replication, the architecture employs data striping at the server level to achieve fine-grain load balancing across multiple servers. Second, a client-pull service model is employed to eliminate the need for interserver synchronization. Third, an admission-scheduling algorithm is proposed to further control the instantaneous load at each server so that linear scalability can be achieved. This paper analyzes the performance of the architecture by deriving bounds for server service delay, client buffer requirement, prefetch delay, and scheduling delay. These performance metrics and design tradeoffs are further evaluated using numerical examples. Our results show that the proposed parallel video server architecture can be linearly scaled up to more concurrent users simply by adding more servers and redistributing the video data among the servers.

Performance evaluation of an adaptive backoff scheme for WLAN

Summary In this paper, a simple self-adaptive contention window adjustment algorithm for 802.11 wireless local area networks (WLAN) is proposed and analyzed. Numerical results show that the new algorithm outperforms the standard 802.11 window adjustment algorithm. Compared with the standard and previously proposed enhancement algorithms, a salient feature of our algorithm is that it performs well in both heavy and light contention cases regardless of the packet sizes and physical versions. Moreover, the adaptive window adjustment algorithm is simpler than previously proposed schemes in that no live measurement of the WLAN traffic activity is needed. Copyright # 2004 John Wiley & Sons, Ltd.

A multiplexing scheme for H.323 voice-over-IP applications

Voice communications such as telephony are delay sensitive. Existing voice-over-IP (VoIP) applications transmit voice data in packets of very small size to minimize packetization delay, causing very inefficient use of network bandwidth. This paper proposes a multiplexing scheme for improving the bandwidth efficiency of existing VoIP applications. By installing a multiplexer in an H.323 proxy, voice packets from multiple sources are combined into one IP packet for transmission. A demultiplexer at the receiver-end proxy restores the original voice packets before delivering them to the end-user applications. Results show that the multiplexing scheme can increase bandwidth efficiency by as much as 300%. The multiplexing scheme is fully compatible with existing H.323-compliant VoIP applications and can be readily deployed.

UVoD: An unified architecture for video-on-demand services

Existing video-on-demand (VoD) systems can be classified into two categories: true-VoD (TVoD) and near-VoD (NVoD). TVoD systems allocate a dedicated channel for every user to achieve short latency. NVoD systems make use of multicast technologies to enable multiple users to share a single channel to reduce system cost. This paper proposes a VoD architecture called UVoD that unifies the existing TVoD and NVoD architectures by integrating unicast with multicast transmissions. A performance model of the system is derived and numerical results show that one can achieve significant performance gain over TVoD (over 500%) under the same latency constraints.

Automatic recovery from disk failure in continuous-media servers

Continuous-media (CM) servers have been around for some years. Apart from server capacity, another important issue in the deployment of CM servers is reliability. This study investigates rebuild algorithms for automatically rebuilding data stored in a failed disk into a spare disk. Specifically, a block-based rebuild algorithm is studied with the rebuild time and buffer requirement modeled. A buffer-sharing scheme is then proposed to eliminate the additional buffers needed by the rebuild process. To further improve rebuild performance, a track-based rebuild algorithm that rebuilds lost data in tracks is proposed and analyzed. Results show that track-based rebuild, while it substantially outperforms block-based rebuild, requires significantly more buffers (17-135 percent more) even with buffer sharing. To tackle this problem, a novel pipelined rebuild algorithm is proposed to take advantage of the sequential property of track retrievals to pipeline the reading and writing processes. This pipelined rebuild algorithm achieves the same rebuild performance as track-based rebuild, but reduces the extra buffer requirement to insignificant levels (0.7-1.9 percent). Numerical results computed using models of five commercial disk drives demonstrate that automatic rebuild of a failed disk can be done in a reasonable amount of time, even at relatively high server utilization (e.g., less than 1.5 hours at 90 percent utilization).

A transparent rate adaptation algorithm for streaming video over the Internet

The lack of end-to-end quality of service support in the current Internet has caused significant difficulties to ensuring playback continuity in video streaming applications. This study addresses this challenge by investigating a new adaptation algorithm to adjust the bit-rate of video data in response to the network bandwidth available to improve playback continuity. Unlike previous works, the proposed algorithm is transparent to the video client, requires no parameter tuning, and yet can outperform existing algorithms. This paper presents this algorithm, evaluates and compares its performance with the best algorithm currently available using extensive trace-driven simulations.

Design, performance analysis, and implementation of a super-scalar video-on-demand system

Despite the availability of video-on-demand (VoD) services in a number of cities around the world, large-scale deployment of VoD services in a metropolitan area is still economically impractical. This study presents a novel super-scalar architecture for building very large-scale and efficient VoD systems. The proposed architecture combines the use of static multicast, dynamic multicast, and intelligent client-side caching to vastly reduce server and network resource requirement. Moreover, in sharp contrast to conventional VoD systems where the system cost increases at least linearly with the system scale, the proposed architecture becomes more efficient as the system scales up and can ultimately be scaled up to serve any number of users while still keeping the startup latency short. This paper presents this new architecture, proposes methods to support interactive playback controls without the need for additional server or client resources, and derives an approximate performance model to relate the startup latency with other system parameters. The performance model is validated using simulation and the architecture is evaluated under various system settings. Lastly, a system implementation is presented and benchmarking results obtained to further verify the architecture, the performance model, and the simulation results.