Exploring wait tolerance in effective batching for video-on-demand scheduling

In a video-on-demand (VOD) environment, batching requests for the same video to share a common video stream can lead to significant improvement in throughput. Using the wait tolerance characteristic that is commonly observed in viewers behavior, we introduce a new paradigm for scheduling in VOD systems. We propose and analyze two classes of scheduling schemes: the Max_Batch and Min_Idle schemes that provide two alternative ways for using a given stream capacity for effective batching. In making a video selection, the proposed schemes take into consideration the next stream completion time, as well as the viewer wait tolerance. We compared the proposed schemes with the two previously studied schemes: (1) first-come-first-served (FCFS) that schedules the video with the longest waiting request and (2) the maximum queue length (MQL) scheme that selects the video with the maximum number of waiting requests. We show through simulations that the proposed schemes substantially outperform FCFS and MQL in reducing the viewer turn-away probability, while maintaining a small average response time. In terms of system resources, we show that, by exploiting the viewers wait tolerance, the proposed schemes can significantly reduce the server capacity required for achieving a given level of throughput and turn-away probability as compared to the FCFS and MQL. Furthermore, our study shows that an aggressive use of the viewer wait tolerance for batching may not yield the best strategy, and that other factors, such as the resulting response time, fairness, and loss of viewers, should be taken into account.     

Channel Allocation under Batching and VCR Control in Video-on-Demand Systems

In order to guarantee continuous delivery of a video stream in an on-demand video server environment, a collection of resources (referred to as a logical channel) are reserved in advance. To conserve server resources, multiple client requests for the same video can be batched together and served by a single channel. Increasing the window over which all requests for a particular video are batched results in larger savings in server capacity; however, it also increases the reneging probability of a client. A complication introduced by batching is that if a batched client pauses, a new stream (which may not be immediately available) needs to be started when the client resumes. To provide short response time to resume requests, some channels are set aside and are referred to as contingency channels. To further improve resource utilization, even when a nonbatched client pauses, the channel is released and reacquired upon resume. In this paper, we first develop an analytical model that predicts the reneging probability and expected resume delay, and then use this model to optimally allocate channels for batching, on-demand playback, and contingency. The effectiveness of the proposed policy over a scheme with no contingency channels and no batching is also demonstrated.     

Efficient support for interactive operations in multi-resolution video servers

In this paper, we present a placement algorithm that interleaves multi-resolution video streams on a disk array and enables a video server to efficiently support playback of these streams at different resolution levels. We then combine this placement algorithm with a scalable compression technique to efficiently support interactive scan operations (i.e., fast-forward and rewind). We present an analytical model for evaluating the impact of the scan operations on the performance of disk-arr ay-based servers. Our experiments demonstrate that: (1) employing our placement algorithm substantially reduces seek and rotational latency overhead during playback, and (2) exploiting the characteristics of video streams and human perceptual tolerances enables a server to support interactive scan operations without any additional overhead.     

Multicast protocols for scalable on-demand download

Previous scalable protocols for downloading large, popular files from a single server include batching and cyclic multicast. With batching, clients wait to begin receiving a requested file until the beginning of its next multicast transmission, which collectively serves all of the waiting clients that have accumulated up to that point. With cyclic multicast, the file data is cyclically transmitted on a multicast channel. Clients can begin listening to the channel at an arbitrary point in time, and continue listening until all of the file data has been received.This paper first develops lower bounds on the average and maximum client delay for completely downloading a file, as functions of the average server bandwidth used to serve requests for that file, for systems with homogeneous clients. The results show that neither cyclic multicast nor batching consistently yields performance close to optimal. New hybrid download protocols are proposed that achieve within 15% of the optimal maximum delay and 20% of the optimal average delay in homogeneous systems.For heterogeneous systems in which clients have widely varying achievable reception rates, an additional design question concerns the use of high rate transmissions, which can decrease delay for clients that can receive at such rates, in addition to low rate transmissions that can be received by all clients. A new scalable download protocol for such systems is proposed, and its performance is compared to that of alternative protocols as well as to new lower bounds on maximum client delay. The new protocol achieves within 25% of the optimal maximum client delay in all scenarios considered.     

Efficient support for interactive scanning operations in MPEG-based video-on-demand systems

In this paper, we present an efficient approach for supporting fast-scanning (FS) operations in MPEG-based video-on-demand (VOD) systems. This approach is based on storing multiple, differently encoded versions of the same movie at the server. A normal version is used for normal playback, while several scan versions are used for FS. Each scan version supports forward and backward FS at a given speedup. The server responds to an FS request by switching from the normal version to an appropriate scan version. Scanning versions are produced by encoding a sample of the raw frames using the same GOP pattern of the normal version. When a scanning version is decoded and played back at the normal frame rate, it gives a perceptual motion speedup. By being able to control the traffic envelopes of the scan versions, our approach can be integrated into a previously proposed framework for distributing archived, MPEG-coded video streams. FS operations are supported using no or little extra network bandwidth beyond what is already allocated for normal playback. Mechanisms for controlling the traffic envelopes of the scan versions are presented. The actions taken by the server and the client's decoder in response to various types of interactive requests are described in detail. The latency incurred in implementing various interactive requests is shown to be within an acceptable range. Striping and disk-scheduling strategies for storing various versions at the server are presented. Issues related to the implementation of our approach are discussed.     

Range Multicast for Video on Demand

We explore a communication paradigm for video on demand, called Range Multicast. This schemeis a shift from the conventional thinking about multicast where every receiver must obtain the same data packet at any time. A range multicast allows new members to join at their specified time and still receive the entire video stream without consuming additional server bandwidth. Clients enjoy better service latency since they can join an existing multicast instead of waiting for the next available server stream. We also present techniques to support video-cassette-recorder-like interactivity in this environment. Unlike existing methods which require clients to cache data in a private buffer, the Range Multicast solution utilizes the shared network storage to make more efficient and cost-effective use of the caching space. Furthermore, since a range multicast can accommodate clients with different play points in the video, a client has a better chance to join an on-going multicast for normal playback after finishing a VCR operation. This strategy avoids the need for a new server stream, and thus further alleviates the server load. Our simulation results confirm the aforementioned benefits.     

Failure recovery algorithms for multimedia servers

In this paper, we present two novel disk failure recovery methods that utilize the inherent characteristics of video streams for efficient recovery. Whereas the first method exploits the inherent redundancy in video streams (rather than error-correcting codes) to approximately reconstruct data stored on failed disks, the second method exploits the sequentiality of video playback to reduce the overhead of online failure recovery in conventional RAID arrays. For the former approach, we present loss-resilient versions of JPEG and MPEG compression algorithms. We present an inherently redundant array of disks (IRAD) architecture that combines these loss-resilient compression algorithms with techniques for efficient placement of video streams on disk arrays to ensure that on-the-fly recovery does not impose any additional load on the array. Together, they enhance the scalability of multimedia servers by (1) integrating the recovery process with the decompression of video streams, and thereby distributing the reconstruction process across the clients; and (2) supporting graceful degradation in the quality of recovered images with increase in the number of disk failures. We present analytical and experimental results to show that both schemes significantly reduce the failure recovery overhead in a multimedia server.     

Video placement and configuration of distributed video servers on cable TV networks

A large-scale, distributed video-on-demand (VOD) system allows geographically dispersed residential and business users to access video services, such as movies and other multimedia programs or documents on demand from video servers on a high-speed network. In this paper, we first demonstrate through analysis and simulation the need for a hierarchical architecture for the VOD distribution network.We then assume a hierarchical architecture, which fits the existing tree topology used in today's cable TV (CATV) hybrid fiber/coaxial (HFC) distribution networks. We develop a model for the video program placement, configuration, and performance evaluation of such systems. Our approach takes into account the user behavior, the fact that the user requests are transmitted over a shared channel before reaching the video server containing the requested program, the fact that the input/output (I/O) capacity of the video servers is the costlier resource, and finally the communication cost. In addition, our model employs batching of user requests at the video servers. We study the effect of batching on the performance of the video servers and on the quality of service (QoS) delivered to the user, and we contribute dynamic batching policies which improve server utilization, user QoS, and lower the servers' cost. The evaluation is based on an extensive analytical and simulation study.     

Increasing multimedia system throughput with consumption-based buffer management

In a multimedia server, multiple media streams are generally serviced in a cyclic fashion. Due to non-uniform playback rates and asynchronous arrivals of queries, there tends to be spare disk bandwidth in each service cycle. In this paper, we study the issue of dynamically using spare disk bandwidth and buffer to maximize the system throughput of a multimedia server. We introduce the concept of minimizing buffer consumption as the criterion to select an appropriate media stream to utilize the spare system resources. Buffer consumption measures not only the amount of buffer but also the amount of time such buffer space is occupied (i.e., the space-time product). Different alternatives to utilizing spare disk bandwidth are examined, including different rate-adjustable retrievals of an already activated stream and prefetching the next waiting stream. For rate-adjustable retrievals, we study buffer consumption-based and remaining-time-based criteria for selecting an active stream to increase retrievals. Simulations are conducted to evaluate and compare different cases. The results show that (1) minimizing buffer consumption is the right criterion for maximizing the system throughput with spare disk bandwidth; (2) in general, prefetching a waiting stream incurs more buffer consumption, and thus is less effective than rate-adjustable retrieval of active streams in maximizing the system throughput; and (3) the advantage of rate-adjustable retrieval over prefetching is especially significant when service cycle time is small.     

Sharing Multicast Videos Using Patching Streams

The access patterns of most information systems follow the 80/20 rules. That is, 80% of the requests are for 20% of the data. A video server can take advantage of this property by waiting for requests and serving them together in one multicast. This simple strategy, however, incurs service delay. We address this drawback in this paper by allowing clients to receive the leading portion of a video on demand, and the rest of the video from an ongoing multicast. Since clients do not have to wait for the next multicast, the service latency is essentially zero. Furthermore, since most services require the server to deliver only a small leading portion of the video, the server can serve many more clients per time unit. We analyze the performance of this approach, and determine the optimal condition for when to use this strategy. We compare its performance to a hardware solution called Piggybacking. The results indicate that more than 200% improvement is achievable.