I/O scheduling for digital continuous media

A growing set of applications require access to digital video and audio. In order to provide playback of such continuous media (CM), scheduling strategies for CM data servers (CMS) are necessary. In some domains, particularly defense and industrial process control, the timing requirements of these applications are strict and essential to their correct operation. In this paper we develop a scheduling strategy for multiple access to a CMS such that the timing guarantees are maintained at all times. First, we develop a scheduling strategy for the steady state, i.e., when there are no changes in playback rate or operation. We derive an optimal Batched SCAN (BSCAN) algorithm that requires minimum buffer space to schedule concurrent accesses. The scheduling strategy incorporates two key constraints: (1) data fetches from the storage system are assumed to be in integral multiples of the block size, and (2) playback guarantees are ensured for frame-oriented streams when each frame can span multiple blocks. We discuss modifications to the scheduling strategy to handle compressed data like motion-JPEG and MPEG. Second, we develop techniques to handle dynamic changes brought about by VCR-like operations executed by applications. We define a suite of primitive VCR-like operations that can be executed. We show that an unregulated change in the BSCAN schedule, in response to VCR-like operations, will affect playback guarantees. We develop two general techniques to ensure playback guarantees while responding to VCR-like operations: passive and active accumulation. Using user response time as a metric we show that active accumulation algorithms outperform passive accumulation algorithms. An optimal response-time algorithm in a class of active accumulation strategies is derived. The results presented here are validated by extensive simulation studies.     

Fast techniques for the optimal smoothing of stored video

Work-ahead smoothing is a technique whereby a server, transmitting stored compressed video to a client, utilizes client buffer space to reduce the rate variability of the transmitted stream. The technique requires the server to compute a schedule of transfer under the constraints that the client buffer neither overflows nor underflows. Recent work established an optimal off-line algorithm (which minimizes peak, variance and rate variability of the transmitted stream) under the assumptions of fixed client buffer size, known worst case network jitter, and strict playback of the client video. In this paper, we examine the practical considerations of heterogeneous and dynamically variable client buffer sizes, variable worst case network jitter estimates, and client interactivity. These conditions require on-line computation of the optimal transfer schedule. We focus on techniques for reducing on-line computation time. Specifically, (i) we present an algorithm for precomputing and storing the optimal schedules for all possible client buffer sizes in a compact manner; (ii) we show that it is theoretically possible to precompute and store compactly the optimal schedules for all possible estimates of worst case network jitter; (iii) in the context of playback resumption after client interactivity, we show convergence of the recomputed schedule with the original schedule, implying greatly reduced on-line computation time; and (iv) we propose and empirically evaluate an “approximation scheme” that produces a schedule close to optimal but takes much less computation time.     

Performance evaluation of smoothing algorithms for transmittingprerecorded variable-bit-rate video

The transfer of prerecorded, compressed variable-bit-rate video requires multimedia services to support large fluctuations in bandwidth requirements on multiple time scales. Bandwidth smoothing techniques can reduce the burstiness of a variable-bit-rate stream by transmitting data at a series of fixed rates, simplifying the allocation of resources in video servers and the communication network. This paper compares the transmission schedules generated by the various smoothing algorithms, based on a collection of metrics that relate directly to the server, network, and client resources necessary for the transmission, transport, and playback of prerecorded video. Using MPEG-1 and MJPEG video data and a range of client buffer sizes, we investigate the interplay between the performance metrics and the smoothing algorithms. The results highlight the unique strengths and weaknesses of each bandwidth smoothing algorithm, as well as the characteristics of a diverse set of video clips     

Impact of video scheduling on bandwidth allocation for multiplexed MPEG streams

We present efficient schemes for scheduling the delivery of variable-bit-rate MPEG-compressed video with stringent quality-of-service (QoS) requirements. Video scheduling is being used to improve bandwidth allocation at a video server that uses statistical multiplexing to aggregate video streams prior to transporting them over a network. A video stream is modeled using a traffic envelope that provides a deterministic time-varying bound on the bit rate. Because of the periodicity in which frame types in an MPEG stream are typically generated, a simple traffic envelope can be constructed using only five parameters. Using the traffic-envelope model, we show that video sources can be statistically multiplexed with an effective bandwidth that is often less than the source peak rate. Bandwidth gain is achieved without sacrificing the stringency of the requested QoS. The effective bandwidth depends on the arrangement of the multiplexed streams, which is a measure of the lag between the GOP periods of various streams. For homogeneous streams, we give an optimal scheduling scheme for video sources at a video-on-demand server that results in the minimum effective bandwidth. For heterogeneous sources, a sub-optimal scheduling scheme is given, which achieves acceptable bandwidth gain. Numerical examples based on traces of MPEG-coded movies are used to demonstrate the effectiveness of our schemes.     

Disk placement for arbitrary-rate playback in an interactive video server

Multimedia data, especially continuous media including video and audio objects, represent a rich and natural stimulus for humans, but require large amount of storage capacity and real-time processing. In this paper, we describe how to organize video data efficiently on multiple disks in order to support arbitrary-rate playback requested by different users independently. Our approach is to segment and decluster video objects and to place the segments in multiple disks using a restricted round-robin scheme, called prime round-robin (PRR). Its placement scheme provides uniform load balance of disks for arbitrary retrieval rate as well as normal playback, since it eliminates hot spots. Moreover, it does not require any additional disk bandwidth to support VCR-like operations such as fast-forward and rewind. We have studied the various effects of placement and retrieval schemes in a storage server by simulation. The results show that PRR offers even disk accesses, and the failure in reading segment by deadline occurs only at the beginning of new operations. In addition, the number of users admitted is not decreased, regardless of arbitrary-rate playback requests.     

Dynamic batching policies for an on-demand video server

In a video-on-demand environment, continuous delivery of video streams to the clients is guaranteed by sufficient reserved network and server resources. This leads to a hard limit on the number of streams that a video server can deliver. Multiple client requests for the same video can be served with a single disk I/O stream by sending (multicasting) the same data blocks to multiple clients (with the multicast facility, if present in the system). This is achieved by batching (grouping) requests for the same video that arrive within a short time. We explore the role of customer-waiting time and reneging behavior in selecting the video to be multicast. We show that a first come, first served (FCFS) policy that schedules the video with the longest outstanding request can perform better than the maximum queue length (MQL) policy that chooses the video with the maximum number of outstanding requests. Additionally, multicasting is better exploited by scheduling playback of the most popular videos at predetermined, regular intervals (hence, termed FCFS-). If user reneging can be reduced by guaranteeing that a maximum waiting time will not be exceeded, then performance of FCFS- is further improved by selecting the regular playback intervals as this maximum waiting time. For an empirical workload, we demonstrate a substantial reduction (of the order of 60%) in the required server capacity by batching.     

Real time video frames allocation in mobile networks using cooperative pre-fetching

In this paper, we study the bandwidth allocation problem for serving video requests in a mobile real-time video player system. One of the main issues to maintain the quality of services (QoS) in mobile video playback is to ensure sufficient number of video frames available at the client side while a video is being played. However, due to the limited bandwidth of a mobile network and variable workload streaming video, this is not easy to achieve in practice. In addition, the communication link between mobile clients and a video server is subject to frequent errors and temporary disconnection. In this paper, we propose to use the notion of buffered bandwidth in addition to the network bandwidth to support real-time playback of videos at mobile clients. Based on this, we designed a bandwidth allocation scheme called Cooperative Pre-Fetching (CP) in which the amount of bandwidth to be allocated to serve a video request depends on the current buffer level of the video at the client relative to the target buffer level of the client. In determining the target buffer level, we consider the errors in communication to the client as well as the other clients who are concurrently served by the system. The buffered video frames at the clients are then used to minimize the impact of error in communications on the overall QoS of video playbacks in the whole system.

Synchronized delivery and playout of distributed stored multimedia streams

Multimedia streams such as audio and video impose tight temporal constraints for their presentation. Often, related multimedia streams, such as audio and video, must be presented in a synchronized way. We introduce a novel scheme to ensure the continuous and synchronous delivery of distributed stored multimedia streams across a communications network. We propose a new protocol for synchronized playback and compute the buffer required to achieve both, the continuity within a single substream and the synchronization between related substreams. The scheme is very general and does not require synchronized clocks. Using a resynchronization protocol based on buffer level control, the scheme is able to cope with server drop-outs and clock drift. The synchronization scheme has been implemented and the paper concludes with our experimental results.     

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.     

Transporting QoS adaptive flows

Distributed audio and video applications need to adapt to fluctuations in delivered quality of service (QoS). By trading off temporal and spatial quality to available bandwidth, or manipulating the playout time of continuous media in response to variation in delay, audio and video flows can be made to adapt to fluctuating QoS with minimal perceptual distortion. In this paper, we extend our previous work on a QoS Architecture (QoS-A) by populating the QoS management planes of our architecture with a framework for the control and management of multilayer coded flows operating in heterogeneous multimedia networking environments. Two key techniques are proposed: i) an end-to-end rate-shaping scheme which adapts the rate of MPEG-coded flows to the available network resources while minimizing the distortion observed at the receiver; and ii) an adaptive network service, which offers “hard” guarantees to the base layer of multilayer coded flows and “fairness” guarantees to the enhancement layers based on a bandwidth allocation technique called Weighted Fair Sharing.     

Traffic characterization algorithms for VBR video in multimedia networks

A network that offers deterministic, i.e., worst case, quality-of-service guarantees to variable-bit-rate (VBR) video must provide a resource reservation mechanism that allocates bandwidth, buffer space, and other resources for each video stream. Such a resource reservation scheme must be carefully designed, otherwise network resources are wasted. A key component for the design of a resource reservation scheme is the traffic characterization method that specifies the traffic arrivals on a video stream. The traffic characterization should accurately describe the actual arrivals, so that a large number of streams can be supported; but it must also map directly into efficient traffic-policing mechanisms that monitor arrivals on each stream. In this study, we present a fast and accurate traffic characterization method for stored VBR video in networks with a deterministic service. We use this approximation to obtain a traffic characterization that can be efficiently policed by a small number of leaky buckets. We present a case study where we apply our characterization method to networks that employ a dynamic resource reservation scheme with renegotiation. We use traces from a set of 25–30-min MPEG sequences to evaluate our method against other characterization schemes from the literature.     

Issues in the design of a storage server for video-on-demand

We examine issues related to the design of a storage server for video-on-demand (VOD) applications. The storage medium considered is magnetic disks or arrays of disks. We investigate disk scheduling policies, buffer management policies and I/O bus protocol issues. We derive the number of sessions that can be supported from a single disk or an array of disks and determine the amount of buffering required to support a given number of users. Furthermore, we propose a scheduling mechanism for disk accesses that significantly lowers the buffer-size requirements in the case of disk arrays. The buffer size required under the proposed scheme is independent of the number of disks in the array. This property allows for striping video content over a large number of disks to achieve higher concurrency in access to a particular video object. This enables the server to satisfy hundreds of independent requests to the same video object or to hundreds of different objects while storing only one copy of each video object. The reliability implications of striping content over a large number of disks are addressed and two solutions are proposed. Finally, we examine various policies for dealing with disk thermal calibration and the placement of videos on disks and disk arrays.     

Efficient support for interactive service in multi-resolution VOD systems

Advances in high-speed networks and multimedia technologies have made it feasible to provide video-on-demand (VOD) services to users. However, it is still a challenging task to design a cost-effective VOD system that can support a large number of clients (who may have different quality of service (QoS) requirements) and, at the same time, provide different types of VCR functionalities. Although it has been recognized that VCR operations are important functionalities in providing VOD service, techniques proposed in the past for providing VCR operations may require additional system resources, such as extra disk I/O, additional buffer space, as well as network bandwidth. In this paper, we consider the design of a VOD storage server that has the following features: (1) provision of different levels of display resolutions to users who have different QoS requirements, (2) provision of different types of VCR functionalities, such as fast forward and rewind, without imposing additional demand on the system buffer space, I/O bandwidth, and network bandwidth, and (3) guarantees of the load-balancing property across all disks during normal and VCR display periods. The above-mentioned features are especially important because they simplify the design of the buffer space, I/O, and network resource allocation policies of the VOD storage system. The load-balancing property also ensures that no single disk will be the bottleneck of the system. In this paper, we propose data block placement, admission control, and I/O-scheduling algorithms, as well as determine the corresponding buffer space requirements of the proposed VOD storage system. We show that the proposed VOD system can provide VCR and multi-resolution services to the viewing clients and at the same time maintain the load-balancing property. Received June 9, 1998 / Accepted April 26, 1999     

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.     

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.     

Design and analysis of permutation-based pyramid broadcasting

Periodic broadcasting can be used to support near-video-on-demand for popular videos. For a given bandwidth allocation, pyramid broadcasting schemes substantially reduce the viewer latency (waiting) time compared to conventional broadcasting schemes. Nevertheless, such pyramid schemes typically have substantial storage requirements at the client end, and this results in set-top boxes needing disks with high transfer rate capabilities. In this paper, we present a permutation-based pyramid scheme in which the storage requirements and disk transfer rates are greatly reduced, and yet the viewer latency is also smaller. Under the proposed approach, each video is partitioned into contiguous segments of geometrically increasing sizes, and each segment is further divided into blocks, where a block is the basic unit of transmission. As in the original pyramid scheme, frequencies of transmission for the different segments of a video vary in a manner inversely proportional to their size. Instead of transmitting the blocks in each segment in sequential order, the proposed scheme transmits these blocks in a prespecified cyclic permutation to save on storage requirements in the client end. Performance analyses are provided to quantify the benefits of the new scheme.     

An experimental analysis of digital video library servers

Much work on video servers has concentrated on movies on demand, in which a relatively small number of titles are viewed and users are given basic VCR-style controls. This paper concentrates on analyzing video server performance for non-linear access applications. In particular, we study two non-linear video applications: video libraries, in which users select from a large collection of videos and may be interested in viewing only a small part of the title; and video walk-throughs, in which users can move through an image-mapped representation of a space. We present a characterization of the workloads of these applications. Our simulation studies show that video server architectures developed for movies on demand can be adapted to video library usage, though caching is less effective and the server can support a smaller user population for non-linear video applications. We also show that video walk-throughs require extremely large amounts of RAM buffering to provide adequate performance for even a small number of users.     

Proxy-assisted periodic broadcast for video streaming with multiple servers

Large scale video streaming over the Internet requires a large amount of resources such as server I/O bandwidth and network bandwidth. A number of video delivery techniques can be used to lower these requirements. Periodic broadcast by a central server combined with proxy caching offers a significant reduction of the aggregate network and server I/O bandwidth usage. However, the resources available to a single server are still limited. In this paper we propose a system with multiple geographically distributed servers. The problem of multiple servers for periodic broadcast is quite different from the problem of object location for multiple web servers. Multiple servers offer increased amount of resources and service availability and may potentially allow a further reduction of network bandwidth usage. On the other hand, the benefit of periodic broadcast mostly comes from high demand videos. With multiple servers holding a video, the demand of the video at each server is reduced. Therefore, it is a challenge to use multiple servers efficiently. We first analyze the dependence of the resource requirements on the number and locations of the servers. Based on the character of the function describing such a dependence, we formulate and solve the problem of video location and delivery, in a way that minimizes resource usage. We explore a trade-off between network and I/O bandwidth requirements. We evaluate our proposed solutions through a number of tests.

A hierarchical architecture for buffer management in integrated services networks

Due to their large bandwidth demand and synchronization requirements, multimedia applications, in general, consume buffers of huge size, which prevents potential customers from using multimedia services. We recognize the problem and propose a hierarchical architecture to reduce the buffer size. The architecture can be applied to both 1- and - applications. We establish the architecture by first determining neighbor sets and then applying a grouping algorithm and a renegotiation process. This architecture can also meet the synchronization requirements of multimedia applications. We evaluate the performance of the architecture through simulations and compare it with that of a direct connection architecture. The result shows that the hierarchical architecture reduces the buffer size significantly without serious penalty to the total bandwidth and without introducing extra hot spots.     

Delivering presentations from multimedia servers

Most multimedia servers reported in the literature are designed to serve multiple and independent video/audio streams. We think that, in future, multimedia servers will also serve complete presentations. Multimedia presentations provide unique opportunities to develop algorithms for buffer management and admission control, as execution-time consumption requirements of presentations are known a priori. In this paper, we examine presentations in three different domains (heavyweight, middleweight, and lightweight) and provide buffer management and admission control algorithms for the three domains. We propose two improvements (flattening and dynamic-adjustments) on the schedules created for the heavyweight presentations. Results from a simulation environment are presented. Received June 9, 1998 / Accepted October 13, 1998