Variable-bit-rate (VBR) compressed video can exhibit significant multiple-time-scale bit-rate variability. In this paper we consider the transmission of stored video from a server to a client across a network, and explore how the client buffer space can be used most effectively toward reducing the variability of the transmitted bit rate. Two basic results are presented. First, we show how to achieve the greatest possible reduction in rate variability when sending stored video to a client with given buffer size. We formally establish the optimality of our approach and illustrate its performance over a set of long MPEG-1 encoded video traces. Second, we evaluate the impact of optimal smoothing on the network resources needed for video transport, under two network service models: deterministic guaranteed service (Chang 1994; Wrege et al. 1996) and renegotiated constant-bit-rate (RCBR) service (Grossglauser et al. 1997). Under both models, the impact of optimal smoothing is dramatic.

Supporting stored video: reducing rate variability and end-to-end resource requirements through optimal smoothing

VBR compressed video is known to exhibit significant, multiple-time-scale bit rate variability. In this paper, we consider the transmission of stored video from a server to a client across a high speed network, and explore how the client buffer space can be used most effectively toward reducing the variability of the transmitted bit rate.We present two basic results. First, we present an optimal smoothing algorithm for achieving the greatest possible reduction in rate variability when transmitting stored video to a client with given buffer size. We provide a formal proof of optimality, and demonstrate the performance of the algorithm on a set of long MPEG-1 encoded video traces. Second, we evaluate the impact of optimal smoothing on the network resources needed for video transport, under two network service models: Deterministic Guaranteed service [1, 9] and Renegotiated CBR (RCBR) service [8, 7]. Under both models, we find the impact of optimal smoothing to be dramatic.

We introduce a specific transport and transmission scheme for video-on-demand (VoD) called constant-rate transmission and transport (CRTT). CRTT establishes a constant bit-rate (CBR) virtual channel between the video provider and the viewer's set-top box (STB) and then transmits cells from the provider into this channel at a constant rate. Since we assume that the number of cells in a frame is variable, CRTT requires that some number of cells be built up in an STB buffer before the commencement of playback. The build up, cell transmission rate, and the set-top memory size must be chosen so that there is no starvation or overflow at the STB. We develop fundamental relationships between these parameters for viable CRTT. We then apply the theory to an MPEG encoding of Star Wars and find that the minimal STB memory far CRTT is 23 Mbytes. We also consider varying the constant rate over a small number of intervals. We find, for example, that for Star Wars approximately 2 Mbytes of set-top memory suffices with 32 constant-rate intervals.

Video-on-demand over ATM: constant-rate transmission and transport

In this paper we present a novel network-conscious approach to the problem of end-to-end video delivery over wide-area networks using proxy servers situated between local-area networks (LANs) and a backbone wide-area network (WAN). We develop a novel and effective video delivery technique called video staging via intelligent utilization of the disk bandwidth and storage space available at proxy servers. We also design several video staging methods and evaluate their effectiveness in reducing the backbone WAN bandwidth requirement. Our results demonstrate that the proposed proxy-server-based, network-conscious approach provides an effective and scalable solution to the problem of the end-to-end video delivery over wide-area networks.

A network-conscious approach to end-to-end video delivery over wide area networks using proxy servers

We introduce a specific transport and transmission scheme for video-on-demand called constant-rate transmission and transport (CRTT). CRTT establishes a CBR virtual channel between the video provider and the viewer's set-top box, and then transmits cells from the provider into this channel at a constant rate. Since we assume that the number of cells in a frame is variable, CRTT requires that some number of cells be built up in a set-top box buffer before the commencement of playback. The build up, cell transmission rate, and the set-top memory size must be chosen so that there is no starvation or overflow at the set-top box. We develop fundamental relationships between these parameters for viable CRTT. We then apply the theory to an MPEG encoding of Star Wars and find that the minimal set-top box memory for CRTT is 23 Mbytes. We also consider varying the constant rate over a small number of intervals. We find, for example, that for Star Wars approximately 2 Mbytes of set-top memory suffices with 32 constant-rate intervals.

Video on demand over ATM: constant-rate transmission and transport

Interframe compression techniques, such as those used in MPEG video, give rise to a coded bit stream where picture sizes differ by a factor of 10 or more. As a result, buffering is needed to reduce (smooth) rate fluctuations of encoder output from one picture to the next; without smoothing, the performance of networks that carry such video traffic would be adversely affected. Various techniques have been suggested for controlling the output rate of a VBR encoder to alleviate network congestion or prevent buffer overflow. Most of these techniques, however, are lossy, and should be used only as a last resort. In this paper, we design and specify an algorithm for lossless smoothing. The algorithm is characterized by three parameters: D (delay bound), K (number of pictures with known sizes), and H (lookahead interval). We present a theorem which guarantees that, if K ≥ 1, the algorithm finds a solution that satisfies the delay bound. (Although the algorithm and theorem were motivated by MPEG video, they are applicable to the smoothing of compressed video in general). To study performance characteristics of the algorithm, we conducted a large number of experiments using statistics from four MPEG video sequences.

http://www.cs.utexas.edu/users/lam/Vita/ACM/LCY94.pdf

An algorithm for lossless smoothing of MPEG video

Interframe coding techniques, such as those used in MPEG video, give rise to a sequence of encoded pictures whose sizes (in number of bits) differ by a factor of ten or more. Buffering is needed to reduce fluctuations in the rate at which video packets are sent to a network connection. We design and specify a lossless smoothing algorithm, characterized by three parameters: D (delay bound), X (number of pictures with known sizes), and H (lookahead interval). We prove a theorem which guarantees that, if K/spl ges/1, the algorithm finds a solution that satisfies the delay bound. We present the algorithm's performance from a large number of experiments conducted using MPEG video traces. Lastly, we discuss algorithm implementation.

Simon Chow

Papers

An algorithm for lossless smoothing of MPEG video

A lossless smoothing algorithm for compressed video