A control-theoretic approach to adapting VBR compressed video fortransport over a CBR communications channel

Future broad-band communications networks are expected to be dominated by video and image traffic. Variable bit-rate (VBR) video compression is generally preferred to constant bit-rate (CBR) compression because constant image quality can be provided. In contrast, CBR transport is preferred to VBR transport from the networking standpoint because of its simplicity. This paper studies the important issue of adapting VBR compressed video for transport over a CBR channel. We focus on temporal traffic smoothing using an elastic buffer. The target image quality and the output rate of the video encoder is controlled by feedback based on the buffer-occupancy level. Previous adaptation schemes are not readily analyzable. An analyzable control-theoretic adaptation framework is proposed. It allows systematic and quantitative investigation of issues such as stability, robustness against scene changes, robustness against image-quality oscillations due to coding-mode switching, and tradeoffs between image-quality and buffer-occupancy (delay) fluctuations. Perhaps more importantly, the framework opens up many new possibilities for further research     

Joint selection of source and channel rate for VBR videotransmission under ATM policing constraints

Variable bit-rate (VBR) transmission of video over ATM networks has long been said to provide substantial benefits, both in terms of network utilization and video quality, when compared with conventional constant bit-rate (CBR) approaches. However, realistic VBR transmission environments will certainly impose constraints on the rate that each source can submit to the network. We formalize the problem of optimizing the quality of the transmitted video by jointly selecting the source rate (number of bits used for a given frame) and the channel rate (number of bits transmitted during a given frame interval). This selection is subject to two sets of constraints, namely, (1) the end-to-end delay has to be constant to allow for real-time video display and (2) the transmission rate has to be consistent with the traffic parameters negotiated by user and network. For a general class of constraints, including such popular ones as the leaky bucket, we introduce an algorithm to find the optimal solution to this problem. This algorithm allows us to compare VBR and CBR under the same end-to-end delay constraints. Our results indicate that variable-rate transmission can increase the quality of the decoded sequences without increases in the end-to-end delay. Finally, we show that for the leaky-bucket channel, the channel constraints can be combined with the buffer constraints, such that the system is identical to CBR transmission with an additional, infrequently imposed constraint. Therefore, video quality with a leaky-bucket channel can achieve the same quality of a CBR channel with larger physical buffers, without adding to the physical delay in the system     

Video aggregation: adapting video traffic for transport overbroadband networks by integrating data compression and statisticalmultiplexing

Future broadband integrated services networks based on the asynchronous transfer mode (ATM) technology are expected to carry information from a large variety of different services and applications. This paper investigates video aggregation, a concept that integrates compression and statistical multiplexing of video information for transport over a communication network. We focus on the transmission of a group of video sessions as a bundle, the practical examples of which include entertainment-video broadcast and video-on-demand (VoD). In this situation, the advantage of constant bit-rate (CBR) transport (which facilitates simple network management and operation) and the advantage of variable bit-rate (VBR) video compression (which yields smoother image quality) can be achieved simultaneously. We show that it is better to integrate compression and statistical multiplexing before the bundle of video traffic enters the network than performing them as independent processes. We present experimental results which indicate the advantages of video aggregation in terms of superior image quality and efficient bandwidth usage     

A rate adaptation transcoding scheme for real-time video transmission over wireless channels

In this work, we investigate the problem of bit-rate adaptation transcoding for transmitting video over burst-error wireless channels, i.e., channels such that errors tend to occur in clusters during fading periods. In particular, we consider a scenario consisting of packet-based transmission with an Automatic Repeat reQuest (ARQ) error control and a feedback channel. With the acknowledgements received through the feedback channel and a statistical channel model, we have an estimate of the current channel state, and effective channel bandwidth. In this paper, we analyze the buffering implications of inserting a video transcoder at the wireless access point with the variable bit-rate channel as the target. We derive the conditions that both the source encoder and transcoder buffers have to meet for preventing the end decoder buffer from underflowing. Furthermore, we propose a bit-rate adaptation algorithm for VBR transcoders used in the wireless access point. Our experimental results demonstrate that the proposed algorithm can accurately control the bit-rate of the transcoded video stream and reduce the number of frames been skipped without violating the end-to-end delay requirement.     

Traffic shaping, bandwidth allocation, and quality assessment forMPEG video distribution over broadband networks

This article provides an overview of residential video delivery systems and presents the applications, benefits, and challenges of using VBR MPEG video encoding in broadband video distribution networks. The network resources required to transmit stored variable-rate MPEG can be reduced by properly analyzing and smoothing the video stream before transmission. A scheduling technique is presented which selects a traffic contract for a pre-encoded MPEG video stream with the criteria of minimizing network resources and maintaining video quality. Several effective bandwidth metrics are discussed and used to model the potential savings in network resources for the shaped streams     

Smoothing, statistical multiplexing, and call admission control forstored video

Variable bit-rate (VBR) compressed video is known to exhibit significant, multiple-time-scale rate variability. A number of researchers have considered transmitting stored video from server to a client using smoothing algorithms to reduce this rate variability. These algorithms exploit client buffering capabilities and determine a “smooth” rate transmission schedule, while ensuring that a client buffer neither overflows nor underflows. We investigate how video smoothing impacts the statistical multiplexing gains available with such traffic, and we show that a significant amount of statistical multiplexing gains can still be achieved. We then examine the implication of these results on network resource management and call admission control when transmitting smoothed stored video using VBR service with statistical quality-of-service (QoS) guarantees. Specifically, we present a uniform call admission control scheme based on a Chernoff bound method that uses a simple, novel traffic model requiring only a few parameters. This scheme provides an easy and flexible mechanism for supporting multiple VBR service classes with different QoS requirements. We evaluate the efficacy of the call admission control scheme over a set of MPEG-1 coded video tracts     

自相似VBR视频源业务流的建模和仿真

该文在对实际VBR MPEG视频源统计特性分析的基础上,参照分形高斯噪声自相似(Fractional Gaussian Noise Self-Similar)模型产生方法,实现了对ATM网络中最主要业务流VBR视频源流的建模,提出了改进方法,使得对实际源的仿真不仅考虑到了长期相关性,同时也兼顾到了短期相关性。仿真结果表明,经改进的自相似VBR视频源模型是一种较理想的模型。     

Picture quality optimization in ABR video services

In transmission of bursty traffic such as video and data, a network with a fixed capacity may have some unused bandwidth. The available bit-rate (ABR) scheme in ATM networks can make use of this spare channel capacity as a lower priority service. ABR has been developed initially for delay-tolerant services such as data. In this paper we demonstrate how with the help of a video transcoder delay-sensitive services such as video may be transmitted over ABR networks. The transcoder further compresses the video bit stream according to the rates dictated by the network with minimum degradation to the picture quality. We have optimised the video quality by combining the classical minimum cell rate (MCR) with the subjectively acceptable minimum quality (SAMQ). The performance of two different coding formats for ABR sources, namely the constant bit-rate (CBR) and variable bit-rate (VBR) video input to the transcoder are compared. We also present techniques for further reduction of congestion at the multiplexing switches to improve video quality and report on the impact of the forward and backward link delays on the ABR performance with a video transcoder.     

Optimal bit allocation for coding of video signals over ATMnetworks

We consider optimal encoding of video sequences for ATM networks. Two cases are investigated. In one, the video units are coded independently (e.g., motion JPEG), while in the other, the coding quality of a later picture may depend on that of an earlier picture (e.g., H.26x and MPEGx). The aggregate distortion-rate relationship for the latter case exhibits a tree structure, and its solution commands a higher degree of complexity than the former. For independent coding, we develop an algorithm which employs multiple Lagrange multipliers to find the constrained bit allocation. This algorithm is optimal up to a convex-hull approximation of the distortion-rate relations in the case of CBR (constant bit-rate) transmission. It is suboptimal in the case of VBR (variable bit-rate) transmission by the use of a suboptimal transmission rate control mechanism for simplicity. For dependent coding, the Lagrange-multiplier approach becomes rather unwieldy, and a constrained tree search method is used. The solution is optimal for both CBR and VBR transmission if the full constrained tree is searched. Simulation results are presented which confirm the superiority in coding quality of the encoding algorithms. We also compare the coded video quality and other characteristics of VBR and CBR transmission     

Design of a fair bandwidth allocation policy for VBR traffic in ATMnetworks

Since real-time variable bit rate (VBR) traffic is inherently bursty, dynamic bandwidth allocation is necessary for ATM streams that carry VBR traffic. In order to provide quality-of-services (QoS) guarantees and to reduce the computational complexity, an hybrid of guaranteed and dynamic adaptive allocation scheme requires to be implemented. Typical dynamic allocations to competing streams are done in the form of linear proportions to the bandwidth requirements. We show that during temporary link congestion such proportional arrangements can give rise to unequal queue growth and, subsequently, degraded QoS. This is found to be true even for streams that belong to the same VBR class and share identical long term traffic characteristics and QoS requirements. In this paper, four allocation algorithms are presented and analyzed in terms of their fairness and QoS potential for real-time VBR traffic. We propose and show that a novel allocation strategy, termed Minmax, solves the mentioned problem of unfairness within a class. By maintaining a fair distribution of buffer length across the streams of a class, the proposed policy can achieve better and fairer QoS performance compared to the traditional methods. We present analytical results, proofs and a simulation study of the described algorithms. Four allocation policies for handling MPEG VBR video streams are simulated in the context of a wireless ATM (WATM) medium access control. The results show that in certain scenarios, the Minmax strategy can reduce losses by an order of magnitude, while decreasing delays substantially     

High-performance prefetching protocols for VBR prerecorded video

We describe several high-performance prefetching protocols for the transport of VBR prerecorded video over a shared channel. The protocols are particularly well suite for offering video on demand over a residential cable network, a residential ADSL network, or over a shared satellite channel. We advocate the use of VBR-encoded video instead of CBR video because, for the same image quality, VBR-encoded video can have a significantly lower average rate. However in order to exploit the efficiency of VBR encoding, it is necessary that the VBR scheme multiplex and prefetch streams so that near 100 percent link utilizations are achieved. Our protocols are based on the observation that there are frequent periods of time during which the network bandwidth is under utilized. During these periods the server can prefetch frames from any of the ongoing videos and send the frames to the buffers in the appropriate clients. Simulation results based on MPEG-encoded traces show that the discussed prefetching protocols compare favorably with other VBR transport protocols     

On the effects of traffic mixtures in direct broadcast satellite networks

In this paper we present a multi‐criterion control simulation in a realistically complex environment of a satellite network, involving non‐symmetric up and downlinks. Direct broadcast satellite (DBS) networks carrying heterogeneous traffic is characterized with challenges, such as high traffic burstiness, wireless channel dynamics, and large, but limited capacity. On the other hand, there are system characteristics that can be leveraged to address these challenges such as in centralized topology, different levels in quality of service (QoS) and priorities, availability of side information about channel conditions, flexibility in delivery of delay insensitive traffic, etc. We have developed an adaptive resource allocation and management (ARAM) system that takes the advantage of such characteristics to maximize the utilization of the available capacity on the forward DBS link, while maintaining QoS in the presence of channel effects and congestion in the network. Since variable‐bit‐rate (VBR) video traffic is given priority over available‐bit‐rate (ABR) data traffic in the ARAM concept, in this paper we investigate the impact of the fraction of VBR load in overall load. Copyright © 2002 John Wiley & Sons, Ltd.     

A multi‐level satellite channel allocation algorithm for real‐time VBR data

Integrated services over a shared satellite channel need a MAC protocol that is able to deal with real‐time traffic without substantially affecting the efficiency of the shared medium. Due to its bursty nature, MPEG coded VBR video transmission is one of the most challenging real‐time applications. In this paper, we analyse the statistical performance of a simple allocation method for VBR traffic by using an accurate statistical model of an MPEG‐2 VBR coded movie, whose traffic exhibits a peak to mean ratio of about 4. The results were obtained both analytically and by simulation, and show that our method is suitable for transmitting non‐interactive video and best‐effort traffic on the same satellite link. The efficiency of the proposed method shows no dependency on the statistical properties of the input traffic. Copyright © 2002 John Wiley & Sons, Ltd.     

A prefetching protocol for continuous media streaming in wirelessenvironments

Streaming of continuous media over wireless links is a notoriously difficult problem. This is due to the stringent quality of service (QoS) requirements of continuous media and the unreliability of wireless links. We develop a streaming protocol for the real-time delivery of prerecorded continuous media from (to) a central base station to (from) multiple wireless clients within a wireless cell. Our protocol prefetches parts of the ongoing continuous media streams into prefetch buffers in the clients (base station). Our protocol prefetches according to a join-the-shortest-queue (JSQ) policy. By exploiting rate adaptation techniques of wireless data packet protocols, the JSQ policy dynamically allocates more transmission capacity to streams with small prefetched reserves. Our protocol uses channel probing to handle the location-dependent, time-varying, and bursty errors of wireless links. We evaluate our prefetching protocol through extensive simulations with VBR MPEG and H.263 encoded video traces. Our simulations indicate that for bursty VBR video with an average rate of 64 kb/s and typical wireless communication conditions our prefetching protocol achieves client starvation probabilities on the order of 10^-4 and a bandwidth efficiency of 90% with prefetch buffers of 128 kbytes     

A Time-Slot Assignment Strategy for ATM Broadband Indoor Wireless Networks

The rapid development of wireless in-building communication systems, has widened the scope of supported applications. Remote terminals may be capable of producing broadband real-time traffic such as variable bit-rate (VBR) video or bursty data transfers. Consequently one of the important issues in indoor broadband wireless networks (IBWN) is employment of an efficient bandwidth management protocol. In such a protocol, both the effects of radio channel behavior and the traffic heterogeneity have to be considered. In this paper we present a bandwidth allocation scheme based on the interplay of the radio channel characteristics and the traffic parameters and requirements. In the proposed scheme the base station visits the terminals in a non-uniform cyclic fashion. The inter-visit interval and the amount of service which is provided to each user during a visit, is determined using the channel and traffic parameters based on an analytical approach. This system is evaluated by simulations and is found to provide improved performance in fulfilling the requirements of multimedia services in IBWN.     

Optimal control of source and channel rate for VBR video transmission over VBR channels

In this letter we propose a control-theoretic approach for the joint source and channel rate control problem of transporting variable bit-rate (VBR) compressed video over VBR channels. Compared with traditional solutions, our solution has no special requirements for the encoder and decoder buffer sizes, and can use much less memory and calculation time to achieve the optimal control. The proposed algorithm can be used in both off-line and on-line coding.     

Smoothing and transporting video in QoS IP networks

Real‐time traffic such as voice and video, when transported over the Internet, demand stringent quality of service (QoS) requirements. The current Internet as of today is still used as a best effort environment with no quality guarantees. An IP‐based Internet that supports different QoS requirements for different applications has been evolving for the past few years. Video streams are bursty in nature due to the instant variability of the video content being encoded. To help mitigate the transport of bursty video streams with minimal loss of information, rate‐adaptive shapers (RASs) are usually being used to reduce the burstiness and therefore help preserve the desired quality. When transporting video over a QoS IP network, each stream is classified under a specific traffic profile to which it must conform, to avoid packet loss and picture quality degradation. In this paper we study, evaluate and propose RASs for the transport of video over a QoS IP network. We utilize the encoding video parameters for choosing the appropriate configuration needed to support the real‐time transport of Variable Bit Rate (VBR) encoded video streams. The performance evaluation of the different RASs is based on the transport of MPEG‐4 video streams encoded as VBR. The performance is studied based on looking at the effect of various parameters associated with the RASs on the effectiveness of smoothing out the burstiness of video and minimizing the probability of packet loss. Copyright © 2005 John Wiley & Sons, Ltd.     

Policing of realistic VBR video traffic in an ATM network

ATM has been accepted by CCITT as the transport mechanism for the future BISDN and will also be widely used in future customer premises networks. Networks based on the ATM principle are expected to provide a very flexible communications infrastructure allowing customers to make effective use of a wide variety of offered services. To provide this flexibility with an acceptable quality of service while operating the network in an economic way, elaborate traffic management functions will be necessary to control the traffic flows within the network. This paper will study one of these functions—the so-called ‘usage parameter control’ or ‘policing’ function—in some detail to illustrate some of the problems that arise and point out possible solutions. The mechanisms chosen to implement the policing function will be the ‘leaky bucket’ mechanism, the ‘jumping window’ mechanism and the ‘moving window’ mechanism. The input streams used to assess the mechanisms represent different types of video communication—videophone, video conference and entertainment video—coded according to different variable bit-rate (VBR) algorithms. In contrast to most of the previous studies, where artificial, statistical traffic sources have been used, the sources used in this paper are directly based on measured ‘real-life’ video data. This ensures that all the statistical properties of the actual traffic stream are preserved and allows identification of the different factors that influence the dimensioning and the performance of the policing mechanism. The results of this study show that the uncertainty about the key parameters at call set-up and the considerable impact of single scenes make the proper dimensioning of policing mechanisms difficult. Furthermore, it seems not to be practical to use the long term mean bit-rate as the key traffic control parameter for these sources. Results indicating that the long-term cell loss ratio is not a sufficient measure for the quality of service are also presented. A comparison of the mechanisms shows that from a performance perspective, the ‘leaky bucket’ mechanism is superior to the two window mechanisms. This work is relevant to evolving standards for both BISDN traffic management and variable bit-rate video coding.     

High performing multimedia transmission approach based on QoS support and admission control over IEEE 802.11e networks

Hybrid coordination function controlled channel access (HCCA) is a medium to enhance quality of service (QoS) via the IEEE 802.11e standard. The main limitation of HCAA is that it is only efficient for constant bit rate (CBR) applications. This is due to the nature of its scheduler that allocates transmission opportunities (TXOPs) based on traffic stream (TS) specifications (TSPECs) that are determined during the traffic setup time. Variable bit rate (VBR) traffics used in HCCA have nondeterministic profile, making it not optimally and efficiently supported by HCCA. The result of this inefficiency is a deterioration of the transmission performance of multimedia data as well as a drop in the number of served QoS video traffics. We propose a novel approach to deal with this issue, which is the feedback‐based admission control unit (FACU). FACU works by optimizing the usage of extra bandwidth to ensure optimal transmission performance of multimedia data. FACU achieves this by exploiting piggybacked information concerning sequential video frames in order to accurately assign the TXOP. The proposed approach is evaluated by utilizing various video sequences. It is demonstrated that FACU maximizes the overall number of video streams and optimizes the overall usage of the network without having any adverse effects on the QoS constraints determined.     

Delivery of low bit rate isochronous streams over the DOCSIS 1.0cable television protocol

The Data Over Cable Systems Interface Specification (DOCSIS) 1.0 protocol enables the delivery of Internet Protocol (IP) traffic over cable TV networks with significantly higher data rates when compared to analogue modems and integrated services digital network (ISDN) links. The availability of greater bandwidth enables the delivery of high quality audio and video services. Such services require bounded delay characteristics. This paper evaluates the capacity and performance characteristics of the DOCSIS 1.0 protocol for the delivery of isochronous streams given the limited quality of service (QOS) features of the protocol and a modem population of up to 500 nodes. Stream rates of up to 64 Kbps with different packet size were considered, suitable for compressed audio, voice and low quality video. Packet concatenation for upstream traffic was also considered. Simulation results indicated that such streams, within the given protocol limitations, can be supported for a particular system population with tradeoffs in terms of system throughput and channel utilisation of up to 50%. The network capacity, in terms of the number of simultaneous streams supported and link utilisation, is significantly affected by packet size. Support for concatenation has limited advantages. Performance benefits from concatenation are more evident when delivering higher rate bitstreams