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PERFORMANCE OF H.263+ SCALABLE VIDEO OVER A DIFFSERV NETWORK Eren Gürses 1, G. Bozdagi Akar 1, Nail Akar 2 1
Department of Electrical And Electronics Engineering, Middle East Technical University, Ankara, Turkey 2 Department of Electrical And Electronics Engineering, Bilkent University, Ankara, Turkey
Abstract Applications requiring QoS, such as interactive and streaming audio/video, are becoming increasingly popular on the Internet. It is not clear yet how the emerging Internet QoS architectures will fulfill the performance requirements of such applications in a heterogeneous and dynamically changing environment. In this paper, we develop a mechanism that uses the temporally scalable and error resilient video coding mode of the H.263+ codec together with the Diffserv QoS architecture. The proposed scalability of transmitted video is shown to allow one to share a congested link with other flows while maintaining video quality for properly provisioned links.
INTRODUCTION The transmission of high quali ty video over the Internet is now becoming a reality due to progresses in video compression, networking technologies, efficient video coders/decoders andincreasing interest in applications such as video on demand, videophone, and videoconferencing. To ful fill different requirements by using one common bitstream in a wide range of video services in heterogenous environments, techniques which can simultaneously support a variety of bitrates tailored to individual services are needed while maintaining end -to-end quality. Coding video in a scalable manner partially solves this problem by offering different rates to different users. For maintaining end -to-end quality, two QoS (Quality of Service) architectures have been proposed by the IETF (Internet Engineering Task Force): the integrated services (IntServ) with the resource reservation protocol (RSVP) and the differentiated services (Diffserv). Diffserv provides a less complicated and scalable solution compared to Intserv, which fits very well to the structure of scalable video coding. Recently, several studies have been done on transmitting scalable video (MPEG -2, H.263+, MPEG-4) over Diffserv networks. In [1], Markopoulou and Hang address the issue of transmission of scalable video (H.263+) in contexts where p acket drops, rather than packet delays, are the primary determinant of application performance. However, in this work only SNR scalability is used and there is no policing algorithm involved at the edge to check the conformance of incoming packets. In [2], Shin et. al. use a relative priority index to represent the relative preference of each packet in terms of loss and delay. Instead of using scalable video, their work is based on full scale video. In [3], Ahmed et. al. investigate the transmission of MPEG -4 bitstreams over a DiffServ network. Their work mostly concentrates on the encapsulation of MPEG -4 packets over RTP/IP and a marking mechanism at the Diffserv edge routers. In this paper, we evaluate the performance of transmitting H.263+ scalable video over a Diffserv network using different SNR and temporal scalabilities, in a realistic scenario in which policing is done at the edge of a Diffserv domain to check conformance. Our
simulation results demonstrate that if the interframes selected by the refe rence picture selection mode of H.263+ are transmitted in the base layer (rather than the enhancement layer), better bandwidth utilization and error resilience is achieved in comparison with the following three cases: i) non -scalable coding ii) SNR scalabi lity iii) temporal scalability without reference picture selection. We also introduce a promotion/demotion policer by which packets are promoted/demoted at the edge of a Diffserv domain according to a novel policing algorithm we propose in this paper.
BACKGROUND In this section we give a brief overview on scalable video and and the Diffserv QoS architecture. Scalable Video Coding An important goal of scalable coding of video is to flexibly support receivers with different access bandwidths, display ca pabilities or display requests to allow video database browsing and multiresolution playback of video content in multimedia environments. Another crucial goal of scalable coding is to provide a layered video bit stream which is amenable for prioritized tra nsmission. Many scalable video -coding techniques have been proposed over the past few years for real -time Internet applications by several video compression standards such as MPEG -2/4 and H.263/263+ [4]. The types of scalability which are defined in these standards can be categorized as temporal, spatial, SNR, object (only for MPEG4) scalability. All these types of scalable video consist of a Base Layer (BL) which is the minimum amount of data needed for decoding the video stream and one or more Enhancement Layers (EL). The EL part of the stream represents additional information. Both the base layer and the enhancement layer can be composed of I -P-B (Intra -Inter (Predicted -Bidirectionally predicted)) pictures which are the three generic picture types used i n the above -mentioned standards. A schematic diagram of scalable video coding using temporal scalability is shown in Figure 1. In this figure, the base layer is composed of the I and P pictures whereas the enchancement layer is composed of P pictures.
Fig. 1. Base and enhancement layers. Other than the temporal scalability, SNR scalability is also widely used especially in applications related to Diffserv and video. One of the drawbacks of this approach is that when one of the EP frames (Enhancement La yer-P frame) is lost, the EP's quality will degrade. Another scalability structure that is suitable for the Diffserv architecture is Fine -Granular Scalability. In FGS, there is no temporal relation among the frames in the EL; this is different
than the tem poral relation among the EP frames in H.263+. Since in FGS the EL is formed of bitplane blocks which are DCT coded, bandwidth may be utilized more efficiently. However because of lack of temporal relation, increase in bitrate occurs especially in cases whe re the BL bitrate is chosen to be small as compared to the total rate. In order to solve the above mentioned problems, we used the Reference Picture Selection mode of H.263+ (Annex N) [6] in this work. This is a simpler version of the temporal scalability mode of H.263+ (Annex O), with backward prediction disabled. Diffserv and Promotion/Demotion Policer Diffserv is essentially a priority dropping mechanism which defines different service classes [5] for applications with different QoS requirements. An e nd-to-end service differentiation is obtained by concatenation of per -domain services and Service Level Agreements (SLAs) between adjoining domains along the path from source to destination. Per domain services are realized by traffic conditioning such as classification, metering, policing, shaping at the edge and simple differentiated forwarding mechanisms at the core of the network. Two of the more popular proposed forwarding mechanisms are Expedited Forwarding (EF) and Assured Forwarding (AF) Per Hop Be haviors (PHB). Since AF may enable service offerings at lesser cost than EF for audio, video, Web and other applications, we used AF PHB (RFC 2597) for transmitting scalable H.263+ bitstream in this work. The AF PHB defines four AF (Assured Forwarding) cla sses: AF1, AF2, AF3, and AF4. Each class is assigned a specific amount of buffer space and bandwidth. Within each AF class, one can specify three drop precedence values: 1, 2, and 3. In the notation Afny represents the AF class number (1, 2, 3, or 4) and r epresents the drop precedence value (1, 2, or 3) within the AFn class. In instances of network congestion, if packets in a particular AF class (for example, AF1) need to be dropped, those packets will be dropped according to the following guideline:
dP(AF11)
