Performance evaluation of transmission schemes for real-time traffic in a high-speed timed-token MAC network

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Abstract

The fast development of high-speed networks makes many real-time multimedia applications possible. These applications usually involve the transmission of massive amounts of digital video data, which requires the application of video image compression technology, under a stringent timing constraint. As a result, the transmission of the variable bit rate (VBR) traffic generated by the compression algorithms plays a very important role in meeting the real-time requirement of the applications. In this paper, we analyze the characteristics of the real-time traffic generated by Moving Pictures Experts Group (MPEG), which is one of the most notable video image compression standards, and evaluate four different transmission schemes for the real-time VBR traffic. In order to have a deterministic behavior and bounded message delays, we choose the timed token medium access control (MAC) protocol for the evaluation of our transmission schemes. Judging from our maximum bandwidth demand analysis, and after a series of intensive simulation experiments, the results reveal that in our best scenario, the performance improvement (PI) can be as high as 422% over the original default transmission scheme in terms of the number of video streams that can be supported in the network.

Introduction

There is growing interest in using high-speed networks for computer communications. In fact, the high transmission rate makes a lot of real-time applications possible. Real-time applications require deterministic behavior and bounded message delays, where the bound is tight compared to the timing requirements of other non-real-time applications. Another reason for the rapid deployment of high-speed networks is due to the heavy demand on bandwidth by multimedia applications. The drastic increase in the use of graphics, voice and video in all aspects of home, office, and industrial applications has become the driving force for switching from low-speed networks to high-speed networks.

In multimedia applications, the biggest constraint on real-time communication is the massive data volume of digital motion videos. To remove this constraint, one approach is to apply video image compression technology. A variety of image compression algorithms have been developed for these variable bit rate (VBR) video services. Among these algorithms, the Moving Pictures Experts Group (MPEG), which is an international standard, is one of the most common and widely accepted algorithms. Hence, a wide variety of inexpensive specialized chips for MPEG are available in the market.

Another approach to achieve real-time communication is to have an effective transmission scheme to process the compressed video frames under stringent timing constraints. To support real-time multimedia applications on top of a high-speed network, the underlying transmission scheme must ensure that most, if not all, frames should reach their destinations before their deadlines. Frames arriving late and missing their deadlines will result in a poorer quality of the picture or jumpy video. In this paper, we exploited the characteristics of the MPEG coded frame sequence and proposed different schemes for improving the real-time support of MPEG transmission over a high-speed network. All the video data we used are captured from various TV programmes. We categorized these video clips according to their workload characteristics – average frame size, maximum frame size, and I:P:B frame size ratio – and studied their effect on the transmission schemes and the underlying network.

In order to satisfy the on-time video frame delivery requirement, a suitable network candidate should provide bounded and deterministic transmission delays. Here, we adopted the timed token medium access control (MAC) protocol because of this important property, which is a necessity for real-time communications, distributed computing systems, and industrial process controls. This is why the timed token MAC protocol has been well received and being adopted in several high-bandwidth network standards over the years. These include the Fibre Distributed Data Interface (FDDI) ANSI Standard, 1987, ANSI Standard, 1989, IEEE 802.4 (Token Bus) (IEEE/ANSI, 1985), the High-Speed Data Bus and the High-Speed Ring Bus (HSDB/HSRB) Aerospace Systems Division, 1988, Cohn, 1989, Uhlhorn, 1991, the Survivable Adaptable Fiber Optic Embedded Network (SAFENET) Green and Marlow, 1989, Kochanski and Paige, 1991, Paige, 1990, and Profibus, a German standard for fieldbus that is widely used in time-critical industrial applications (Ming and Koppenhoefer, 1996). Besides, many distributed real-time applications also use the timed token MAC protocol as their backbone network. Some of our previous studies Ng and Liu, 1991, Ng and Liu, 1993, Ng, 1993a, Ng, 1993b have shown that the timed token MAC protocol is suitable for distributed real-time communication and distributed multimedia applications within a local area network (LAN). Furthermore, our recent studies Ng, 1999, Ng and Lee, 1999 have shown that the real-time network performance can be improved by adopting different transmission schemes in a timed token MAC network.

On the other hand, the VBR video encoding schemes as well as the MPEG-I video transmission over a computer network have been studied extensively. Ott et al. (1992) and Lam et al. (1994) proposed smoothing schemes for VBR video. Reibman and Berger (1992) and Reininger et al. (1993) studied the problem of transporting/multiplexing VBR/MPEG video over ATM networks. Pancha and El Zarki, 1992, Pancha and El Zarki, 1993 studied the MPEG-I video coding standard for transmission of VBR video and the performance of variable bandwidth allocation schemes for VBR MPEG-I video. Furthermore, there were also extensive studies on MPEG video characterization and modeling by Ismail et al., 1995, Izquierdo and Reeves, 1995, Krunz et al., 1995.

Reader should notice that by MPEG standard, we are referring to the MPEG-I standard as specified by ISO/IEC, 1993, ISO/IEC, 1995. Therefore, from here and onward, we will omit the suffix and use MPEG instead of MPEG-I. However, reader should note that our proposed transmission schemes are also applicable to MPEG-II, and other VBR video streams. In fact, these transmission schemes can be generalized and are applicable to other VBR real-time communication applications and distributed time critical industrial controls.

The remainder of this paper is organized as follows: Section 2 describes the network model and the timed token MAC protocol. Section 3 briefly describes the MPEG video streams and their classification. Section 4 describes and analyses the proposed transmission schemes. Section 5 describes the simulation experiments, performance metrics, and presents the simulation results. Finally, Section 6 provides a conclusion of this study.

Section snippets

Network model

We consider a network with x stations connected by point-to-point links forming a ring. The MAC is controlled by token-polling from station to station. Synchronous messages arrive or are ready to be transmitted at regular intervals (periodic) and are associated with deadline constraints. The main idea of the timed token MAC protocol was presented by Grow (1982) and further studied by Ulm (1982).

It was shown in (Agrawal et al., 1994) that any arbitrary ring network where a node may have zero,

MPEG video stream

MPEG is a compression algorithm for the efficient transport and the storage of massive amounts of digital data. However, MPEG compression is also suitable for transmitting video frames over a computer network. The basic idea of this compression scheme is to predict motion from frame to frame in the temporal direction, and then to use discrete cosine transforms (DCT) to organize the redundancy in the spatial directions ISO/IEC, 1993, ISO/IEC, 1995, Mitchell et al., 1997. In MPEG standard, there

The transmission schemes and analysis

In this section, the first two schemes are the default transmission schemes for transmitting the real-time video and stored video. These schemes act as reference points when compared with other proposed schemes. For the rest of the section, we propose transmission schemes in order to improve the transmission of MPEG video streams over a timed token MAC network. These schemes are designed to regulate the network traffic at the sending station, to smooth out the burstiness of the VBR streams, or

Simulation and results

Although for each of our proposed scheme, we derive its maximum bandwidth demand and show that all of them perform better than the default Real-Time Scheme (summarized in Table 10 in Appendix A). However, because of the VBR in a MPEG video stream, and the number of streams per system, it is difficult to tell which transmission scheme works the best. Therefore, we have constructed a series of experiments to find out the PI among the proposed schemes as compared with the default Real-Time Scheme.

Conclusion

For a summary of this performance study, we first categorized the MPEG streams into 4 different types according to their I:P:B frame size ratios. By examining the bandwidth demand and the transmission sequence, we proposed various schemes to improve the support of a timed token MAC network for MPEG video transmission. In particular, four transmission schemes were proposed. The Startup Phase Regulating Scheme is to regulate the transmission so that the worst case phasing is avoided. Since the

Acknowledgements

The work reported in this paper was supported in part by the Hong Kong RGC Earmarked Research Grant under RGC/97-98/54, and by the FRG of HKBU under FRG/96-97/II-103.

Joseph Kee-Yin Ng received a B.S. in Mathematics and Computer Science, a M.Sc. in Computer Science, and a Ph.D. in Computer Science from the University of Illinois at Urbana-Champaign in the years 1986, 1988, and 1993, respectively. Dr. Ng is currently an associate professor in the Department of Computer Science at Hong Kong Baptist University. His research areas include: Real-Time Networks, Multimedia Communication, ATM Delay Analysis, High-Speed Network Simulation, and Distributed Systems

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  • Joseph Kee-Yin Ng received a B.S. in Mathematics and Computer Science, a M.Sc. in Computer Science, and a Ph.D. in Computer Science from the University of Illinois at Urbana-Champaign in the years 1986, 1988, and 1993, respectively. Dr. Ng is currently an associate professor in the Department of Computer Science at Hong Kong Baptist University. His research areas include: Real-Time Networks, Multimedia Communication, ATM Delay Analysis, High-Speed Network Simulation, and Distributed Systems Performance Evaluation. Dr. Ng is a member of the IEEE and the IEEE Computer Society since 1991, and has been an exco-member (1993–95), General Secretary (1995–1997), and Vice-Chair (1997-present) of the IEEE, Hong Kong Section, Computer Chapter. He is also a member of the ACM, and EUROMICRO.

    Victor Chung-sing Lee received his B.Sc. (Hons) in Information Technology in 1990 from City Polytechnic of Hong Kong. He received his M.Phil in Computer Science in 1994 from the same institute and the Ph.D degree in Computer Science in 1997 from City University of Hong Kong. Dr. Lee is currently a research fellow in Department of Computer Science at City University of Hong Kong. His research interests include: High-Speed Networks, Real-Time Databases and Mobile Computing. He is also a member of IEEE and ACM.

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