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Real-Time Systems Symposium, 1997. Proceedings., The 18th IEEE

Date 2-5 Dec. 1997

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Displaying Results 1 - 25 of 33
  • Proceedings Real-Time Systems Symposium [front matter]

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    Freely Available from IEEE
  • Formal modeling and analysis of an audio/video protocol: an industrial case study using UPPAAL

    Page(s): 2 - 13
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    A formal and automatic verification of a real-life protocol is presented. The protocol, about 2800 lines of assembler code, has been used in products from the audio/video company Bang & Olufsen throughout more than a decade, and its purpose is to control the transmission of messages between audio/video components over a single bus. Such communications may collide, and one essential purpose of the protocol is to detect such collisions. The functioning is highly dependent on real-time considerations. Though the protocol was known to be faulty in that messages were lost occasionally, the protocol was too complicated in order for Bang & Olufsen to locate the bug using normal testing. However using the real-time verification tool UPPAAL, an error trace was automatically generated, which caused the detection of "the error" in the implementation. The error was corrected and the correction was automatically proven correct, again using UPPAAL. A future, and more automated, version of the protocol, where this error is fatal, will incorporate the correction. Hence, this work is an elegant demonstration of how model checking has had an impact on practical software development. The effort of modeling this protocol has in addition generated a number of suggestions for enriching the UPPAAL language. Hence, it's also an excellent example of the reverse impact. View full abstract»

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  • Efficient verification of real-time systems: compact data structure and state-space reduction

    Page(s): 14 - 24
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    During the past few years, a number of verification tools have been developed for real-time systems in the framework of timed automata (e.g. KRONOS and UPPAAL). One of the major problems in applying these tools to industrial-size systems is the huge memory-usage for the exploration of the state-space of a network (or product) of timed automata, as the model-checkers must keep information on not only the control structure of the automata but also the clock values specified by clock constraints. In this paper, we present a compact data structure for representing clock constraints. The data structure is based on an O(n/sup 3/) algorithm which, given a constraint system over real-valued variables consisting of bounds on differences, constructs an equivalent system with a minimal number of constraints. In addition, we have developed an on-the-fly, reduction technique to minimize the space-usage. Based on static analysis of the control structure of a network of timed automata, we are able to compute a set of symbolic states that cover all the dynamic loops of the network in an on-the-fly searching algorithm, and thus ensure termination in reachability analysis. The two techniques and their combination have been implemented in the tool UPPAAL. Our experimental results demonstrate that the techniques result in truly significant space-reductions: for six examples from the literature, the space saving is between 75% and 94%, and in (nearly) all examples time-performance is improved. Also noteworthy is the observation that the two techniques are completely orthogonal. View full abstract»

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  • On-the-fly symbolic model checking for real-time systems

    Page(s): 25 - 34
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    This paper presents an on-the-fly and symbolic algorithm for checking whether a timed automaton satisfies a formula of a timed temporal logic which is more expressive than TCTL. The algorithm is on-the-fly in the sense that the state-space is generated dynamically and only the minimal amount of information required by the verification procedure is stored in memory. The algorithm is symbolic in the sense that it manipulates sets of states, instead of states, which are represented as boolean combinations of linear inequalities of clocks. We show how a prototype implementation of our algorithm has improved the performances of the tool KRONOS for the verification of the FDDI protocol. View full abstract»

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  • A better polynomial-time schedulability test for real-time fixed-priority scheduling algorithms

    Page(s): 36 - 45
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    The problem of scheduling real-time periodic task has been studied extensively since its first introduction by C.L. Liu and J.W. Layland in their classic paper (1973). Due to several merits of the fixed-priority scheduling scheme, a lot of research work has focused on the analysis of fixed-priority scheduling algorithms. For the case that the deadlines of the executions of all the tasks coincide with the ends of their corresponding periods. Liu and Layland derived a worst-case utilization bound for a task set to be schedulable by the rate-monotonic (RM) algorithm. A. Burchard et al. (1995) presented another schedulability condition for RM, which has a higher utilization bound under a certain task condition. Although their closed-form utilization bounds provide a convenient way for testing the schedulability of a task set under the RM algorithm, the schedulability test using their bounds is too pessimistic since a lot of task sets with total utilizations larger than their bounds (and less than or equal to 1) are still schedulable by RM. In this paper, we propose a polynomial-time schedulability test and prove that it is better than Liu and Layland's and Burchard's utilization bounds in the sense that as long as the total utilization of a task set is less than or equal to their bounds, our schedulability test will always answer positively for the schedulability of the task set under RM and even if a feasible task set has a total utilization larger than their bounds, our schedulability test will still answer positively with a high probability. We also show how to generalize our polynomial-time schedulability test to handle general task sets scheduled by arbitrary fixed-priority scheduling algorithms. View full abstract»

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  • Combining (/sub m//sup n/)-hard deadlines and dual priority scheduling

    Page(s): 46 - 57
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    The problem of effectively scheduling soft tasks whilst guaranteeing the behaviour of hard tasks has been addressed in many papers and a large number of techniques have been proposed. The dual priority mechanism is an intuitively simple method with low overheads. A hard task is assigned two priorities. Upon invocation, the task starts executing with a low priority and it is promoted to a high priority at a time that will guarantee that its deadline is met. Soft tasks are assigned medium priorities; they can thus preempt any hard task that is executing before its promotion time. To increase the capacity for soft tasks, and therefore the effectiveness of the real-time system, hard tasks may be assigned a (/sub m//sup n/)-hard (read n in m) temporal constraint. This implies that the task must meet n deadlines in any m invocations. This paper addresses the combination of such constraints and dual priority scheduling. This approach reduces the gap between dynamic priority and fixed priority scheduling with the goal of reducing the average response time of soft tasks. View full abstract»

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  • Real-time queueing network theory

    Page(s): 58 - 67
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    This paper presents real-time queueing network theory, the extension of real-time queueing theory introduced by J.P. Lehoczky (1996) to Jackson queueing networks. This theory describes the behavior of standard Jackson queueing networks when the tasks have end-to-end deadlines. This theory is focused on describing the instantaneous multidimensional profiles of the lead-times of all the tasks in all the nodes of the network. The earliest end-to-end deadline first policy is used at all nodes in the network, although the theory permits other scheduling policies and different policies at different nodes. The real-time network problem is formulated in general, but it is analyzed under heavy traffic conditions where the traffic intensity at each node approaches 1. In this case, the multivariate queue length process converges to a Brownian network, a multivariate Brownian motion constrained to the first orthant. The conditional multivariate lead-time profiles at each of the nodes given the queue lengths converge to a deterministic profile. This characterization is borne out by simulation, and is used to design and evaluate queue control policies to reduce task lateness. View full abstract»

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  • Jitter concerns in periodic task systems

    Page(s): 68 - 77
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    A model for periodic tasks is proposed that explicitly incorporates jitter-the uncertainty in the arrival times of individual frames. Feasibility-analysis of systems of such tasks is studied in the context of dynamic-priority, preemptive, uniprocessor scheduling. From a computational complexity perspective, the problem is shown to be no more difficult than feasibility analysis in systems of periodic tasks that do not exhibit jitter. Several feasibility analysis algorithms are presented and proven correct. View full abstract»

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  • Adaptive fault tolerance and graceful degradation under dynamic hard real-time scheduling

    Page(s): 79 - 89
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    Static redundancy allocation is inappropriate in hard real-time systems that operate in variable and dynamic environments, (e.g., radar tracking, avionics). Adaptive fault tolerance (AFT) can assure adequate reliability of critical modules, under temporal and resource constraints, by allocating just as much redundancy to less critical modules as can be afforded thus gracefully reducing their resource requirement. We propose a mechanism for supporting adaptive fault tolerance in a real-time system. Adaptation is achieved by choosing a suitable redundancy strategy for a dynamically arriving computation to assure required reliability and to maximize the potential for fault tolerance while ensuring that deadlines are met. The proposed approach is evaluated using a real-life workload simulating radar tracking software in AWACS early warning aircraft. The results demonstrate that our technique outperforms static fault tolerance strategies in terms of tasks meeting their timing constraints. Further, we show that the gain in this timing-centric performance metric does not reduce the fault tolerance of the executing task below a predefined minimum level. Overall, the evaluation indicates that the proposed ideas result in a system that dynamically provides QoS guarantees along the fault-tolerance dimension. View full abstract»

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  • A low-cost processor group membership protocol for a hard real-time distributed system

    Page(s): 90 - 98
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    Processor group membership protocols implement a service that allow processors to agree on which processors are operational. Implementations of group membership for hard real-time systems have concentrated on either reducing failure detection latency or minimizing message complexity. Instead, we present a protocol that uses shared resources-processor time and network bandwidth-as a small, bounded tax imposed on existing broadcast message traffic. In doing so, the group membership protocol can easily be taken into account by any schedulability analysis. View full abstract»

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  • Efficient spare-resource allocation for fast restoration of real-time channels from network component failures

    Page(s): 99 - 108
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    Since real-time applications usually require not only timeliness but also fault-tolerance, it is essential to incorporate fault-tolerance into real-time communication services that are indispensable to distributed real-time applications. The techniques for failure recovery in datagram communication are not adequate for real-time communication, because they cannot provide recovery-delay guarantees. To ensure fast recovery of a real-time channel from network component failures, we need to reserve network resources (spare resources) along a backup route before failures actually occur. The focus of this paper is on minimizing the amount of spare resources while meeting the fault-tolerance requirement. Specifically, we present resource sharing mechanisms and backup-route selection algorithms, and evaluate their efficiency with extensive simulations. View full abstract»

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  • Wait-free object-sharing schemes for real-time uniprocessors and multiprocessors

    Page(s): 111 - 122
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    Several new wait-free object-sharing schemes for real-time uniprocessors and multiprocessors are presented. These schemes have characteristics in common with the priority inheritance and priority ceiling protocols, but are nonblocking and implemented at the user level. In total, six new object-sharing schemes are proposed: two for uniprocessors and four for multiprocessors. Breakdown utilization experiments are presented that show that the multiprocessor schemes entail less overhead than lock-based schemes. View full abstract»

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  • More optimism about real-time distributed commit processing

    Page(s): 123 - 133
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    In (Gupta et al., 1996), we proposed a new commit protocol, OPT, specially designed for use in distributed firm-deadline real-time database systems. OPT allows transactions to optimistically borrow uncommitted prepared data in a controlled manner. This controlled borrowing reduces the data inaccessibility and the priority inversion that is inherent in real-time commit processing. Experimental evaluations showed the new OPT protocol to be highly successful, as compared to the classical distributed commit protocols, in minimizing the number of missed transaction deadlines. In this paper, we extend and improve upon this prior work in the following ways. First, we consider parallel distributed transactions whereas the previous study was restricted to sequential transactions. Second, we evaluate the extent to which OPT's real-time performance is adversely affected by those cases where its optimism turns out to be misplaced. This is achieved by comparing OPT's performance with that of Shadow-OPT, a protocol that augments OPT with the shadow transaction approach and ensures that the right decision about access to uncommitted data is always eventually made. In all of our experiments, which considered a wide range of workloads and system configurations, the difference between OPT and Shadow-OPT never exceeded ten percent. Moreover, the difference was reduced to less than two percent when OPT was enhanced with a simple healthy lenders heuristic. Finally, we compare the performance of OPT to that of an alternative priority, inheritance-based approach to addressing priority inversion during commit processing. View full abstract»

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  • A novel approach to multiprogrammed multiprocessor synchronization for real-time kernels

    Page(s): 134 - 143
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    In order to solve the problem of inopportune preemption in multiprogrammed multiprocessor synchronization, two strategies which are applicable to real-time systems have been investigated: preemption-safe locking and wait-free synchronization. Both of them, however, have a problem for use in the implementation of a real-time kernel. Preemption-safe locking has the drawback that the preemption cost becomes large; while wait-free operations on complex data structures are generally very inefficient and are not practical. We propose a novel approach to multiprogrammed multiprocessor synchronization, called the SPEPP (Spinning Processor Executes for Preempted Processors) synchronization, with which the preemption cost can be reduced to almost zero, while operations on complex data structures can be realized with reasonable efficiency. This paper presents the two algorithms of the SPEPP synchronization and their extensions, and demonstrates its effectiveness through the performance measurements of real-time kernels implemented with the SPEPP synchronization algorithms. View full abstract»

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  • Similarity-based load adjustment for real-time data-intensive applications

    Page(s): 144 - 153
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    How to exploit application semantics to improve the performance of a real-time data-intensive application has been an active research topic in the past few years. Weaker correctness criteria and semantics-based concurrency control algorithms were proposed to provide more flexibility in reordering read and write events. Distinct from the past work, this paper exploits the tradeoff between data consistency and system workload. The definition of similarity is combined with the idea of transaction skipping to provide a theoretical foundation for reducing the workload of a transaction system. We also propose guidelines to adjust the execution frequencies of a static set of transactions and prove their correctness. The strengths of this work were verified by simulation experiments on an air traffic control example (Peng et al., 1997). View full abstract»

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  • Real-time filesystems. Guaranteeing timing constraints for disk accesses in RT-Mach

    Page(s): 155 - 165
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    Traditional real-time systems have largely avoided the use of disks due to their relative slow speeds and their unpredictability. However, many real-time applications including multimedia systems and real-time database applications benefit significantly from the use of disks to store and access real-time data. We investigate the problem of obtaining guaranteed timely access to files on a disk in a real-time system. Our study focuses on several aspects of this problem of providing a real-time filesystem. First, we consider the use of two real-time disk scheduling algorithms: earliest deadline scheduling and just-in-time scheduling, a variation of aperiodic servers for the disk. The latter algorithm is designed to improve disk throughput that can be hurt when a real-time scheduling algorithm such as EDF is applied directly. Admission control policies with practically acceptable properties of performance and usability are provided. Next, we design and implement a real-time filesystem on the RT-Mach microkernel-based system running a real-time shell. The new interface we develop is based on RT-Mach's resource reservation paradigm and provides guaranteed and timely access for multiple concurrent applications requiring disk bandwidth with different timing and volume requirements. Finally, we perform a detailed performance evaluation of the real-time filesystem including its raw performance. We show the following positive but rather surprising result: our real-time scheduling filesystem not only provides guaranteed and timely access but also does so at relatively high levels of throughput. Traditional disk scheduling algorithms offer completely unacceptable file access latencies for real-time applications and do so only at slightly higher throughput. View full abstract»

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  • Implementation and evaluation of real-time Java threads

    Page(s): 166 - 175
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    Java has many benefits such as security in distributed environments, reusability of code, and portability because it is architecture neutral. From those characteristics, Java is beginning to be used in many new environments. Even though Java provides various advantages, it still has problems which must be solved. One issue is that there are often real-time constraints that should be met in those applications. Current Java execution environment and language specification cannot satisfy those requirements. Our research focuses on the current limits of the Java language and its execution environment, and to seek the possibility for real-time using Java and the technologies we have available today. To investigate issues in real-time Java, we have implemented a prototype real-time Java environment which provides real-time Java threads and a real-time synchronization mechanism. Then we evaluated its performance. The results indicate that timing requirements are better met and unbounded priority inversion is avoided in our system. View full abstract»

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  • Early detection of timing constraint violation at runtime

    Page(s): 176 - 185
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    As real time applications become more complex and distributed, monitoring for timing constraint compliance becomes more important in facilitating the enforcement of conditional guarantees and for recovery purposes. C.E. Chodrow et al. (1991) described a O(n/sup 3/) satisfiability checking algorithm for timing constraint monitoring at each check point, where n is the number of time terms in the timing constraint specification. We show that a timing violation can be caught as early as possible by deriving and monitoring a minimum set of timing constraints from the timing constraint specification. We show that only O(n) time is needed in the worst case for checking at each check point. An implementation based on the results reported herein appears in a companion paper (A.K. Mok and G. Liu, 1997). View full abstract»

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  • Enhanced analysis of cache-related preemption delay in fixed-priority preemptive scheduling

    Page(s): 187 - 198
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    We propose an enhanced technique for analyzing, and thus bounding cache related preemption delay in fixed priority preemptive scheduling focusing on instruction caching. The proposed technique improves upon previous techniques in two important ways. First, the technique takes into account the relationship between a preempted task and the set of tasks that execute during the preemption when calculating the cache related preemption delay. Second, the technique considers phasing of tasks to eliminate many infeasible task interactions. These two features are expressed as constraints of a linear programming problem whose solution gives a guaranteed upper bound on the cache related preemption delay. The paper also compares the proposed technique with previous techniques. The results show that the proposed technique gives up to 60% tighter prediction of the worst case response time than the previous techniques. View full abstract»

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  • A cache-aware scheduling algorithm for embedded systems

    Page(s): 199 - 209
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    The paper presents a methodology for scheduling real time tasks in embedded systems where the task layout is known at design time and does not change at execution time (static systems) and where the cache miss costs are significant when compared to the normal execution time of the tasks. The scheduling model assumes a time driven dispatching of the application tasks which are ordered in a pre defined sequence. Building such a sequence in a way that is not only efficient but accounts for optimal cache sequencing is the aim of our method. The refinement of the schedule towards an optimal solution is done by simulated annealing techniques. The evaluation of the schedules is done by considering the effects of instruction caching when evaluating the computation time of the tasks. View full abstract»

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  • Scalable hardware earliest-deadline-first scheduler for ATM switching networks

    Page(s): 210 - 218
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    A fast, scalable hardware earliest deadline first (EDF) link scheduler for ATM switching network is developed. This EDF scheduler is a fast hardware solution suitable for real time scheduler on nodes in ATM switching networks up to 2.5 Gbps switching speed (scheduling within 0.17 /spl mu/s), capable of performing simultaneous input and output operations within two clock cycles (mostly in one clock cycle). The designed hardware is efficient since the architecture employs the minimum size EDF priority queue, combined with variable size FIFO queues for channels implemented with a two port memory buffer. Early traffic can be simply checked and delayed. Also, it is scalable with respect to the number of channels C and the total number of buffers N. Moreover, deadline folding technique eliminates the need to extend the deadline resolution. Simulation studies and layout design demonstrate the efficiency and utility of the proposed architecture. View full abstract»

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  • Formal development of a real-time kernel

    Page(s): 220 - 229
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    The formal development of a simple real time operating system kernel is described. The kernel provides a set of operations that allows a restricted Ada 95 tasking model to be supported, suitable for fixed priority real time systems. The requirements for the kernel are expressed in terms of the computational model using RTL, and the abstract specification of the kernel is validated against this. The development of an implementation from this specification is then described, with the PVS proof system used to verify each step in the development process. View full abstract»

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  • A constraint-based approach for specification and verification of real-time systems

    Page(s): 230 - 239
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    We develop a general constraint logic programming (CLP) based framework for specification and verification of real time systems. Our framework is based on the notion of timed automata that have traditionally been used for specifying real time systems. In our framework, a user models the ordering of real time events as the grammar of a language accepted by a timed automata, the real time constraints on these events are then captured as denotations of the grammar productions specified by the user. The grammar can be specified as a Definite Clause Grammar (DCG), while the denotations can be specified in constraint logic. The resulting specification can hence be regarded as a constraint logic program (CLP), and is executable. Many interesting properties of the real time system can be verified by posing appropriate queries to this CLP program. A major advantage of our approach is that it is constructive in nature, i.e., it can be used for computing the conditions under which a property will hold for a given real time system. Our framework also suggests new types of formalisms that we call constraint automata and timed push down automata. View full abstract»

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  • Guidelines for automated implementation of executable object oriented models for real-time embedded control systems

    Page(s): 240 - 251
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    We present our experiences in applying real time scheduling theory to embedded control systems designed using ROOM (Real time Object Oriented Modeling) methodology. ROOM has originated from the telecommunications community and has been successfully applied to many commercial systems through the supporting case tool ObjecTime. It is particularly suitable for modeling reactive real time behavior. Furthermore, it provides many other advantages through the use of object orientation, and the use of executable models from which code may be generated quickly and efficiently. Since many real time embedded control systems have significant reactive, event driven behavior, it is attractive to use ROOM methodology to develop such systems. However, the ROOM methodology does not provide tools to specify and analyze the temporal behavior as is required for the hard real time components of embedded systems, and for which the real time scheduling theory provides an analytical basis. We show how real time scheduling theory may be applied to ROOM models using a cruise control example to illustrate. The biggest challenge comes from minimizing the adverse effects of priority inversions. Our results are very encouraging, and we show that not only is it possible to apply real time scheduling theory, but that it can be done very efficiently provided certain guidelines are followed in the design and implementation of the ROOM model. View full abstract»

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  • Optimal smoothness results and approximation techniques for real-time VBR video traffic smoothing

    Page(s): 253 - 263
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    We study the problem of real time VBR video traffic smoothing. One fundamental difficulty in this problem is that at any time during transmission, the information of most future video frames is not available and thus it is unlikely that global optimization can be achieved in the smoothing process. In order to measure the effectiveness of real time video smoothing methods, we first propose a benchmark algorithm which achieves optimality on some of the smoothness parameters in the smoothing results. Based on this algorithm, we found that significant discrepancy exists between the results produced by some of the existing smoothing methods and the smoothness upper bounds. With this observation, we then focus on devising an algorithm which improves the smoothing results. Experimental results show that our algorithm makes noticeable improvements in some of the smoothness parameters compared to existing smoothing methods. View full abstract»

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