The problem of scheduling multiple streams of real-time customers is addressed in this paper. The paper first introduces the notion of (m; k)-firm deadlines to better characterize the timing constraints of real-time streams. More specifically, a stream is said to have (m; k)-firm deadlines if at least m out of any k consecutive customers must meet their deadlines. A stream with (m; k)-firm deadlines experiences a dynamic failure if fewer than m out of any k consecutive customers meet their deadlines. The paper then proposes a priority-based policy for scheduling N such streams on a single server to reduce the probability of dynamic failure. The basic idea is to assign higher priorities to customers from streams that are closer to a dynamic failure so as to improve their chances of meeting their deadlines. The paper proposes a heuristic for assigning these priorities. The effectiveness of this approach is evaluated through simulation under various customer arrival and service patterns....

Large-scale wireless sensor networks represent a new generation of real-time embedded systems with significantly different communication constraints from traditional networked systems. This paper presents RAP, a new real-time communication architecture for large-scale sensor networks. RAP provides convenient, high-level query and event services for distributed micro -sensing applications. Novel location-addressed communication models are supported by a scalable and light-weight network stack. We present and evaluate a new packet scheduling policy called velocity monotonic scheduling that inherently accounts for both time and distance constraints. We show that this policy is particularly suitable for communication scheduling in sensor networks in which a large number of wireless devices are seamlessly integrated into a physical space to perform real-time monitoring and control. Detailed simulations of representative sensor network environments demonstrate that RAP significantly reduces the end-to-end deadline miss ratio in the sensor network.

This paper focuses on bridging the gap between theory and practice in the management of host CPU and link resources for real-time communication. Using our implementation of real-time channels, a paradigm for real-time communication in packet-switched networks, we illustrate the tradeoff between resource capacity and channel admissibility, which determines the number and type of real-time channels that can be accepted for service and the performance delivered to best-effort traffic. We demonstrate that this tradeoff is affected significantly by the choice of implementation paradigms and the grain at which CPU and link resources are multiplexed amongst active channels. To account for this effect, we extend the admission control procedure for real-time channels originally proposed using idealized resource models. Our results show that practical considerations significantly reduce channel admissibility compared to idealized resource models. Further, the optimum choice of multiplexinggrain ...

The transfer of prerecorded, compressed video requires multimedia services to support large fluctuations in bandwidth requirements on multiple time scales. Bandwidth smoothing techniques can reduce the burstiness of a variable-bit-rate stream by prefetching data at a series of fixed rates, simplifying the allocation of resources in video servers and the communication network. Given a fixed client-side prefetch buffer, several bandwidth smoothing algorithms have been introduced that are provably optimal under certain constraints. This paper presents a comprehensive performance evaluation of bandwidth smoothing algorithms, based on a collection of metrics that relate directly to the server, network, and client resources necessary for the transmission, transport, and playback of prerecorded video. Due to the scarcity of available trace data, we have constructed a video capture testbed and generated a collection of twenty full-length, motion-JPEG encoded video clips. Using these ...

In this paper we address the issue of guaranteeing the end-to-end deadlines of hard real-time connections in an ATM network. In an ATM network, a set of hard realtime connections can be admitted only if the end-to-end delays of cells belonging to individual connections are not more than their deadlines. We systematically decompose an ATM network into constant delay and variable delay servers to facilitate the delay analysis. Effective traffic description is the key part of such a process. We propose a comprehensive traffic description function that provides adequate information about the worst case traffic behavior of connections anywhere in the network. We also study some simple approximations of this function that perform reasonably well in practice. We analyze and compare the performance of ATM networks with FCFS and WRR link scheduling policies under different loading conditions. 1 Introduction ATM networks are being increasingly considered for use in mission-critical systems. A ...

Parallel machines have the potential to satisfy the large computational demands of emerging real-time applications. These applications require a predictable communication network, where time-constrained traffic requires bounds on latency or throughput while good average performance suffices for best-effort packets. This paper presents a router architecture that tailors low-level routing, switching, arbitration and flowcontrol policies to the conflicting demands of each traffic class. The router implements deadline-based scheduling, with packet switching and table-driven multicast routing, to bound end-to-end delay for time-constrained traffic, while allowing best-effort traffic to capitalize on the low-latency routing and switching schemes common in modern parallel machines. To limit the cost of servicing time-constrained traffic, the router shares packet buffers and link-scheduling logic between the multiple output ports. Verilog simulations demonstrate that the design meets the perfo...

We propose a work-conserving scheduling mechanism for providing deterministic performance guarantees in ATM networks. The most attractive feature of the proposed mechanism, which we call Carry-Over Round Robin (CORR), is its simplicity. It is an extension of weighted round robin scheduling. We have derived closed form bounds for worst case end-to-end delay when CORR is used in conjunction with composite leaky bucket, moving window and jumping window regulators. We have also analyzed the fairness properties of CORR scheduling. Our results show that albeit its simplicity, CORR is very competitive with some of the more complex scheduling disciplines such as Packet-by-Packet Generalized Processor Sharing and Self Clocked Fair Queuing, both in terms of delay performance and fairness. Besides the scheduling mechanism and its analysis, one of the important contributions of this study is the characterization of traffic envelopes defined by composite shapers consisting of multiple lea...

Parallel machines have the potential to satisfy the large computational demands of real-time applications. These applications require a predictable communication network, where time-constrained traffic requires bounds on throughput and latency while good average performance suffices for best-effort packets. This paper presents a new router architecture that tailors low-level routing, switching, arbitration, flow-control, and deadlock-avoidance policies to the conflicting demands of each traffic class. The router implements bandwidth regulation and deadline-based scheduling, with packet switching and table-driven multicast routing, to bound end-to-end delay and buffer requirements for time-constrained traffic, while allowing best-effort traffic to capitalize on the low-latency routing and switching schemes common in modern parallel machines. To limit the cost of servicing time-constrained traffic, the router includes a novel packet scheduler that shares link-scheduling logic across the ...

In order to realize real-time communication over Ethernet or fast Ethernet, one must be able to bound the medium access time within an acceptable limit. The multiple access nature of an Ethernet makes it impossible to guarantee a deterministic medium access time (hence packet delivery deadlines) to individual stations. However, one can bound the medium access time statistically by limiting the packet arrival rate at the medium access control (MAC) layer. While focusing on automated manufacturing systems as the main application, this paper considers the connection admission control (CAC) problem for statistically bounding the medium access time of Ethernet. Specifically, a packet is guaranteed to have a medium access time smaller than a predefined bound with a certain probability if the instantaneous packet arrival rate is kept below a certain threshold. Through an analysis, we first derived such a threshold. In order to keep the packet arrival rate under the given threshold, we employ a middleware which (i) resides between the transport layer and Ethernet datalink layer and (ii) smooths packet streams between them. The implementation of this middleware requires only a minimal change in the OS kernel without any modification to the current standard of Ethernet MAC protocol or TCP or UDP/IP stack. In order to solve the CAC problem, we derived the probability of transmitting a packet successfully upon each trial by modeling the MAC protocol, 1-persistent CSMA/CD, and the collision resolution protocol, Binary Exponential Backoff, of Ethernet. Using a simulation study, we have shown this analytic model to provide a reasonably accurate estimate of packet-loss (or deadline-miss) rate over Fast Ethernet. Finally, we implemented the middleware on the Linux OS, experimentally dem...

. Emerging parallel real-time and multimedia applications broaden the range of performance requirements imposed on the interconnection network. This communication typically consists of a mixture of different traffic classes, where guaranteed packets require bounds on latency or throughput while good average performance suffices for the best-effort traffic. This paper investigates how multicomputer routers can capitalize on low-latency routing and switching techniques for besteffort traffic while still supporting guaranteed communication. Through simulation experiments, we show that certain architectural features are best-suited to particular performance requirements. Based on these results, the paper proposes and evaluates a router architecture that tailors low-level routing, switching, and flow-control policies to the unique needs of best-effort and guaranteed traffic. Careful selection of these policies, coupled with fine-grain arbitration between the classes, allows the guaranteed ...