This report extends the current analysis associated with static priority pre-emptive based scheduling to address the wider problem of analysing schedulability of a distributed hard real-time system; in particular it derives analysis for a distributed system where tasks with arbitrary deadlines communicate by message passing and shared data areas. A simple TDMA protocol is assumed, and analysis developed to bound not only the communications delays, but also the delays and overheads incurred when messages are processed by the protocol stack at the destination processor. The report illustrates how a windowbased analysis technique can be used to find the worst-case response times of a distributed task set. An extended example illustrating the application of the analysis is presented.

Abstract-We study the problem of guaranteeing synchronous message deadlines in token ring networks where the timed to-ken medium access control protocol is employed. Synchronous bandwidth, defined as the maximum time for which a node can transmit its synchronous messages every time it receives the token, is a key parameter in the control of synchronous message transmission. To ensure the transmission of synchronous messages before their deadlines, synchronous capacities must be properly allocated to individual nodes. We address the issue of appropriate allocation of the synchronous capacities. Several synchronous bandwidth allocation schemes are analyzed in terms of their ability to satisfy deadline constraints of synchronous messages. We show that an inappropriate allocation of the syn-chronous capacities could cause message deadlines to be missed, even if the synchronous traffic is extremely low. We propose

The dramatically increased bandwidths and processing capabilities of future high-speed networks make possible many distributed real-time applications, such as sensor-based applications and multimedia services. Since these applications will have traffic characteristics and performance requirements that differ dramatically from those of current data-oriented applications, new communication network architectures and protocols will be required. In this paper we discuss the performance requirements and traffic characteristics of various real-time applications, survey recent developments in the areas of network architecture and protocols for supporting real-time services, and develop frameworks in which these, and future, research efforts can be considered.

We study the problem of guaranteeing synchronous message deadlines in communication networks where the timed token medium access control protocol is employed. Synchronous capacity, defined as the maximum time for which a node can transmit its synchronous messages every time it receives the token, is a key parameter in the control of synchronous message transmission. To ensure the transmission of synchronous messages before their deadlines, synchronous capacities must be properly allocated to individual nodes. In this paper, we develop and analyze an optimal synchronous capacity allocation scheme. An optimal scheme can allocate the synchronous capacities in such a way that the synchronous message deadlines are guaranteed if there exists any allocation scheme that can do so. The optimality of the allocation scheme proposed in this paper is formally proved and the bounds for its Worst Case Achievable Utilization are derived. Key Words: Hard Real-Time, Distributed System, FDDI, Timed Token...

In a distributed hard real-time system, communications between tasks on different processors must occur in bounded time. The inevitable communication delay is composed of both the delay in transmitting a message on the communications media, and also the delay in delivering the data to the destination task. This paper derives schedulability analysis bounding the media access delay and the delivery delay. Two access protocols are considered: a simple timed token passing approach, and a real-time priority broadcast bus. A simple delivery approach is considered where the arrival of a message generates an interrupt --- the so-called `on demand' approach. 1. INTRODUCTION A hard real-time system is often composed from a number of periodic and sporadic tasks which communicate their results by passing messages; in a distributed system these messages are sent between processors across a communications device. In order to guarantee that the timing requirements of all tasks are met, the communica...

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 ...

One common way of constructing hard real-time systems is to use a number of periodic and sporadic tasks assigned static priorities and dispatched at run-time according to the preemptive priority scheduling algorithm. Most analysis for such systems attempts to find the worst-case response time for each task by assuming that the worst-case scheduling point is when all tasks in the system are released simultaneously. Often, however, a given set of hard real-time tasks will have offset constraints: a number of tasks sharing the same periodic behaviour will be constrained to execute at fixed offsets in time relative to each other. In this situation the assumption of a simultaneous release of all tasks can lead to pessimistic scheduling results. In this paper we derive good response time bounds for tasks with offset information, giving an optimal priority ordering algorithm. 1.

The CAC algorithm must efficiently determine if a new connection can be admitted by verifying that its QoS requirements can be met without violating those of previously admitted connections. In hard real-time systems, the QoS requirements are specified in terms of endto -end cell deadlines and no cell loss due to buffer overflow. A CAC algorithm must account for interdependencies among connections caused by statistical multiplexing of cells in ATM networks. Furthermore, arbitrary topology of the network may lead to cyclic dependencies among various connections. We present an efficient CAC algorithm that addresses the above issues. The algorithm uses a traffic descriptor called the maximum traffic rate function to effectively compute bounds on end-to-end delays of connections and buffer requirements within the network. Our work differs from most previous work in that it does not require traffic restoration inside the network. Email : famitava,ksanjay,zhao@cs.tamu.edug Fax : 409-847-85...

Reliable and timely delivery of messages between processing nodes is essential in distributed real-time systems. Failure to deliver a message within its deadline usually forces the system to undertake a recovery action which introduces some cost (or overhead) to the system. This recovery cost can be very high, especially when the recovery action fails due to lack of time or resources. Proposed in this paper is a scheme to minimize the expected cost incurred as a result of messages failing to meet their deadline. The scheme is intended for distributed real-time systems, especially with a point--to--point interconnection topology. The goal of minimizing the expected cost is achieved by sending multiple copies of a message through disjoint routes and thus increasing the probability of successful message delivery within the deadline. However, as the number of copies increases, the message traffic on the network increases, thereby increasing the delivery time for each of the copies. There i...

For fault-tolerant real-time distributed systems, the probability that a message is not delivered within its real-time deadline must be small enough that it does not adversely affect system reliability. We investigate the delivery of messages for the Totem Protocol, a reliable ordered broadcast protocol that we have developed for fault-tolerant distributed systems with physical broadcasts over a local-area network. The total order on broadcast messages, constructed by the Totem Protocol, supports the maintenance of consistency of replicated information as, for example, in a replicated database. We present a methodology for determining the probability of satisfying bounds on the latency from message origination to ordered delivery in the presence of communication faults.