Nicholas Malcolm

The timed-token protocol for real-time communications

The timed-token protocol is a token-passing protocol in which each node receives a guaranteed share of the network bandwidth. Partly because of this property, the timed-token protocol has been incorporated into a number of network standards, including the IEEE 802.4 token bus, the Fiber Distributed Data Interface (FDDI), and the Survivable Adaptable Fiber Optic Embedded Network (Safenet). Networks based on these standards are becoming increasingly popular in new generation real-time systems. In particular, the IEEE 802.4 standard is included in the Manufacturing Automation Protocol (MAP), which has been widely used in computer-integrated manufacturing and industrial applications. Meeting message deadlines requires proper control of medium access. In the timed-token protocol, access to the communication medium is controlled by a token that is passed among the nodes in a circular fashion. Messages are segregated into two separate classes: synchronous and asynchronous. Synchronous messages, used for real-time communication, can have deadline constraints and thus are given a guaranteed share of the network bandwidth. The authors focus on meeting the deadlines of synchronous messages.<<ETX>>

Hard real-time communication in multiple-access networks

With the increasing use of distributed hard real-time systems, the ability of computer networks to handle hard real-time message traffic is becoming more important. For traditional networks, maximizing the throughput or minimizing the average message delay is the most important performance criteria. In the hard real-time domain, however, concern focuses on satisfying the time constraints of individual messages. This paper examines recent developments in hard real-time communication in local area multiple-access networks. Two general strategies are used in hard real-time communication: the guarantee strategy and the best-effort strategy. In the former, messages are guaranteed to meet their deadlines during normal operation of the network. In the best-effort strategy, the network will attempt to send messages before their deadlines, but no guarantees are given. Real-time message traffic can be distinguished according to whether it is best suited for the guarantee strategy or the best-effort strategy. Although this paper concentrates on multiple-access networks, many of the concepts presented and lessons learned are also applicable to other types of networks.

Guaranteeing synchronous messages with arbitrary deadline constraints in an FDDI network

Issues related to guaranteeing synchronous messages with arbitrary deadline constraints in a fiber distributed data interface (FDDI) network are addressed. It is shown that several network parameters must be set carefully if message deadlines are to be satisfied. Message deadlines can only be met if sufficient synchronous bandwidth is allocated to each mode. Thus, proper synchronous bandwidth allocation is essential if deadlines are to be guaranteed. The target token rotation time (TTRT) determines both the speed of token circulation and the network utilization available to user applications. TTRT should also be chosen carefully to ensure that the token circulates fast enough while maintaining a high available utilization. Sufficient buffer space must be provided for outgoing messages, otherwise messages could be lost due to buffer overflow. An integrated method for allocating the synchronous bandwidth and selecting TTRT so that the time constraints of synchronous messages with arbitrary deadlines are guaranteed to be met is proposed and analyzed.

Hard real-time communications with weighted round robin service in ATM local area networks

In this paper, we address issues related to providing guaranteed real-time communication in ATM local area networks. We concentrate on output link scheduling because it plays a critical role in meeting message deadlines. We are particularly interested in the weighted round robin scheduling policy because of its simple design and implementation. To use weighted round robin scheduling for hard real-time applications, the weights must be properly allocated to each of the connections. We propose and analyze two weight allocation schemes. The first scheme is heuristic, and is easy to understand and implement. The second scheme is optimal. That is, it can always guarantee a set of hard real-time connections whenever it is possible to do so. We evaluate and compare the system performance in terms of its admission probability-the probability that deadlines of all connections in a randomly chosen connection set can be met. We find that the optimal weight allocation scheme indeed performs the best. However, the heuristic scheme performs closely to the optimal scheme over a wide range of loading conditions.

Advances in hard real-time communication with local area networks

Recent developments in real-time communications in local-area networks are reviewed. Issues relating to the transmission of both synchronous and asynchronous messages are examined. Priority-driven protocols, global reservation protocols, and protocols with bounded access times are discussed.<<ETX>>

Performance evaluation of a bandwidth allocation scheme for guaranteeing synchronous messages with arbitrary deadlines in an FDDI network

We study the performance of FDDI networks in terms of their guarantee probability, i.e., the probability that a set of synchronous messages are guaranteed to meet their deadlines. Traditional techniques such as queuing analysis cannot be directly used to derive the guarantee probability. To counter this problem, we develop a new geometric model of schedulability. Based on this model, we obtain a numerical method to compute the exact values of the guarantee probability. A closed-form approximation for the guarantee probability is also derived, and is shown to be relatively accurate and computationally efficient. The network performance is then systematically examined in terms of the guarantee probability. We find that there is a high probability that a randomly chosen message set can be guaranteed even when the real-time traffic is increased beyond the worst case achievable utilization bound. Hence, FDDI networks are applicable for real-time applications in a wide range of loading conditions.<<ETX>>

Version selection schemes for hard real-time communications

Version selection schemes for hard real-time communications in multiaccess networks are studied. Two classes of version selection schemes are proposed: those that are based on message time constraints and those that are based on the network traffic. The authors discuss the design and implementation of both classes of version selection schemes used in conjunction with a window-based media access protocol. Integrated version selection schemes that combine both the timing- and the traffic-based approaches are discussed. The simulation results show that as the network load increases, the version selection schemes are effective at reducing the message loss by trading off the proportion of long version messages sent.<<ETX>>

Performance evaluation of admission policies in ATM based embedded real-time systems

We study the effect of the output link scheduling discipline of an ATM switch on the ability of an ATM LAN to admit real-time connections. Three output link scheduling policies are studied: first come first served (FCFS), round robin (RR), and packet-by-packet generalized processor sharing (PGPS). We derive connection admission criteria for the three scheduling policies. To evaluate the performance of the three scheduling policies, we introduce the metric of admission probability. The admission probability gives the probability that a randomly chosen set of real-time connections will be admitted into the network. The admission probability allows system designers to study the performance of different scheduling policies over a wide range of network loads. We observe that the performance of the three scheduling policies is sensitive to message deadlines. When the deadlines are small, PGPS outperforms both RR and FCFS, and RR outperforms FCFS. When the deadlines are large, all three scheduling policies perform the same. We also note that although PGPS is better than RR and FCFS most of the time, its improved performance is achieved at the cost of high implementation complexity and run time overheads.<<ETX>>

Real-time communication in FDDI-based reconfigurable networks

We report our ongoing research in real-time communication with FDDI-based reconfigurable networks. The original FDDI architecture was enhanced in order to improve its fault-tolerance capability while a scheduling methodology, including message assignment, bandwidth allocation, and bandwidth management is developed to support real-time communication. As a result, message deadlines are guaranteed even in the event of network faults.<<ETX>>