Testing the Schedulability of Synchronous Traffic for the Timed Token Medium Access Control Protocol

One of the key issues related to guaranteeing synchronous message deadlines in a timed token network (such as fiber distributed data interface) where the timed token medium access control protocol is used is the schedulability test of synchronous traffic (i.e., testing whether or not all synchronous messages can be transmitted before their deadlines, under a given setting of network parameters). Much work has been done on how to assign network parameters appropriately in order to guarantee timely transmission of synchronous traffic. As a result quite a few synchronous bandwidth allocation schemes and some good guidelines on selection of the target token rotation time have been proposed. In contrast, limited research has been conducted on how to effectively test whether or not given network parameters can guarantee timely transmission of all synchronous messages (of a considered synchronous message set) before their deadlines. The previous testing methods for synchronous message schedulability only provide a sufficient (but not necessary) test and therefore fail to always keep effective for any synchronous message set considered. In this paper, we propose two testing methods for determining the schedulability of a synchronous message set with message deadlines no longer than periods. The proposed tests perform better than any previous test in the sense that they are both sufficient and necessary. Some numerical examples are given to compare different testing methods, all of which have demonstrated the superiority of the proposed tests to other existing testing methods.     

On the Effectiveness of the FDDI-M Medium Access Protocol for Real-Time Traffic

FDDI networks have been widely deployed to support real-time traffic such as voice and video communications. However FDDI's medium access control (MAC) protocol can transmit synchronous messages up to at most one half of the total bandwidth of the FDDI network. In a recent paper by K. Shin and Q. Zheng [Shin95], a modification to the FDDI MAC protocol, called FDDI-M, has been proposed to double a ring's ability to support synchronous traffic. In this paper we present an analytical study of the timing properties of the FDDI-M protocol. Using the Worst Case Achievable Utilization (WCAU) as the performance metric, we first evaluate the performance of various synchronous bandwidth allocation (SBA) schemes in guaranteeing synchronous message deadlines. It is found that, in comparison with FDDI, the FDDI-M protocol results in a higher WCAU for the normalized proportional SBA scheme. However, for the local SBA schemes, the WCAU values remain zero. Next, other performance metrics such as throughput and percentage of frames missing deadlines are examined using simulation. A series of simulations using actual MEPG video traffic confirms that FDDI-M does provide superior synchronous transmission characteristics.     

Communication Response Time in P-NET Networks: Worst-Case Analysis Considering the Actual Token Utilization

Fieldbus networks aim at the interconnection of field devices such as sensors, actuators and small controllers. Therefore, they are an effective technology upon which distributed computer-controlled systems (DCCS) can be built. DCCS impose strict timeliness requirements to the communication network. In essence, by timeliness requirements we mean that traffic must be sent and received within a bounded interval, otherwise a timing fault is said to occur. P-NET is a multi-master fieldbus standard based on a virtual token passing scheme. In P-NET each master is allowed to transmit only one message per token visit, which means that in the worst-case the communication response time could be derived considering that the token is fully utilized by all stations. However, such analysis can be proved to be quite pessimistic. In this paper, we propose a more sophisticated P-NET timing analysis model, which considers the actual token utilization by different masters. The major contribution of this model is to provide a less pessimistic, and thus more accurate, analysis for the evaluation of the worst-case communication response time in P-NET fieldbus networks.     

分布式实时数据库的通讯延迟模型与协议

本文通过建立分布式实时数据库系统中的一个网络通讯延迟模型,对它的时间开销进行了分析,指出实现实时通讯的可行性.同时给出了通讯协议ECSMA/CD(extend carrier sense multiple access/collission detection)的算法,为数据或消息的实时传送提供了一种机制.