Paulo Pedreiras

The FTT-CAN protocol: why and how

The requirement for flexible operation is becoming increasingly important in modern industrial systems. This requirement has to be supported at all system levels, including the field level in process industry, as well as the cell and machine control levels in manufacturing industry, where fieldbus-based communication systems are commonly found. Furthermore, typical applications at these levels require both time- and event-triggered communication services, in most cases under stringent timing constraints, to convey state data in the former case and alarms and management data in the latter. However, neither the requirement for flexible operation under guaranteed timeliness nor for joint support of time and event-triggered traffic are efficiently fulfilled by most of existing fieldbus systems. This paper presents a new protocol, flexible time-triggered communication on controller area network, which fulfills both requirements: it supports time-triggered communication in a flexible way as well as being an efficient combination of both time- and event-triggered traffic with temporal isolation. These types of traffic are handled by two complementary subsystems, the synchronous and the asynchronous messaging systems, respectively. The paper includes a justification for the new protocol as well as its description and worst case temporal analysis for both subsystems. This analysis shows the capability of the protocol to convey real-time traffic of either type.

Scheduling within temporal partitions: response-time analysis and server design

As the bandwidth of CPUs and networks continues to grow, it becomes more attractive, for efficiency reasons, to share such resources among several applications with the minimum level of interference. This can be achieved using temporal partitions, with each application assigned to its own partition and executing as if it was executing alone on a resource with lower bandwidth. The partitions are associated to servers that execute the application tasks according to a given application-level scheduler. On the other hand, the set of servers is scheduled by a system-level scheduler. This paper addresses the particular case of fixed priorities-based application-level schedulers together with a periodic server model at the system level. It starts with an adequate response time analysis based on the notion of server availability for a known server. Then it addresses the inverse problem of designing a server with minimum system-level resource requirements to fulfill the application time constraints. In this context, the paper shows that response time based schedulability tests with linear time bounds do not need to consider all tasks but just a small subset, which may lead to substantial speed-ups. The proposed method goes a step further with respect to other recent works in the literature by considering a more complete task model, effectively computing the server parameters and establishing a better trade-off concerning complexity and tightness.

FTT-Ethernet: a flexible real-time communication protocol that supports dynamic QoS management on Ethernet-based systems

Ethernet was not originally developed to meet the requirements of real-time industrial automation systems and it was commonly considered unsuited for applications at the field level. Hence, several techniques were developed to make this protocol exhibit real-time behavior, some of them requiring specialized hardware, others providing soft-real-time guarantees only, or others achieving hard real-time guarantees with different levels of bandwidth efficiency. More recently, there has been an effort to support quality-of-service (QoS) negotiation and enforcement but there is not yet an Ethernet-based data link protocol capable of providing dynamic QoS management to further exploit the variable requirements of dynamic applications. This paper presents the FTT-Ethernet protocol, which efficiently supports hard-real-time operation in a flexible way, seamlessly over shared or switched Ethernet. The FTT-Ethernet protocol employs an efficient master/multislave transmission control technique and combines online scheduling with online admission control, to guarantee continued real-time operation under dynamic communication requirements, together with data structures and mechanisms that are tailored to support dynamic QoS management. The paper includes a sample application, aiming at the management of video streams, which highlights the protocols ability to support dynamic QoS management with real-time guarantees.

The FTT-ethernet protocol: merging flexibility, timeliness and efficiency

Despite having been designed to interconnect office equipment such as computers and printers, since its early daysEthernet has also been considered for use in the industrial domain. However, it was not originally developed to meet the requirements of real-time industrial automation systems and it was commonly considered unsuited for applications at the field level, i.e.to interconnect sensors, actuators and controllers. Therefore, along its 30 years of existence, several proposals have been presented to make this protocol exhibit real-timebehaviour. Nevertheless, these proposals either require specialised hardware, or are suited to soft-real-time operation only, or are bandwidth or response-time inefficient. This paper presents an overview about the work previously done towards real-time communication on Ethernet. Then, it presents a new protocol, FTT-Ethernet, which relies on common network adapters and on a new transmission control named master/multi-slave that efficiently supports hard-real-time operation in a flexible way.

Enhancing real-time communication over cots ethernet switches

Switched Ethernet arose in the last decade as a means to increase global throughput with parallel switching paths, segment the network and create isolated collision domains, thus reducing the non-determinism of the original shared Ethernet. However, COTS Ethernet switches still suffer from a few drawbacks that affect negatively their real-time communication capabilities. For example, there can be overflows in ports queues with consequences across ports, priority levels and virtual LANs, and the number of priorities is too short for any kind of priority-based scheduling. Moreover, switches present extra latencies and jitter due to the need to interpret frame addresses and also due to different internal architectural solutions. In this paper we propose using the Flexible Time-Triggered communication paradigm to enhance the temporal behavior of Ethernet switches with respect to periodic streams. We explain the system architecture and we present a formulation of the global periodic traffic scheduling problem handled by the FTT master. Simulation and experimental results show the advantages of using such synchronized framework.

Online QoS Management for Multimedia Real-Time Transmission in Industrial Networks

A growing number of industrial applications incorporate multimedia information processing. These multimedia applications are commonly distributed and subject to time constraints that must be met across networks without creating intolerable interference over typical control flows. However, multimedia traffic, in general, and video streaming, in particular, have specific characteristics that conflict with the operational framework of conventional real-time protocols. In particular, video compressors generate highly variable bit-rate streams that mismatch the constant-bit-rate channels typically provided by real-time protocols, severely reducing the efficiency of network utilization. This paper focuses on low-latency multimedia transmission over Ethernet with dynamic quality-of-service (QoS) management. We propose a multidimensional mechanism that controls, in an integrated way, both the compression parameters and the network bandwidth allocated to each stream. The goal is to provide the best possible QoS to each stream, recomputing the compression levels and network bandwidth whenever significant events, such as channel setup/teardown, or structural changes happen. This paper also presents novel QoS metrics based both on the image quality and network parameters. Several experiments with prerecorded video streams illustrate the advantages of the proposed approach and the convenience of the metrics.

The FTT-CAN protocol for flexibility in safety-critical systems

A new communication protocol for distributed embedded systems attempts to find a compromise between the often-opposing goals of system flexibility and safety.

Combining operational flexibility and dependability in FTT-CAN

The traditional approaches to the design of distributed safety-critical systems, due to fault-tolerance reasons, have mostly considered static cyclic table-based traffic scheduling. However, there is a growing demand for operational flexibility and integration, mainly to improve efficiency in the use of system resources, with the network playing a central role to support such properties. This calls for dynamic online traffic scheduling techniques so that dynamic communication requirements are adequately supported. Nevertheless, using dynamic traffic management mechanisms raises additional problems, in terms of fault-tolerance, related with the weaker knowledge of the future system state caused by the higher level of operational flexibility. Such problems have been recently addressed in the scope of using flexible time-triggered CAN (FTT-CAN) in safety-critical applications in order to benefit from the high operational flexibility of this protocol. This paper gathers and reviews the main mechanisms that were developed to provide dependability to the protocol, namely, master replication and fail-silence enforcement.

Coordinating distributed autonomous agents with a real-time database: the CAMBADA project

Interest on using mobile autonomous agents has been growing, recently, due to their capacity to cooperate for diverse purposes, from rescue to demining and security. However, such cooperation requires the exchange of state data that is time sensitive and thus, applications should be aware of data temporal coherency. In this paper we describe the architecture of the agents that constitute the CAMBADA (Cooperative Autonomous Mobile roBots with Advanced Distributed Architecture) robotic soccer team developed at the University of Aveiro, Portugal. This architecture is built around a real-time database that is partially replicated in all team members and contains both local and remote state variables. The temporal coherency of the data is enforced by an adequate management system that refreshes each database item transparently at a rate specified by the application. The application software accesses the state variables of all agents with local operations, only, delivering both value and temporal coherency.

Multi-level hierarchical scheduling in ethernet switches

The complexity of Networked Embedded Systems (NES) has been growing steeply, due to increases both in size and functionality, and is becoming a major development concern. This situation is pushing for paradigm changes in NES design methodologies towards higher composability and flexibility. Component-oriented design technologies, in particular supported by server-based scheduling, seem to be good candidates to provide the needed properties. As a response we developed a multi-level hierarchical server-based architecture for Ethernet switches that provides composability and supports online adaptation and reconfiguration. This paper extends our work, presenting the associated response-time based schedulability analysis, necessary for the admission control procedure. Additionally, we have derived the temporal complexity of the analysis, which is shown to be O(n2), where n is the number of higher priority components associated with a given server. Finally, we present a proof-of-concept implementation and a set of experimental results that validates the analysis.