E. Nett

Otto-von-Guericke University Magdeburg

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Michael Mock

TU Dresden

M. Gergeleit

RheinMain University of Applied Sciences

Jozef Kaiser

Central European Institute of Technology

Stefan Schemmer

University Hospital Magdeburg

Reinhold Kröger

Centre Européen d’Etudes de Sécurité et d’Analyse des Risques

Weijia Jia

Beijing Normal University

Leandro Buss Becker

University of Manchester

Andrea Bondavalli

University of Florence

R. Schumann

University of Bonn

K.E. Grosspietsch

Asklepios Klinik Sankt Augustin

Cooperative Institutions

University of Miami

International Center for Advanced Studies

Center for Information Technology

University of Wisconsin–Madison

Otto-von-Guericke University Magdeburg

Gesellschaft Fur Mathematik Und Datenverarbeitung

FZI Research Center for Information Technology

UW Health University Hospital

RheinMain University of Applied Sciences

Cornell University

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Continuous clock synchronization in wireless real-time applications

Michael Mock R. Frings E. Nett Spiro Trikaliotis

Continuous clock synchronization avoids unpredictable instantaneous corrections of clock values. This is usually achieved by spreading the clock correction over the synchronization interval. In the context of wireless real time applications, a protocol achieving continuous clock synchronization must tolerate message losses and should have a low overhead in terms of the number of messages. The paper presents a clock synchronization protocol for continuous clock synchronization in wireless real time applications. It extends the IEEE 802.11 standard for wireless local area networks. It provides continuous clock synchronization, improves the precision by exploiting the tightness of the communication medium, and tolerates message losses. Continuous clock synchronization is achieved with an advanced algorithm adjusting the clock rates. We present the design of the protocol, its mathematical analysis, and measurements of a driver level implementation of the protocol on Windows NT.

Self-clocking signal

Clock drift

Digital clock manager

Clock domain crossing

Network Time Protocol

Master clock

Vector clock

Timing failure

Data synchronization

10.1109/reldi.2000.885400

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Citations (147)

Reliable real-time cooperation of mobile autonomous systems

Stefan Schemmer E. Nett Michael Mock

Autonomous systems are expected to provide increasingly complex and safety-critical services that will, sooner or later, require the cooperation of several autonomous systems for their fulfillment. In particular, coordinating the access to shared physical and information technological resources will become a general problem. Scheduling these resources is subject to strong real-time and reliability requirements. In this paper, we present an architecture that allows autonomous mobile systems to schedule shared resources in real-time using their own wireless distributed infrastructure. In our architecture, there is a clear separation between the application-specific scheduling part that is modeled as a function of the global state and the communication part that is used to provide the global state. By isolating the more error-prone communication part within a communication hardcore, the reliability of the overall system is increased and the locally executed scheduling function can be designed with primary focus on the application-specific real-time requirements.

Real-time communication

10.1109/reldis.2001.970774

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Citations (17)

Enhancing O-O Middleware to Become Time-Aware

Springer eBooks (2001)

E. Nett M. Gergeleit Michael Mock

Implementation

Structuring

Legacy system

10.1007/978-1-4615-1695-8_6

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Citations (2)

Recovery-management in the RelaX distributed transaction layer

R. Schumann Reinhold Kröger Michael Mock E. Nett

Transactions are especially valuable in distributed systems, since they isolate the programmer from the effects of both concurrency and failures. In implementing transactions at the system level, flexibility has to be introduced into the transaction concept. Specifically, the premature release of data objects has to be addressed. To assure recoverability, resulting dependencies between transactions are stored by the system in a distributed data structure called a recovery graph. The storing of redundant information in the recovery graphs at the different sites reduces the complexity of the commit protocol, and a chase protocol used to abort transactions can be derived which excludes infinite chasing. The redundant information can be distributed almost for free because it can be piggybacked on messages. The new commit/abort protocols will be used in the RelaX project (reliable distributed applications support on Unix), which carries on work done at GMD in the PROFEMO project on distributed transaction mechanisms.< >

Commit

Compensating transaction

Two-phase commit protocol

Serializability

Programmer

Abort

Distributed database

10.1109/reldis.1989.72745

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Citations (21)

A generic log-service supporting fast recovery in distributed fault-tolerant systems

B. Wirz E. Nett

Logs are an important facility for fault-tolerant distributed systems since they allow to reliably store information that is needed to provide a global consistent system state also in the presence of failures. The authors focus on the problem of fast recovery after a node crash. The approach is mainly based on minimizing the number of log records to be retrieved. This is achieved by periodically discarding obsolete information in a very efficient manner without effecting the normal logging procedure. The main idea behind is that the generic Log-Service provides a high level interface to the application which allows the Log-Service itself to interpret the semantics of log records without consulting the application during run-time. In addition, the authors are able to reduce the overhead in analyzing the log contents during restart by scanning the log only once and only forward.

10.1109/apads.1993.588858

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Citations (9)

Adaptive software fault tolerance policies with dynamic real-time guarantees

E. Nett Hermann Streich P. Bizzarri Andrea Bondavalli F. Tarini

Real time applications with high dependability requirements demand fault tolerance strategies. While for small systems with static behaviour policies, worse case execution times can be used, this is not true for more complex systems, in which worst case execution times are partially unknown or differ drastically from their average execution time. In such cases often only a minimum of quality can be achieved. The paper proposes to combine fault tolerant policies described by the FERT (Fault tolerant Entity for Real Time) notation with the dynamic scheduling scheme TPS (TaskPair Scheduling). TPS alleviates FERT's precondition of completely known WCETs and provides a flexible implementation base to enable an easy mapping of FERT strategies to a runtime system. In a first step, a significant subset of FERT is investigated, which implies: the recovery block scheme, N Version programming, and imprecise computations. TPS is utilised to guarantee different levels of quality, tailored to the application and the required level of fault tolerance, while guaranteeing that a common deadline is met.

Software fault tolerance

Precondition

10.1109/words.1996.506264

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Citations (16)

Reliable real-time communication in cooperative mobile applications

IEEE Transactions on Computers (2003)

E. Nett Stefan Schemmer

Embedded systems are expected to provide increasingly complex and safety-critical services that will, sooner or later, require the cooperation of several such systems for their fulfillment. In particular, coordinating the access to shared physical and information technological resources will become a general problem. Examples are mobile robots in industrial automation or car-to-car coordination for future traffic control applications. In such applications, cooperation is subject to strong real-time and reliability requirements. In this paper, we present an architecture that allows autonomous mobile systems to schedule shared resources in real-time using their own wireless distributed infrastructure. In this architecture, there is a clear separation between the application-specific scheduling part and the application independent communication part that constitutes the real-time and reliability hardcore of the system. The latter provides clock synchronization, real-time atomic multicast, and real-time group membership based on an IEEE 802.11 standard wireless LAN. An application prototype shows how the architecture can be used in future mobile cooperative applications.

Real-time communication

10.1109/tc.2003.1176984

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Citations (51)

A high performance reliable atomic group protocol

Weijia Jia Jiannong Cao E. Nett Jozef Kaiser

A novel and efficient group communication (multicast) protocol, based on a single logical-clock token ring approach, is described. The protocol is highly efficient and it guarantees total ordering and atomicity of multicast messages for asynchronous distributed systems. Unlike other logical token-ring algorithms, the protocol does not have a problem of token loss. The optimized fault-tolerant algorithms of the protocol can handle process failures and network partitioning. The experiment results of the implemented protocol in a local area network of workstations have demonstrated that its performance is better than any existing solutions in the same environment, especially, for achieving message total ordering and atomic (safe) delivery.

Token ring

Atomicity

Token passing

Token bus network

10.1109/icpads.1996.517585

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Citations (1)

An adaptive approach to object-oriented real-time computing

E. Nett M. Gergeleit Michael Mock

Real time computing is becoming an enabling technology for many important distributed applications such as flexible manufacturing, multimedia, robotics and process control. Traditionally, real time systems have been realized as isolated embedded systems. Unfortunately, this approach no longer suffices for future complex systems in the application scenarios mentioned above. In this situation, the use of an object oriented design paradigm greatly reduces the complexity of the system while improving reusability and manageability. Furthermore, the surrounding IT infrastructure is more and more accessible through object oriented interfaces (e.g. CORBA). In addition, object oriented modeling allows reflection of the dynamic characteristics of the applications mentioned above by instantiating objects dynamically. In order to meet the real time requirements in such an environment, static scheduling is not sufficient since many non predictable resource conflicts influence execution times. Therefore, the most distinguishing requirement of these complex heterogeneous systems is the need of the computing system to dynamically adapt to dynamically changing conditions. Little work has been done on integrating object oriented system design with resource allocation algorithms that are flexible enough to cope with this new requirement. The paper presents an approach for an adaptive object oriented system with integrated monitoring, dynamic execution time prediction and scheduling. It explains how this approach is applied to CORBA and C++.

Reusability

10.1109/isorc.1998.666806

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Citations (36)

Scheduling solutions for supporting dependable real-time applications

F. Sandrini Felicita Di Giandomenico Andrea Bondavalli E. Nett

This paper deals with tolerance to timing faults in time-constrained systems. TAFT (Time Aware Fault-Tolerant) is a recently devised approach which applies tolerance to timing violations. According to TAFT, a task is structured in a pair, to guarantee that deadlines are met (although possibly offering a degraded service) without requiring the knowledge of task attributes difficult to estimate in practice. Wide margin of actions is left by the TAFT approach in scheduling the task pairs, leading to disparate performances; up to now, poor attention has been devoted to analyse this aspect. The goal of this work is to investigate on the most appropriate scheduling policies to adopt in a system structured in the TAFT fashion, in accordance with system conditions and application requirements. To this end, all experimental evaluation will be conducted based on a variety of scheduling policies, to derive useful indications for the system designer about the most rewarding policies to apply.

Processor scheduling

Task Analysis

10.1109/isorc.2000.839519

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Citations (4)