Dennis Christmann

Realization of Try-Once-Discard in Wireless Multihop Networks

In networked control systems, the Try-Once-Discard (TOD) protocol is of high interest because its properties can be characterized by Lyapunov functions. This feature makes it practical to incorporate TOD into Lyapunov-based design of linear and nonlinear control systems, yielding a self-contained theory for system stabilization. In previous work, candidates for TOD realizations for single-hop (wired and wireless) networks have been proposed. However, it has been a hitherto open question whether TOD can be realized in wireless multihop networks. In fact, it is far from obvious how dynamic value-based competition with deterministically bounded maximum delay, as required by TOD, is achievable in wireless multihop networks. In this paper, we give a positive answer to this question, by presenting a functionally complete realization of TOD in wireless multihop networks. Our solution is based on highly accurate multihop tick synchronization, and applies an algorithm for collision-protected network-wide value arbitration with deterministic delay. We provide experimental evidence for the feasibility of our solution on existing micro controller platforms, and assess our TOD realization in a batch reactor scenario.

Multi-hop Clock Synchronization in Wireless Ad-Hoc Networks

In this paper, we introduce Black Burst Clock Synchronization (BBCS), a novel protocol for multi-hop time synchronization in wireless ad-hoc networks, located at MAC level. BBCS is based on the exchange of synchronized tick and time frames, which are protected against collisions by a special encoding using black bursts. It provides a deterministic upper bound for clock offset that only depends on maximum network diameter, and on the used transceiver hardware. BBCS has low complexity in terms of communication, computation, storage, structure, and energy consumption. It provides low and deterministic convergence delay, and is robust against node movements and node failures. In this work, we introduce BBCS, provide a formal analysis of its properties, and evaluate the required overhead for clock-synchronizing a multi-hop wireless ad-hoc network.

Flexible and energy-efficient duty cycling in wireless networks with MacZ

In wireless networks formed of battery-powered nodes, energy consumption is a major concern. On the communication side, energy can be saved by switching transceivers to idle or power-down mode when not needed. To this end, duty cycling protocols located at MAC level have been devised. These protocols have in common that they define modified CSMA/CA schemes, and that they assume the availability of some time synchronization mechanism. This strongly limits their potential to reduce energy consumption, as well as their flexibility regarding activity phases. In this paper, we propose a more flexible and energy-efficient solution for duty cycling, which is based on our experimental wireless MAC protocol MacZ. Flexibility is strengthened by the possibility to define weakly periodic activity periods. Energy-efficiency is improved by the possibility to reserve and use exclusive transmission slots, thereby switching transceivers to active mode only when needed. Both measures are supported by an underlying protocol for accurate, deterministic tick and time synchronization. We assess our solution and compare it to a well-known duty cycling protocol based on real-life scenarios.

The Arbitrating Value Transfer Protocol (AVTP) - Deterministic Binary Countdown in Wireless Multi-Hop Networks

We present the Arbitrating Value Transfer Protocol (AVTP), a deterministic binary countdown algorithm for multi-hop wireless networks. AVTP enables deterministic arbitration among contending nodes within a given hop radius, which may be chosen to reach network-wide arbitration, thereby exceeding the capabilities of existing protocols. In addition, the deterministic agreement on data values among all nodes is supported. Even taking into account the limitations of actual hardware devices, AVTP provides small and constant arbitration delays, and has low and bounded complexity. It does not require bidirectional links, and is robust against topology changes. Applications of AVTP include deterministic and robust leader election with bounded delay.Finally, we have implemented and evaluated AVTP on Imote2 sensor nodes.

Automatic topology discovery in TDMA-based ad hoc networks

In wireless networks, topology information is often of great importance. Knowledge about the communication and interference topology enables more efficient and reliable use of the wireless medium, and some wireless protocols even require knowledge about nodes in sensing range. Manually measuring and configuring topology information of ad hoc networks is very cumbersome. This paper presents ATDP (Automatic Topology Discovery Protocol), a protocol for the automatic detection of communication, interference, and sensing topologies in wireless TDMA networks. Furthermore, the gathered information is distributed across the network using exclusively assigned TDMA time slots, thus providing all nodes with a consistent view of the network topologies. Experiments with our implementation on Imote2 nodes show that ATDP reliably detects topologies of multihop networks. The experiments also show that consideration of the detected interference links in a TDMA schedule significantly improves packet delivery rates and thus reliability of the network.

Priority scheduling in SDL

In real-time systems, the capability to achieve short or even predictable reaction times is essential. In this paper, we take a pragmatic approach by proposing priority-based scheduling in SDL combined with a mechanism to suspend and resume SDL agents. More specifically, we define adequate syntactical extensions of SDL and show that they are compliant with the formal SDL semantics. We have implemented all proposed extensions in our SDL tool chain, consisting of SDL compiler, SDL runtime environment, and environment interfacing routines, thereby being compatible with model-driven development processes with SDL. In a series of runtime experiments on sensor nodes, we show that compared to customary SDL scheduling policies, priority scheduling with suspension of SDL agents indeed achieves significantly shortened reaction times.

Real-time signaling in SDL

SDL is a formal specification language for distributed systems, which provides significant, yet limited real-time expressiveness by its notion of time (now) and its timer mechanism. In our current work, we are investigating various ways to augment this expressiveness, by proposing language extensions and exploiting degrees of freedom offered by SDLs formal semantics. This paper presents some recent results of our work: a mechanism for real-time signaling, which can be roughly characterized as a generalization of SDL timers. More specifically, we propose to add the possibility of specifying a time interval for the reception of ordinary SDL signals, by stating their time of arrival and expiry. This extension can be used, for instance, to specify time-triggered scheduling, which is required in many real-time systems. In the paper, we present the concept of real-time signaling, propose a syntactical extension of SDL, define its formal semantics, outline our implementation, show excerpts of a control application, and report on measurement results.

Virtual Prototyping with Feral - Adaptation and Application of a Simulator Framework

Nowadays, complexity and extent of embedded functionality grows rapidly, leading to distributed solutions. Distributed embedded systems can, for instance, be found in timeand safety-critical domains, such as avionics and automotive. Suitable approaches to develop these systems are virtual prototyping and model-based development. Virtual prototyping enables early testing and evaluation of systems in realistic simulated environments; model-based development is applied to design, implement, and deploy the embedded functionality itself. In this paper, we briefly survey our modular simulator framework Feral, which provides a generic solution to virtual prototyping by rapid coupling of diverse simulators, including simulators for model-based specifications. We demonstrate the adaptation of Feral by incorporating several simulators, in particular, existing simulators of Simulink and SDL models, ns-3, and our newly developed simulators for CAN and FlexRay, which are current communication technologies in the automotive domain. We then apply Feral to an adaptive cruise control system, evaluating different design alternatives in a real world scenario. In particular, we evaluate early design decisions by substantiating them with facts, e. g., regarding the performance of a particular system architecture or communication technology.

Black-Burst-Based Quality-of-Service Routing (BBQR) for Wireless Ad-Hoc Networks

In this paper, we present Black-Burst-based Quality-of-Service Routing (BBQR), a novel QoS routing protocol for wireless ad-hoc networks. BBQR solves two core problems of QoS routing. First, there is the problem that concurrent route discoveries may fail due to mutual blocking of resources. BBQR avoids this problem by serializing route discoveries, using a distributed, yet highly efficient arbitration scheme. Secondly, there is the problem that routing packets may be lost due to frame collisions on the wireless medium, which may lead to failure of route discoveries, too. BBQR avoids this problem through synchronized route discovery phases and the use of collision-resistant frame encodings with black bursts. Further roperties of BBQR are that route discoveries have constant duration and low and bounded communication complexity.

Model-driven development of time-critical protocols with SDL-MDD

Contention-based medium access in wireless networks suffers from the problem of frame collisions. In previous work, we have introduced new transfer protocols for the network-wide transmission of bit sequences that overcome the problem of destructive collisions. In this paper, we present the model-driven development of these protocols with SDL-MDD and its SDL tool chain. On the one hand, we show how to formally specify low-level functionality and time-critical behavior - network-wide deterministic arbitration - using the available constructs of a high-level design language. On the other hand, we show the embedding of this high-level design into our SDL execution environment that is extended to support time-critical requirements of the introduced transfer protocols.