Leveraging Synchronous Transmissions for the Design of Real-time Wireless Cyber-Physical Systems
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Date
2020Type
- Doctoral Thesis
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Abstract
Cyber-Physical Systems (CPS) refer to systems where some intelligence is
embedded into devices that interact with their environment; that is, collecting
information from the physical space, processing that information, and taking
actions that affect the environment. Automatically turning the heating on
when room temperature gets cold is one of the simplest example of CPS.
Things get more complex when applications are distributed between low-power
devices that should operate autonomously for multiple years. Then, performing
reliable and energy efficient wireless communication becomes paramount.
Moreover, applications often specify deadlines; that is, maximal tolerable delays
between the execution of distributed tasks. Systems that guarantee to meet
such deadlines are called real-time systems. Wireless CPS capable of providing
real-time guarantees while using low-power communication technology are
desirable but they are particularly challenging to design. In the past few years, a
technique known as synchronous transmissions (ST) has been shown to enable
reliable and energy efficient communication in low-power multi-hop networks.
In a nutshell, ST consists in letting multiple devices transmit a packet during the
same time interval; communication is likely to be successful if the transmissions
are well synchronized, hence the name of synchronous transmissions. ST can
be leveraged to realize any multi-hop broadcast – a one-to-all communication –
in a given time; a very interesting property for designing real-time systems.
While the potential of ST is recognized by the low-power wireless academic
community, this technique has not yet been leveraged for the design of CPS.
We identify at least three issues that limit the adoption of ST in this domain:
(i) ST is difficult to use due to stringent time synchronization requirements:
in the order of μs. There is a lack of tools to facilitate the implementation
of ST by CPS engineers, which are often not wireless communication experts.
(ii) There are only few examples showcasing the use of ST for CPS applications
and academic works based on ST tend to focus on communication rather
than applications. Convincing proof-of-concept CPS applications are missing.
(iii) The inherent variability of the wireless environment makes performance
evaluation challenging. The lack of an agreed-upon methodology hinders
experiment reproduciblility and limits the confidence in the performance claims.
Consequently, we developed support tools and methods to facilitate the
evaluation of wireless protocols and the implementation of CPS based on ST.
Furthermore, we leveraged ST to design two CPS solutions targeting different
classes of real-time applications. This dissertation presents these contributions.
In Chapter 2, we propose to design and analyze performance evaluation
experiments for networking protocols using a concrete, rational, and statistically
sound methodology. We implement this methodology in a framework called
TriScale which allows to make performance claims with quantifiable levels
of confidence. Furthermore, we leverage the TriScale framework to propose
the first formalized definition of reproducibility for networking experiments.
Chapter 3 presents Baloo, a flexible design framework for network stacks
based on ST. Users implement their protocol through the programming
interface offered by Baloo while the framework handles the complex low-
level operations; e.g., meeting the time synchronization requirements of ST.
We show that Baloo is flexible enough to implement a wide variety of commu-
nication protocols while introducing only limited memory and energy overhead.
Finally, we design and implement two wireless CPS based on ST:
– the Distributed Real-time Protocol (DRP) uses contracts to maximize the
flexibility of execution between distributed tasks (Chapter 4);
– Time-Triggered Wireless (TTW ) statically co-schedules all task executions
and packet transfers to minimize end-to-end latency (Chapter 5).
We demonstrate that real-time guarantees can be provided in a reliable and
energy efficient manner. Furthermore, TTW supports update rates of tens of
ms, which is sufficient to perform distributed closed-loop control of inverted
pendulums – a fundamental benchmark for control and robotic applications.
With this dissertation, we showcase that ST is suitable to meet the requirements
of real-time wireless CPS. Furthermore, we facilitate the implementation of
such systems with Baloo, a design framework that makes ST accessible to
the non-expert. Finally, TriScale provides an important building block to
confidently evaluate the performance of networking protocols – an essential
building block of wireless CPS. Building on TriScale, it would be useful to define
benchmark problems representative of different classes of applications to serve
as baseline for the evaluation of future wireless CPS solutions. Ultimately, we
must transition from proof-of-concepts to real-world wireless CPS applications;
this would be further facilitated by porting Baloo to newer and more
powerful platforms, thereby pushing the limits of achievable performance levels. Show more
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https://doi.org/10.3929/ethz-b-000401717Publication status
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TIK-SchriftenreiheVolume
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ETH ZurichOrganisational unit
03429 - Thiele, Lothar (emeritus) / Thiele, Lothar (emeritus)
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