99 research outputs found
Fault Tolerant Gradient Clock Synchronization
Synchronizing clocks in distributed systems is well-understood, both in terms
of fault-tolerance in fully connected systems and the dependence of local and
global worst-case skews (i.e., maximum clock difference between neighbors and
arbitrary pairs of nodes, respectively) on the diameter of fault-free systems.
However, so far nothing non-trivial is known about the local skew that can be
achieved in topologies that are not fully connected even under a single
Byzantine fault. Put simply, in this work we show that the most powerful known
techniques for fault-tolerant and gradient clock synchronization are
compatible, in the sense that the best of both worlds can be achieved
simultaneously.
Concretely, we combine the Lynch-Welch algorithm [Welch1988] for
synchronizing a clique of nodes despite up to Byzantine faults with
the gradient clock synchronization (GCS) algorithm by Lenzen et al.
[Lenzen2010] in order to render the latter resilient to faults. As this is not
possible on general graphs, we augment an input graph by
replacing each node by fully connected copies, which execute an instance
of the Lynch-Welch algorithm. We then interpret these clusters as supernodes
executing the GCS algorithm, where for each cluster its correct nodes'
Lynch-Welch clocks provide estimates of the logical clock of the supernode in
the GCS algorithm. By connecting clusters corresponding to neighbors in
in a fully bipartite manner, supernodes can inform each other
about (estimates of) their logical clock values. This way, we achieve
asymptotically optimal local skew, granted that no cluster contains more than
faulty nodes, at factor and overheads in terms of nodes and
edges, respectively. Note that tolerating faulty neighbors trivially
requires degree larger than , so this is asymptotically optimal as well
An algorithm for clock synchronization with the gradient property in sensor networks
We introduce a distributed algorithm for clock synchronization in sensor
networks. Our algorithm assumes that nodes in the network only know their
immediate neighborhoods and an upper bound on the network's diameter.
Clock-synchronization messages are only sent as part of the communication,
assumed reasonably frequent, that already takes place among nodes. The
algorithm has the gradient property of [2], achieving an O(1) worst-case skew
between the logical clocks of neighbors. As in the case of [3,8], the
algorithm's actions are such that no constant lower bound exists on the rate at
which logical clocks progress in time, and for this reason the lower bound of
[2,5] that forbids constant skew between neighbors does not apply
Modelling Clock Synchronization in the Chess gMAC WSN Protocol
We present a detailled timed automata model of the clock synchronization
algorithm that is currently being used in a wireless sensor network (WSN) that
has been developed by the Dutch company Chess. Using the Uppaal model checker,
we establish that in certain cases a static, fully synchronized network may
eventually become unsynchronized if the current algorithm is used, even in a
setting with infinitesimal clock drifts
A Case for Time Slotted Channel Hopping for ICN in the IoT
Recent proposals to simplify the operation of the IoT include the use of
Information Centric Networking (ICN) paradigms. While this is promising,
several challenges remain. In this paper, our core contributions (a) leverage
ICN communication patterns to dynamically optimize the use of TSCH (Time
Slotted Channel Hopping), a wireless link layer technology increasingly popular
in the IoT, and (b) make IoT-style routing adaptive to names, resources, and
traffic patterns throughout the network--both without cross-layering. Through a
series of experiments on the FIT IoT-LAB interconnecting typical IoT hardware,
we find that our approach is fully robust against wireless interference, and
almost halves the energy consumed for transmission when compared to CSMA. Most
importantly, our adaptive scheduling prevents the time-slotted MAC layer from
sacrificing throughput and delay
Adaptive Synchronization of Robotic Sensor Networks
The main focus of recent time synchronization research is developing
power-efficient synchronization methods that meet pre-defined accuracy
requirements. However, an aspect that has been often overlooked is the high
dynamics of the network topology due to the mobility of the nodes. Employing
existing flooding-based and peer-to-peer synchronization methods, are networked
robots still be able to adapt themselves and self-adjust their logical clocks
under mobile network dynamics? In this paper, we present the application and
the evaluation of the existing synchronization methods on robotic sensor
networks. We show through simulations that Adaptive Value Tracking
synchronization is robust and efficient under mobility. Hence, deducing the
time synchronization problem in robotic sensor networks into a dynamic value
searching problem is preferable to existing synchronization methods in the
literature.Comment: First International Workshop on Robotic Sensor Networks part of
Cyber-Physical Systems Week, Berlin, Germany, 14 April 201
PALS: Distributed Gradient Clocking on Chip
Consider an arbitrary network of communicating modules on a chip, each
requiring a local signal telling it when to execute a computational step. There
are three common solutions to generating such a local clock signal: (i) by
deriving it from a single, central clock source, (ii) by local, free-running
oscillators, or (iii) by handshaking between neighboring modules. Conceptually,
each of these solutions is the result of a perceived dichotomy in which
(sub)systems are either clocked or asynchronous. We present a solution and its
implementation that lies between these extremes. Based on a distributed
gradient clock synchronization algorithm, we show a novel design providing
modules with local clocks, the frequency bounds of which are almost as good as
those of free-running oscillators, yet neighboring modules are guaranteed to
have a phase offset substantially smaller than one clock cycle. Concretely,
parameters obtained from a 15nm ASIC simulation running at 2GHz yield
mathematical worst-case bounds of 20ps on the phase offset for a
node grid network
Kalman filter based ranging and clock synchronization for ultra wide band sensor networks
This Thesis presents the design, implementation, and validation of a Kalman filterbased
range estimation technique to precisely calculate the inter-node ranges of Ultra
Wide Band (UWB) modules. In addition to that the development and validation of
an improved global clock synchronization framework is presented.
Noise characteristics of relative time measurements of a stationary UWB anchor pair
are first analyzed using an Allan deviation plot. To track the propagation of the
imprecise clocks on low cost UWB transceiver platforms, Kalman filters are used in
between every anchor pair. These filters track the variation of a remote anchor’s
hardware clock relative to it’s own hardware clock, while estimating the time of flight
between the anchor pair as a filter state. While adhering to a simple round robin
transmission schedule, both inbound and outbound message timestamp data are used
to update the filter. These measurements have made the time of flight observable in
the chosen state space. A faster relative clock filter convergence has been achieved
with the inclusion of the clock offset ratio as a measurement additional to the timestamps.
Furthermore, a modified gradient clock synchronization algorithm is used to achieve
global clock synchronization throughout the network. A correction term is used in the
gradient clock synchronization algorithm to enforce the global clock rate to converge
at the average of individual clock rates while achieving asymptotic stability in clock
rate error state. Stability of the original and modified methods for time invariant
hardware clocks are compared using eigenvalue tests. Experiments are conducted
to evaluate synchronization and ranging accuracy of the proposed range estimation
approach
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