413 research outputs found
Reachability of Communicating Timed Processes
We study the reachability problem for communicating timed processes, both in
discrete and dense time. Our model comprises automata with local timing
constraints communicating over unbounded FIFO channels. Each automaton can only
access its set of local clocks; all clocks evolve at the same rate. Our main
contribution is a complete characterization of decidable and undecidable
communication topologies, for both discrete and dense time. We also obtain
complexity results, by showing that communicating timed processes are at least
as hard as Petri nets; in the discrete time, we also show equivalence with
Petri nets. Our results follow from mutual topology-preserving reductions
between timed automata and (untimed) counter automata.Comment: Extended versio
A PROTOCOL SUITE FOR WIRELESS PERSONAL AREA NETWORKS
A Wireless Personal Area Network (WPAN) is an ad hoc network that consists of devices that surround an individual or an object. Bluetooth® technology is especially suitable for formation of WPANs due to the pervasiveness of devices with Bluetooth® chipsets, its operation in the unlicensed Industrial, Scientific, Medical (ISM) frequency band, and its interference resilience. Bluetooth® technology has great potential to become the de facto standard for communication between heterogeneous devices in WPANs.
The piconet, which is the basic Bluetooth® networking unit, utilizes a Master/Slave (MS) configuration that permits only a single master and up to seven active slave devices. This structure limitation prevents Bluetooth® devices from directly participating in larger Mobile Ad Hoc Networks (MANETs) and Wireless Personal Area Networks (WPANs). In order to build larger Bluetooth® topologies, called scatternets, individual piconets must be interconnected. Since each piconet has a unique frequency hopping sequence, piconet interconnections are done by allowing some nodes, called bridges, to participate in more than one piconet. These bridge nodes divide their time between piconets by switching between Frequency Hopping (FH) channels and synchronizing to the piconet\u27s master.
In this dissertation we address scatternet formation, routing, and security to make Bluetooth® scatternet communication feasible. We define criteria for efficient scatternet topologies, describe characteristics of different scatternet topology models as well as compare and contrast their properties, classify existing scatternet formation approaches based on the aforementioned models, and propose a distributed scatternet formation algorithm that efficiently forms a scatternet topology and is resilient to node failures.
We propose a hybrid routing algorithm, using a bridge link agnostic approach, that provides on-demand discovery of destination devices by their address or by the services that devices provide to their peers, by extending the Service Discovery Protocol (SDP) to scatternets.
We also propose a link level security scheme that provides secure communication between adjacent piconet masters, within what we call an Extended Scatternet Neighborhood (ESN)
Line Search for an Oblivious Moving Target
Consider search on an infinite line involving an autonomous robot starting at
the origin of the line and an oblivious moving target at initial distance from it. The robot can change direction and move anywhere on the line
with constant maximum speed while the target is also moving on the line
with constant speed but is unable to change its speed or direction. The
goal is for the robot to catch up to the target in as little time as possible.
The classic case where and the target's initial distance is unknown
to the robot is the well-studied ``cow-path problem''. Alpert and Gal gave an
optimal algorithm for the case where a target with unknown initial distance
is moving away from the robot with a known speed . In this paper we design
and analyze search algorithms for the remaining possible knowledge situations,
namely, when and are known, when is known but is unknown, when
is known but is unknown, and when both and are unknown.
Furthermore, for each of these knowledge models we consider separately the case
where the target is moving away from the origin and the case where it is moving
toward the origin. We design algorithms and analyze competitive ratios for all
eight cases above. The resulting competitive ratios are shown to be optimal
when the target is moving towards the origin as well as when is known and
the target is moving away from the origin
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