6,645 research outputs found
Is Our Model for Contention Resolution Wrong?
Randomized binary exponential backoff (BEB) is a popular algorithm for
coordinating access to a shared channel. With an operational history exceeding
four decades, BEB is currently an important component of several wireless
standards. Despite this track record, prior theoretical results indicate that
under bursty traffic (1) BEB yields poor makespan and (2) superior algorithms
are possible. To date, the degree to which these findings manifest in practice
has not been resolved.
To address this issue, we examine one of the strongest cases against BEB:
packets that simultaneously begin contending for the wireless channel. Using
Network Simulator 3, we compare against more recent algorithms that are
inspired by BEB, but whose makespan guarantees are superior. Surprisingly, we
discover that these newer algorithms significantly underperform. Through
further investigation, we identify as the culprit a flawed but common
abstraction regarding the cost of collisions. Our experimental results are
complemented by analytical arguments that the number of collisions -- and not
solely makespan -- is an important metric to optimize. We believe that these
findings have implications for the design of contention-resolution algorithms.Comment: Accepted to the 29th ACM Symposium on Parallelism in Algorithms and
Architectures (SPAA 2017
Parallel algorithm with spectral convergence for nonlinear integro-differential equations
We discuss a numerical algorithm for solving nonlinear integro-differential
equations, and illustrate our findings for the particular case of Volterra type
equations. The algorithm combines a perturbation approach meant to render a
linearized version of the problem and a spectral method where unknown functions
are expanded in terms of Chebyshev polynomials (El-gendi's method). This
approach is shown to be suitable for the calculation of two-point Green
functions required in next to leading order studies of time-dependent quantum
field theory.Comment: 15 pages, 9 figure
On time-dependent AdS/CFT
We clarify aspects of the holographic AdS/CFT correspondence that are typical
of Lorentzian signature, to lay the foundation for a treatment of
time-dependent gravity and conformal field theory phenomena. We provide a
derivation of bulk-to-boundary propagators associated to advanced, retarded and
Feynman bulk propagators, and provide a better understanding of the boundary
conditions satisfied by the bulk fields at the horizon. We interpret the
subleading behavior of the wavefunctions in terms of specific vacuum
expectation values, and compute two-point functions in our framework. We
connect our bulk methods to the closed time path formalism in the boundary
field theory.Comment: 19 pages, v2: added reference, JHEP versio
Gauge-Invariant Renormalization Group at Finite Temperature
We propose a gauge-invariant version of Wilson Renormalization Group for
thermal field theories in real time. The application to the computation of the
thermal masses of the gauge bosons in an SU(N) Yang-Mills theory is discussed.Comment: 23 pages, latex2e, 1 EPS figure. The discussions of BRS identities
and of the RG kernel have been modified. Final version, to appear on Nucl.
Phys.
Phase Coherence in Quantum Brownian Motion
The quantum theory of Brownian motion is discussed in the Schwinger version
wherein the notion of a coordinate moving forward in time is replaced by
two coordinates, moving forward in time and moving backward
in time. The role of the doubling of the degrees of freedom is illustrated for
the case of electron beam two slit diffraction experiments. Interference is
computed with and without dissipation (described by a thermal bath). The notion
of a dissipative interference phase, closely analogous to the Aharonov-Bohm
magnetic field induced phase, is explored.Comment: 12 pages, LaTeX, 2 Figure
1+1 Dimensional Compactifications of String Theory
We argue that stable, maximally symmetric compactifications of string theory
to 1+1 dimensions are in conflict with holography. In particular, the finite
horizon entropies of the Rindler wedge in 1+1 dimensional Minkowski and anti de
Sitter space, and of the de Sitter horizon in any dimension, are inconsistent
with the symmetries of these spaces. The argument parallels one made recently
by the same authors, in which we demonstrated the incompatibility of the
finiteness of the entropy and the symmetries of de Sitter space in any
dimension. If the horizon entropy is either infinite or zero the conflict is
resolved.Comment: 11 pages, 2 figures v2: added discussion of AdS_2 and comment
Byzantine Multiple Access Channels -- Part I: Reliable Communication
We study communication over a Multiple Access Channel (MAC) where users can
possibly be adversarial. The receiver is unaware of the identity of the
adversarial users (if any). When all users are non-adversarial, we want their
messages to be decoded reliably. When a user behaves adversarially, we require
that the honest users' messages be decoded reliably. An adversarial user can
mount an attack by sending any input into the channel rather than following the
protocol. It turns out that the -user MAC capacity region follows from the
point-to-point Arbitrarily Varying Channel (AVC) capacity. For the -user MAC
in which at most one user may be malicious, we characterize the capacity region
for deterministic codes and randomized codes (where each user shares an
independent random secret key with the receiver). These results are then
generalized for the -user MAC where the adversary may control all users in
one out of a collection of given subsets.Comment: This supercedes Part I of arxiv:1904.1192
Byzantine Multiple Access Channels -- Part II: Communication With Adversary Identification
We introduce the problem of determining the identity of a byzantine user
(internal adversary) in a communication system. We consider a two-user discrete
memoryless multiple access channel where either user may deviate from the
prescribed behaviour. Owing to the noisy nature of the channel, it may be
overly restrictive to attempt to detect all deviations. In our formulation, we
only require detecting deviations which impede the decoding of the
non-deviating user's message. When neither user deviates, correct decoding is
required. When one user deviates, the decoder must either output a pair of
messages of which the message of the non-deviating user is correct or identify
the deviating user. The users and the receiver do not share any randomness. The
results include a characterization of the set of channels where communication
is feasible, and an inner and outer bound on the capacity region. We also show
that whenever the rate region has non-empty interior, the capacity region is
same as the capacity region under randomized encoding, where each user shares
independent randomness with the receiver. We also give an outer bound for this
randomized coding capacity region.Comment: arXiv admin note: substantial text overlap with arXiv:2105.0338
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