549 research outputs found
Performance evaluation of flooding in MANETs in the presence of multi-broadcast traffic
Broadcasting has many important uses and several mobile ad hoc networks (MANETs) protocols assume the availability of an underlying broadcast service. Applications, which make use of broadcasting, include LAN emulation, paging a particular node. However, broadcasting induces what is known as the "broadcast storm problem" which causes severe degradation in network performance, due to excessive redundant retransmission, collision, and contention. Although probabilistic flooding has been one of the earliest suggested approaches to broadcasting. There has not been so far any attempt to analyse its performance behaviour in MANETs. This paper investigates using extensive ns-2 simulations the effects of a number of important parameters in a MANET, including node speed, pause time and, traffic load, on the performance of probabilistic flooding. The results reveal that while these parameters have a critical impact on the reachability achieved by probabilistic flooding, they have relatively a lower effect on the number of saved rebroadcast packets
Renormalization-group improved fully differential cross sections for top pair production
We extend approximate next-to-next-to-leading order results for top-pair
production to include the semi-leptonic decays of top quarks in the
narrow-width approximation. The new hard-scattering kernels are implemented in
a fully differential parton-level Monte Carlo that allows for the study of any
IR-safe observable constructed from the momenta of the decay products of the
top. Our best predictions are given by approximate NNLO corrections in the
production matched to a fixed order calculation with NLO corrections in both
the production and decay subprocesses. Being fully differential enables us to
make comparisons between approximate results derived via different (PIM and
1PI) kinematics for arbitrary distributions. These comparisons reveal that the
renormalization-group framework, from which the approximate results are
derived, is rather robust in the sense that applying a realistic error estimate
allows us to obtain a reliable prediction with a reduced theoretical error for
generic observables and analysis cuts
Probing the top-quark width through ratios of resonance contributions of
We exploit offshell regions in the process
to gain access to the top-quark width. Working at next-to-leading order in QCD
we show that carefully selected ratios of offshell regions to onshell regions
in the reconstructed top and antitop invariant mass spectra are,
\emph{independently} of the coupling , sensitive to the top-quark
width. We explore this approach for different centre of mass energies and
initial-state beam polarisations at colliders and briefly comment on
the applicability of this method for a measurement of the top-quark width at
the LHC.Comment: 31 pages, 10 figures, 2 table
Resummation and Matching of -quark Mass Effects in Production
We use a systematic effective field theory setup to derive the
production cross section. Our result combines the merits of both fixed 4-flavor
and 5-flavor schemes. It contains the full 4-flavor result, including the exact
dependence on the -quark mass, and improves it with a resummation of
collinear logarithms of . In the massless limit, it corresponds to a
reorganized 5-flavor result. While we focus on production, our
method applies to generic heavy-quark initiated processes at hadron colliders.
Our setup resembles the variable flavor number schemes known from heavy-flavor
production in deep-inelastic scattering, but also differs in some key aspects.
Most importantly, the effective -quark PDF appears as part of the
perturbative expansion of the final result where it effectively counts as an
object. The transition between the fixed-order (4-flavor) and
resummation (5-flavor) regimes is governed by the low matching scale at which
the -quark is integrated out. Varying this scale provides a systematic way
to assess the perturbative uncertainties associated with the resummation and
matching procedure and reduces by going to higher orders. We discuss the
practical implementation and present numerical results for the
production cross section at NLO+NLL. We also provide a comparison to the
corresponding predictions in the fixed 4-flavor and 5-flavor results and the
Santander matching prescription. Compared to the latter, we find a slightly
reduced uncertainty and a larger central value, with its central value lying at
the lower edge of our uncertainty band.Comment: 54 pages, 16 figures. Final version to be published in JHEP (one ref
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Performance evaluation of adjusted probabilistic broadcasting in MANETs
Appropriate use of a probabilistic broadcasting method in MANETs can decrease the number of rebroadcasts, and as a result reduce the opportunity of contention and collision among neighbouring nodes. In this paper we evaluate the performance of adjusted probabilistic flooding by comparing it to "simple" flooding as used with the ad hoc on demand distance vector (AODV) routing protocol as well as a fixed probabilistic approach. The results reveal that the adjusted probabilistic flooding exhibits superior performance in terms of both reachability and saved rebroadcast
Improving route discovery in on-demand routing protocols using local topology information in MANETs
Most existing routing protocols proposed for MANETs use flooding as a broadcast technique for the propagation of network control packets; a particular example of this is the dissemination of route requests (RREQs), which facilitate route discovery. In flooding, each mobile node rebroadcasts received packets, which, in this manner, are propagated network-wide with considerable overhead. This paper improves on the performance of existing routing protocols by reducing the communication overhead incurred during the route discovery process by implementing a new broadcast algorithm called the adjusted probabilistic flooding on the Ad-Hoc on Demand Distance Vector (AODV) protocol. AODV [3] is a well-known and widely studied algorithm which has been shown over the past few years to maintain an overall lower routing overhead compared to traditional proactive schemes, even though it uses flooding to propagate RREQs. Our results, as presented in this paper, reveal that equipping AODV with fixed and adjusted probabilistic flooding, instead, helps reduce the overhead of the route discovery process whilst maintaining comparable performance levels in terms of saved rebroadcasts and reachability as achieved by conventional AODV\@. Moreover, the results indicate that the adjusted probabilistic technique results in better performance compared to the fixed one for both of these metrics
On the performance of probabilistic flooding in mobile ad hoc networks
This paper investigates using extensive simulations the effects of a number of important system parameters in a typical MANETs, including node speed, pause time, traffic load, and node density on the performance of probabilistic flooding. The results reveal that most of these parameters have a critical impact on the reachability and the number of saved rebroadcast messages achieved by probabilistic flooding, prompting the need for dynamically adjusting nodal retransmission probabilities depending on the current state of the network
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