61 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
Strategic Contention Resolution in Multiple Channels
We consider the problem of resolving contention in communication networks
with selfish users. In a \textit{contention game} each of identical
players has a single information packet that she wants to transmit using one of
multiple-access channels. To do that, a player chooses a
slotted-time protocol that prescribes the probabilities with which at a given
time-step she will attempt transmission at each channel. If more than one
players try to transmit over the same channel (collision) then no transmission
happens on that channel. Each player tries to minimize her own expected
\textit{latency}, i.e. her expected time until successful transmission, by
choosing her protocol. The natural problem that arises in such a setting is,
given and , to provide the players with a common, anonymous protocol (if
it exists) such that no one would unilaterally deviate from it (equilibrium
protocol).
All previous theoretical results on strategic contention resolution examine
only the case of a single channel and show that the equilibrium protocols
depend on the feedback that the communication system gives to the players. Here
we present multi-channel equilibrium protocols in two main feedback classes,
namely \textit{acknowledgement-based} and \textit{ternary}. In particular, we
provide equilibrium characterizations for more than one channels, and give
specific anonymous, equilibrium protocols with finite and infinite expected
latency. In the equilibrium protocols with infinite expected latency, all
players transmit successfully in optimal time, i.e. , with
probability tending to 1 as .Comment: The results of this work are included in the 11th International
Symposium on Algorithmic Game Theory (SAGT 2018) and the 16th Workshop on
Approximation and Online Algorithms (WAOA 2018
A 3-player protocol preventing persistence in strategic contention with limited feedback
In this paper, we study contention resolution protocols from a game-theoretic
perspective. In a recent work, we considered acknowledgment-based protocols,
where a user gets feedback from the channel only when she attempts
transmission. In this case she will learn whether her transmission was
successful or not. One of the main results of ESA2016 was that no
acknowledgment-based protocol can be in equilibrium. In fact, it seems that
many natural acknowledgment-based protocols fail to prevent users from
unilaterally switching to persistent protocols that always transmit with
probability 1. It is therefore natural to ask how powerful a protocol must be
so that it can beat persistent deviators.
In this paper we consider age-based protocols, which can be described by a
sequence of probabilities of transmitting in each time step. Those
probabilities are given beforehand and do not change based on the transmission
history. We present a 3-player age-based protocol that can prevent users from
unilaterally deviating to a persistent protocol in order to decrease their
expected transmission time. It is worth noting that the answer to this question
does not follow from the results and proof ideas of ESA2016. Our protocol is
non-trivial, in the sense that, when all players use it, finite expected
transmission time is guaranteed. In fact, we show that this protocol is
preferable to any deadline protocol in which, after some fixed time, attempt
transmission with probability 1 in every subsequent step. An advantage of our
protocol is that it is very simple to describe, and users only need a counter
to keep track of time. Whether there exist -player age-based protocols that
do not use counters and can prevent persistence is left as an open problem for
future research.Comment: arXiv admin note: substantial text overlap with arXiv:1606.0658
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