40 research outputs found
Decentralised Learning MACs for Collision-free Access in WLANs
By combining the features of CSMA and TDMA, fully decentralised WLAN MAC
schemes have recently been proposed that converge to collision-free schedules.
In this paper we describe a MAC with optimal long-run throughput that is almost
decentralised. We then design two \changed{schemes} that are practically
realisable, decentralised approximations of this optimal scheme and operate
with different amounts of sensing information. We achieve this by (1)
introducing learning algorithms that can substantially speed up convergence to
collision free operation; (2) developing a decentralised schedule length
adaptation scheme that provides long-run fair (uniform) access to the medium
while maintaining collision-free access for arbitrary numbers of stations
Future Evolution of CSMA Protocols for the IEEE 802.11 Standard
In this paper a candidate protocol to replace the prevalent CSMA/CA medium
access control in Wireless Local Area Networks is presented. The proposed
protocol can achieve higher throughput than CSMA/CA, while maintaining
fairness, and without additional implementation complexity. Under certain
circumstances, it is able to reach and maintain collision-free operation, even
when the number of contenders is variable and potentially large. It is backward
compatible, allowing for new and legacy stations to coexist without degrading
one another's performance, a property that can make the adoption process by
future versions of the standard smooth and inexpensive.Comment: This paper has been accepted in the Second IEEE ICC Workshop 2013 on
Telecommunication Standards: From Research to Standard
Learning-Based Constraint Satisfaction With Sensing Restrictions
In this paper we consider graph-coloring problems, an important subset of
general constraint satisfaction problems that arise in wireless resource
allocation. We constructively establish the existence of fully decentralized
learning-based algorithms that are able to find a proper coloring even in the
presence of strong sensing restrictions, in particular sensing asymmetry of the
type encountered when hidden terminals are present. Our main analytic
contribution is to establish sufficient conditions on the sensing behaviour to
ensure that the solvers find satisfying assignments with probability one. These
conditions take the form of connectivity requirements on the induced sensing
graph. These requirements are mild, and we demonstrate that they are commonly
satisfied in wireless allocation tasks. We argue that our results are of
considerable practical importance in view of the prevalence of both
communication and sensing restrictions in wireless resource allocation
problems. The class of algorithms analysed here requires no message-passing
whatsoever between wireless devices, and we show that they continue to perform
well even when devices are only able to carry out constrained sensing of the
surrounding radio environment
Decentralized Constraint Satisfaction
We show that several important resource allocation problems in wireless
networks fit within the common framework of Constraint Satisfaction Problems
(CSPs). Inspired by the requirements of these applications, where variables are
located at distinct network devices that may not be able to communicate but may
interfere, we define natural criteria that a CSP solver must possess in order
to be practical. We term these algorithms decentralized CSP solvers. The best
known CSP solvers were designed for centralized problems and do not meet these
criteria. We introduce a stochastic decentralized CSP solver and prove that it
will find a solution in almost surely finite time, should one exist, also
showing it has many practically desirable properties. We benchmark the
algorithm's performance on a well-studied class of CSPs, random k-SAT,
illustrating that the time the algorithm takes to find a satisfying assignment
is competitive with stochastic centralized solvers on problems with order a
thousand variables despite its decentralized nature. We demonstrate the
solver's practical utility for the problems that motivated its introduction by
using it to find a non-interfering channel allocation for a network formed from
data from downtown Manhattan
Towards a Collision-Free WLAN: Dynamic Parameter Adjustment in CSMA/E2CA
Carrier Sense Multiple Access with Enhanced Collision Avoidance (CSMA/ECA) is
a distributed MAC protocol that allows collision-free access to the medium in
WLAN. The only difference between CSMA/ECA and the well-known CSMA/CA is that
the former uses a deterministic backoff after successful transmissions.
Collision-free operation is reached after a transient state during which some
collisions may occur. This article shows that the duration of the transient
state can be shortened by appropriately setting the contention parameters.
Standard absorbing Markov Chain theory can be used to describe the behaviour of
the system in the transient state and to predict the expected number of slots
to reach the collision-free operation.
The article also introduces CSMA/E2CA, in which a deterministic backoff is
used two consecutive times after a successful transmission. CSMA/E2CA converges
quicker to collision-free operation and delivers higher performance than
CSMA/CA in harsh wireless scenarios with high frame error rates.
To achieve collision-free operations when the number of contenders is large,
it may be necessary to dynamically adjust the contention parameter. The last
part of the article suggests an approach for such parameter adjustment which is
validated by simulation results
Decentralised Algorithms for Wireless Networks.
Designing and managing wireless networks is challenging for many
reasons. Two of the most crucial in 802.11 wireless networks are:
(a) variable per-user channel quality and (b) unplanned, ad-hoc deployment
of the Access Points (APs). Regarding (a), a typical consequence
is the selection, for each user, of a different bit-rate, based on
the channel quality. This in turn causes the so-called performance
“anomaly”, where the users with lower bit-rate transmit for most of
the time, causing the higher bit-rate users to receive less time for
transmission (air time). Regarding (b), an important issue is managing
interference. This can be mitigated by selecting different channels
for neighbouring APs, but needs to be carried out in a decentralised
way because often APs belong to different administrative domains, or
communication between APs is unfeasible. Tools for managing unplanned
deployment are also becoming important for other small cell
networks, such as femtocell networks, where decentralised allocation
of scrambling codes is a key task