1,085 research outputs found
Scheduling for next generation WLANs: filling the gap between offered and observed data rates
In wireless networks, opportunistic scheduling is used to increase system throughput by exploiting multi-user diversity. Although recent advances have increased physical layer data rates supported in wireless local area networks (WLANs), actual throughput realized are significantly lower due to overhead. Accordingly, the frame aggregation concept is used in next generation WLANs to improve efficiency. However, with frame aggregation, traditional opportunistic schemes are no longer optimal. In this paper, we propose schedulers that take queue and channel conditions into account jointly, to maximize throughput observed at the users for next generation WLANs. We also extend this work to design two schedulers that perform block scheduling for maximizing network throughput over multiple transmission sequences. For these schedulers, which make decisions over long time durations, we model the system using queueing theory and determine users' temporal access proportions according to this model. Through detailed simulations, we show that all our proposed algorithms offer significant throughput improvement, better fairness, and much lower delay compared with traditional opportunistic schedulers, facilitating the practical use of the evolving standard for next generation wireless networks
Nap: Practical Micro-Sleeps for 802.11 WLANs
In this paper, we revisit the idea of putting interfaces to sleep during
'packet overhearing' (i.e., when there are ongoing transmissions addressed to
other stations) from a practical standpoint. To this aim, we perform a robust
experimental characterisation of the timing and consumption behaviour of a
commercial 802.11 card. We design Nap, a local standard-compliant
energy-saving mechanism that leverages micro-sleep opportunities inherent to
the CSMA operation of 802.11 WLANs. This mechanism is backwards compatible and
incrementally deployable, and takes into account the timing limitations of
existing hardware, as well as practical CSMA-related issues (e.g., capture
effect). According to the performance assessment carried out through
trace-based simulation, the use of our scheme would result in a 57% reduction
in the time spent in overhearing, thus leading to an energy saving of 15.8% of
the activity time.Comment: 15 pages, 12 figure
Improving Performance for CSMA/CA Based Wireless Networks
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) based wireless networks are becoming increasingly ubiquitous. With the aim of supporting rich multimedia
applications such as high-definition television (HDTV, 20Mbps) and DVD (9.8Mbps), one of the technology trends is towards increasingly higher bandwidth. Some recent IEEE 802.11n proposals seek to provide PHY rates of up to 600 Mbps. In addition to increasing bandwidth, there is also strong interest in extending the coverage of CSMA/CA based wireless networks. One solution is to relay traffic via multiple intermediate stations if the sender and the receiver are far apart. The so called “mesh” networks based on this relay-based approach, if properly designed, may feature both “high speed” and “large coverage” at the
same time. This thesis focusses on MAC layer performance enhancements in CSMA/CA based networks in this context.
Firstly, we observe that higher PHY rates do not necessarily translate into corresponding increases in MAC layer throughput due to the overhead of the CSMA/CA based MAC/PHY layers. To mitigate the overhead, we propose a novel MAC scheme whereby transported information is partially acknowledged and retransmitted. Theoretical analysis and extensive simulations show that the proposed MAC approach can achieve high efficiency (low MAC
overhead) for a wide range of channel variations and realistic traffic types.
Secondly, we investigate the close interaction between the MAC layer and the buffer above it to improve performance for real world traffic such as TCP. Surprisingly, the issue
of buffer sizing in 802.11 wireless networks has received little attention in the literature yet it poses fundamentally new challenges compared to buffer sizing in wired networks. We propose a new adaptive buffer sizing approach for 802.11e WLANs that maintains a high
level of link utilisation, while minimising queueing delay.
Thirdly, we highlight that gross unfairness can exist between competing flows in multihop mesh networks even if we assume that orthogonal channels are used in neighbouring
hops. That is, even without inter-channel interference and hidden terminals, multi-hop mesh networks which aim to offer a both “high speed” and “large coverage” are not achieved. We propose the use of 802.11e’s TXOP mechanism to restore/enfore fairness. The proposed approach is implementable using off-the-shelf devices and fully decentralised (requires no message passing)
Coding in 802.11 WLANs
Forward error correction (FEC) coding is widely used in communication systems to correct transmis-
sion errors. In IEEE 802.11a/g transmitters, convolutional codes are used for FEC at the physical
(PHY) layer. As is typical in wireless systems, only a limited choice of pre-speci¯ed coding rates is
supported. These are implemented in hardware and thus di±cult to change, and the coding rates are
selected with point to point operation in mind.
This thesis is concerned with using FEC coding in 802.11 WLANs in more interesting ways that are
better aligned with application requirements. For example, coding to support multicast tra±c rather
than simple point to point tra±c; coding that is cognisant of the multiuser nature of the wireless
channel; and coding which takes account of delay requirements as well as losses. We consider layering
additional coding on top of the existing 802.11 PHY layer coding, and investigate the tradeo® between
higher layer coding and PHY layer modulation and FEC coding as well as MAC layer scheduling.
Firstly we consider the joint multicast performance of higher-layer fountain coding concatenated
with 802.11a/g OFDM PHY modulation/coding. A study on the optimal choice of PHY rates with and
without fountain coding is carried out for standard 802.11 WLANs. We ¯nd that, in contrast to studies
in cellular networks, in 802.11a/g WLANs the PHY rate that optimizes uncoded multicast performance
is also close to optimal for fountain-coded multicast tra±c. This indicates that in 802.11a/g WLANs
cross-layer rate control for higher-layer fountain coding concatenated with physical layer modulation
and FEC would bring few bene¯ts.
Secondly, using experimental measurements taken in an outdoor environment, we model the chan-
nel provided by outdoor 802.11 links as a hybrid binary symmetric/packet erasure channel. This
hybrid channel o®ers capacity increases of more than 100% compared to a conventional packet erasure
channel (PEC) over a wide range of RSSIs. Based upon the established channel model, we further
consider the potential performance gains of adopting a binary symmetric channel (BSC) paradigm for
multi-destination aggregations in 802.11 WLANs. We consider two BSC-based higher-layer coding
approaches, i.e. superposition coding and a simpler time-sharing coding, for multi-destination aggre-
gated packets. The performance results for both unicast and multicast tra±c, taking account of MAC
layer overheads, demonstrate that increases in network throughput of more than 100% are possible
over a wide range of channel conditions, and that the simpler time-sharing approach yields most of
these gains and have minor loss of performance.
Finally, we consider the proportional fair allocation of high-layer coding rates and airtimes in 802.11
WLANs, taking link losses and delay constraints into account. We ¯nd that a layered approach of
separating MAC scheduling and higher-layer coding rate selection is optimal. The proportional fair
coding rate and airtime allocation (i) assigns equal total airtime (i.e. airtime including both successful
and failed transmissions) to every station in a WLAN, (ii) the station airtimes sum to unity (ensuring
operation at the rate region boundary), and (iii) the optimal coding rate is selected to maximise
goodput (treating packets decoded after the delay deadline as losses)
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