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A Cross-Layer Backpressure Architecture for Wireless
Multihop Networks
Abstract:
Contemporary wireless multihop networks operatemuch below their capacity due to the
poor coordinationamong transmitting nodes. In this paper, we present XPRESS,a cross-layer
backpressure architecture designed toreach thecapacity of wireless multihop
networks. Instead of a collection ofpoorly coordinated wireless routers, XPRESS turns a
mesh networkinto a wireless switch. Transmissions over the network arescheduled using a
throughput-optimal backpressure algorithm.Realizing this theoretical concept entails
several challenges, whichwe identify and address with a cross-layerdesign and
implementationon top of our wireless hardware platform. In contrast toprevious work, we
implement and evaluate backpressure scheduling over a TDMA MAC protocol, as it was
originally proposed intheory. Our experiments in an indoor testbed show that XPRESS
can yield up to 128% throughput gains over 802.11.
EXISTING SCHEME
Existing city-wide meshnetworks are often split into smaller 10- to 20-node
subnetworks,each communicating with a different gateway and operating over a separate
channel. Existing techniques for interferenceestimation can be broadly classified as passive
or active.Passive approaches require monitors deployed throughoutthe wireless network to
collect traffic traces.I n a different approach, severalsystems have been built on top of
existing MAC protocols,such as 802.11. Akyolet al. modify the 802.11 contentionwindow to
prioritize links with a higher differential backlog.

2. PROPOSED SCHEME
We implement and evaluate backpressure scheduling over a TDMA MAC protocol, as it
was originally proposed intheory. Our experiments in an indoor testbed show that
XPRESScan yield up to 128% throughput gains over 802.11.ropose a passive technique to
learn abouttransmission conflicts. This approach exploits several exposedterminals unused
by 802.11, but it does not address hiddeninterferers. The propose an online approachwhere
APs periodically silence their clients and run a quickinterference tests. ropose centralized
architectures toschedule AP transmissions. XPRESS shares the centralizedphilosophy of
these architectures, but also has fundamentaldifferences. propose 802.11backpressure
implementations to reduce the end-to-end delay.This issue can beaddressed by combining
the backpressure algorithm with thenetwork utility maximization (NUM) framework,
originallyproposed for wireline network.
CONCLUSION
We presented the design and implementation of XPRESS,a backpressure architecture for
wireless multihop networks.Our design leverages a centralized controller for
obtainingthroughput-optimal scheduling. In contrast to previous work,we integrated
backpressure scheduling with a TDMA MACprotocol to allow precise timing in
transmissions. Moreover,we introduced a novel interference estimation technique andan
efficient speculative backpressure scheduler. Our resultsin an indoor testbed confirm that
XPRESS achieves the fullnetwork capacity and up to 128% gains over 802.11. In ourfuture
work, apart from the topics discussed , wealso intend to evaluate XPRESS in larger
networks. We believeour work opens up interesting avenues in wireless networksystem
design, showing that optimal centralized routing andscheduling are feasible for small- to
medium-sized wirelessmultihop networks. For larger networks, we believe that the
design primitives of the XPRESS cross-layer protocol stack canlead to newdistributed
wireless architectures beyond 802.11.
SYSTEM CONFIGURATION:-
HARDWARE CONFIGURATION:-

3.  Processor - Pentium –IV
 Speed - 1.1 Ghz
 RAM - 256 MB(min)
 Hard Disk - 20 GB
 Key Board - Standard Windows Keyboard
 Mouse - Two or Three Button Mouse
 Monitor - SVGA
SOFTWARE CONFIGURATION:-
 Operating System : Windows XP
 Programming Language : JAVA
 Java Version : JDK 1.6 & above.