| 摘要(英) |
Conventional wireless network relies on base stations or access
points relaying messages between mobile units and to/from wired
infrastructure.
The mobile ad hoc network (MANET) distinguishes itself
from the conventional wireless network by eliminating the
dependence on base stations for relaying messages.
In a MANET, a mobile host has to play the role of a router and
cooperate with others to exploits the multihop
communication ability for delivering messages among mobile hosts.
Due to the mobility and the unreliable nature of wireless
transmission, supporting even the most primitive broadcast and
unicast operations represents a great challenge in the mobile ad
hoc network.
In this dissertation, we address the issues of broadcast storm problem,
route maintenance problem, and implementing an ad hoc routing protocol
in a MANET.
The broadcast operation is very useful for route discovery,
naming, addressing, and helping multicast operations in
all kinds of networks.
In a MANET, without the knowledge of the network topology, a broadcast is
often accomplished by straightforward flooding.
Through analyses and simulations, three major problems associated with
flooding are identified, that is, redundancy, contention,
and collision.
Collectively we refer to these problems as the broadcast storm
problem.
Two directions are suggested to alleviate the problem,
that is, to reduce the possibility of redundant rebroadcasts, and
to differentiate the timing of rebroadcasts.
Following these directions, we develop five rebroadcast schemes, called
probabilistic, counter-based,
distance-based, location-based, and
cluster-based schemes, to facilitate MANET broadcasting.
In these schemes, various threshold mechanisms are used to
intelligently reduce the number of redundant rebroadcasts.
Simulation results show great improvement of efficiency as
compared with the flooding scheme.
In addition, adaptive schemes are developed, which dynamically adjust
thresholds according to hosts’’ neighborhood states, to achieve better
efficiency without sacrificing reliability.
To deliver a message, a mobile host in a MANET must initiate a
route discovery procedure if the route to the destination does
not exist or is broken.
In existing routing protocols, the route discovery is often achieved by
flooding route request packets throughout the network, which often
incurs high overhead.
Different strategies are used in these protocols to preserve and fix a route
when a link breakage occurs for reducing the re-route overhead.
We address two issues, route deterioration and route
breakage, of the route maintenance problem.
Generic route optimization and local route recovery
schemes are proposed to improvement the performance of existing routing
protocols.
Specifically, we show how to apply the schemes to DSR, SSA, AODV,
and ZRP routing protocols.
Simulation results show that the control packet overhead is
reduced, and the data packet bandwidth is improved.
Since a route to some destination may be unavailable or broken at the time
when the route is needed, an operation system that supports networking
in a MANET environment must be capable of dealing with such conditions and
hiding temporary network jitters from applications.
In our implementation work, the original TCP/IP layer of Linux operation
system is enhanced to support packet buffering, error hiding, and to cooperate
with a routing daemon implementing a DSR-like routing protocol.
We achieve the goal to allow existing network applications to run
smoothly in a MANET without modifying the applications.
With our implementation, we perform tests on multi-hop routing to
measure the route discovery delay and data throughput, and to evaluate the
collision caused by the flooding of route request packets. |
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