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do not contribute to the community. Secondly, in such an ad hoc P2P wire-
less environment, energy e ciency is beyond doubt a crucial factor in the
system design due to the fact that mobile wireless devices are inevitably en-
ergy limited. Indeed, it is challenging to tackle the energy e ciency problem
of mobile devices in a judicious manner and it has intrigued researchers for
years, e.g., [Bharghavan et al., 1994, Singh et al., 1998, Singh and Raghaven-
dra, 1998]. This motivates the need for a new energy e cient topology control
for effectively supporting P2P file sharing applications.
Topology control, in a traditional sense, comprises two components: neigh-
bor discovery and network organization [Rajaraman, 2002]. In neighbor dis-
covery, one has to detect network nodes in its proximity, and construct a
neighbor set in which it could find possible next-hops to establish communi-
cation linkages. On the other hand, network organization involves the decision
of which communication links to establish with neighboring nodes. Typically,
it involves the use of power management schemes such as sleeping and trans-
mit power control. The former disables some communication links temporarily
and the latter adjusts the transmission range.
In summary, traditionally, the objective of topology control is the preser-
vation of network connectivity while improving the e ciency of transmissions.
However, Leung and Kwok pointed out that connectivity should be consid-
ered at the application level. Specifically, their suggested schemes are aimed
at achieving an e cient connectivity among mobile devices in order to better
serve the file sharing application. Indeed, their idea is that the underlying
network layer (or even link layer) connections should be constructed in such
a way that the file sharing application's performance is improved. The metric
that they use to judge performance is the file success ratio.
Many energy e cient protocols have been proposed in the literature. Nev-
ertheless, previous researchers usually neglect the importance of considering
the difference of remaining energy levels between individual nodes. Indeed, in
the pioneering work by Singh [Singh et al., 1998], it only uses metrics for the
total energy consumption in the whole path from server-peer (the peer who
shares files) to client-peer (the peer who requests files).
The wireless topology control scheme proposed by Leung and Kwok has
two components [Leung and Kwok, 2008]. In the first component, called Ad-
jacency Set Construction, the topology control system finds out which nodes
could be the next-hop when a node has to communicate with another node
which is n hops away (n≥2). Specifically, the design rationale is that the
construction of neighbor set is related to: (1) contribution levels of different
nodes in the P2P file sharing network, (2) the popularity of file resources
owned by individual nodes, (3) aggressiveness of the file requesting node, and
(4) remaining energy levels of nodes. Our protocol not only takes energy ef-
ficiency into consideration but also controls the topology of the file sharing
network to introduce fairness.
In the second component, called Community-Based Asynchronous
Wakeup, the topology control system forms “virtual communities” among mo-
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