presents a power saving technique for multi-hop ad hoc wireless
networks that reduces energy consumption without significantly
diminishing the capacity or connectivity of the network.
It builds on the observation that when a region of a shared-channel
wireless network has a sufficient density of nodes, only a
small number of them need be on at any time to forward traffic
for active connections.
The technique is a distributed, randomized algorithm where
nodes make local decisions on whether or sleep, or to join
a forwarding backbone as a coordinator. Each node bases its
decision on an estimate of how many of its neighbors will
benefit from it being awake and the amount of energy available
to it. We give a randomized algorithm where coordinators rotate
with time, demonstrating how localized node decisions lead
to a connected, capacity-preserving global topology.
Minimizing energy consumption is an important challenge
in mobile networking. Significant progress has been made
on low-power hardware design for mobile devices that the wireless
network interface is often a device’s single largest consumer
of power. Since the network interface may often be idle, turning
the radio off when not in use could save this power. In practice,
however, this approach is not straightforward: a node must
arrange to turn its radio on not just to receive packets addressed
to it, but also to participate in any higher-level routing
and control protocols. The requirement of cooperation between
power saving and routing protocols is particularly acute in
the case of multi-hop ad hoc wireless networks, where nodes
must forward packets for each other. Coordination of power
saving with routing in ad hoc wireless networks is the subject
of this paper.
A good power-saving coordination technique for wireless
ad-hoc networks ought to have the following characteristics.
allow as many nodes as possible to turn their radio receivers
off mot of the time, since even an idle receive circuit
can consume almost as much energy as an active transmitter.
On the other
hand, it should forward packets between any source and
destination with minimally more delays than if all nodes
were awake. This implies that enough nodes must stay awake
to form a connected backbone.
the backbone formed by the awake nodes should provide
about as much total capacity as the original network,
since otherwise congestion may increase. This means that
paths that could operate without interference in the original
network should be represented in the backbone.
Each node in
the network makes periodic, local decisions on whether to
sleep or stay awake as a coordinator and participate in the
forwarding backbone topology. To preserve capacity, a
node decides to volunteer to be a coordinator if it discovers
that two of its neighbors cannot communicate with each other
directly or through an existing coordinator. To keep the number
of redundant coordinators low and rotate this role amongst
all nodes, each node delays announcing its willingness with
a random delay that takes two factors into account: the amount
of remaining battery energy, and the number of pairs of neighbors
it can connect together. This combination ensures, with high
probability, a capacity-preserving connected backbone at any
pint in time, where nodes tend to consume energy at about
the same rate. The power saving technique does all this using
only local information, consequently scaling well with the
number of nodes.
The network adaptively elects “Coordinators” from all nodes
and the coordinators stay awake continuously and perform multi-hop
packet routing within the ad hoc network, while other
nodes remain in power-saving mode and periodically check if
they should wake up and become a coordinator.
It can be
achieved by the four ways.
* It ensures that enough coordinators are elected so that
every node is in radio range of at least one coordinator.
* It rotates the coordinators in order to ensure that all
nodes share the task of providing global connectivity roughly
* It attempts to minimize the number of nodes elected as coordinators,
thereby increasing network lifetime, but without suffering
a significant loss of capacity or an increase in latency.
* Finally, It elects coordinators using only local information
in decentralized manner-each node only consults state stored
in local routing tables during the election process.
Periodically, a non-coordinator node determines if it should
become a coordinator or not. The following coordinator eligibility
rule in Span ensures that the entire network is covered with
eligibility rule: if two neighbors of a non-coordinator
node cannot reach each other either directly or via
one or two coordinators, the node should become a coordinator.
While this election algorithm does not yield the minimum
number of coordinators required to merely maintain connectedness,
it forms a network that roughly contains a coordinator
in every populated radio range in the entire network
topology. Since packets will be routed through coordinators,
this topology ought to yield good capacity.
occurs where multiple nodes discover the lack of a coordinator
at the same time, and all decide to become a coordinator.
We resolve contention by delaying coordinator announcements
with a randomized back-off delay. Each node chooses
a delay value, and delays the HELLO message that announces
the node’s volunteering as a coordinator for that amount
of time. If at the end of the delay, the node has not
received any HELLO messages from other potential coordinators,
it sends the HELLO message. Otherwise, it reevaluates
its eligibility based on any HELLO messages received,
and makes its announcement if and only if the eligibility
rule still holds.
Each coordinator periodically checks if it should withdraw
as a coordinator. A node should withdraw if every pair of
its neighbors can reach each other directly or via some other
coordinators. However, in order to also ensure fairness,
after a node has been a coordinator for some period of time,
it withdraws if every pair neighbor nodes can reach each other
via some other neighbors, even if those neighbors are not currently
coordinators. This rule gives other neighbors a chance to become
To prevent temporary loss of connectivity between a coordinator’s
withdrawal message and the announcement from a new coordinator,
a node continues to serve as a coordinator for a short period
of time even after announcing its withdrawal. This ‘grace period’
allows the routing protocol to continue to use the old coordinator
until a new coordinator is elected.
presents a distributed coordination technique for multi-hop
ad hoc networks that reduces energy consumption without significantly
diminishing the capacity or connectivity of the network.
In ad hoc network, coordinator is elected from all nodes and
it rotates them in time. The coordinator may stay awake and
perform multi-hop packet routing within the ad hoc network,
while other nodes remain in power-saving mode and periodically
check if they should awaken and become a coordinator.
Each node uses a random back off to decide whether to become
a coordinator. This delay is a function of the number of other
nodes in the neighborhood that can be bridged using this node,
and the amount of energy it has remains the same. The implementation
of the power saving technique periodically wakes up the nodes
and makes to listen advertisements and increases the cost.
This warrants investigation into a more robust and efficient
power saving technique in MAC layer that minimizes the amount
of time each node in power saving mode.
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Morries. “Span: An energy-efficient coordination algorithm
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Report: Massachusetts Institute of Technology 8.5 (2002):
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