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small, the impact of performing broadcast is always heavy on energy consump-
tion, especially in a sensor network. For this reasons in this paper we present
a recovery procedure that does not uses broadcast at all, and we show that its
performances are at least equal to it. We call this procedure
active recovery
.In
this new solution, if a node
n
i
holding, among the others, the coordinate
∗
01001
∗
discover that the node
n
f
with coordinate
∗
0100
∗
is no longer available it will
start a search for
, the search is performed using a special type of message
(recovery message)
m
r
. Each node that is crossed by a recovery message knows
that each coordinate with prefix equal to the destination (
∗
0100
∗
in this case)
must not be used to evaluate the distance to it. This little variation tries to help
the system getting out of the subtree generated by the node failure. The orphan
node will try in sequence each of its neighbors to deliver the recovery message
and the search will stop either when
m
r
is delivered or when all the neighbors
have been contacted. The rest of the recovery procedure is the same as in [17].
Once the coordinate has been found, the holding node fixes the relationship with
the affected node by giving it a new coordinate, in our case through
n
4
and
n
7
.
∗
0100
∗
d
1
*11(n
5
)
*1010100(n
3
)
d
5
*1
a)
d
6
*101(n
5
)
d
3
*110(n
1
)
*101010(n
4
)
d
7
*1010
(n
6
)
d
2
*1110(n
1
)
*10101011(n
3
)
d
4
*10101(n
7
)
*1101(n
3
)
d
5
*1
*1101 = *101
d
6
*101(n
5
)
*1101 = *101
d
7
*1010(n
6
)
*1101 = *101
d
1
*11(n
5
)
*1010100(n
3
)
*1101 = *101
b)
d
3
*110(n
1
)
*101010(n
4
)
d
2
*1110(n
1
)
*10101011(n
3
)
d
4
*10101(n
7
)
*1101(n
3
)
*11 = *1010
Fig. 2.
Effects of Node Failure (
d
3
) during Routing of a Packet from Node
d
1
to Node
d
4
4 Lazy Recovery of Node Failures
W-Grid also include a
lazy recovery
feature. Besides active recovery we also let
the nodes try to fix failures situations not solved by exploiting the trac during
queries routing.
Lazy recovery act as follows: whenever a node cannot recovery trough active
recovery sets itself in a
recovery failed
state. When a node is in this state it
will first of all notice all its neighbors about it and its neighbors will do the
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