Database Reference
In-Depth Information
and d 2 . Each event is inserted in the distributed database implicitly generated
by W-Grid, reporting for instance date and time of occurrence. Without loss of
generality we can define a domain for T and P let us say Dom ( d 1 )=[
40 , 60]
and Dom ( d 2 ) = [700 , 1100]. We present two examples: (i) an exact-match and
(ii) a range query submitted to the network.
(i) Return the times at which sensors surveyed a temperature of 26 Cel-
sius degrees and a pressure of 1013mbar. The linearization [25,24] of the two-
dimensional data values results in a binary string which indicates the path to be
followed in the network to get to the sensor storing the data. Then, any sensor
can be taken as starting point for the query to get to the destination. In this
case the result of the linearization is 1 :
c t = 11011000
As described in [25,24] the length of the destination string can be adjusted,
without affecting the hops that were previously covered, during the routing if
we find that sensors with longest string exist.
(ii) Return the times at which sensors surveyed a temperature ranging from
26 to 30 Celsius degrees and pressure ranging from 1013 to 1025mbar. After
calculating the correspondent binary string for the four corners of the range
query, namely:
(26,1013) (26,1025) (30,1013) (30,1025)
c 1 = *110110 00 c 2 = *110110 01
c 3 = *110110 10 c 4 = *110110 11
allwehavetodoisqueryingsensorswhosecoordinateshave
110110 as prefix.
3 Active Recovery of Node Failures
In ad-hoc networks nodes usually have scarce resource and they especially suffer
of power constraints. This can lead to nodes failures that could affect routing
eciency. In W-Grid robustness is guaranteed by multiple coordinates at each
node and by the adopted routing metric. In fact, it is possible to route through
different paths. If a broken path is discovered the packet can change direction
(e.g. next hop) and follow a different path, according to another coordinate.
Whenever a nodes detects that it can not contact one of its father(s) it must
start a recovery procedure and find its closest existing relative. In [17] we forced
the orphan node to perform a ”local broadcast” searching for the parent of the
missing coordinate. The term ”local broadcast” was used since it is very likely
that the searched coordinate will be close to the broadcasting node considering
that it is a close relative. However, even if the broadcast packet time-to-live is
1 By standardizing 26 and 1013 to their domains we get 0,76 and 0,78 respectively.
We multiply both of them by 2 4 to get a string of length 8. The binary conversion
of the multiplications are 1010 and 1100 respectively. Then, by crossing bit by bit
the two string we get * 11011000 .
 
Search WWH ::




Custom Search