Biomedical Engineering Reference
In-Depth Information
Fig. 6.18 Experimental
variation of sensor
initialization delay with
distance for different
communication scenarios
220
Scenario 1
Scenario 2
Scenario 3
Scenario 4
200
180
160
140
120
100
80
60
40
20
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Distance (m)
T
ID
¼
N
T
RTS
þ
T
CTS
ð
Þ þ
T
BER
ð
6
:
1
Þ
where T
ID
is the sensor initialization delay, T
RTS
is the time difference between a
beacon reception and transmission of an RTS by a sensor node, T
CTS
is the time
difference between transmission of an RTS message and successful reception of a
CTS message or maximum time-out time in case of an unsuccessful CTS reception
by a sensor node, N
¼
1
;
2
;
3...10 is the number of initialization attempts and T
BER
is the time taken to negotiate a final PPB value for data transmission. Sensor
initialization delay is measured at sensor nodes using a timer to measure the time
difference between the reception of the first beacon message and successful ini-
tialization of the sensor. This information is then transferred to a computer ter-
minal at the end of initialization using a data packet. Figure
6.18
depicts the
variation of average sensor initialization delay with distance for the variable PPB
experiments in different scenarios shown in Table
6.1
. Average initialization delay
is calculated by averaging the initialization delay recorded by all four sensors over
three experiments conducted for each separation distance.
It can be seen from Fig.
6.18
that the initialization delay increases with the
separation distance in general. This variation can be explained by the increase of
BER and propagation delay with increasing separation distance. High BER values
cause the initialization request messages to be erroneous, which results in
increased number of initialization attempts. However, sensor initialization delay
for all the scenarios showed only a little variation from each other up to a distance
of 1.1 m. This is due to the fact that all the initialization messages use the max-
imum allowable PPB value of 100 PPB for sensor initialization. Hence, the effect
of BER on initialization delay affects equally for all the scenarios within this
separation distance. A rapid increase in initialization delay can be observed above
a separation distance of 1.1 m; after which BER can no longer be contained closer
to 10
-4
due to the maximum PPB limitation of 100 PPB. Scenario 2 showed the
highest deviation in initialization delay above the separation distance of 1.1 m.
