747 sensor network operation-1-187-4

30 SENSOR DEPLOYMENT, SELF-ORGANIZATION, AND LOCALIZATION

	140
		Span
	120	SCARE
elected
	100
of coordinators
	80
	60
Number
	40
	20
	50	100	150	200	250	300

Number of nodes

Figure 2.9 Number of coordinators selected with an increase in nodes.

Section 2.2.5. SCARE selects almost the same number of coordinators as in the ideal case. This behavior is different from the behavior of SCARE in Figure 2.9 as here the nodes are placed in a regular fashion and not randomly deployed. Random deployment results in SCARE selecting more nodes as coordinators to cover the entire grid and still maintain connectivity. Any self-configuration algorithm should have minimal control message overhead. In Figure 2.13, we compare the number of control messages used by SCARE and Span for the self-configuration. SCARE uses a smaller number of control messages compared to Span because it takes advantage of the random initialization of the nodes. This leads to a partial configuration of the network; hence SCARE uses fewer number of control messages to achieve self-configuration.

	10000
	9500
Coverage	9000
	8500
	8000
					Span
					SCARE
	7500
	50	100	150	200	250	300

Number of nodes

Figure 2.10 Coverage versus number of nodes for SCARE and Span.

2.2 SCARE

31

	60					SCARE
as coordinators	55					Span
	50
	45
	40
elected
	35
of nodes	30
	25
Fraction	20
	15
	10	100	150	200	250	300
	50

Number of nodes

Figure 2.11 Fraction of nodes selected as coordinators in SCARE and Span.

Figure 2.14 shows the effects of mobility on packet loss rate for both Span and SCARE. Nodes follow the random way-point model described in the previous subsubsection. Packet loss rate is calculated as the ratio of the number of lost packets to the number of packets actually sent. We note that the packet loss rates for both these methods are comparable.

Figure 2.15 shows the fraction of surviving nodes as a function of simulation time for both SCARE and Span. SCARE uses fewer control messages and consumes less energy for self-configuration and reconfiguration of the network. The number of surviving nodes falls below 80% at 765 for SCARE compared to 700 for Span.

	50
					Ideal
					SCARE
coordinators	45
	40
of selected	35
	30
Number
	25
	20
	50	100	150	200	250	300

Number of nodes

Figure 2.12 Coordinators selected in SCARE versus an ideal number of coordinators selected based on square tiling.

32 SENSOR DEPLOYMENT, SELF-ORGANIZATION, AND LOCALIZATION

	1400
	1200		Span
			SCARE
messages	1000
of control	800
	600
number
	400
Total
	200
	0
	0	100	200	300	400	500	600	700	800	900	1000

Simulation time (s)

Figure 2.13 Number of control messages used for self-configuration.

Effect of Location Estimation Error on Results We next investigate how errors in distance estimation affect the performance of SCARE. Since nodes use distance estimation only to determine their eligibility to go to the sleep state, we do not expect SCARE to be significantly affected because of moderate errors in distance estimates.

To measure this feature of SCARE quantitatively, we ran simulations by introducing artificial errors in distance estimation. We modeled such errors by shifting the location of each node by a random amount in the range [x ± e, y ± e], where e is either 10 or 20% of the radio range of a node and [x, y] is the location of a sensor node. Nodes use these

	16 x 10−3
								SCARE
	14							Span
	12
rate	10
loss
Packet	8
	6
	4
	2	200	300	400	500	600	700	800	900	1000
	100

Pause time (s)

Figure 2.14 Packet loss rate as a function of pause time.

							2.2	SCARE	33
						SCARE
	1					Span
nodes	0.8
of surviving	0.6
Fraction	0.4
	0.2
	0
	0	200	400	600	800	1000	1200

Simulation time (s)

Figure 2.15 Fraction of nodes remaining with time for Span and SCARE.

artificial locations rather than their real location to estimate the distance between them and a coordinator node. We refer to this scheme as either SCARE-10 or SCARE-20.

Figure 2.16 shows the results of these simulations. The simulations using SCARE-10 and SCARE-20 are based on incorrect estimation of the distance from the coordinators by the nodes. Consequently, the number of coordinators is different from the case when there is no error. However, the increase in the number of coordinators is negligible, while the decrease in coverage is found to be minimal. In the case of SCARE-10, the increase is only 3% for a small number of nodes and negligible (<0.2%) for a large number of nodes.

	50
	48
elected	46
	44
coordinators	42
	40
	38
of
Number	36
	34
					SCARE
	32				SCARE10
					SCARE20
	30
	50	100	150	200	250	300

Number of nodes

Figure 2.16 Effect of error in distance estimation on SCARE.