# Energy-Efficient Region Shift Scheme to Support Mobile Sink Group in Wireless Sensor Networks
## 4 Performance Evaluation  
We compare the performance of our RSMGP with that of GMR [16], which is a traditional multicast scheme in WSNs and RingRouting [9], which is an efficient mobility support scheme for individual sinks. M-Geocasting [5] is compared as a primitive geocasting scheme to support mobile sink groups. In addition, we select ERMG [12] as the most recent study dealing with region discovery. We implemented three protocols in Network Simulator Qualnet Network Simulator (4.0, Scalable Network Technologies, Culver City, CA, USA) [17]. Sensor nodes follow the specification of MICA2 [18] and their transmission range is about 40 m. Simplified IEEE 802.15.4 was used as the PHY/MAC layer protocol. The size of the sensor network is set to 1500 m \(\times\) 1500 m where 2500 nodes are randomly distributed. Initially, for simulation, the number of member sinks in a sink group is 10; the number of source node selected randomly is 20. Average energy consumption is defined as average consumption of twenty times of transmission. Transmitting and receiving power consumption rates of the sensors are 3.12 \(\upmu\)J and 2.34 \(\upmu\)J, respectively. The group moved following the _random waypoint_ mobility model [19]. Default speed of the sink group is set to 3m/s and default radius of the group region is set to 100 m.  
Figure 7 shows average energy consumption for the number of member sinks of a group. If the number of member sinks increases, GMR consumes much energy in proportion to the number of sinks because they request all member sinks to report their location to the source. In RingRouting, although the ring structure rendezvous area effectively provides destination locations, the energy consumption increases due to individual location update and data dissemination. M-Geocasting consumes less energy for group location update than GMR since all member sinks do not need to update location to a source in M-Geocasting. ERMG also shows a similar tendency with M-Geocasting but with less energy consumption. The reason is that region discovery is conducted in the large expected area. The energy consumption of RSMGP does not significantly increase since our scheme calculates a group region using only a few member sinks and does not update the location. Figure 8 presents average energy consumption for the speed of mobile sink group. If the speed increases, the energy consumption of both GMR and RingRouting increases due to frequent location updates to a source according to increasing movement speed. M-Geocasting consumes less energy than both schemes at high speeds because all member sinks do not need location updates to a source in M-Geocasting. However, as the speed increases, region discovery frequency by flooding becomes  
Figure 6: Combining locations of deviated sinks for interval time between data dissemination. high. ERMG blocks frequent querying to some degree by cell unit communication. RSMGP efficiently supports the group mobility because it reduces calculation frequency and control messages. Figure 9 shows energy consumption for the radius of group area. In M-Geocasting and ERMG, the longer radius makes the mobile sink group stay in the calculated group region for a long time, but makes sensor nodes consume more energy for flooding. At first, therefore, energy consumption decreases until the radius is 100 m since recalculation rate for region decreases, but energy consumption rather increases as it increases the radius greater than 100 m due to flooding cost increasing rapidly. RSMGP, however, is not influenced by the radius of group region because only some sinks deviating group region is the factor that constructs the calculated group area. Consequently, in RSMGP, the energy consumption is almost similar regardless of the radius of group area as well as lower than that of M-Geocasting. Figure 8: Energy consumption impacted by moving speed of a sink group. Figure 7: Energy consumption impacted by the number of member sinks. Figure 10 illustrates data delivery ratio for the number of member sinks of a group.