## 5 Experimental Section
### Impact Factors Analysis
#### 5.3.1 Network Density  
From Figure 6 we could see that the number of message transmissions of all schemes goes up with the number of nodes. The Epidemic has the largest message transmissions, the number increases from 6.54E+4 to about 1.25E+6. Messages are exchanged among any two encountered nodes at Epidemic, while in other schemes the messages are selectively forwarded among the nodes. At the PDA, PROXY and ADG schemes messages are first selectively forwarded towards the proxy nodes, and then picked up by the \(MC\). The former dominates the total cost of message transmissions, so the number of total message transmissions are very close. They have the lowest total message transmissions of the compared schemes, and the number is less than 25% of that in Epidemic when there are 60 nodes at the network. The performance of PROPHET lies in the middle, the number of transmissions is about 64%-70% of that at Epidemic at our simulation setting. For the proposed ADG scheme, ordinary nodes would sent their sensed data to the proxy nodes before the data are picked up by \(MC\). There would be fewer messagesexchanges among ordinary nodes in a sparse network because nodes would have fewer encounters. When there are more nodes, e.g., 60, the message transmission would goes up to about 2.65*E+5 at ADG, which is about 13 times of that when there are 10 node.  
Figure 7 shows the impact of network density to the data coverage. Epidemic and PROPHET has the smallest data gathering rate. Their data coverage decreases a little bit with the number of nodes, and the overall data coverage is less than 10.2%. This is mainly because of the limited data gathering period, which is about 12.5% of the total epoch. For other schemes their data coverage firstly goes up with the number of nodes, and reaches as high as about 72%, 52% and 32.5% for ADG, PDA, and PROXY respectively when 40 nodes are deployed. Proxy nodes would have more contact opportunities and receive data from ordinary nodes when there are more nodes within the network at these schemes. Yet ADG has higher data coverage because \(MC\) could collect the data from a set of proxy nodes while \(MC\) has only one data gathering location at PDA. Although there are multiple proxy nodes at the PROXY scheme, \(MC\) might lose the track of the proxy nodes as they keeps moving within the data gathering period. So the data at some proxy nodes could not be upload to the \(MC\), which harms the data coverage. For the ADG scheme, when there are more nodes in the network, e.g., more than 40 nodes, there would be too many proxy nodes that act as intermediate storage to be visited by the \(MC\). However, because the data gathering period is limited due to energy concern, \(MC\) is not able to collect all the data from the proxy nodes at one epoch, which decreases the data coverage. The data coverage is about 43% when there are 60 nodes and with one data collector.  
However, it is easy to extend ADG using multiple collectors, or having multiple data gathering rounds at each epoch to handle the scalability problem of the data gathering. Figure 8 shows the performance results that use two data collectors. The amount of total transmissions is roughly the same with that at Figure 6, but the data coverage increases with the number of nodes due to the increased chance of contacts and meta-data exchanges within the network. It gathers as high as 82 percent of the sensed data when there are 60 nodes in the network. Generally, the number of collectors and the number of  
Figure 6: Number of nodes _vs._ total message transmissions.  
data gathering rounds could be tuned according to the network density and the data generating rate of the network.