## 5 Network Costs
### Labor Costs  
The deployment of WSN nodes and their maintenance requires spending time and effort. The labor costs are related to the time preparing for and spent in the field as well as the transportation costs to and from the field. An estimation of the labor costs is presented here by translating the time spent and distance travelled into a dollar cost. Deployment time includes the time spent checking the network status, changing more batteries, replacing broken or faulty equipment, adding new nodes to the network, updating the network's gateway, and installing environmental sensors. The time to deploy a new node depends on the node type. A relay node requires less time than a regular node because it does not have additional sensors to set up and install. Once a location is determined, a relay node may be readily switched on and placed in the field to begin operation in the network. A regular node requires additional activities such as burying sensors into the soil and/or attaching sap flow sensors to a tree, which are typically done while the more is switched off. Following deployment, each node in the network also requires regular maintenance. By far, the most recurrent maintenance task is the replacement of discharged batteries. Between May 2010 and the end of April 2014, we performed replacement of batteries 128 times out of 154 field visits, as seen in Figure 16. However, not every field visit to change node batteries is successful (e.g., faulty equipment or faulty batteries), requiring multiple successive field visits to restore network connections. The number of field visits during the Sluggo era (see Figure 4) occurred more frequently due, in part, to network expansion (_i.e._, deployment phases) and node relocations (as described in Section 3) and issues regarding the rechargeable batteries. It was found that the rechargeable AA batteries do not uniformly hold their charge, resulting in varied battery charge levels after recharging. A screening process, where only batteries that maintain adequate charge following a recharging were used in the network, resulted in longer battery life as indicated by fewer dots in Figure 16 from mid-2011 until the end of 2012. Network maintenance was reduced during 2013 after some initial bugs in the CTP application. These problems were solved in November 2013 and regular field visits were resumed as nodes started to deplete their batteries. Based on the battery maintenance schedule in Figure 16, the average time (and standard deviation) between battery changes for each node (about 58 days) is presented in Figure 17. The red line indicates the average time between field visits (_i.e._, considering all the nodes in the network) to be about 12 days. Each field visit is recorded in a log where the time spent at the field can be retrieved. The labor costs are obtained by computing the total time spent in the field plus some additional time for preparation before each trip. Preparation time may include charging and sorting batteries, pre-programming motes, preparing mote enclosures, and building and testing environmental sensors. Transportation costs are also included, accounting for the physical distance travelled between the university and the WSN site. Based on the field log, the average time spent on site is estimated to be about 6 hours per visit. To account for preparation time before field visits and the occasions where there are additional field workers, this time was adjusted by a constant multiplier of 1.6 (_i.e._, a total of 9.6 hours per visit). A cost of $15/h was assumed (based on graduate and undergraduate students' salary). The transportation cost was computed by multiplying the round trip distance (about 20 miles) by a constant transformation factor of $0.565/mile. Table 8 shows the summary of the cumulative labor costs up through April 2014 (approximately $23,900) and the average cost per visit for the same time period (approximately $155). Figure 16: Battery changing schedule from initial network deployment until the end of April 2014.