## 3 Survey Results
### Time Synchronization  
Time synchronization has been addressed as one of the core challenges of sensor networks. Therefore, its use in deployments is analyzed and statistics are shown in Table 16.  
\begin{table}
\begin{tabular}{c l c c c c} \hline \hline
**Nr** & **Codename** & **Time-sync used** & **Accuracy, \(\mu\)sec** & **Advanced time-sync** & **Self-made time-sync** \\ \hline
1 & Habitats & n & - & - & - \\
2 & Minefield & y & 1000 &? &? \\
3 & Battlefield & y &? & n & y \\
4 & Line in the sand & y & 110 & n & y \\
5 & Counter-sniper & y & 17.2 (1.6 per hop) & y & y \\
6 & Electro-shepherd & n & - & - & - \\
7 & Virtual fences & n & - & - & - \\
8 & Oil tanker & y &? & n & y \\
9 & Enemy vehicles & n & - & - & - \\
10 & Trove game & n & - & - & - \\
11 & Elder RFID & n & - & - & - \\
12 & Murphy potatoes & n & - & - & - \\
13 & Firewxnet & y & \(>\)1000 & n & y \\
14 & AlarmNet & n & - & - & - \\
15 & Ecuador Volcano & y & 6800 & y & n \\
16 & Pet game & n & - & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 16: Deployments: time synchronization.  
Reliable routing is possible if at least one of two requirements holds:  
1. A 100% duty cycle is used on all network nodes functioning as data routers without switching to sleep mode.
2. Network nodes agree on a cooperative schedule for packet forwarding; time synchronization is required.  
Therefore, no effective duty cycling and multi-hop routing are possible without time synchronization.  
Time synchronization is used in 38% of deployments, while multi-hop routing is used in 57% of cases (the remaining 19% use no duty-cycling).  
Although very accurate time synchronization protocols do exist [81], simple methods, including GPS, are used most of the time, offering accuracy in millisecond, not microsecond range.  
Only one of deployments used a previously developed time synchronization approach (not including GPS usage in two other deployments); all the others use custom methods. The reason is that despite many published theoretical protocols, no operating system provides an automated and easy way to \"switch on\" time synchronization.  
_Design rule 25_: Time synchronization provided by the operating system would be of a high value, saving sensor network designers time and effort for custom synchronization development.  
\begin{table}
\begin{tabular}{l l c c c c} \hline \hline
**Nr** & **Codename** & **Time-sync used** & **Accuracy, \(\mu\)sec** & **Advanced time-sync** & **Self-made time-sync** \\ \hline
17 & Plug & n & - & - & - \\
18 & B-Live & n & - & - & - \\
19 & Biomotion & y &? & n & y \\
20 & AID-N & n & - & - & - \\
21 & Firefighting & n & - & - & - \\
22 & Rehabil & n & - & - & - \\
23 & CargoNet & n & - & - & - \\
24 & Fence monitor & n & - & - & - \\
25 & BikeNet & y & 1 ms? & n, GPS & n \\
26 & BriMon & y & 180 & n & y \\
27 & IP net &? &? &? &? \\
28 & Smart home &? &? &? &? \\
29 & SVATS & y, not implemented & - & - & - \\
30 & Hitchhiker & y &? & n & y \\
31 & Daily morning & n & - & - & - \\
32 & Heritage & y & 732 & y & y \\
33 & AC meter & n & - & - & - \\
34 & Coal mine & n & - & - & - \\
35 & ITS & n & - & - & - \\
36 & Underwater & y &? &? & y \\
37 & PipeProbe & n & - & - & - \\
38 & Badgers & n & - & - & - \\
39 & Helens volcano & y & 1 ms? & n, GPS & n \\
40 & Tunnels & n & - & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 16: _Cont._