## 1 Introduction
In this paper, we propose the Application-based Partial Initialization (API), whose design principles are outlined as follows:  
* Partial initialization. This protocol initializes as many nodes as required to execute a given application. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Previous Work** & **Main Protocol** & & **Initialization Method** \\ \cline{2-4} [PERSON] et al. (2007) [5] & & **Channel Access** & **Procedure** \\ [PERSON] et al. (2010) [6] & & Random access & SYNC broadcast among nodes \\ [PERSON] et al. (2011) [7] & & Undefined & Corresponding message broadcast among nodes \\ [PERSON] et al. (2012) [8] & & Undefined & Short INIT flooding from sink to sensors under tree topology \\ [PERSON] et al. (2012) [9] & & Random access + back-off & An initiator broadcasts Pilot, and others send \\ [PERSON] et al. (2013) [4] & & Scheduled & Corresponding message broadcast among nodes \\ [PERSON] et al. (2016) [10] & & TDMA & Corresponding message broadcast among nodes \\ [PERSON] et al. (2017) [11] & & TDMA & An initiator broadcasts SYNC, and others send \\ [PERSON] et al. (2018) [12] & & Scheduled & Ping broadcast among nodes \\ [PERSON] et al. (2000) [13] & & Scheduled & An initiator broadcasts IRES, and others send \\ [PERSON] et al. (2015) [14] & & Undefined & An initiator broadcasts Polling, and others send \\ [PERSON] et al. (2017) [15] & & Undefined & Heillo flooding \\ [PERSON] et al. (2011) [16] & & Modeling discovery & ALOHA & Corresponding message broadcast among nodes \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of previous works on underwater initialization methods. * Limited repetition. In a UWASN, communication failures frequently occur due to bad channel conditions, including natural or artificial noises as well as the Doppler effect [17]. This poor underwater environment makes it difficult for nodes to be initialized at once. Thus, the same process needs to be repeated in order to satisfy the application requirement. * Simplicity. The method of initializing nodes is designed simply to be compatible with any network protocol. Accordingly, this protocol adopts the round-robin-based message exchange algorithm, which does not require scheduling, time synchronization, or retransmission. Since there have been no API-like protocols for a UWASN, we analyze the feasibility of the API by the following two steps:  
* First, we investigate whether the API can obtain data whose statistics are similar to data that requires full initialization (FI). To do this, we perform a simulation that generates a random data set of all nodes periodically and chooses only some nodes randomly according to the application requirement. At each period, we calculate the statistics of the accumulated data sets of the API and the FI, respectively, and check their similarity. Using the results, we can conclude that partial initialization can be used instead of full initialization, provided that the similarity in the data statistics between the API and FI converges and exists within the target range. * Second, we check how much the API can improve in performance over the FI. Intuitively, we can predict that the API can guarantee better performance than the FI in some aspects since the API can have a smaller number of active nodes. However, it is difficult to determine the degree of which the API can outperform the FI since it varies depending on the performance metrics. We consider two performance metrics: (1) the initialization delay and (2) the message overhead. The initialization delay measures how long the initialization takes to satisfy the requirement of a given application. The message overhead calculates the amount of the messages required for initialization, which is closely related to the energy efficiency. Furthermore, we execute simulations extensively by varying the application requirement, the randomness of the data, and the bit error rate of the acoustic channels to check the performance superiority of the API to the FI.