## 4 Stray Node Detection Protocol and Method of Avoiding Message Collision  
We propose a detection method. The area covered by each transmitter is limited by its transmission power. Any node that goes out of that area is regarded as a stray node. As shown in Figure 3, the transmission power of each node is adjusted so that the radius of the area covered by its transmitter  
Figure 1: Position of the proposed transmission medium multi-access contention avoidance technology.  
Figure 2: Assumed usage.  
is r. Nodes are classified into one leader node and normal nodes. Let r\({}^{\prime}\) be the distance between a normal node and the leader node. If r < r\({}^{\prime}\), that normal node is regarded as a stray node.  
Normal nodes located within a spherical area of radius r with the leader node at the center can communicate by one hop. Normal nodes that are outside the spherical area need at least two hops to communicate with the leader node, or cannot communicate with the leader node (timeout). If this situation is detected, it is determined that it is likely there is a stray member somewhere.  
The round trip time (RTT) can be used to estimate the distance. However, calculation of RTT requires a node that has received a message to send back a reply immediately. This can cause collisions, which make it difficult to communicate. Therefore, the distance is estimated from the number of hops in this paper.  
Two different communication protocols are considered for stray node detection. The first is an autonomous response-type protocol, in which each normal node sends a message to the leader node at a certain interval. Communication takes place automatically without requiring any user operation. A drawback of this protocol is that message transmission attempts occur frequently.  
The second is a request-response-type protocol, in which the leader node requests communication any time it wants to check whether there is a stray node. While it requires users to initiate communication, message transmission attempts take place less frequently. In this research, messages are broadcast communication using CSMA/CA. This means that no attempt is made to confirm the arrival of a message by receiving an Ack message. The characteristics of the two protocols are summarized in Table 1.  
The assumed protocol stack is shown in Figure 4. The Underwater SND (Stray Node Detection) at the application layer implements the proposed protocol. CSMA/CA that works in ad-hoc communication mode is adopted in the data link layer. These have been implemented based on the OSI reference model so that they can support a variety of applications, such as verbal communication and image transfer.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Item** & **Autonomous Response-Type** & **Request-Response-Type Stray** \\  & **Stray Node Detection** & **Node Detection** \\ \hline User operation & Not required & Required \\ Frequency of communication attempts & High & Low \\ Communication initiating node & normal node & leader node \\ \hline \hline \end{tabular}
\end{table}
Table 1: Characteristics of the autonomous response-type and the request-response-type protocols.  
Figure 3: Overview of the stray node detection protocol.  
Information elements (IEs) that are set in messages used in the stray node detection protocol are defined as follows.  
1. group ID.
2. source node ID.
3. message type (REQUEST message or REPLY message).
4. sequence number.
5. number of hops.