## 4 System Model and Description of the Proposed Protocols
### Proposed Protocol 2 (BF-SPR-Three)
#### 4.2.1 Network Configuration  
Before explaining the network architecture of BF-SPR-Three, a few terms need to be defined here. The nodes which perform data forwarding including their own sensed data and the data they received from higher depth nodes, are known as relay nodes and the nodes which lie on the layer or coinciding with the layers are considered as cross nodes. The proposed protocol BF-SPR-Three consists of sinks, relay nodes, cross nodes varying from (150-450) and surface gateways. Basically, the sink works as an embedded unit and housed with both acoustic and radio modems. The radio modem is used for the communication among sinks and base stations, while the acoustic modem is used for underwater communication and data collection among sinks and underwater sensor nodes (using multi-hop). Moreover, we assumed that:  
1. Firstly, sinks have high energy. Moreover, they are connected with each other to balance the load of DPs on respective sinks. 2. Secondly, if the data is successfully delivered from any of the relay nodes to any of the sinks, then it means that data is successfully delivered to the control station. 3. In the end, we have not considered the movement of nodes in a vertical direction, which is almost negligible, whereas we have only considered the mobility of the nodes in the horizontal direction (due to water currents). Initially, all the sensor nodes are randomly distributed in the underwater network and they are in sleeping mode (in this mode, nodes are basically alive, consuming a negligible amount of energy; however, they are not participating in the transmission process). When a node (during sleeping mode) receives a DP, then this node changes its status from sleeping to the active mode (in this mode, nodes actively participate in the transmission process and consume energy during packets reception and transmission). If any node is not participating in the transmission process, then that nodes get back to the sleeping mode again. In addition, the key step in the proposed protocol is binary tree generation. This tree generation procedure is adopted for multi-path transmission. Let us consider the LMPC routing protocol scenario (as shown in Figure 8). In LMPC, the UWSN is distributed into layers. Moreover, the binary tree generation helps the routing protocol in reliable data delivery. Then, from every sensor node, different copies of DP are transmitted. This process remains continuous until the DP reaches the destined sink. While forwarding the DP from sea depth to lower depth, different noises including shipping noises add up with data and produce bit error in the packets. If this hindrance exceeds a certain limit, it ultimately drops the DP. Due to this reason, multiple copies of DP are required for reliable data delivery. By keeping these parameters in mind, we have implemented a new routing protocol named BF-SPR-Three. The beacon mechanism is similar for EP-VIR-Three routing protocol. In BF-SPR-Three, the complete shortest and the efficient path is computed using the 'BF' algorithm. The network environment is distributed into unequal layers (closer near the sea surface and distanced at the sea bottom (see Figure 10)). In the current protocol, the sensor node (source) generates a single copy of the data. As the data is advanced to the lower depth layer using relay nodes (selected from the BF algorithm), then the current forwarder (source node) checks the next forwarder node (destination node) regarding whether it is a cross-node or a simple relay node. Upon DPs reception, if the next forwarder node is a relay node, then it unicasts the DP. On the contrary, a cross-node uses IP multicasting technology (a bandwidth preserving technology that minimizes the traffic of DPs by instantaneously delivering a single stream of data to potentially thousands of corporate receivers) to transmit the DP. In current work, the IP of the neighbor nodes is their unique ID. In the end, these DPs are directed towards surface gateways and ultimately data reaches the sink.