## 5 BN-MAC Protocol Design
### Inter-Synchronized Transmission Schedule  
BN-MAC is used with WSNs that consist of different regions. The previous section highlights how to access the channel and forward the data inside regions. This section explains how to set the schedules within and outside regions. Each region of the WSN includes a BN. Inter-synchronized transmission is performed from one region to other regions. The BN receives intraregional data packets within the region, and the BN forwards the inter data packets outside of the region. When communicating within the region, the BN first broadcasts three \"hello\" messages to warn the nearest region nodes to prepare for receiving the BN indication signal (BNIS). The BN does not wait to receive an acknowledgment from all of the region nodes. If the BN receives a single acknowledgment from one of the nearest nodes, it assumes that the \"hello\" message has been delivered successfully. Thus, if any node is unable to obtain the \"hello\" message, the neighbor node informs other nodes of the schedule exchange time. In this manner, each node knows the BNIS. The BNIS consists of the current time, the next distribution time, the next collection time, and the schedule for obtaining intraregional data packets from the nodes of the region, as shown in Figure 9.  
Figure 9: Inter synchronized transmission schedule with the region node and BNs. The BNIS also has the responsibility of exchanging traffic slots between the source and the destination and describing the related offset time. Once the BN announces its schedule for the nodes of the region, all of the nodes are responsible for following the given schedule. At the end of the scheduled time of the region nodes, the BN synchronizes with another BN of a region to exchange an interregional synchronous schedule to send and receive data communication. After the contention period starts, the node responsible for the data exchange requests the schedule-slot for the next scheduled distribution time. The nodes only remain active during the BNIS. When the BN intends to communicate with another BN of a region, the BN begins the interregional synchronized transmission schedule by using carrier sensing. The BN forwards the message of request-to-send (RTS). In response, the BN receives a clear-to-send (CTS) message from the BN of the other region. There is no hidden terminal problem in BN-MAC because the BNs of all regions broadcast the messages to provide each BN with the schedule of every region. Through this process, all of the BNs know the other BNISs. After receiving the CTS, the transmitter of the BN forwards the broadcast source inter frame (BSIF) to another region. (BSIF is the frame used by BN to synchronize for sending the data to another BN of adjacent region). The receiver BN receives the broadcast destination inter frame (BDIF) during the interval with the CTS and RTS and acknowledges the received packets (BDIF is the frame received by BN after sending BSIF frame, it means BN is allowed to send the data to another BN of adjacent region). We tested the intercommunication performance of BN-MAC and other hybrid protocols in terms of throughput and average energy consumption. We use varying numbers of transmitting nodes at a low duty cycle. Figure 10 presents the average energy consumption for each transmitter node, illustrating that BN-MAC is superior to the other hybrid MAC protocols at a low duty cycle. As mentioned above, BN-MAC has an intra-semi-synchronous transmission schedule that follows the low-duty-cycle mechanism as well as the inter-synchronized transmission schedule that supports the low duty cycle under heavy traffic. BN-MAC is also outperforms the Z-MAC, A-MAC, Speck-MAC, ADC-SMAC, LPRT-MAC protocols at a low duty cycle under heavy traffic. BN-MAC also consumes  
Figure 10: Energy consumption during heavy traffic using a low duty cycle. less energy over a heavy traffic load using the low duty cycle. Figure 11 presents the potential increase in throughput obtained via BN-MAC during heavy traffic at a low duty cycle.