*3.2. TDMA Protocol*

Figure 4 is a TDMA diagram proposed in this paper. The concept is to predetermine the communication time slot and turn off the wireless communication function when nodes are idle. The nodes enter the sleep mode for the rest of the time to save power. First, Trigger Time (TTrig.) is the synchronous trigger signal sent by the bridge. Next, Sensing Time (Tsensing) is the time interruption required by the node for sensing, and the time length depends on the processing time required by the installed sensors. Waiting for Require Time (TWRn) is the interval required by the nth node to wait for a bridge command. Response Time (TRes.n) is required to reply to the sensing information packet. Moreover, Delay Time (TDelay) is the slot for packet parsing, wireless communication reception, transmission mode switching, and radio wave calibration for the nodes and bridges. Finally, Acquisition Time (TAcq.) indicates the period from the start of the triggering to the end of the node polling.

The node ID is assigned before the network constructed, and the sequence of the TDMA time slot is dependent on this ID. When a connected sensor node is turned off, the bridge skips the slot after a couple reconnections. The reservation slot is reserved in the last portion of acquisition duration, and it is reserved for the connection of a new sensor node with lowest data rate and highest power. If the node is checked by the bridge, the time slot will be arranged into the dedicated ID slot in acquisition duration.
