# Resilient Green Cellular IoT for Landslide Monitoring Using Voice Channels
## 3 VRITHI: System Design and Characteristics
### Communication Phases in VRTHI  
As aforementioned, VRTHI uses both the control and data channel simultaneously for real-time collaboration between IoT nodes. Every communication sequence between two IoT nodes goes through four phases: (1) Node Initialization, (2) Control Channel Initialization, (3) Node Collaboration, and (4) Connection Closure. Each of these phases are briefly discussed below:  
* _Node Initialization phase:_ in this phase, the node is powered up, and the modem of the node is connected to the cellular network. At this point, the node is assigned a signal bearer by the cellular provider. A bearer is a tunnel that connects a specific service from the Telecom network to the user equipment (UE) viz modem. Different services, such as voice, Internet data, video call, etc., have different bearers from the Telecom network to the UE. Signal bearers are used by the Telecom network to signal the UE for services, such as UE registration, incoming call signaling, etc. Voice bearers are used to carry voice traffic between the UEs, and data bearers are used to connect the UE to the Internet. The voice bearer is given the highest priority and guaranteed bit rate compared to other bearers by the Telecom network. As landslides are more susceptible during rainy conditions, such a priority for voice traffic is more helpful for the landslide early warning system.
* _Control Channel Initialization phase:_ The nodes enter this phase if they require to start collaborative real-time communication with other IoT nodes. In this phase, the UE uses the control channel to connect to other node, and a dedicated voice bearer VB_0 is created between the nodes by the Telecom network. The entire communication sequence between two IoT nodes is illustrated in Figure 6. The voice bearer is shaded in brown color, the data bearer in blue color, and the signal bearer in black color. In the figure, the control channel Initialization is shown as the message sequence (2).
* _Node Collaboration phase:_ In this phase, an IoT node uses the control channel to exchange the control parameters. Protocol message sequence (3) to (6), in Figure 6, are exchanged during this phase. This protocol message exchange is iterative until the entire sensor data is communicated between the nodes. The block marked as loop in Figure 6 takes care of changing the IP addresses and ports at run-time. In brief, the message sequence (3) is used by the modem to signal its interest in establishing the IP data connection. In sequence (4), the IoT nodes mutually exchange their source IPs (assigned by cellular provider), aounce, and CRC through voice bearer VB_0. The noure is used for a simple XoR mechanism to differentiate between each exchanged message, and CRC is used to detect errors during the conversion of payload into voice codec. In sequence (5), additional parameters, such as acknowledgment or no acknowledgment, TCP or UDP protocol, and the number of ports to open, are exchanged. After successful exchanges of these messages, both the nodes agree to start mutual communication of IP data. It is only at this stage that one of the nodes starts to listen on the data port, and the IP message sequence (6) is sent to that port by creating data bearer DB_0.
* In the _Connection Closure_ phase, one of the peer IoT nodes participating in the real-time communication indicates the end of conversation in the control channel. Hence, the control channel and data channel are closed at both the ends. The voice and data bearers,VB_0 and DB_0 allocated by the Telecom network are released.  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline  & **1209 Hz** & **1336 Hz** & **1477 Hz** & **1633 Hz** \\ \hline
697 Hz & 1 & 2 & 3 & A \\ \hline
770 Hz & 4 & 5 & 6 & B \\ \hline
852 Hz & 7 & 8 & 9 & C \\ \hline
941 Hz & * & 0 & \# & D \\ \hline \end{tabular}
\end{table}
Table 3: DTMF Frequency table.