# Study of Data Transfer in a Heterogeneous LoRa-Satellite Network for the Internet of Remote Things
## 3 Heterogeneous Network Architecture
### Heterogeneous LoRa-Iridium Network Architecture  
We considered a network architecture (Figure 2) to study the applicability of the proposed GDEP method. Elements of the architecture included:  
1. Endpoint data collection devices. One or several sensors can be connected to these devices; data transmission is performed according to the LoRa specification.
2. Terrestrial LoRaWAN gateway.
3. Terrestrial LoRa-Iridium gateway. MQTT broker is co-located with this element.
4. Iridium satellite system for hybrid terrestrial-satellite network organization.
5. Terrestrial Iridium gateway.
6. Terrestrial Iridium-MQTT gateway.
7. General data collection and analysis system.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Format** & \begin{tabular}{c} **Requires Predefined** \\ **Data Structure** \\ \end{tabular} & \begin{tabular}{c} **Has Embedded** \\ **Compression** \\ \end{tabular} & \
\begin{tabular}{c} **Allows Binary** \\ **Data Transmission** \\ \end{tabular} &
\begin{tabular}{c} **Use Data Optimization** \\ **Techniques** \\ \end{tabular} \\ \hline JSON & No & No & No & No \\ BSON & No & No & Yes & No \\ CBOR & No & No & Yes & Yes \\ MsgPack & No & No & Yes & Yes \\ JSONNC & No & Yes & Yes & Yes \\ Protobuf & Yes & No & Yes & Yes \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of data presentation formats.  
Figure 2: Heterogeneous long range (LoRa)-Iridium network architecture.  
We performed the task decomposition and defined the main stages of information transfer from endpoint data collection devices (1) to the data collection and analysis system (7).  
Each of the endpoint data collection devices (1) can have one or multiple sensors (e.g., temperature, vibration, light), and transmit data in the LoRa binary format to the LoRaWAN gateway (2). The LoRaWAN gateway (2) acts as a transparent bridge passing the information to a connected MQTT service through a broker. This information is encoded in the JSON format. The number of published MQTT messages corresponds to the number of received packets on the LoRa transmission channel. The LoRa-Iridium gateway (3) retrieves messages from the LoRaWAN gateway (2) through an MQTT broker. We assumed there were no constraints on the channel width and on the energy efficiency of transmission between (2) and (3) as they were equipped with a stationary power supply and communicated with each other via a high-speed connection.  
We selected MQTT as one of the standard and widely used protocols for data transfer in the IoT. The protocol was initially developed at IBM [48] and Arcom (now Cirrus Link) for economic data transfer from oil pipelines via a satellite channel. In 2010, protocol specifications were published, and in the fall of 2014, MQTT version 3.1.1 became an open standard for the Organization for the Advancement of Structured Information Standards (OASIS). Two papers [49; 50] provide comparisons of session layer protocols outlining the efficiency of MQTT for IoT.  
The main task of the LoRa-Iridium gateway (3) is to convert messages coming from the terrestrial IoT network into the appropriate format of the SBD service.  
Each SBD message is transmitted via the satellite constellation (4) and the Iridium terrestrial gateway (5) to the Iridium MQTT gateway (6), which decodes messages back from SBD to JSON and publishes them through another MQTT broker to the data collection system (7).