## 3 Long-Range, Low-Power IoT Communication Protocols and Their Security
### C-UNB/Sigfox  
The Cooperative-Ultra Narrow Band (C-UNB) standard is a cellular IoT technology specified by 3 GPP in the technical report TR45.820 [59]. A similar technology called triple-diversity UNB (3D-UNB) was developed by a French company named Sigfox, which uses the ISM spectrum. In the United States, it uses the 902 MHz band. As illustrated in Figure 5, both C-UNB and Sigfox are LPWA networks that provide wide-area connectivity, low data rates, and ad-hoc access protocols to serve a large number of IoT devices. **Architecture. ** At the physical layer, C-UNB uses macro-channels, such as 200 kHz GSM channels, or the sidebands of LTE channels, similar to NB-IoT. In this way, C-UNB uplink channels are created by dividing a 200 kHz macro-channel into narrowband channels, or micro-channels. The bandwidth of each micro-channel is just a few hundred a Hertz, being one of the slowest data rates in LPWA, and for this reason, it is called ultra- narrowband. Assuming that the uplink bit rate is estimated to be 300 bps and the modulation scheme is Differential Binary Phase Shift Keying (D-BPSK), the required bandwidth of each micro-channel is 600 Hz, i.e., two times the bit rate. The C-UNB downlink also employs micro-channels. The downlink bit rate is higher, at 600 bps. Similarly, the Sigfox radio link has micro-channels with 600 Hz of bandwidth both in the downlink and uplink, where the downlink uses Gaussian Frequency Shift Keying (GFSK) and the uplink uses D-BPSK modulation. The total bandwidth needed in the uplink and downlink is 192 kHz for each macro-channel, which provides 320 micro-channels in the uplink and downlink. The triple diversity in the 3D-UNB of Sigfox means diversity in space, time, and frequency. Frequency diversity comes from such a large number of micro-channels, which allows hundreds of devices to transmit at the same time. In the capacity analysis done in [48], Sigfox connections showed less than a 1% failure rate for outdoor users, assuming that each user had eight IoT devices. Time diversity means that the devices transmit the same message three times, one after the other, each on a different random micro-channel. Space diversity means that base stations in adjacent cells listen to the same 192 kHz macro-channel. Thus, packet collisions are avoided [72]. IoT devices, or end-points, transmit mostly in the uplink because the end-points sleep to save battery. Whenever a Sigfox end-point needs to send data, it randomly selects a \begin{table}
\begin{tabular}{c c c} \hline \hline
**Application** & **Message Frequency** & **Message Size** \\ \hline Waste management & 1 per hour & 50 bytes \\ Bike renting & 4 per hour & 50 bytes \\ Gas meter & 4 per hour & 100 bytes \\ Parking & 1 per hour & 100 bytes \\ Water meter & 1 per day & 200 bytes \\ Electricity meter & 1 every 4 h & 300 bytes \\ Pollution monitoring & 1 per hour & 1000 bytes \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Smart city applications that utilize NB-IoT, according to 3 GPP. micro-channel to transmit data. Thus, both C-UNB and 3D-UNB networks use a contention-based data link protocol, similar to unslotted ALOHA [50]. When an uplink packet is received, the base station sends an acknowledgment using the same micro-channel of the uplink message. If a collision is detected, i.e., two devices transmit in the same micro-channel and no acknowledgment is received, then the frame is re-transmitted. In the C-UNB specification, the uplink frame header has a repetition counter, which can have values equal to one, two, or three [72]. For mobile autonomous reporting (MAR) applications, it is assumed to need at most two repetitions. Three repetitions are said to be normal for exception reports (e.g., alarms). For Sigfox networks, the maximum payload sizes are fixed at twelve bytes in the uplink. No IP headers are used, i.e., the application data are encapsulated directly by the frame header. The end-point device is identified using a four-byte identification number.