# MAC Layer Protocols for Internet of Things: A Survey
## 3 Long Range MAC Layer Protocols
### SigFox  
SigFox is a technology that brings a new network and information strategy to IoT. Named by its developer, a group from Labge, France with the same name, SigFox is an IoT player with a network operator business model. Used by applications that require low data rates, SigFox is also classified as a Low Throughput Network (LTN) protocol, as defined by ETSI ERM TG28. Based on an Ultra Narrow Band (UNB) technology, Sigfox uses a 100 Hz transmission band in ETSI and ARIB (Japanese regulatory body Association of Radio Industries and Businesses) regions and a 600 Hz transmission band in FCC (Americas and Oceania) regions. This characteristic allows a data rate of 100 bps and 600 bps at ETSI and FCC regions, respectively [118; 119]. SigFox protocol stack is composed of three main layers: Frame, MAC and Physical layers. Figure 23 depicts the comparison between SigFox and the OSI reference model. Sigfox operates on the Sub-GHz ISM band carrier using 863-870 MHz in ETSI and ARIB (Europe and Japan) and 902-928 MHz in FCC (Americas and Oceania). The bit rate depends on the bandwidth of each region. In ETSI and ARIB, the bandwidth used for the transmission is 100 Hz so, in this case, the bit rate is 100 bps. In the FCC regions, the 600 Hz transmission bandwidth occupancy permits a 600 bps transmission data rate [115]. The regulations in the FCC, ETSI, and ARIB regions determine the permitted transmission power in each case. In the ETSI regions, the maximum power transmission is 14 dBm per device for a maximum of two seconds of emission time, and a total of 140 messages per day. Sigfox Base Transmission Station (BTS) can transmit up to 26 dBm at a maximum. The regulations in the FCC region (Latin and Americas, United States of America and Asia) defines that each device can transmit up to 22 dBm with a maximum time transmission of 0.346 seconds, and a total of 140 messages per day. The FCC regulated BTSs can transmit up to 30 dBm. To overcome challenges with interference from the transmission medium, as well as transmission collisions that are inherent in the process, diversity comes as an efficient tool. Since there is no need to synchronize the network elements, devices can use an initial random frequency for transmission and then send two replicas randomly at different frequencies and at random time-slots. This simultaneous time and frequency diversity mechanism increases the robustness against interferences while increasing reception efficiency. Different from mobile cellular systems, a Sigfox terminal is not attached to a single base station. With proper deployment, the message sent by a Sigfox terminal can be received by several base stations. Figure 22: LoRa and LoRaWAN protocol stack. This feature is called spatial diversity that, coupled with the time and frequency diversity of repetitions, are strong points in Sigfox protocol conception technology. SigFox systems use D-BPSK (Differential Binary Phase Shift Keying) modulation to uplink and GFSK (Gaussian frequency shift keying) modulation to downlink. D-BPSK transmits at 1 bps using only 1 Hz of the operational band, which means that one single signal uses only a small part of the operational band, and therefore brings higher efficiency in the spectrum medium access. Applications that demand low transmission data rates, with lower cost devices, high availability, and easy implementation coupled with the advantages of high link-budget, drive the use of D-BPSK modulation. Its physical layer employs a proprietary frequency hopping and frame repetition pattern to avoid interference of signals and it is responsible to determine how SigFox signals will be transmitted. The physical layer handles the modulation used, the bit rate and transmission power control, as well as the radio resource occupation. UNB technology is identified by using a legacy Aloha protocol-based medium access technique called Random Frequency and Time Division Multiple Access (RFTDMA).