# MAC Layer Protocols for Internet of Things: A Survey
## 4 Discussion and Open Issues
### Long Range Protocols  
In the case of the LPWANs presented, the main parameters that differ are the distance range of the protocols discussed. Improvements, adaptations, and innovations have occurred mostly at the PHY layer while the MAC layer was adjusted as needed. Figure 25 presents a comparison of the long-range protocols studied in this survey. With no power control and baud rate adjustment mechanisms, the LoRa modulation itself becomes inefficient for applications that demand frequent transmissions and high transmission rates (tens of kilobits or more). Due to these characteristics, LoRa is automatically surpassed by its evolution, the LoRaWAN. LoRaWAN improves the MAC layer with the power control mechanism by dividing the operational mode of the devices into classes and using adaptive data rates (ADR). SigFox comes as a great choice for applications that do not require high data rates and require low power consumption. The transmission efficiency presented is due to the high robustness of the link, with a link budget greater than 150 dBm in some regions. This ensures robustness against interference, thanks to the narrow band occupied in the transmission. Its disadvantage is the low transmission rate as Sigfox takes a long time to transmit a message. This characteristic imposes hard operational conditions for a mobile supported application. In SigFox, LoRa, and LoRaWAN, the uplink capacity depends on spectrum occupation and quantity of messages transmitted simultaneously to uplink. These systems capacity is not limited by data traffic load. Figure 25 illustrates long range protocols characteristics. Figure 24: Comparison of the short range protocols. Existing cellular network technologies are now heavily challenged by new IoT applications incoming their technological eco-system. Cellular networks are designed to operate with the human voice traffic profile that has a known and predictable pattern. With the advent of Internet traffic, mobile networks have had to adapt to meet an instant and floating demand for this service. Based on the adaptations made from LTE network, NB-IoT networks are eminent solutions for harnessing the existing mobile cellular network infrastructure. NB-IoT emerges as a strong alternative given the available and well consolidated mobile network infrastructure. The adaptations made in LTE to support NB-IoTIoT services offer a new data traffic profile, different from the user services that demand for high transmission data rates. As data traffic from H2H applications such as audio, video, and voice tends to migrate to higher capacity systems such as 5G, NB-IoT capabilities become more widely available. This is an excellent alternative to the telecommunication operators since they have a 4G infrastructure and it is not compatible with the upcoming 5G. Then, telecommunication operators can re-use the available network infrastructure for IoT, based on NB-IoT, and perform its investments for 5G. Figure 25: Long-range protocol characteristics. Challenged by IoT applications, it is not enough to adapt. Now, it is necessary to re-invent in order to support a new data traffic profile coming from objects which have completely different traffic, range, and time-based characteristics. Without the predefined parameter definitions regarding data transfer delay, NB-IoT has its limitations when used by systems that require some time dependency parameter. To meet this demand, LTE CAT-M technology, has brought to this new service profile an immediate deployed solution. Although they seem to be competitors, LTE CAT-M and NB-IoT protocols support services with different requirements for data transfer time requisites. They can become a complementary solution. Even though they are treated as networks for long distances, the differences between LP-WAN technologies and cellular mobile network technologies are very clear. The strong differences between the architectures of the PHY and MAC layers of these two types of networks make it difficult to have a fair comparison between them.