# MAC Layer Protocols for Internet of Things: A Survey
## 4 Discussion and Open Issues
### Short Range Protocols  
This study demonstrated the diversity of patterns in the MAC layer of short range protocols, regarding their application characteristics. RFID and NFC technologies do not have many features in common although both were developed for short-range communications. For example, applications for identification and tracking uses RFID more commonly while NFC has been popularized among smartphones for exchanging media content and e-business devices for payment machines. There is a constant evolution of these technologies, adapting to new demands. Retaining miniaturization, energy consumption, and transmission efficiency characteristics.  
Different features sometimes prevent a direct protocol comparison due to strong differences in the design and construction of their PHY and MAC layers. However, the PHY layer of the standard IEEE 802.15.4 is used as a base for many other short range protocols. In addition, protocols that present different solutions do not compensate the drawbacks of the IEEE 802.15.4, but rather they meet the requirements of the different applications. The comparison between BLE and IEEE 802.15.4 elucidates the lower power consumption of BLE against IEEE 802.15.4. Regarding throughput and energy efficiency, IEEE 802.11a indicates some advantages over IEEE 802.15.4. Using the PHY layer of IEEE 802.15.4 protocol, the Wireless-HART protocol implements different facilities in its MAC layer such as TDMA as the medium access technique and frequency hopping that increases its capacity [121; 122].  
The Z-Wave protocol presents the PHY and MAC layers with a different combination. Its MAC layer, standardized by ITU G.9959, presents the CSMA-CA inheritance middle access technique, security mechanisms, medium access optimization (such as backoff time, frame validation, and topology flexibility). These features allow high data rate performance with the range that the sub-GHz ISM band can provide.  
Operating in the range of TVWS (470-790 MHz), the Weightless protocol needs to compensate the transmission bandwidth affected by spectral pollution. It does this by using complex modulation techniques such as Phase Shift Keying derivations and quadrature amplitude modulation options. Its transmission performance is also reinforced by the joint use of multiple access mechanisms such as the FHMA and TDD medium.  
The different variations of the IEEE 802.11 protocol have been following the demands for wireless communications. The massive use of its variations that have evolved is a consequence of the demand for H2H services that require high transmission rates, with very dense networks. The IEEE 802.11 ah variant has come to improve the performance and robustness of data communication. It is due to various mechanisms implemented in the MAC layer, such as Short MAC headers, short beacons, power saving mode, regular restricted access window, among others. In addition, it contributes to good transmission range performance, a reduction of channel usage compared to the previous IEEE 802.11 standards and new techniques for multiple antenna features.  
The differences between applications and technical characteristics makes it difficult to compare short distance coverage protocols (Bluetooth, Bluetooth Low Energy, IEEE 802.11 ah and IEEE 802.15.4) with long distance coverage protocols. Then, the next sub-section discusses the IoT long range protocols. Figure 24 presents a comparison of short range protocols studied in this survey.