# MAC Layer Protocols for Internet of Things: A Survey
## 3 Long Range MAC Layer Protocols
### LTE--Long Term Evolution  
Long Term Evolution enhanced Machine-Type Communication (LTE eMTC) standards-based technologies support CAT-0 and CAT-M modes. While LPWAN LTE CAT-0 is commonly used to implement M2M/IoT, CAT-M reduces complexity keeping the coverage aspect using existing mobile cellular network infrastructure [96; 97]. LTE eMTC counts on the same mobile technology benefits as security, privacy, data reliability and device identification [98]. With the applications involving the communication information generated by the human-to-human (H2H) services, the transmission data rate in the cellular networks increased considerably in the last decade. However, the traffic generated by M2M communication has different characteristics from those generated by H2H services in actual mobile technologies. M2M devices send more traffic than receive traffic when compared to H2H devices. For example, (H2H) service traffic has specific concentration characteristics at certain times of the day, while M2M traffic may present a uniform characteristic. In applications, such as measuring, M2M traffic tends to be periodic and mobility is short when compared to (H2H) communication devices. Thus, service quality requirements between M2M and H2M may be very different [99; 100]. 3 GPP offered the appropriateness of LTE to permit MAC connection and PHY links through LTE networks and to optimize the technologies and mechanisms related to radio access. It provides LTE networks updates do deal with MTC and IoT requirements as follows:  
Figure 19: NB-IoT key parameters. * CAT-0, version 12 (R12): Since Category 1 (CAT-1) has low transmission capacity, a new category has become necessary to support the new challenges of MTC and IoT. The Category 0, or CAT-0 of R12 have lower hardware complexity when compared to CAT-1 [101]. * CAT-M, version 13 (R13): With the objective of new complexity reduction techniques, CAT-M was proposed in R13 [8]. * CAT-N, version 13 (R13): As major IoT devices, or MTCs, typically deal with long distance coverage to transmit few data bytes, CAT-N has been incorporated into the LTE specifications to support the required functionality. The main objectives of CAT-N is to improve distance coverage reducing its complexity and, as a consequence, a greater battery life cycle [89]. In order to fulfill the prerequisites necessary for LTE networks to be able to attend MTC services, 3 GPP worked on the R12 to minimize power consumption and cost with new data traffic profile, hardware simplification and spectrum adjustments. LTE OFDMA modulation and SC-FDMA coding is maintained. 3 GPP LTE Release 13 is being studied for IoT applications due to the fact that it uses a lower transmission bandwidth than the LTE terminals of previous releases. Release 13 works with freedom of spectrum occupancy within the LTE carrier using only 1.4 MHz of the 20 MHz available LTE carriers. A 15 dB link budget enhancement can guarantee longer distance coverage and better obstacle penetration factor [89; 102; 103; 89; 104]. LTE eMTC technology presents the same layers observed in the LTE protocol stack, shown in Figure 20. Briefly, the layers can be described as follows [90]:  
* Non-Access Stratum (NAS): Works between Device and Network Core and is used for control, authentication and mobility management purposes. * Radio Resource Control (RRC): In an eNodeB base station, this layer takes handover decisions, sends broadcast messages containing system information and controls the measurements of the device (UE) parameters. * Packet Data Control Protocol (PDCP): Responsible for the compression and decompression of the user IP packet headers. It also performs data encryption, both on the user data plane and on the control plane data plane. * Radio Link Control (RLC): Used to format and transport data between the Device and eNodeB base station, transfer upper layer Protocol Data Units (PDUs), error correction, concatenation, segmentation, and reassembly of Service Data Unit (SDUs). * MAC: It performs physical transport channel mapping.