# MAC Layer Protocols for Internet of Things: A Survey
## 4 Discussion and Open Issues
### Open Issues  
In this rapidly evolving scenario, the emergence of new features, interoperability and performance aspects to be studied or validated is constant. Considering the discussion presented here and the characteristics of the protocols, the following open research topics are identified:  
* Regardless of the technology used in any solution or application, the information exposure always raises questions regarding each protocol security requirements, emphasizing that each solution requires different aspects of security. Beyond the scope of fixed or mobile WSNs security [123], from this study, the use of AES encryption by most protocols is clear. Some protocols that do not use it have some other methods to ensure data privacy at the MAC layer level such as proprietary coding or the vendor specific modulation type. Proprietary technologies like Sigfox and Wireless-Hart ensure this security by keeping their development features closed, limiting access to their techniques. Based on the information presented, it is clear that, in terms of security, there is a need to treat data at upper layers and have security embedded in the solution code, aspects that deserve more attention.
* The ability of terminals to roam between technologies is also an untreated topic. The design of hybrid systems such as multi-protocol gateways seems to be an unavoidable consequence of these protocol heterogeneities [124]. The rapid evolution of hardware solutions induces the development of multi-protocol platforms and systems [125], multi-band radio interfaces with adaptive and opportunistic techniques such as 5G. These protocols are required to be inter-operable in different layers, allowing performance comparisons. For example, in areas of roaming between different protocol technologies, systems and network operators, technical alliances necessary to obtain a good cross-border roaming, including the security aspect.
* With the current evolution, devices need several connectivity technologies. The same application can use more than one connectivity technology and terminal roaming between technologies can happen abruptly. Once the technology roaming aspect has become part of the scenario, the need for a verification of existing tools or solutions will arise to bring the interconnectivity standardization to the MAC layer. Thus, leaving the application layer and its applications, operating transparently and inherently to the protocol technology being used.
* Since there are such heterogeneity, connectivity, and interconnection, there are coexistence, dispute for resources, and interference. Although numerous works have been done on the performance of these protocols, little has been published on practical comparisons in identical scenarios. This comparison can point out the capabilities offered by each protocol and investigate their characteristics in a co-existent environment. As ISM bands are preferred by LP-WAN protocols, their coexistence in the same scenario calls for more studies so that more can be learned about the inter-technology interferences that can degrade their qualities.  
* For more critical environments and applications, such as healthcare and some industrial environments, reliability may be more important than other features. Highly reliable services may sacrifice other characteristics such as latency. Studies of data \"acknowledgment\" mechanisms demand flexibility in the packet sizes used and network responsiveness to these characteristics.
* Some techniques or applications, primarily for management platform purposes, are still dependent on broadcast and multicast functions inherited from IP networks that still need to be adapted or evolved. Broadcast, multicast, and control channels are also emerging aspects that are having their applications questioned without many specific studies.