LoRaWAN is a wireless technology for Low Power Wide Area Network (LPWAN). Today, it is considered as one of the most serious alternatives for IoT thanks to its low cost, low power consumption equipments and its open business model. LoRaWAN specifications proposes interesting solutions regarding MAC layer operations to deliver the best communication performances to connected things. Despite its crucial impact on the overall performances, few research consider the LoRaWAN MAC layer. This paper presents LoRaWAN MAC layer operations and services based on LoRaWAN Alliance technical specifications. It provides a description of MAC layer operations and modes in a thorough manner. In addition, it proposes an overview of recent studies related to LoRaWAN performances and stands out the major challenges to be addressed in order to enhance the performances of data exchanges.