Age of Information for Frame Slotted Aloha

Frame slotted Aloha (FSA) is the de facto MAC layer standard protocol in many ultra-low-power IoT applications, such as Radio Frequency Identification (RFID) and Machine to Machine (M2M) communications. As the age of information (AoI) is an emerging and critical metric for quantifying the freshness of the status update information collected in time-sensitive IoT applications, systematic analysis of AoI for FSA is called for. However, very limited work has been done on this topic despite its both theoretical and practical implications for the operation and optimization of FSA. To fill this void, this paper delivers a comprehensive analysis of AoI for four versions of FSA, namely synchronous and asynchronous FSA with and without retransmission. The core technique of our analysis is to model the AoI for FSA as Markov chains to derive statistics on the delay and inter-delivery time. Our central results consist of the lower bounds of AoI, the exact AoI expressions in the four FSA protocols and the optimum frame length for the AoI of FSA. Our analysis reveals the impact of the arrival rate and the protocol parameters on AoI, and also shows that the retransmission would improve AoI when the arrival rate is small.