A survey of hybrid MAC protocols for machine-to-machine communications

Machine-to-machine (M2M) communications are envisaged to present several interesting applications in the coming years. However, they have major setbacks hurdling their massive deployment. A major challenge is ensuring the network and wireless medium robustly accommodate the millions of devices which will be part of this communication paradigm while gracefully accommodating their diverse quality of service requirements. In this regard, medium access control schemes play a vital role. Traditional contention-based protocols are constrained in their performance under high loads while scheduling-based schemes may be faced with limited scalability and poor delay guarantees. Similarly, tree-based contention resolution schemes have some finite delay associated with them. Recently, hybrids of the aforementioned schemes have been studied in literature to solve the problems associated with traditional medium access schemes. This paper surveys the state-of-the-art hybrid schemes for M2M presenting a classification of recent protocols. Furthermore, we give a summary of major areas of interest such as problem solved, approach used and trade-offs. Salient differences in the protocols are identified with insights into opportunities for improvement. Our findings reveal general design guidelines and interesting future directions.