  In the modern world, the connectivity-as-we-go model is gaining popularity.
Internet-of-Things (IoT) envisions a future in which human beings communicate
with each other and with devices that have identities and virtual
personalities, as well as sensing, processing, and networking capabilities,
which will allow the developing of smart environments that operate with little
or no human intervention. In such IoT environments, that will have
battery-operated sensors and devices, energy efficiency becomes a fundamental
concern. Thus, energy-efficient (EE) connectivity is gaining significant
attention from the industrial and academic communities. This work aims to
provide a comprehensive state-of-the-art survey on the energy efficiency of
medium access control (MAC) protocols for cellular IoT. we provide a detailed
discussion on the sources of energy dissipation at the MAC layer and then
propose solutions. In addition to reviewing the proposed MAC designs, we also
provide insights and suggestions that can guide practitioners and researchers
in designing EE MAC protocols that extend the battery life of IoT devices.
Finally, we identify a range of challenging open problems that should be solved
for providing EE MAC services for IoT devices, along with corresponding
opportunities and future research ideas to address these challenges.
