  This article addresses the market-changing phenomenon of the Internet of
Things (IoT), which relies on the underlying paradigm of machine-to-machine
(M2M) communications to integrate a plethora of various sensors, actuators, and
smart meters across a wide spectrum of businesses. The M2M landscape features
today an extreme diversity of available connectivity solutions which -- due to
the enormous economic promise of the IoT -- need to be harmonized across
multiple industries. To this end, we comprehensively review the most prominent
existing and novel M2M radio technologies, as well as share our first-hand
real-world deployment experiences, with the goal to provide a unified insight
into enabling M2M architectures, unique technology features, expected
performance, and related standardization developments. We pay particular
attention to the cellular M2M sector employing 3GPP LTE technology. This work
is a systematic recollection of our many recent research, industrial,
entrepreneurial, and standardization efforts within the contemporary M2M
ecosystem.
