  Cellular networks have been engineered and optimized to carrying
ever-increasing amounts of mobile data, but over the last few years, a new
class of applications based on machine-centric communications has begun to
emerge. Automated devices such as sensors, tracking devices, and meters - often
referred to as machine-to-machine (M2M) or machine-type communications (MTC) -
introduce an attractive revenue stream for mobile network operators, if a
massive number of them can be efficiently supported. The novel technical
challenges posed by MTC applications include increased overhead and control
signaling as well as diverse application-specific constraints such as ultra-low
complexity, extreme energy efficiency, critical timing, and continuous data
intensive uploading. This paper explains the new requirements and challenges
that large-scale MTC applications introduce, and provides a survey on key
techniques for overcoming them. We focus on the potential of 4.5G and 5G
networks to serve both the high data rate needs of conventional human-type
communications (HTC) subscribers and the forecasted billions of new MTC
devices. We also opine on attractive economic models that will enable this new
class of cellular subscribers to grow to its full potential.
