  Connected vehicles are expected to play a major role in the next future to
improve safety and traffic efficiency on the road and short-range technologies
have been defined to enable the direct exchange of information. To this aim,
two solutions are currently the subject of a debate that goes beyond the
technician, i.e., IEEE 802.11p and sidelink cellular-vehicle-to-anything
(C-V2X). Tested and mature for deployment the first, possibly more efficient
the second. In both cases, one of the main aspects is the management of channel
congestions, which can cause serious packet losses and have a critical impact
on the reliability of applications. Congestions can be managed through
different approaches, including the control of transmission power, packet
generation frequency, and the adopted modulation and coding scheme. Congestion
management has been well studied in IEEE 802.11p, with consolidated algorithms
included in the standards, whereas it appears somehow as a new topic looking at
C-V2X. In this work, a review of the main congestion control mechanisms and a
discussion of their applicability and efficiency in the two technologies is
provided. This topic is addressed without focusing on specific algorithms and
with the aim to provide general guidelines as a starting point for new
proposals.
