difference between the peak value of the reception probability
and the one obtained using the lowest possible value for CS t
( −95 dBm in this case) can be significant, reaching almost
10% in the scenario with the highest density. Second, the
optimal carrier sense threshold varies with the vehicular
density, increasing when the number of neighbors becomes
larger. This confirms the ideas formulated earlier in this
chapter and shows the necessity of an adaptive mechanism
for physical carrier sense control.
In this chapter, different techniques for congestion control
in vehicular ad hoc networks have been described. These
mechanisms can be divided into two classes: protocol agnostic
solutions, which could work with any MAC protocol, and
IEEE 802.11 dedicated solutions, designed specifically for the
protocols in this family.
The chapter begins with a discussion on generic
mechanisms, such as adaptive beaconing frequency, data rate
control or transmission power adjustment. The most
significant studies in each category are presented and the
feasibility of these ideas is commented.
However, the focus of the chapter is on two parameters
with a central role in IEEE 802.11, namely the CW of the
back-off mechanism and the carrier sense threshold.
Adaptive mechanisms for both of these parameters are
presented and their performance is compared with the
current version of the IEEE 802.11p standard. The
improvement brought by choosing the correct value for these
parameters is an important argument supporting the design
of efficient mechanisms and their integration in the IEEE