  Wireless networks have become the main form of Internet access. Statistics
show that the global mobile Internet penetration should exceed 70\% until 2019.
Wi-Fi is an important player in this change. Founded on IEEE 802.11, this
technology has a crucial impact in how we share broadband access both in
domestic and corporate networks. However, recent works have indicated
performance issues in Wi-Fi networks, mainly when they have been deployed
without planning and under high user density. Hence, different collision
avoidance techniques and Medium Access Control protocols have been designed in
order to improve Wi-Fi performance. Analyzing the collision problem, this work
strengthens the claims found in the literature about the low Wi-Fi performance
under dense scenarios. Then, in particular, this article overviews the MAC
protocols used in the IEEE 802.11 standard and discusses solutions to mitigate
collisions. Finally, it contributes presenting future trends in MAC protocols.
This assists in foreseeing expected improvements for the next generation of
Wi-Fi devices.
