  Vehicle-to-vehicle (V2V) communication plays a pivotal role in intelligent
transport systems (ITS) with cellular-vehicle to everything (C-V2X) and IEEE
802.11p being the two competing enabling technologies. This paper presents
multi-dimensional discrete-time Markov chain (DTMC) based models to study the
medium access control (MAC) layer performance of the IEEE 802.11p standard and
C-V2X Mode 4. These models are coupled with an appropriate DTMC based queuing
model, and traffic generators for periodic cooperative awareness messages
(CAMs) and event-driven decentralized environmental notification messages
(DENMs). Closed-form solutions for the steady-state probabilities of the models
are obtained, which are then utilized to derive expressions for several key
performance metrics. An application for a highway scenario is presented to
provide numerical results and to draw insights on the performance. In
particular, a performance comparison between IEEE 802.11p and C-V2X Mode 4 in
terms of the average delay, the collision probability, and the channel
utilization is presented. The results show that IEEE 802.11p is superior in
terms of average delay, whereas C-V2X Mode 4 excels in collision resolution.
The paper also includes design insights on possible future MAC layer
performance enhancements of both standards.
