  We analyse the performance of the IEEE 802.11bd MAC protocol, with Enhanced
Distributed Channel Access (EDCA) and repeated transmissions, in terms of the
MAC access delay of packets pertaining to safety-related events. We outline
Markov chain models for the contention mechanism of priority-based access
categories, and derive the associated steady-state probabilities. Using these
probabilities, we characterise the delay experienced by the packet in the MAC
layer. Further, we characterise the reliability of the protocol in terms of the
likelihood that a packet is delivered within a critical time interval.
Numerical computations are conducted to understand the impact of various system
parameters on the MAC access delay. The analysis indicates that the MAC access
delay depends on various system parameters, some of which are influenced by the
traffic scenario and nature of safety-critical events. Motivated by this, we
used our analysis to study the delay and reliability of the 802.11bd MAC
protocol specific to the context of platooning of connected vehicles subject to
interruptions by human-driven motorised two wheelers. We observe that while the
delay performance of the protocol is as per the QoS requirements of the
standard, the protocol may not be reliable for this specific application. Our
study suggests that it is desirable to co-design vehicular communication
protocols with prevalent safety-related traffic applications.
