  Life-critical warning message, abbreviated as warning message, is a special
event-driven message that carries emergency warning information in
Vehicle-to-Everything (V2X). Three important characteristics that distinguish
warning messages from ordinary vehicular messages are sporadicity, crowding,
and ultra-time-criticality. In other words, warning messages come only once in
a while in a sporadic manner; however, when they come, they tend to come as a
crowd and they need to be delivered in short order. This paper puts forth a
medium-access control (MAC) protocol for warning messages. To circumvent
potential inefficiency arising from sporadicity, we propose an override network
architecture whereby warning messages are delivered on the spectrum of the
ordinary vehicular messages. Specifically, a vehicle with a warning message
first sends an interrupt signal to pre-empt the transmission of ordinary
messages, so that the warning message can use the wireless spectrum originally
allocated to ordinary messages. In this way, no exclusive spectrum resources
need to be pre-allocated to the sporadic warning messages. To meet the crowding
and ultra-time-criticality aspects, we use advanced channel access techniques
to ensure highly reliable delivery of warning messages within an ultra-short
time in the order of 10 ms. In short, the overall MAC protocol operates by
means of interrupt-and-access.
