Constant MAX_CAD_ATTEMPTS 

pub const MAX_CAD_ATTEMPTS: u8 = 5;

CAD attempts a frame gets before it is dropped, from Channel Access § Backoff Procedure: one initial attempt and four retries. Public because every UMSH transmitter shares the procedure — a host relaying frames onto a segment contends with the same neighbours as the MAC does, and a second opinion about how long to persist would just be a second, wrong answer.