Reducing Age-of-Information tails with bridge guarding during delayed reconfiguration in semi-persistent wireless scheduling

Age-critical wireless applications depend not only on average delay, but also on rare tail events that make status information stale when the network is under stress. In semi-persistent scheduling (SPS) systems, a controller may detect a burst of urgent traffic, but the new resource configuration is not applied immediately. This control-plane delay creates a short reconfiguration gap. During this gap, the system can accumulate adaptation debt, and the largest Age of Information (AoI) values can become concentrated in a small part of time. This study examines this effect through a mathematical and discrete-event model with two traffic classes: age-critical status updates and best-effort (BE) background packets. Age-critical sources use a per-source latest-update replacement queue, where a newer status update replaces an older pending one. BE packets are represented by an aggregate backlog served from the residual capacity left after age-critical allocation. An inertia index relates the effective adaptation time to the traffic burst time scale, and a tail-localization measure quantifies how much AoI tail risk appears inside reconfiguration gaps. The proposed bridge guard protects the command-to-activation interval with a short budget-limited increase in age-critical service. It reallocates a bounded share of the common service pool using information available when the reconfiguration command is issued without anticipating future bursts. Across the evaluated parameter grid, bridge guarding reduces the gap-conditioned AoI p99 from 130.55 ms to 112.27 ms and the tail risk ratio from 31.85 to 5.08, with a measured guard-time fraction of 1.27%. A paired-seed design and a common scheduler-level service model isolate the effect of pending-interval protection. The evaluation covers fixed scheduling, delayed reconfiguration, reactive guarding, periodic reconfiguration, static partial overprovisioning, an oracle upper bound, fast queue-plane service, earliest-deadline-first (EDF), and max-age scheduling. The results establish bridge guarding as a resource-efficient operating point for tail-reliable age-critical scheduling under non-negligible control-plane delay.