  This paper focuses on achieving optimal multi-user channel access in
distributed networks using a reconfigurable intelligent surface (RIS). The
network includes wireless channels with direct links between users and RIS
links connecting users to the RIS. To maximize average system throughput, an
optimal channel access strategy is proposed, considering the trade-off between
exploiting spatial diversity gain with RIS assistance and the overhead of
channel probing. The paper proposes an optimal distributed Carrier Sense
Multiple Access with Collision Avoidance (CSMA/CA) strategy with opportunistic
RIS assistance, based on statistics theory of optimal sequential observation
planned decision. Each source-destination pair makes decisions regarding the
use of direct links and/or probing source-RIS-destination links. Channel access
occurs in a distributed manner after successful channel contention. The
optimality of the strategy is rigorously derived using multiple-level pure
thresholds. A distributed algorithm, which achieves significantly lower online
complexity at $O(1)$, is developed to implement the proposed strategy.
Numerical simulations verify the theoretical results and demonstrate the
superior performance compared to existing approaches.
