In long-haul fiber networks, rings carry traffic such that any single fiber cut still leaves a working path. Building a ring through a chosen set of POPs (points of presence) means finding a directed Hamilton circuit on the eligible-fiber graph. Carriers like AT&T, NTT, and Lumen run nightly optimization to refresh ring designs as link costs change. The same problem hides inside robotic arm tours (visit each weld, return to home), PCB drilling sequencing (each via, exactly once), and the canonical traveling salesman problem (Hamilton + edge weights — TSP is just Hamilton with an objective). Erlang's actor heritage is no accident here: AT&T's Bell Labs heritage and Ericsson's telecom-switch design both leaned on Erlang precisely for routing problems shaped like this.
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Telco failover routing
When a telecom carrier provisions a SONET/OTN ring, it is solving a Hamilton circuit. Every site on the ring, exactly once, with a directed traffic flow.