A read-only counter tracking how many times a valid-length packet arriving at the port contained at least one invalid symbol, incrementing at most once per carrier event and unaffected by collisions elsewhere on the repeater. Rising symbol-error counts point to a physical-layer problem such as electrical noise or a marginal cable or transceiver on that specific port, distinct from collision-related issues. If this counter climbs steadily on port 6 while collision counters stay flat, an admin would suspect a bad cable or connector on that port rather than a topology issue.