This sets whether this port frames its optical signal as SONET or SDH. SONET and SDH use different overhead byte conventions despite similar underlying rates, so this setting on this specific port must match what the connected equipment or carrier network actually uses, or the two ends will fail to recognize each other's framing even though the raw optical signal is otherwise compatible. A circuit terminating on European carrier equipment would typically need this port set to sdh, while a North American circuit would use sonet, since mismatching the two prevents the ports from framing correctly against each other.