A read-write enumeration, defaulting to fcs16, setting the size of the Frame Check Sequence used on this SONET interface, either 16 or 32 bits. A larger 32-bit FCS provides stronger error detection than the 16-bit default, which matters for high-throughput links where undetected bit errors become statistically more likely, but both ends of the link must agree on the FCS size used. An admin running a high-speed SONET link that needs stronger error detection might set this to fcs32(2), provided the far-end equipment supports the same setting.