csOc3CrcType is a read-write setting choosing between a 16-bit and a 32-bit CRC for frame checking on this OC3 port, and it only takes effect when the port is running Packet over SONET (POS). The two ends of a POS link must agree on CRC width or every frame will appear corrupted to the receiver, so this is a first check when a freshly configured POS circuit won't pass traffic despite a clean optical signal. For example, if a newly installed OC3 POS link to a partner router shows a stream of FCS errors, an admin checks whether csOc3CrcType matches the crc32 setting the partner is actually using.