A read-write enumeration selecting the interface's transmit clock source: loopTiming uses the recovered receive clock as the transmit clock, while localTiming uses the device's own internal clock source. In a SONET network only one end of a link should be the timing master using localTiming, while the other recovers and reuses that timing via loopTiming, so setting this incorrectly on both ends can cause frame slips and errors. A CPE-side SONET interface would typically be set to loopTiming so it derives its clock from the carrier's network, while the carrier's own equipment provides localTiming.