csOc3PosSentFrameUnderrunAborted is a read-only 64-bit counter of the number of transmit frames on this OC3 port that were aborted because the transmit FIFO ran empty, or underran, before the frame finished sending. A rising underrun count points to the sending device not feeding data into the transmit buffer fast enough, typically an internal bus, memory, or CPU bottleneck on the local router rather than anything wrong with the fiber or the far end. For example, if csOc3PosSentFrameUnderrunAborted starts climbing during periods of high CPU load on the router, an admin would suspect the device itself can't keep the OC3 transmit FIFO fed, rather than suspecting a fiber or far-end problem.