This field reports the number of link down events on the port, exposed here as cienaCesPmExtendedTxRxStatsBinLinkDown. On this Ciena CES port's PM data, each increment marks a transition of this specific port's link state to down, so pair this with LinkUp on the same port — equal counts moving together mean the link is genuinely cycling rather than one side simply being stuck; and this sits in the currently active extended Tx/Rx interval bin — its sibling StartTime/ClearTime fields mark exactly which window is being counted, so a change here reflects activity happening in the live collection interval, not something already filed away in history. For example, link-down transitions climbing from 0 to just 0 on this port would already be worth a closer look, since a healthy link should hold this counter at zero.