This field reports the number of link down events on the port, exposed here as cienaCesPmExtendedTxRxTotalStatsBinLinkDown. 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 is the running grand-total counter for the extended Tx/Rx PM group, tallied since the bin's own StartTime (in practice since the PM instance was created or last reset), so treat it as a lifetime tally rather than a per-interval number. For example, link-down transitions climbing from 0 to just 32 on this port would already be worth a closer look, since a healthy link should hold this counter at zero.