If the Ethernet interface has a cable connected and is receiving a valid signal, the physical link shows an “up” status and there are no active alarms or defects. If the interface is not connected or is not receiving a valid signal, the output shows a physical link down status and an active LINK alarm and defect.
Source: http://kb.juniper.net/InfoCenter/index?page=content&id=KB26486
What is that 'valid signal'?
Best Answer
Link Integrity Test pulses.