Loopback is a common operation in routine maintenance and troubleshooting.

In an outloop (external loopback), the transmitted signal is sent out of the NE and then looped back into its receive port, so signals flow into the local WDM NE.

Hardware loopback on optical ports uses pigtail jumpers, and on electrical ports uses self-loop cables, both performed manually.

When performing optical hardware loopback, adding an optical attenuator is necessary to prevent excessive return power from damaging the transmitter.