ETH-TST (Ethernet Test Signal) and ETH-LCK (Ethernet Lock Signal) protocols are defined in Y.1731. ETH-TST is used to perform one-way on-demand in-service or out-of-service diagnostics tests. This includes verifying bandwidth throughput, frame loss, bit errors, etc.

ETH-LCK is used to communicate the administrative locking of a MEP and consequential interruption of data traffic forwarding towards the MEP expecting this traffic. It allows a MEP receiving frames with ETH-LCK information to differentiate between a defect condition and an administrative locking action at lower level MEP.