This parameter selects how the Circuit Emulation Service (CES) synchronizes its timing to the network—either by using standard ATM timing or proprietary loop timing when the line itself is running in loop mode. Network engineers set this based on the circuit type—proprietary loop timing accommodates older TDM equipment that needs to recover clock from the data stream itself, while standard timing works with modern ATM. Setting this to "loop timing" mode on an E1/T1 emulation circuit tells the CES to extract timing from the incoming signal rather than the ATM network, required for some legacy telephone systems.