This part of IEC 61290-4 applies to erbium-doped fibre amplifiers (EDFAs) and optically amplified elementary sub-systems. It applies to OAs using active fibres (optical fibre amplifiers, OFAs), containing rare-earth dopants. These amplifiers are commercially available and widely deployed in service provider networks. The object of this part of IEC 61290-4 is to provide the general background for EDFA transients and related parameters, and to describe a standard test method for accurate and reliable measurement of the following transient parameters: • Channel addition/removal transient gain overshoot and transient net gain overshoot • Channel addition/removal transient gain undershoot and transient net gain undershoot • Channel addition/removal gain offset • Channel addition/removal transient gain response time constant (settling time)