This Recommendation presents an overview of the operational measurements, defined in Recommendation E.438, that can be performed by test calls and used to assess the performance of N-ISDN 64 kbit/s circuit-switched UDI bearer services. Correspondent measurements (excluding transmission quality measures), performed by exchanges on live traffic, are specified in Recommendation M.3650. These Recommendations should be used by administrations and by ROAs for monitoring and improvement analysis of ISDN services and networks in operations. The measured values can be used both to set guaranteed performance levels in Service Level Agreement with the customers and Service Quality Agreements with other administrations, and to verify, analyse and improve the performance level currently achieved.