The free-run accuracy must be considered in the case of ATM because the interface function is not higher (as is the case with PMA with +/- 50 ppm and BA with +/- 100 ppm) than the free-run accuracy standard of the clock generators with +/- 32 ppm for stratum 4 or +/- 50 ppm for ISO 11 573 class III.

The STM1 or OC3 interface has been designed for frequency deviations of +/- 15 or 20 ppm.

The OpenScape 4000 standard clock generators meet these requirements in free-run mode. The trimming range is also affected in that it is automatically limited by an ATM interface board. Synchronization on a (drifting) local exchange could be stopped with the error message "BAD CLOCK" to prevent a private ATM interface from failure.