The accuracy of free-run mode in clock generators for normal ISDN PABX networks is important for systems which are to operate as the master of a digital island, i.e. for plesiochronous or pseudosynchronous private networks. As free-run accuracy does not have to be very high in the first case, the small clock generator with +/- 32 ppm is sufficient. The standard clock generator (class 38) offers +/- 10 ppm and the networking clock generator (class 68, 78: +/- 5 ppm with AICB or with an external clock box: 1 x 10 to the power of minus nine to 10 to the power of minus eleven). This accuracy is guaranteed for fifteen years under all operation conditions. Note the exact definition of free-run accuracy when comparing models.