The STMA board provides the first ATM interface in a system. If an STMA board is configured, the security equipment narrows the trimming range of the clock generator during startup to approx. +/- 15 to 20 ppm. The clock generator ensures that any private ATM interface behind a PABX system always works. The OpenScape 4000 clock generator can therefore cancel synchronization onto the master (e.g. the local exchange) and issue the message "Trimming range exceeded" although the frequency departure is still within the ISDN standards. As with a lot of error messages from the OpenScape 4000 clock generator, the error does not lie within the OpenScape 4000 system but outside it.

Together with the ATM, many new systems currently enter the market which originated from the data world and do therefore disregard "clocking" as it is required by the ISDN/PCM world. In future there will be more and more ATM systems also suitable for the CBO. The present ATM systems must be checked to see if they can transmit clocking for a CBO or not. If the result is positive, the next question arises: whether the ATM systems have a priority list for various ports, i.e. can be regarded as an own hierarchy level like a OpenScape 4000 system. If the result is negative, connections via the ATM system must be treated like analog connections, i.e. they are not mentioned in the network synchronization plan. This must lead to appropriate conclusions since there are fewer substitute routes, for instance, for synchronization. In future, however, there will be no differences between transmissions via PDH, SDH and/or ATM.