The route clocking source should be investigated or determined in the case of non-clock-transparent routes. For leased lines, this can be the master for both systems at the connection's endpoint. However, make sure that the connection always has an external clocking source. Avoid a return to clock-transparent operation during a failure without informing the network management centers involved, since this procedure results in a forbidden clock loop that goes unnoticed at first. Structured leased lines always have an external clocking source. Check the clocking source and routes to which the clock is distributed for every multiplexer in the network.

External clocking can generally be assumed when using SDH technology. The network synchronization plan is then drawn up in compliance with the switching and the transmission equipment. Check for SDH multiplexers if the interfaces are set to "retiming" and if the "retiming" is to be accepted by the OpenScape 4000 system. This function requires the connection of the SDH multiplexer's clocking port (T4) to the OpenScape 4000's front reference port using a special cable. For information on the advantages of transferring the "retiming" function to the PBX, see Section 46.5.7, "SDH".