Layer 1 of a trunk connection must be permanently active for continuous synchronization of a PBX. With OpenScape 4000 systems, interfaces to the clock master are only started after the clock generator reports sufficient synchronism. For instance, in France the S0 interface cannot be set to permanent activity. The timers on the trunk side, on the other hand, are set for insufficient intervals for the OpenScape 4000 system to respond in order to synchronize.

There may be cases in other networks where it is necessary for interfaces to the clock master to be immediately operable although synchronism is still insufficient. Therefore the AMO REFTA was prepared to assign the parameter "Ready despite asynchronous mode" to any interface if necessary. This parameter is not set by default. If it is set for an interface, this interface enters the "Ready" mode immediately, even after an HRST, without waiting for the "Synchronous" message from the clock generator.

If OpenScape 4000 systems are networked with S0 connections, the parameter SMD must be set to "NO" (despite priority = 0) in the AMO SYSPAR. This sets the layer 1 of this BA to permanent activity. Both sides of the connection must be set to "Trunk".