(see Figure 64)

Metering begins in the local exchange (level=0) and commences at level=1 since most private networks are connected to the public switched network and receive their synchronous clock from it. All systems with a digital connection to the local exchange that they use for synchronization are assigned level = 1.

All systems which are connected to level = 1 systems that they used for synchronization are assigned to hierarchy level = 2.

All systems which are connected to level = 2 systems that they use for synchronization are in hierarchy level = 3.

All systems which are connected to level = 3 systems that they use for synchronization are in hierarchy level = 4.

This can continue up to level 4 if systems on the same level are to be interconnected. The number of levels can exceed four in systems that are simply "cascaded". The OpenScape 4000 standard clock generators allow about seven or eight levels without any loss of synchronism under the usual boundary conditions e.g. jitter, drift and phase hits. The configuration of more than five levels is not recommended, however, as propagation times in the network are excessive for connections to public switched networks.

Once all systems have been assigned to a level, the resulting number of levels must be determined: If there are more than two levels with tie trunks, networking clock generators should be used on the first level. If there are more than three levels with tie trunks, the network clock generators should be used on the first and second level. If the systems are only cascaded without tie trunks, a network clock generator should be used from the fifth level in the first level and on the sixth level in the first and second level.

( --> Selection of clock generators)