The limit of four levels has been introduced so that double faults on the synchronization path can be sustained with no loss of synchronism. This limitation is justified by the fact that the drift must not become too great; the drift is not attenuated in the different stages, but can in fact even be increased.

There are other important (quality) criteria independent of network synchronization, which can lead to a limitation of levels.

The "round trip delay", for instance, must be taken into account without any special measures for echo suppression. Then a maximum of five levels is reasonable.