Currently, network operators are increasingly migrating to an Ethernet - based IP backhaul network, because it can accommodate the soaring demand for broadband data service. However, Ethernet - based IP network is unlike SDH/SONET this legacy synchronous networks, it works asynchronously. Its clock is free running, normally it is around ±100ppm. However, the Ethernet - based IP network has to worked with legacy synchronous networks and technologies. It because the WAN network has been dominated by SDH/SONET these synchronous networks historically, and also the legacy mobile technologies such as the circuit switching to 2G/3G voice needs to be supported as well. Moreover, the new mobile technologies such as 4G/LTE base stations also need higher time and phase synchronisation accuracy. So in order to let Ethernet - based IP backhaul asynchronous network to work together with other synchronous networks, it introduces two methods to synchronise: SyncE; and PTP.
An Example Network with Mixed Synchronous and Asynchronos Equipment