This process has to be repeated for every frame, since they can vary in length. Other technologies can be asynchronous. Some other technique allows us to figure out where things begin and end. Every device is on the same clock. Cells are transmitted on regular intervals, full or empty.
Cells are always the same size.
Once we start together, we never need to synchronize again. Once set, always kept open and all cells follow the same route in the same order. ATM and emulation. ATM can emulate other protocols. This helps insure compatibility with older technologies.