In short:
Asynchronous transmission uses start and stop bits/signals for every byte send. If no data is beeing send, the line is not used. Normally used in low-speed applications.
Synchronous transmssion sends continuously signals over the line with predeterminated bitcombination if it isnt data. Always a signal that is used for sync. Even if there arnt data to send, a predetermend bitpattern is send. Normally used i high-speed applications.
To sum up:
Asynchronous recevier runs clock async/unsynched with respect to tthe incoming signal.
Synchronous receciver operats in sync with the recevied signal.
This communication is about the encoding over a line.
PPP is an encapsulationprotocol that can be used on both asynch and synch transmission. PPP is a data link layer protocol of OSI. This is all about how to transfer data over a line.
