The opposite is isochronous or synchronous serial communication, where a separate clock signal is transferred, or can be retrieved from a self-clocking line code method or from a bit synchronization bit pattern in the beginning of large data frames.
The opposite is isochronous or synchronous serial communication, where a separate clock signal is transferred, or is retrieved from a self-clocking line code method such as 8B/10B encoding used in Firewire and Manchester code used in Ethernet. Alternatively it may be retrieved from a bit synchronization bit pattern in the beginning of large data blocks or data frames.