When a stream of bits, or symbols is to be transmitted over a noisy channel some bandwidth must be devoted to synchronization. Many techniques hide this pretty well but it can always be found. There are at least these three obstacles to synchronization:
Inbound channels may not be protected against, or may even be required by the application. In Keykos it may be possible in some applications to isolate as well as confine the transmitter.
The most obvious way to synchronize on many systems is with the system clock. Computing or waiting for one second is a good way to send a bit it both ends have access to a clock. It is feasible on some systems to deny the system clock to the covert transmitter.