There is no clock directly encoded in the signal (in contrast with signaling
protocols such as Manchester encoding) – the start transition provides the
only temporal information in the data stream. The transmitter and receiver
each independently maintain clocks running at (a multiple of) an agreed frequency
– commonly, and inaccurately, called the baud rate. These two clocks
are not synchronized and are not guaranteed to be exactly the same frequency,
but they must be close enough in frequency (better than 2%) to recover the
data.