Modems send letters of the alphabet as series of eight zeroes and ones. The modem
always precedes each character with a zero. The transition to zero helps resynchronize
the clock of the receiving modem so it can sample at the exact centre of each incoming
bit. Modems idle by sending ones (marking). The zero (space) bit signifies the start of
a character. See the essay More Than You Wanted To Know About RS232C .