The moment a byte is written into SBUF, it is framed with the start and stop bits and transferred serially via the TxD line...
When the bits are received serially via RxD, the 8051 deframes it by eliminating the stop and start bits, making a byte out of the data received, and then placing it in SBUF...
As for as 8051 is concerned, The machine cycle frequency will be = 11.0592 / 12 = 921.6 kHz,
and 921.6 kHz / 32 = 28,800 Hz is frequency by UART to timer 1 to set baud rate...
(a) 28,800 / 3 = 9600 where -3 = FD (hex) is loaded into TH1
(b) 28,800 / 12 = 2400 where -12 = F4 (hex) is loaded into TH1
(c) 28,800 / 24 = 1200 where -24 = E8 (hex) is loaded into TH1
Notice that dividing 1/12 of the crystal frequency by 32 is the default
value upon activation of the 8051 RESET pin.
And you are correct too whenever there is overflow of Timer 1 [MODE-2 {8-bit auto reload mode}], it will transmit a bit of data..
28,800 Hz is frequency by UART to timer 1 to set baud rate...