Serial transmission sends one bit at atime over a single transmission line. The cost of communication hardware is considerably reduced since only a single wire or channel is require for the serial bit transmission which also slows the speed of transmission. Telephone lines use serial transmission for digital data, thus modems are connected to the computer via a serial port. A serial port is a socket on a computer used to connect the serial interface to a serial line or bus. A serial interface is a data channel that transfers digital data serially; it is typically implemented as a card that plug into a expansion slot on a computer motherboard. Serial interface have multiple lines but only one is used for data. An external serial bus carries serial data to any device connected to it, e.g. Ethernet. Serial transmission can be either synchronous or asynchronous.
Serial transmission technology is increasingly used for the transmission of digital data. A large number of up-to-date communications networks apply serial transmission. The numerous applications include computer networks for office communications, building and manufacturing automation, and finally, Internet. The transmission of a stream of bits from one device to another across a transmission media involves a great deal of corporation and agreement between the two sides. One of the most fundamental requirements is synchronization. The receiver must know the rate at which bits are being received so that it can sample the medium at appropriate intervals to determine the value of each received bit.
For achieving the desired synchronization, there are two approaches. Synchronous and asynchronous transmissions are two different methods of transmission synchronization.