On digital equipment, serial data can be sent from one component to another over a single wire pair. Serial data is sent one piece at a time until all of the data has been sent to the receiver. Parallel data transmission, on the other hand, sends the data in groups. A good example of serial data has been noded already.

Serial data can be sent using two different modes.

Synchronous Mode

In a synchronous serial data transmission, the sending equipment sends a synchronizing message to the receiver. The receiver matches the clock signal. Data is then sent serially over the medium until the entire message is received. The receiver breaks the data down into data words according to its synchronized clock.

    Time 0 1 2 3 4 5 6 7 8 9 A B C D E F
INITIALIZE 1 0 1 1 1 0 1 0 0 1 1 1 1 0 1

After the initialize command, the sender and receiver clocks are ticking off intervals at the same time. The receiver counts off time intervals and breaks the data down into words. Networks use synchronous serial data transmission.

Asynchronous Mode

Asynchronous mode serial data transmissions do not synchronize the clocks of the sender and the receiver. The sender encodes a start and stop bit into the message.

    Time 0 1 2 3 4 5 6 7 8
    SEND X 1 0 1 1 0 1 1 Y

In this instance, the the X is the start bit and the Y is a stop bit. Since asynchronous transmissions are relatively short, the clock for the sender and receiver are assumed to be accurate enough for short messages. The word shown above is sent bit by bit until the entire message is transmitted. The receiver, when the start bit arrives, checks the data to see if it is a 1 or a 0 at set intervals. When the stop bit arrives, the message is complete and (optionally) the parity is checked for errors.

Telephone modems use asynchronous transmissions.

In a side-by-side comparison, synchronous transmissions are faster because they do not have to send a start and stop bit.