Digital signals are also called square waves . The internal data bus is a Parallel bus. In Parallel data transmission each bit uses a separate wire. This serves to inform the receiver when data is available. In addition, another line may be used by the receiver to inform the sender that the data has been used, and it's ready for the next data.
Parallel transmission is obviously faster than serial, because more than one bit is sent at a time.
Parallel transmission is good only for short links, and examples are found in all computers. The address, data and control buses which interface the processor to other peripherals inside the computer are all parallel buses. Because of the wide number of hardware manufacturers, a standard is essential in order to connect one computer to another computer of a different type. There are recognized and widely accepted standards governing how data is to be transmitted, whether asynchronously, parallel, or synchronously. Standards govern the format of the data, and also specify the hardware details like voltages to use, bit durations, speeds etc.