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Every item from text to images to sound can be reduced to a numerical representation. These numbers can be written in any base but computers work best with binary (base-2). These numbers must then be converted to a physical form so that they can be manipulated and moved around. This form is usually as electrical, magnetic or electromagnetic impulses. This process is known as data transmission

Data transmission occurs between a transmitter and receiver over some transmission medium. The connection between the transmitter and receiver is known as a communication channel. Transmission media can be guided or unguided, but in both cases data is still transmitted as electromagnetic waves. In guided media, the data follows a physical path set by cables of some sort, i.e. optical fibres or coaxial cables. Unguided media does not guide the signals, but provide means of transmission, such as propagation through air, vacuum or sea water in the form of radio waves of varying frequency.

The transmitted signals decrease in strength over larger distances. If this is not corrected by some means, it makes the data harder to recover.

In serial data transmission, single bits are sent one after another on a single wire by varying the voltage on this wire. Serial data communication is used to network two computers using a crossover cable to join the network card in one computer with the network card in another computer. A crossover cable is a particular kind of two-way serial cable. Two-way communication requires two signal lines. The Universal Serial Bus (USB) is another example of serial data transmission.

The circuit to the left is a simple circuit for sending binary digits serially. In its current position, the lamp receives 5 volts, but in the other position, it will receive 0 volts.

Serial data transmission is used for long-distance communication,