In information systems and information theory, digital data, as a sequence of discrete symbols each of them able to take on only a certain number of states from an alphabet, such as digits or letters, is said to be “digital data”. An example is a book, which may contain a string of alphabetic characters. Digital data can also be a network of interconnecting nodes in a computer network itself. In computer engineering, digital data is said to be “state machines” – that is to say, they are the representations of certain real state functions on a computer hardware device. A good representation of digital data therefore is a finite series of states and transformations from one input to another output in a finite succession.