An abstraction layer (or abstraction level) is a way of hiding the implementation details of a particular set of functionality. Software models that use layers of abstraction include the OSI 7 Layer model for computer network protocols, the OpenGL graphics drawing library, and the byte stream input/output (I/O) model originated by Unix and adopted by MSDOS, Linux, and most other modern operating systems. In the Unix operating system, most types of input and output operations are considered to be streams of bytes being read from a device or being written to a device. This stream of bytes model is used for file I/O, socket I/O, and terminal I/O in order to provide device independence. In order to read and write to a device at the application level, the program calls a function to open the device which may be a real device such as a terminal or a virtual device such as a network port or a file in a file system. The device's physical characteristics are mediated by the operating system which in turn presents an abstract interface that allows the programmer to read and write bytes from/to the device.