The USB [Universal Serial Bus] specification defines the mechanical and electrical characteristics, as well as the protocol layer of the interface. The cable and connector are precisely defined. USB defines two types of hardware, hubs and functions, as well as terminal devices. In theory, up to 127 devices can be linked together in a star bus topology in which the only limitation is the 7-bit address. Connections are possible between the hub and function or terminal device or between hub and hub, whereby direct connections between terminal devices are not possible.