digital audio players, radio-frequency identification tags, GPS, and interactive whiteboards. Mark Weiser proposed three basic forms for ubiquitous computing devices: Tabs: a wearable device that is approximately a centimeter in size Pads: a hand-held device that is approximately a decimeter in size Boards: an interactive larger display device that is approximately a meter in size Ubiquitous computing devices proposed by Mark Weiser are all based around flat devices of different sizes with a visual display. Expanding beyond those concepts there is a large array of other ubiquitous computing devices that could exist. Some of the additional forms that have been conceptualized are: Dust: miniaturized devices can be without visual output displays, e.g. micro electro-mechanical systems (MEMS), ranging from nanometres through micrometers to millimetres. See also Smart dust. Skin: fabrics based upon light emitting and conductive polymers, organic computer devices, can be formed into more flexible non-planar display surfaces and products such as clothes and curtains, see OLED display. MEMS device can also be painted onto various surfaces so that a variety of physical world structures can act as networked surfaces of MEMS. Clay: ensembles of MEMS can be formed into arbitrary three dimensional shapes as artefacts resembling many different kinds of physical object (see also tangible interface). In Manuel Castells' book The Rise of the Network Society, Castells puts forth the concept that there is going to be a continuous evolution of computing devices. He states we will progress from stand-alone microcomputers and decentralized mainframes towards pervasive computing. Castells' model of a pervasive computing system, uses the example of the Internet as the start of a pervasive computing system. The logical progression from that paradigm is a system where that networking logic becomes applicable in every realm of daily activity, in every location and every context. Castells envisages a system where billions of miniature, ubiquitous inter-communication devices will be spread worldwide, "like pigment in the wall paint". Ubiquitous computing may be seen to consist of many layers, each with their own roles, which together form a single system: Layer 1: Task management layer Monitors user task, context and index Map user's task to need for the services in the environment To manage complex dependencies Layer 2: Environment management layer To monitor a resource and its capabilities To map service need, user level states of specific capabilities Layer 3: Environment layer To monitor a relevant resource To manage reliability of the