Layer 1 devices view network topologies as physical (as opposed to logical). Physical topologies include a bus, ring, or star. The data transfer medium has a physical and electrical component. This includes cables, jacks, patch panels, punch blocks, hubs, and multi-station access units (MAUs). Data is represented as binary expressions on a computer network. At the physical layer, data is represented as binary (a series of ones and zeros). A binary expression is a combination of bits, where a bit is either a 1 or a 0. These bits are represented by voltage (copper wiring) or light (fiber optic cable). When voltage is present or absent on a wire, it represents either a binary 1 or binary 0.