Gabinets are metallic structures that contain storage devices and a computer processor while protecting them from external electromagnetic radiation and damage. They come in various sizes for different uses like desktops, servers, or media centers/HTPCs and include drive bays for components like optical drives, card readers, and mainboards. Transfer speeds for ports have increased over generations of USB from 1.5 Mbps originally to up to 3.2 Gbps for USB 3.0. Expansion cards add functionality for graphics, audio, ports and networking.