The RB411U is a versatile yet very small device. It includes all - a miniPCI slot (for 802.11 wireless cards), a USB 2.0 port and a miniPCIe slot for connecting a 3G card (does not support 802.11 wireless cards). A SIM card slot for 3G is also present on the RB411U.
RB411U includes RouterOS - the operating system, which can be a router, firewall, bandwidth manager, a mobile 3G router, and more - all at the same time.
You can use the USB port to attach a secondary storage device, or a USB 3G modem. The RB411U mounting holes are in the same location as other RB411 models, so you can easily mount it into any supported outdoor case/antenna.