The RB411U is a versatile but very small unit. It includes everything: a miniPCI slot (for 802.11 wireless cards), a USB 2.0 port and a miniPCIe slot to connect 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, 3G mobile router, and more - all at the same time.
You can use the USB port to connect a secondary storage device or 3G USB modem. The mounting holes of the RB411U are in the same location as the other RB411 models, allowing you to easily mount it in any outdoor box/antenna taken in.