From my experience, if you're running a back-to-base alarm system, you shouldn't need a filter/splitter.
If you have a modem router like mine, basically all you need to do is plug in all the wires and you're set.
A seperate router requires you to first connect the ADSL line to the modem, then connect the modem to the router via USB or Ethernet. Then connect your computers to that router via antenna, USB or Ethernet, and you can start surfing the web.