X10 is definately a POWER LINE standard. NOT Twisted pair.

Most security systems use twisted pair which has to be separately installed, is expensive (due to the labour) and can be cut very easily.

X10 is also not reliable (as mentioned by Alfredo).

Write your own protocol (which is intrinsicaly more secure because only you know what it is) and use a decent error checking/reduction method, eg. always start each byte with a preamble that must be the right length or the receiving device ignores the whole byte, and send the same byte several times to make sure it gets through.

Perhaps another standard you may look at to get more ideas would be the DCC (Digital Command Control) standard for digital control of model trains. You can find this standard online at http://www.nmra.org/standards...ndards-DCC