Hmm, go and read chapter five of lcamtuf's book "Silence on the Wire," part of which is incidentally available as a book sample: http://www.nostarch.com/downlo

Hmm, go and read chapter five of lcamtuf's book "Silence on the Wire," part of which is incidentally available as a book sample: http://www.nostarch.com/download/silence_ch05.pdf http://www.nostarch.com/download/silence_ch05.pdf wherein he gives this scenario a fair analysis (discussing signal encoding schemes, timing, and a DIY kit).