jslove@starch.enet.dec.com ("J. Spencer Love; 237-2751; SHR1-3/E29 06-Nov-1990 1006") (11/06/90)
> When there isn't an alarm the phone company polls your box about twice a > minute to make sure that he's alive and that the phone line is uncut. I can immediately see a way to defeat this. Short the line near the house. If the loop is at all long, the resistance of the wires between the short and the central office will fairly reliably prevent detection of the short, which will look like the line is off-hook. I *guarantee* that an alarm box squeal will not get past a short. One thing that would get past a nonmalicious short in the event of an alarm would be to ground (unbalance) one side of the line (a similar technique is used by pay telephones), but once the short is in place the wires between the short and the house could be safely cut. In fact, a 600 ohm resistor could thwart both short and open detection. No special gear or knowledge is needed to use this strategy, which can be applied at the side of your house unless you also wire your yard for intruder detection (help, officer, there's a killer poodle pissing in my flower bed). A separate security company leased line, on the other hand, has at least the possibility of being concealed. In the ISDN era, special hardware will be required to simulate a working line, since a continuous digital dialogue is possible over the 16Kbit call-control channel even when two other conversations are in progress over the same wires. A short and cut wires look the same at the link level. To defeat Mission Impossible burglars, public key encryption can be used between the house and the security company, a la "smart cards". The security company can require a different response to each challenge and the transformation can be kept secret. However, it'll be several years (at least) before ISDN is widely available in the U.S. to residential subscribers. -- Spencer