Answer Machine Defeat

[SteveGlover]

Hi!

Im developing system that comprises a Burglar alarm panel that connects to a standard phone line using a low speed modem. A remote PC (+modem) can phone up the panel for remote diagnostics, programming, etc.

In the UK its common to use the same phone line for both the Burglar alarm panel and fax machine (phone lines are expensive!), so I need a means of defeating the fax machine (stop it answering the call).

The usual way seems to be to put the fax machine on say 5 rings to answer. The PC can dial the phone line and let it ring just once or twice and then hang up. The fax machine won't answer the call (it's on 5 rings), but the Burglar alarm panel recognises the short ring sequence and starts a 60 second timer. The next time that the phone rings within this 60 second window it answers the call immediately and so defeats the fax machine.

In theory this should work, but the problem is that I can't figure out a reliable and robust way ensuring that the PC modem only lets the line ring once or twice before hanging up. I could time it on a typical modem and typical line, but its hardly likely to be reliable across different modems and different phone line providers. The PC could be connected to a mobile phone, which often take a little longer to establish a call.

Has anyone encountered the same problem before? Any ideas on how to determine how many times the "far end" has rung?

Thanks

 

[pansophic]

It is impossible to determine how many times that the far end has rung. That is because the ring back generator (the tone that the caller hears) and the ring generator (the sine wave sent down the line to ring the phone) are two asynchronous devices. They have nothing to do with each other, and may or may not be in sync. You can make an educated guess though. Especially if you do something like 3 consecutive calls within say 40 seconds. 1 or 2 rings per call. On your end, listen for 2 ring back tones, then disconnect to call again.

I have seen numerous scenarios to work out this problem, but the fax/data/voice switch boxes are by far the most ingenious. They actually answer the phone call, but continue to provide a ring back tone to the caller. They listen for the CNG tone that is generated by a calling fax (once every 4 seconds I believe) and then route the call based on this. If they hear the tone, then they generate a ring voltage toward the fax. If they don't then they generate a ring voltage toward the voice/modem device. They are highly effective when the caller is using the fax's autodial feature, but fail miserably when someone decides to dial manually. They also must be powered and aren't exactly free.


pansophic