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BOCA PRO16 Reference Guide
Callback Security
As mentioned earlier, your modem allows you to specify a modem
command with every user name and password. This feature allows you
to implement modem callback. Callback means that after a caller has
successfully connected, your modem will hang up and then dial the
caller.
Even if an unauthorized caller successfully connects to your modem,
that caller will have to be using the phone line which your modem is
going to call back.
Three commands are required to implement callback. First, you must
hang up your modem with a H command.
Before your modem can call the caller back, it must wait for the caller’s
modem to hang up and return to local command state. S Register 27
contains a value representing the number of seconds your modem will
wait before commencing dialing. The default value for this register is 0
— your modem will begin to dial as soon as it receives a dial command.
It is necessary, therefore, to place another value in S Register 27 to allow
the caller’s modem sufficient time to hang up before your modem begins
to dial. A 30-second delay should be sufficient time to allow most
modems to hang up, so place an S27=30 in the command.
Using a 30-second delay means the telephone exchange, in most cases,
will disconnect the call. This ensures unauthorized callers cannot ‘fool’
callback security by not hanging up after your modem hangs up.
Alternatively, contact your telephone company — some telephone
companies offer an automatic disconnect facility, which assumes a caller
cannot remain off-hook after the modem has hung up.
Place a D command and the caller’s phone number in the command.
Assuming the caller’s phone number is 1234567, you will now have a
command which looks like this:
H S27=95 D1234567
After the user has called back and has completed the call, your modem
will hang up, automatically issue an ATZ command to restore all the
stored settings and return to local command state, ready to answer
incoming calls.
Security and Encryption
Security &
Encryption
pro16ref.pm6 6/14/96, 2:36 PM40
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