Black Box MD1276A, User Manual

Page: 36 / 289

Page 36
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 unauthorised 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 dialling. 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 unauthorised 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 been 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. The ATZ command will effect some settings - to save your configuration, issue
a &W after you initialize the modem.
+
UK Modems. Telephone exchanges in the UK may not hangup after 30 seconds.
Consult the supplier of your telephone line for a possible solution.
+ If security is enabled, the answering modem will not assert DCD or DSR until a
valid user name and password are received. If \Q5 is selected, CTS will not be
asserted until a valid user name and password are received.