More on modems...
Hayes, a long time ago, set down a standard
for modem commands, which started to be followed by a lot of
Modem Manufacturers and modems following this standard were called
‘Hayes Compatible’. This complete set of Modem Commands,
which follows the Hayes Standard, is called the AT Command set.
Here, AT stands for Attention. It basically
tends to say to the Modem, ‘OK, Mr Modem, you better pay
attention as I have some commands for you to execute”. The
‘AT’ precedes all Modem commands with a few exceptions,
which we will discuss later.
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HACKING TRUTH: A very simple and basic example of an AT command,
would be, the one which is issued when you dial into your ISP.
You, see when you click on the ‘Connect’ Button, the
DUN software sends the following command to your modem:
ATDT or ATDP command followed by the number
you want to dial and enter.
Now, let us analyze various parts of the
above command. The ‘AT’ tells the modem to pay attention.
The second part tells the modem, what kind of dialing system
to use. For Example, DT tells it to use dial by tone, while DP
tells it to use pulse dial. And the last part is the number you
want to dial.
NOTE: To issue commands to your modem, you
need to be in Command mode of your communication package.
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Your modem accepts commands only when it
is in the command mode. A modem is always either in the command
mode, or the on-line mode. When you boot up, your modem is by
default in command mode. So, when your modem is in command mode,
then all AT commands are considered to be commands, while in
online mode, all commands are considered to be as data packets.
Now, we come to an interesting part. The
above paragraph tells us that when we connect to the Internet,
our modem is in the online mode and thus cannot accept any commands.
So, this means that even if you know the IP Address of a person
and send them a modem command string, then the modem will only
treat it as normal data and will not react to it, as it is in
online mode. So, for the modem to start accepting and start reacting
to AT commands, it has to be switched to the command mode. This
is where the escape sequence comes in.
When the modem is in the online mode, then
it can be brought to the command mode by sending it the escape
characters i.e. +++. Passing the escape characters will switch
the modem to the command mode and it will start reacting to AT
commands. On giving the escape character, the actual AT command
need not have the string ‘AT’ and it need not end with
‘enter’.
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HACKING TRUTH: The modem can be returned
to the on-line state after the escape sequence has been issued.
To return the modem to the on-line state, you have to give the
ATO command.
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This means, that if you know the IP Address
of someone, then if you send the +++ string to it followed by
the AT Modem command, then you could practically control it,
remotely. You could disconnect the modem, change the Dial type,
force it to dial a particular number etc etc.
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