2.3 Modem Configuration
2.3.2 Manual configuration of modems with standard commands

You can manually configure the modem instead of having the router force a configuration on it. On the modem side, you can use the standard configuration commands to do the following:

  • Perform hardware flow control

  • Lock DTE speed to ensure that the modem will always communicate with the access server at the specified speed; as an example, when you use an async interface, you lock the speed to its theoretical maximum of 115.2 Kbps; the router speed command sets both transmit and receive speeds

  • Hang up when you quit a session

  • Have the CD signal truthfully reflect the carrier state

To manually configure your modem, you most likely do a reverse Telnet to your modem and apply some AT commands. In general, each modem vendor has its own modem command set that differs from other vendors' command sets. However, the following modem commands are common among most vendors:

Warning: Because the Caller ID function activates on the second ring, hackers typically target modems that answer on the first ring. If a modem takes more than one ring to answer, many hackers don't pursue the matter. You are, therefore, advised to set your modem to at least ATS0 = 2, thus pretending that your line subscribes to Caller ID.

Lab Activity    
  You are working for the Atlanta Bread Company and would like to setup out-of-band management on your router. You will properly configure an asynchronous connection on your router and dialin from a remote location.

AT Command Sets
AT commands stands for attention commands that are used to address the modem and force a certain behavior from it. The following is a listing of different command sets that exist and examples of commands used in the modem modules of Cisco 2600 and 3600 series:

  • Alphabetic command set -- Q0 enables output of result codes.

  • Caret command set -- ^V Displays the modem current Flash memory bootstrap revision.

  • Dollar command set -- $Bn sets the serial port rate to n bps, for example $B 115200.

  • Percent command set -- %Bn sets the modem port data rate, for example %B 33600.

  • Ampersand command set -- &D3 sets the modem on-hook and resets when detecting an on-to-off transition of DTR.

  • Parenthesis command set -- )M1 autoadjusts the power level for cellular modems.

  • Asterisk command set -- * I displays the modem identifier.

  • Hyphen command set -- -DL redials the last number dialed.

  • Colon command set -- :Dn sets manual dial.

  • @ Character command set -- @E gives detailed modem call status.

  • Backslash command set -- \F displays stored phone numbers.

  • # Character command set -- #E0 enables the escape code sequence in answer mode.

AT commands can be found at the following web link.

Web Links
Colon Command Set