The 201 detects digital/analog phone lines before a modem is connected, thereby protecting an analog modem from damage caused by a digital line. Just insert the phone line into the 201 or plug the 201 with attached adapter cable to the wall jack, then you may easily identify if the line is safe by checking polarity and line current. The 201 has a line noise filter to block radio frequency interference (RFI) that can corrupt data received over modems and interrupt internet and Email connections. With the 201, the modem on your notebook computer will be completely protected and can reduce the headache of Internet, Email, and fax interruptions caused by RFI.