Dial-up Modem Basics and Manufacturers

“Modem” is short for “modulator-demodulator.”

A dial-up modem is used to establish an internet connection using the existing plain old telephone exchange via a telephone line in a home or office. This setup is shown in the figure below. A dial-up modem will have two ports: one is a voice port where the telephone line is connected, and the other is a data port where the PC is connected.

It provides a lower data rate connection compared to other modem technologies but requires less investment if one has an analog telephone line. The disadvantage of dial-up internet is that if one uses the internet, the same telephone line cannot be used for making or receiving voice calls.

ITU-T (formerly CCITT) has classified various standards based on the data rate/modulation type the dial-up modem supports. These are referred to as the V series of standards. A few are summarized below.

A dial-up modem converts digital data into an analog form compatible with being transported via an analog telephone line.

Standards

• ITU V.21 - Audio FSK, 300 bps
• ITU V.22 - PSK, 1200 bps
• V.32 - 9600 bps
• V.34 - 28.8 Kbps and 33.6 Kbps options, backward compatible with V.32
• V.90 - 56 Kbps
• V.92 - Transmits at the same speed as V.90 with some additional features

Protocols Supported

PPP (Point-to-Point Protocol) is used in dial-up modems to establish a connection with the ISP’s server. It has more features compared to its predecessor, SLIP (Serial Line Internet Protocol). PPP is defined in RFC 1661.

PPP is regarded as a data link layer protocol and is interfaced with the network layer of the system to establish the TCP/IP connection with the peer entity.

Dial-up Modem Manufacturers

The following table lists dial-up modem manufacturers: