2.1 Modem Functions
2.1.8 Modem modulation standards
The function of a modem is to convert digital signals (DTE-to-DCE) into analog signals (DCE-to-DCE), and vice versa. The ITU-T has defined and introduced several modem modulation standards over the years. Various modem manufacturers, however, have also marketed their own proprietary versions of modems. Interoperability among various types of modems, and sometimes even from the same vendor's modems, can be a challenge.

With proper configuration, V.34 modems can intelligently adapt to line conditions during a transition. Two communicating modems will initially attempt to set up a call at 28.8 Kbps. If line conditions do not allow a transmission at this speed, the modems fall back to the next-highest speed in steps of 2.4 Kbps (possibly down to 2.4 Kbps, if necessary). Alternatively, if line conditions improve, the modems can increase the speed.

Older modems negotiate a fixed transmission rate during handshaking, but communications continue at the same speed after that. If line quality deteriorates below a certain threshold, the connection is lost. Older modems cannot take advantage of any increased bandwidth later, when the line quality improves. The V.32bis standard was finalized in July 1991; V.34 standards were finished in June 1994.

The V.34 annex 12 standard supports a 33.6-Kbps transmit and receive operation. When compression is used, up to 133.8 Kbps is possible if the PC can handle this speed.

The newest standard is the V.90, 56-Kbps standard. Most modem manufacturers have a V.90 product, even though the actual maximum data rate allowed is 53 Kbps within the United States.

The access server is unaware of modulations because it is directly involved only with DTE-to-DCE communication. However, the access server-to-modem speed must account for modulation speed and compression ratio for optimal end-to-end performance. Some common international and proprietary modem standards are shown in the Figure.