GAO V.90 software module implements the V.90 ITU-T recommendation for modems that can connect at data rates of up to 56 Kbps. The standard specifies a pair
of high-speed modems, one digital and the other analog, which are designed for Internet use. The V.90 Server Modem is a digital modem with a maximum bit rate of 56 Kbps transmit and 33.6 Kbps
receive. The V.90 Client Modem is an analog modem with a maximum bit rate of 56 Kbps receive, 33.6 Kbps transmit.
Most ISPs are digitally connected both to the Internet and to the telephone company's central office (CO). When this is the case, there is a clear digital
connection downstream from the ISP's modem to the CO's line card that serves the user and contains a digital to analog converter.
The result of having this digital connection is that an analog to digital conversion, and therefore quantization noise, is avoided between the ISP and CO.
Without the limits imposed by quantization noise, it is possible to achieve downstream connection speeds of up to 56 Kbps downstream. Because most users are not digitally connected to the CO, an
analog to digital conversion and the associated quantization noise cannot be avoided in the upstream direction. This means that V.34 modulation techniques must be used and upstream speeds are still
limited to 33.6 Kbps.