The ITU-T Recommendation (1999) for full-duplex (FDX) asynchronous dial-up modems operating over the public switched telephone network (PSTN) at transmission rates up to 56 kbps downstream and 33.6 kbps upstream over a 4-kHz analog voice grade channel. In order to achieve this level of performance, a 56 kbps modem configuration requires that only one transmission link be analog.The end user dial-up connection is through a V.90 modem over an analog local loop to the PSTN. The connection through the PSTN must be entirely digital, including the originating and terminating central offices (COs), all tandem offices, and all transmission facilities.The local loop connection at the corporate intranet site, Internet service provider (ISP), or other site also must be digital, such as T-carrier, E-carrier or ISDN. This configuration limits the transmissions to only a single D-to-A-to-D (digital-to-analog-to-digital) conversion process, which limits the amount of quantizing noise associated with the PCM encoding process.The V.90 standard was known as V.last, as it was expected to be the last conventional modem standard developed by the ITU-T.V.92 was developed shortly thereafter, in 2000. Note: Although V.90 modem technology is fully capable of achieving a downstream transmission rate of 64 kbps, the T-carrier systems in the North American PSTN use a bit-robbing convention that reduces the reliable maximum transmission rate to 56 kbps. Further, FCC limitations on amplitude levels restrict the downstream rate to 53.5 kbps in the United States. See also analog, asynchronous, bit robbing, channel, CO, dial-up circuit, digital, downstream, Ecarrier, encode, FDX, Internet, intranet, ISDN, ISP, ITU-T, modem, PCM, PSTN, quantizing noise, tandem switch, T-carrier, transmission rate, upstream, V.92, voice grade, and V series.
An ITU standard (1998) for an analog modem that communicates at 56 Kbps downstream and 33.6 Kbps upstream. It was designed for ISPs and online services that are digitally attached to the telephone system via T1 and T3 circuits. In practice, the downstream link is not faster than 45 Kbps in these PCM modems, so called because they use pulse code modulation downstream and standard V.34 upstream. Initially, two incompatible technologies competed in this arena: x2 from U.S. Robotics and K56Flex from Rockwell and Lucent. Such modems can be upgraded to V.90 if they contain software-upgradable memory chips. See V.92, V.34 and channel bonding.