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V.90 is the ITU Telecommunication Standardization Sector (ITU-T) telecommunications standard for modems. It was introduced in 1998 and allowed 56 Kbps downloads without the need to demodulate an analog signal. It also allowed 33.6 Kbps uploads requiring an analog signal to modulate over a 4 KHz analog voice grade channel.
The V.90 standard provides for full-duplex asynchronous transmissions, but in order to achieve speeds of up to 56 Kbps for downloads, transmissions must be put through a fully digital public switched telephone network (PSTN) that originates and terminates at the telephone company offices, all tandem offices and all transmission facilities.
V.90 is also designed as a standard to digitally attach Internet service providers and online services to the telephone system. Typically, these services were provided through T1 or T3 connections.
This standard was also known as V.last because it was expected to be the last standard developed by ITU-T. However, V.92 was presented in 1999 as an enhanced version of V.90.
Techopedia Explains V.90
The V.90 standard was developed by two companies – Rockwell and U.S. Robotics (now 3Com) as a way to combine two competing technologies. The V.90 telecommunications standard was capable of downstream transmission rates of up to 64 Kbps; however, a bit-robbing convention by the North American PSTN reduced this speed to 56 Kbps. Additional limitations imposed by the U.S. Federal Communications Commission reduced it further, to 53.5 Kbps.
With the V.90 standard, downloads do not require demodulation of downstream data. Instead, modems decode the data of multibit voltage pulses. However, upstream data still requires digital-to-analog modulation.
V.90 has advantages and disadvantages. It is often compared to the Integrated Services Digital Network (ISDN) standard. V.90 required no additional fees from the local phone company or installation charges, but the ISDN standard has maximum transmission speeds of 128 Kbps, twice those of V.90 modems, even without the North American PSTN and FCC limitations described above. In addition, ISDN allows both voice and data transmissions on the same line.
