V.90 - 56Kbps Modems

This allows up to 56 Kbps from the Server/ISP end to the user, and up to 28.8 Kbps from the user to the ISP.

Actually, there is a 20-year old FCC rule that prevents these modems from operating at speeds higher than 53 Kbps. The rules were designed to prevent interference to people's telephone conversations. But technological improvements have made the modern-day phone system less susceptible to signal interference. So, the current power limits on modems are no longer needed, FCC officials said in September of 1998.

How it works
When using an analog modem to access a server via the Public Switched Telephone Network (PSTN), an analog-to-digital conversion occurs at the the source central office (CO). If there is also an analog modem at the server end, a digital-to-analog conversion takes place when the signal leaves the destination CO. The net result of these conversions is a corrupted or reconstructed signal that differs from the original.

It is primarily the quantization noise produced by the analog-to-digital conversion at the CO that ultimately restricts upstream network speed to the V.34 speed of 33.6 Kbps. If the server end has a digital connection, this noisy conversion for is not needed when data is sent from the server to the client. This is the premise upon which V.90 rests.

There were two protocols in contention for becoming the ITU-T V.90 standard, which drew upon parts of both: One technology, called K56Flex(TM), is from Lucent Technologies and Rockwell Semiconductor. The other, called x2 is from US Robotics and 3COM.

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112 Kbps
Today there are products and services available that combine two 56 Kbps links to provide 112 Kbps of bandwidth.

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V.92
There is a new modem standard currently going through standardization. It is called V.92. V.92 provides the following improvements over V.90:

  • Reduces initial handshake process by10 to 12 seconds
  • Lets users put the data connection on hold for up to 16 minutes while switching to a voice call
  • Defines asymetric speeds, but allows a faster upstream speed— up to 48 Kbps. .