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Rockwell Corporation's K56flex modem chipset gave users the capability to receive data on ordinary phone lines at 56 Kbps (thousand bits per second). K56flex was similar in capability to US Robotics' x2 modem and transmission technology. K56flex and X2 contributed to and were replaced by the ITU-TS V.90 standard. Modems built with K56flex can be upgraded to the V.90 standard by getting a software update from the modem manufacturer. (Note that Rockwell does not build modems itself, but only the chipsets for them.) Conexant, a Rockwell subsidiary compnay, now builds V.90 chipsets.
56 Kbps transmission technologies exploit the fact that most telephone company offices are interconnected with digital lines. Assuming your Internet connection provider has a digital connection to its telephone company office, the downstream traffic from your local Internet access provider can use a new transmission technique on your regular twisted pair phone line that bypasses the usual digital-to-analog conversion. A K56flex (or V.90) modem doesn't need to demodulate the downstream data. Instead, it decodes a stream of multi-bit voltage pulses generated as though the line was equipped for digital information. (Upstream data still requires digital-to-analog modulation.)
Unlike Integrated Services Digital Network, the 56 Kbps technologies do not require any additional installation or extra charges from your local phone company. On the other hand, the maximum transmission speed of ISDN is twice that of V.90 at 128 Kbps. You also have the flexibility of combining digital and voice transmission on the same line.