for 53.3-Kbps downloads, but you're still limited to a maximum of V.34 upload speeds.
For example, only the best local lines can support the demands of 56-Kbps operation. Poor-quality local loops generally result in download speeds in the low 40-Kbps range.
Another bottleneck that's often overlooked may be inside your system: your antiquated serial port. If you're using a standard 16550 universal asynchronous receiver/transmitter (UART) and an external modem, you're limiting your baud rate to 115.2 Kbps -- regardless of the capability of your modem. Add in data compression of over 2-to-1, and you've saturated your serial port.
Fortunately, a number of high-speed serial-port products are available to address this problem. When required during our testing, we used the Digi AccelePort 4r -- PCI DB25, from Digi International, to
provide access of up to 230 Kbps. Lava Computer claims that its LavaPort-PnP port can support a baud rate as high as 460 Kbps.
The accompanying graph compares a 56-Kbps modem's theoretical data throughput to the best and worst data rates delivered by the modems we tested. In the first test, we used an incompressible file that ideally should have delivered a full 53.3 Kbps over clean lines. Instead, the slowest modem reached just 47 percent of that goal, and the fastest just over 81 percent.
Next, we switched to a file that was designed to support 3-to-1 compression under V.42bis. Our worst and best results were 41 percent and 60 percent of theoretical capacity, respectively. In both cases, we used a high-speed serial port good for transfer rates of up to 230 Kbps.
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56-Kbps Reality Check
