I've discovered long ago that the best alternative have been the use of an external modem. Use of such a device regains the processing cycles lost through the use of an internal "soft modems" and allows your machine(s) to operate more effectively.
The connection rates are quite acceptable, stable and reliable. An external modem handles all of the flow controls, all A/D conversions and throughput much better than an internal.
Quality in internal modems can be rather questionable - as well as with external modems. Price does not always represent the "best" external modem but rather shop and compare on features, chipsets, compression scheme, etc and possibility of newer, future firmware releases for external modems. All modems are not created equal.
The highest per-centage of internal modems are classified as "soft" or WINmodems. Many of these are instable by nature, require the PC OS to configure a "soft" or virtual comm port.
By preference (and server preference as well), I'd use an external comm port and modem. THey are just plain easier to use, tweak and configure.
As with any modem, quality of phone lines, quality of service, and number of devices connected in your home or small office will always affect modems in general.
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