2.2 External vs. Internal
A modem for a PC may be either internal, external serial, or external USB. The internal one is installed inside of your PC (you must remove screws, etc. to install it). An external one just plugs in to a cable: USB cable (USB modem) or to the serial port (RS-232 serial modem). As compared to external serial modems, the internal modems are less expensive, are less likely to to suffer data loss due to buffer overrun, and usually use less electricity. An internal modem obviously doesn't use up any desk space.
External serial modems are usually easier to install and usually has less configuration problems provided the serial port you'll connect it to is configured OK. External USB modems are more likely to be winmodems and are reportedly usually more difficult to deal with than external serial modems. External modems have lights which may give you a clue as to what is happening and aid in troubleshooting. The fact that the serial port and modem can be physically separated also aids in troubleshooting. External modems are easy to move to another computer. If you need to turn the power off to reset your modem (this is seldom necessary) then with an external you don't have to power down the entire PC.
Unfortunately, most external serial modems have no switch to turn off the power supply when not in use and thus are likely to consume a little electricity even when turned off (unless you unplug the power supply from the wall). Each watt they draw usually costs you over $1/yr. Another possible disadvantage of an external is that you will be forced to use an existing serial port which may not support a speed of over 115,200 bps (although as of late 2000 most new internal modems don't either --but some do). For details Can't Set a High Enough Speed
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