A modem for a PC may be either internal or external. The internal one is installed inside of your PC (you must remove screws, etc. to install it) and the external one just plugs into a serial port connector on a PC. Internal modems are less expensive, are less likely to to suffer data loss due to buffer overrun, usually use less electricity, and use up no space on your desk.
External modems are much easier to install and require less configuration. They 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.
Unfortunately most external 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 costs you about $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 k (although as of late 1998 most new internal modems don't either --but some do). If a new internal modem had a 16650 UART it would put less load on the CPU (but almost none do as of late 1998).
Internal modems present a special problem for Linux, but will work just as well as external modems provided you avoid the high percentage of them that will work only for MS Windows, and also provided that you spend time (sometimes a lot of time) to configure them correctly. Some of the modems which will work only under MS Windows are, unfortunately, not labeled as such. If you buy a new one, make sure that you can return it for a refund if it will not work under Linux.
While most new modems are plug-and-play you have various ways to deal with them:
There are many Linux users that say that it's a lot simpler just to get an external modem and plug it in. But since new peripherals are mostly PnP today, you may eventually need to deal with it, so why delay the inevitable? Still, the most expedient (and expensive) solution is an external modem (if you have a free serial port).
Many external modems are labeled "Plug and Play" (PnP) but they should all work fine as non-PnP modems. Since you usually plug the modem into a serial port which has its own IRQ number and IO address, the modem needs no PnP features to set these up. However, the serial port itself may need to be configured (IRQ number and IO address) unless the default configuration is OK.
How can an external modem be called PnP since it can't be configured by PnP? Well, it has a special PnP identification built into it that can be read (thru the serial port) by a PnP operating system. Such an operating system would then know that you have a modem on a certain port and would also know the model number. Then you might not need to configure application programs by telling them what port the modem is on (such as /dev/ttyS2 or COM3). But since you don't have such a PnP operating system you will need to configure your application program manually by giving it the /dev id (such as /dev/ttyS2).
Connecting an external modem is simple compared to connecting most other devices to a serial port that require various types of "null modem" cables. Modems use straight through cable, with no pins crossed over. Most computer stores should have these. Make sure you get the correct gender. If you are using the DB9 or DB25 serial port at your computer, it will always be male which means that the connector on the cable should be female. Hook up your modem to one of your serial ports. If you are willing to accept the default IRQ and IO address of the port you connect it to, then you are ready to start your communication program and configure the modem itself.
An internal modem is installed in a PC by taking off the cover of the PC and inserting the modem card into a vacant slot on the motherboard. There are modems for the ISA slots and others for the PCI slots. While external modems plug into the serial port (via a short cable) the internal modems have the serial port built into the modem. In other words, the modem card is both a serial port and a modem.
Setting the IO address and IRQ for a serial port was formerly done by jumpers on the card. These are little black rectangular "cubes" about 5x4x2 mm in size which push in over pins on the card. Plug-and-Play modems (actually the serial port part of the modems) don't use jumpers for setting these but instead are configured by sending configuration commands to them (via IO address space on the ISA bus inside the computer). Such configuration commands can be sent by a PnP BIOS, the isapnp program (for the ISA bus only) or by a PnP operating system. The configuring of them is built into Windows 95/98 OSs. Under Linux you have a choice of ways (none of which is always easy) to io-irq configure them:
Software modems turn over much (or even almost all) of the work of the modem to the main processor (CPU) chip of your computer (such as a Pentium chip). Complex proprietary software programs (drivers) do this work on the CPU. A majority of internal modems made after about mid-1998 don't work with Linux since they are software modems which only work under Windows and are often called "winmodems". Although a few volunteers were willing to try writing Linux drivers for these modems, specs were not made available so this couldn't be done. Prior to about 2000, no software modem could be used with Linux due to no drivers for them under Linux.
Then finally in late 1999 two software modems appeared that could work under Linux. Lucent Technologies supplied a Linux binary-only code to support it's PCI software modems, but bugs were reported in the first version. PC-TEL introduced a new software modem for Linux. Will other companies follow these leads and thus create "linmodems"? For a list of modems which work/don't_work under Linux see modem list A project to get winmodems to work under linux is at http://linmodems.org
If code is made available to operate a "winmodem" under Linux, then one may call it a "linmodem". Is it still a "winmodem"? Perhaps it is since it also works under MS Windows. The term "Winmodem" is a trademark for a certain type of "winmodem".
Here is some more precise terminology regarding software modems. HSP (Host Signal Processor) means that the host processor (your CPU chip) creates the code needed to produce the electrical signal on the phone line. The modem itself just creates whatever electrical waveshape the CPU tells it to. In contrast to this, a "controllerless" modem can create the waveshapes on its own (but can't control the modem). It contains no facilities to deal with bytes being sent and received. It can't compress strings of bytes; it can't check for errors; it can't put them into packets. In other words it can't control the modem but instead has the CPU do all this work using software. The Rockwell HCF (Host Controlled Family) does this. If the software that does all this could be ported to Linux and then there wouldn't be this problem. Besides the above, a modem which doesn't simulate a serial port will not work under Linux.
How do you determine if an internal modem will work under Linux? First see if the name or description of it indicates it's a software modem: HSP, HCF, HSF, controllerless, host-controlled, host-based, and soft-... modem. If it's a software modem it will only work for the rare cases (so far) where a Linux driver is available. If you don't know the model of the modem and you also have Windows on your Linux PC, click on the "Modem" icon in the "Control Panel". Then check out the modem list on the Web mentioned 4 paragraphs above. If the above doesn't work (or isn't feasible), you can look at the package it came in (or a manual) find the section on the package that says something like "Minimum System Requirements" or just "System Requirements". It may be in fine print. Read it closely. If Windows is listed as one of the requirements then it will likely not work under Linux.
Otherwise, it may work under Linux if it fails to state explicitly that you must have Windows. By saying it's "designed for Windows" it may only mean that it fully supports Microsoft's plug-and-play which is OK since Linux uses the same plug-and-play specs (but it's harder to configure under Linux). Being "designed for Windows" thus gives no clue as to whether or not it will work under Linux. You might check the Website of the manufacturer or inquire via email. I once saw a web-page that specifically stated that one model worked under Linux while implying that another model didn't.
Besides the problems of getting a driver, what are the pros and cons of software modems. Since the software modem uses the CPU to do much of its work, the software modem requires less on-board electronics and thus costs less. At the same time, the CPU is heavily loaded by the modem which may result in slower operation. This is especially true if other CPU-intensive tasks are running at the same time. Of course when you're not using the software modem there is no degradation in performance at all. Is the cost savings worth it? In some cases yes, especially if you seldom use the modem or are not running any other CPU intensive tasks when the modem is in use. Thus there are cases where use of a software modem is economically justified. The savings in modem cost could be used for a better CPU which would speed things up a little. But the on-board electronics of a modem can do the job more efficiently than a general purpose CPU. So if you use the modem most of the time it's probably better to avoid software modems (and then you can use a less powerful CPU :-).
A PCI modem card is one which inserts into a PCI-bus slot on the motherboard of a PC. Unfortunately, it seems that most PCI modems will not work under Linux but efforts are underway to support some of them. See PCI Bus Yet Supported
Such modems use DSP's (Digital Signal Processors) which are programmed by algorithms which must be downloaded from the hard disk to the DSP's memory just before using the modem. Unfortunately, the downloading is done by Dos/Windows programs so one can't do it from Linux. Ordinary modems that work with Linux often have a DSP too (and may mention this on the packaging), but the program that runs it is stored inside the modem. This is not a "DSP modem" in the sense of this section and should work OK under Linux. An example of a DSP modem is IBM's Aptiva MWAVE.
If a DSP modem modem simulates a serial port, then it is usable with Linux which communicates with modems via the serial port. If you also have Dos/Windows on the same PC you may be able to use the modem: You first install the driver under DOS (using DOS and not Window drivers). Then start Dos/Windows (make sure the modem gets initialized) and without turning off the computer, go into Linux. One way to get to Linux is to use loadlin.exe which is a DOS program that will boot Linux from DOS (See Config-HOWTO). Another way is to just press CTRL-ALT-DEL. The modem remains on the same com port (same IO address) that it used under DOS.
Modems that require Rockwell RPI (Rockwell Protocol Interface) drivers can still be used with Linux even thought the driver software works only under Windows. This is because the Windows software which you don't have does only compression and error correction. If you are willing to operate the modem without compression and error correction then it's feasible to use it with Linux. To do this you will need to disable RPI by sending the modem (via the initialization string) a "RPI disable" command each time you power on your modem. On my modem this command is +H0. Not having data compression available may not be much of a handicap since most long files which you download from the Internet are already compressed and attempts at further compression may only slow things down a bit.