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2. Modems for a Linux PC

2.1 External vs. Internal

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 usually easier to install and usually 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. 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 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

Internal modems present a special problem for Linux, but will work just as well as external modems provided you avoid the ones that will work only for MS Windows. Configuring them ranges from very easy (automatically) to difficult depending on both the modem, your skills, and how easy it is to find info about your modem --info that is not all in this HOWTO. Some of the modems which will work only under MS Windows due to lack of Linux drivers (for software modems). If you buy a new one that you're not sure will work under Linux, try to get an agreement that you can return it for a refund if it doesn't work out.

While most new modems are plug-and-play you have various ways to deal with the PnP configuring:

  • The serial driver does it all for you (more likely for a PCI modem)
  • Use the "isapnp" program
  • Let a PnP BIOS do the configuring
The last 2 items of the above have shortcomings. Isapnp documentation is difficult to understand although reading the Plug-and-Play-HOWTO (long) will aid in understanding it. If you want the PnP BIOS to do the configuring, all you need to do is to make sure that it knows that you don't have a PnP operating system. But it may not do it correctly and you may need to find out what it's done see What is set in my serial port hardware?.

Many Linux users still say that it's a lot simpler just to get an external modem and plug it in. But if you get the right internal modem it may be just as easy.

2.2 Is a Driver Needed ?

ISA (and some PCI) hardware modems (including all external modems) don't really need any driver. The serial port the the modem resides on does need a driver and it's supplied either as a Linux serial module or compiled into the kernel. Any driver for a PCI Modem should install automatically since they are detected by system software. Software modems do need a driver. The drivers for MS Windows are *.exe programs which will not run under Linux. So you must use a Linux driver (if it exists). See Software-based Modems (winmodems, linmodems)

2.3 External Modems

PnP External Modems

Many external modems are labeled "Plug and Play" (PnP) but they should all work fine as non-PnP modems. While the serial port itself may need to be configured (IRQ number and IO address) unless the default configuration is OK, an external modem uses no such IRQ/IO configuration. You just plug the modem into the serial port.

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 id number. If it's a software modem, it could try to locate a driver for it. It could also tell application programs what port your modem is on. (such as /dev/ttyS2 or COM3). But since you don't have such a PnP operating system you may need to configure your application program manually by giving it the /dev id (such as /dev/ttyS2). Some programs can probe for a modem on various ports.

Cabling & Installation

Connecting an external modem is simple compared to connecting most other devices to a serial port that require various types of "null modem" cables (which will not work for modems). Modems use straight through cable, with no pins crossed over. Most computer stores should have this. Make sure you get the correct gender and number of pins. 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.

What the Lights (LED's) Mean (for some external modems)

  • TM Test Modem
  • AA Auto Answer (If on, your modem will answer an incoming call)
  • RD Receive Data line = RxD
  • SD Send Data line = TxD
  • TR data Terminal Ready = DTR (set by your PC)
  • RI Ring Indicator (If on, someone is "ringing" your modem)
  • OH Off Hook (If off, your modem has hung up the phone line)
  • MR Modem Ready = DSR ??
  • EC Error Correction
  • DC Data Compression
  • HS High Speed (for this modem)

2.4 Internal Modems

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 PCI slots, other modems for the older ISA slots, and ARM software "modems" for the new small AMR slot. Some new PC don't have any ISA slots. Only some newer PCs will have ARM 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 over the bus inside the computer. Such configuration commands can be sent by a PnP BIOS, by the isapnp program (for the ISA bus only), by setpci (for the PCI bus), or by newer serial device drivers for certain modems. Under Linux you may have a choice of how to configure the ones that don't get io-irq configured by the serial driver.

  1. ISA bus: Use "isapnp" which may be run automatically at every boot-time
  2. Let a PnP BIOS do it, and then maybe tell setserial the IO and IRQ
  3. PCI bus: Use lspci -vv to look at it and setpci to configure.

See Quick Install for more details, especially for the PCI bus.

2.5 Software-based Modems (winmodems, linmodems)

Introduction to software modems (winmodems)

Software modems turn over some (or even almost all) of the work of the modem to the main processor (CPU) chip of your computer (such as a Pentium chip). This requires special software (a modem driver) to do the job. Until late 1999, such software was released only for MS Windows and wouldn't work with Linux. Even worse was that the maker of the modem kept the interface to the modem secret so that no one could write a Linux driver for it (even though a few volunteers were willing to write Linux drivers).

But things have improved some since then so that today (late 2001) many such modems do have a linux driver. There is no standard interface so that different brands/models of software-modems need different drivers (unless the different brands/models happen to use the same chipset internally).

Another name for a software modem (used by MS) is "driver-based modem". The conventional hardware-based modem (that works with Linux) doesn't need a modem driver (but does use the Linux serial driver) After about mid-1998 most new internal modems were software modems.

Software modems fall into 2 categories: linmodems and winmodems. Winmodems will only work under MS Windows. Linmodems will work under Linux (but formerly were mostly winmodems so some still call them "winmodems"). The term "Winmodem" is also a trademark for a certain model of "winmodem" but that's not the meaning of it in this document.


In late 1999, two software-based modems appeared that could work under Linux and were thus called "linmodems". Lucent Technologies (LT) unofficially released a Linux binary-only code to support most of its PCI modems. PC-TEL (includes "Zoltrix") introduced a new software-based modem for Linux. After that, interest increased for getting winmodems to work under linux. There is a GPL'ed driver for Intel's (Modem Silicon Operations) MD563x HaM chipset (nee Ambient division of Cirrus Logic). As of mid-2001 there are also drivers for: Conexant HSF, Motorola SM56, ESS (ISA only), and IBM's Mwave for Thinkpads 600+.

What percent of software modems now (2001) work under Linux? Well, there's a lot of modem chips not supported: Lucent/Agere ARM (Scorpio), 3COM/US Robotics, Conexant HCF, some SmartLink (3 different chipsets), Ambient HSP, and possibly others. But there might be support for some of these by the time you read this. So it seems that over half the software modem chips are now supported (as of late 2001).

Be warned in advance that determining if your modem is a linmodem may or may not be easy. You may need to first find out what chipset you have and who makes it. Just knowing the brand and model number of your modem may not be sufficient. There are complex ways to find this out using say "lspci" (more than once) and then looking up the chip maker using the long modem number. This requires checking a database or searching the Internet. It's not always simple. It could happen that you will put a fair amount of effort into this only to get the bad news that your modem isn't supported. See Linmodem-HOWTO for more details.

Linmodem sites and documentation

Software-based modem types

There are two basic types of software modems. In one type the software does almost all of the work. The other is where the software only does the "control" operations (which is everything except processing the digital waveshapes --to be explained later). Since the hardware doesn't do the control it's called a "controllerless" modem. The first type is an all-software modem (sometimes just called a software modem).

For both of these types there must be analog hardware in the modem to generate an electrical waveshape to send out the phone line. It's generated from a digital signal (which is sort of a "digital waveshape"). It's something like the digital electronics creates a lot of discrete points on graph paper and then the modem draws a smooth curve thru them. There must also be hardware to convert the incoming waveshape to digital. This is just analog-to-digital conversion (and conversely). It's done by a codec (coder-decoder).

Then this digital waveshape must be converted to a data byte stream. This is known as demodulation, while turning data bytes into a digital waveshape is known as modulation. The modem can't just send an incoming data byte stream to the PC but must first do decompression, error correction, and convert from serial to the parallel bus of the computer. Likewise for an outgoing data byte stream.

The difference between the two types of software-based modems is where the digital modulation takes place. In the all-software modem this modulation is done in the CPU using a Host Signal Processor (HSP). In the controllerless modem it's done in the modem but all other digital work is done by the CPU. This other digital work consists of dealing with AT-commands, data compression, error correction, and simulating a serial port. In the all-software modem, there are still two items handled by hardware: the A/D conversion of waveshapes by the codec and echo cancellation.

For example the Rockwell HCF (Host Controlled Family) is an all-software modem. If the software that does these tasks could be ported to Linux and then there wouldn't be a major problem.

Is this modem a software modem?

How do you determine if an internal modem is a software modem? First see if the name, description of it, or even the name of the MS Windows driver for it indicates it's a software modem: HSP, HCF, HSF, controllerless, host-controlled, host-based, and soft-... modem. If it's one of these modem it will only work for the cases where a Linux driver is available. Since software modems cost less, a low price is a clue that it's a software modem.

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 (see Web Sites. 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 it requires a Pentium CPU, then almost all of it's work is done by software and it's not likely to work under Linux. If it requires a 486 CPU (or better) then it's likely a host-controlled modem that will work only if there exists a Linux driver for it. Saying that it only works with Windows is also bad news. However, even in this case there may be a Linux driver for it.

Otherwise, it may be a hardware modem 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. Some manufacturers are specifically stating that certain models work under Linux.

Should I get a software modem?

Only if you know there is a Linux driver for it that works OK. 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 some (or all) of its work, the software modem requires less on-board electronics and thus costs less. At the same time, the CPU work load is increased by the modem which may result in slower operation.

The percentage of loading of the CPU by the modem depends on both what CPU you have and whether or not it's an all-software modem. For a modern CPU and a modem that only uses the CPU as a controller, there's little loss of performance. In most other cases, you will not suffer a loss of performance if there are no 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 many cases yes, especially if you don't use the modem much and/or are not running any other CPU intensive tasks when the modem is in use. 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 (except that it's not efficient at all when it's not in use). So if you use the modem a lot it's probably better to avoid all-software modems (and then you can use a less powerful CPU :-).

2.6 PCI Modems

A PCI modem card is one which inserts into a PCI-bus slot on the motherboard of a PC. While many PCI winmodems will not work under Linux (no driver available) other PCI modems will work under Linux. The Linux serial driver is being modified to support certain PCI modem cards (but not winmodems). If the Linux serial driver supports it then the driver will set up the PnP configuration for you. See PCI Bus Support Underway If no special support is in the Linux serial driver it may still work OK but you have to do some work to configure it.

2.7 AMR Modems

These are all winmodems that insert into a special AMR (Audio Modem Riser) slot on the motherboard. Audio cards are sometimes used in this slot. The slot's main use is for HSF type modems where the CPU does almost all of the work. This results in a small modem card and thus a short AMR slot. Such a "modem" is actually little more than a codec which transforms digital signals representing an analog voltage wave into the analog wave itself (and conversely). Linux supports at least one of them.

2.8 USB Modems

USB = Universal Serial Bus. Some USB modems work with Linux and some don't. Linux has support for modems that conform to the USB Communication Device Class Abstract Control Model (= USB CDC ACM). There's a module for ACM named acm.o. See the /usb/acm... document in the kernel documentation directory (/usr/share/doc/kernel-doc-2.4.x in Debian, perhaps /usr/doc/kernel... in some distributions). The ACM serial port for the first (0th) such modem is (with devfs) is: /dev/usb/acm/0. Otherwise it might be /dev/usb/ttyACM0.

2.9 Which Internal Modems might not work with Linux

MWave and some DSP Modems

Note that there's now a Linux driver for the ACP (Mwave) modem used in IBM Thinkpads 600+. See the mini-HOWTO: ACP-Modem.

While hardware modems used use DSPs (Digital Signal Processors) some of these DSPs are programmed by a driver which must be downloaded from the hard disk to the DSPs memory just before using the modem. Unfortunately, such downloading is normally done by Dos/Windows programs (which doesn't work for Linux). But there has been substantial success in getting some of these modems to work with Linux. For example, there is a Linux driver available to run a Lucent (DSP) modem.

Ordinary modems that work fine with Linux (without needing a driver for the modem) often have a DSP too (and may mention this on the packaging), but the program that runs the DSP is stored inside the modem. These work fine under Linux. An example of a DSP modem that has problems working under Linux is the old IBM's Aptiva MWAVE.

One way to get some DSP modems to work with Linux is to boot from DOS (if you have it on your Linux PC). You first install the driver under DOS (using DOS and not Window drivers). Then start Dos/Windows and start the driver for the modem so as to program the DSP. Then without turning off the computer, start Linux.

One may write a "batch" file (actually a script) to do this. Here is an example but you must modify it to suit your situation.

rem mwave is a batch file supplied by the modem maker
call c:\mww\dll\mwave start
rem loadlin.exe is a DOS program that will boot Linux from DOS (See
rem Config-HOWTO).
c:\linux\loadlin f:\vmlinuz root=/dev/hda3 ro

One may create an icon for the Window's desktop which points to such a batch file and set the icon properties to "Run in MSDOS Mode". Then by clicking on this icon one sets up the modem and goes to Linux. Another possible way to boot Linux from DOS is to press CTRL-ALT-DEL and tell it to reboot (assuming that you have set things up so that you can boot directly into Linux). The modem remains on the same com port (same IO address) that it used under DOS.

The Newcom ifx modem needs a small kernel patch to work correctly since its simulation of a serial port is non-standard. The patch and other info for using this modem with Linux is at http://quinine.pharmacy.ohio-state.edu/~ejolson/linux/newcom.html.

Rockwell (RPI) Drivers

Some older Rockwell chips need Rockwell RPI (Rockwell Protocol Interface) drivers for compression and error correction. They can still be used with Linux even though the driver software works only under MS Windows. This is because the MS 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 old modem this command was +H0. Not having data compression available makes it slower to get webpages but is just as fast when downloading files that are already compressed.

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