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Special Report:

Beware Soft Modems

When you buy your next modem, you could innocently buy a software-based modem, not realizing the limitations of what you're buying.


Why soft modems are different

Software-based modems use fewer chips compared to traditional modems. The work normally done by the missing chips is transferred to software running on the host computer's main processor (the Pentium, PowerPC, etc.).

Modems consist of two major components:

  • A datapump performs the basic modulation/demodulation tasks for which modems are named
  • A controller provides the modem's identity: this is where the protocols for hardware error correction, hardware data compression, and basic modulation protocols (such as V.34, x2 or K56flex) exist. The controller is also responsible for interpreting AT commands.

A traditional modem implements both features in hardware, as chips inside the modem.

A controllerless modem, such as the U.S. Robotics (now 3Com) Winmodem, still has a hardware datapump, but implements the controller function as software.

An HSP modem dispenses with both the controller and the datapump, and uses software to provide both functions. Short for host signal processor, HSP modems transfer the work normally done by the missing chips to software running on the host computer's main processor (the 486, Pentium, PowerPC, etc.)


How do you know if it's a software modem?

There are a number of clues that your modem is software based:

  • U.S. Robotics Winmodems are the most popular software-based modems. USR also sells their Winmodems as OEM products to other vendors. For instance, the IBM 56K Internet Kit is an OEM Winmodem.
  • Rockwell's 14.4 RPI modems are software-based, as are Lucent's K56flex LT Winmodems, and Apple's GeoPort Telecom Adapters for Power Macintoshes. The GeoPorts for Quadra AVs are slightly different, in that they use the host computer's digital signal processor (DSP), rather than the host computer's CPU.
  • If a modem is advertised as Windows-only, it is probably software-based.
  • The phrase "controlless modem" isn't just a clue: it's an absolutely accurate description.


The cons

There are a number of flaws in the soft modem concept. In all cases, your host computer has to do more work to make up for the modem's lack of hardware. One RPI modem user reported slow transfer rates and dropped connections until he upgraded his PC from eight to sixteen megabytes of RAM (which improves performance in Windows). IBM's minimum requirements for the 56K Internet Kit modem is a 486, but some features require a 90 MHz Pentium, and DSVD features require a 133 MHz Pentium.

Note that U.S. Robotics Winmodems and other controllless modems have an on-board DSP that takes care of basic modulation and de-modulation tasks, so the load on the host computer is minimal. A number of USR Winmodem users have written in to say that they have been very pleased with the performance of their modem, and hadn't noticed any slowdown on their computer.

The other big disadvantage of software-based modems is that the software needed to make them work is operating-system dependent. Apple's GeoPort Telecom Adapters require the Mac operating system, so they won't work with MKlinux or Be OS. Other soft modems are generally Windows-only, so they can't be used with Linux, Solaris, OpenStep, or OS/2 unless someone writes special drivers. In contrast, any external Hayes-compatible modem with a full chip set can be used with any operating system on any computer with a serial port.

Even if you don't plan to run Linux on your PC, you may want to play modem-to-modem games like Doom that run in DOS. DOS games generally won't work with Winmodems, unless DOS drivers are provided. Major exception: Quake will still work, because Quake runs in a DOS shell under Window, and uses the Windows network drivers.

Winmodem configuration can be messy, because Winmodems use a range of memory addresses instead of a fixed memory address. Winmodems require a DLL file that loads into memory at startup, using some of Windows' resources.


The pros

If software-based have these problems, why do they continue to sell? In part, the answer is cost. By omitting a few chips, manufacturers can sell Winmodems for twenty to fifty dollars less than traditional modems. In an era of thousand dollar PCs and handheld consumer devices, cost is an enormous issue.

Soft modems are also popular for features like DSVD (digital simultaneous voice and data) and answering machine features, which are easier to implement with a soft modem.

Soft modems have some real benefits in small devices, such as laptop and hand held computers. They weigh less than traditional modems and take up less space. They also consume less electricity, which makes them ideal for battery-powered devices.

The upgradeability of soft modems is mixed. RPI modems aren't upgradeable to anything beyond a 14.4. After years of delays, Apple finally released a free 33.6 software upgrade for the 14.4 Power Mac GeoPorts, but not the Quadra AV GeoPorts. Apple has no plans for a 56K upgrade. U.S. Robotics had a free software-based 56K upgrade for some Sportster 33.6 Winmodems, but not others..

Upgradeability is one area where purely software-based HSP modems have an advantage over traditional modems and controllerless modems. Because almost every aspect of the modem is in software, the modem is highly upgradeable.

For another perspective on the pros and cons of soft modem use, see Tom Herbert's "Integrating a Soft Modem."

The pitch

In general, I recommend avoiding HSP modems. Get a good old-fashioned modem with all of its chips intact. Nothing else offers the same level of reliability, compatibility, and performance. The small savings of buying a Winmodem doesn't compensate for the load it shifts to your expensive new Pentium. And if you're an online gamer, you definitely want to avoid Winmodems.

Winmodems have their advantages. If you plan to only use Windows, and if gaming isn't your thing, a Winmodem can save you money, and it may offer more telephony features than a similarly-priced modem.

- Les Jones


Lee Schneider wrote in with some notes on his Multiwave modem, many of which apply to Winmodems in general:


1.These modems require a faster processor to connect at high speeds. A P100 is marginal for a 28.8 to 33.6 connection. A P133 is better and anything faster works fine. A P75 will generally not connect at 28.8. (Forget a 486)

2.They work fine with AMD K5 CPUs and probably other non Intel CPU's as well.

3.A 56K-type connection would probably require a 166MMX or better CPU.

4.They have a DSP model which probably puts a smaller load on the CPU, but may be less upgradable due to limitations in the DSP.

5.I have personally not had any problems with my 33.6PnP, but I am aware of situations where video card drivers cause conflicts in the form of frequent Internet disconnects. In one case I contacted Multitech about a conflict with the Mystique Win95 driver. It took a while, but they eventually e-mailed me a new driver that now works fine (for a friend). I have also experienced problems with CL5436/46 drivers. In that case a different version solved the problem.

6.These modems are full featured at very good prices.

7.They use a virtual comm port. The best way (in my view) to install them is to DISABLE comm 2 in the BIOS, let Win95 do its thing during installation and then go into the Device Mgr. in Win95 and change to comm 2(IRQ3/2F8).


Embedded Systems Programming - "Integrating a Soft Modem"

Windows Sources - U.S. Robotics Sportster Winmodem review


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