Almost all newer 56K modems use either a 3COM chipset, a Rockwell chipset, or a Lucent chipset. Computer manufacturers do not make modems, they buy 56K modems and sometimes rename them as their own. Compaq computers, for example, have 56K modems that use either the Lucent chipset(56K DF) or the Rockwell chipset(HCF), while some Gateway computers use the 3COM chipset.
Even most modem manufacturers do not make their own 56K chipsets. They simply buy the chips and build their own modems, making modifications to specific components of the modems to add additional features like distinctive ring, or caller id.
This might bring up the question: Why a is there such a huge difference
Answer: There are many components that make up a modem, not just the chipset. When you buy a hi-end 56K modem, you are paying for the class "A" circuitry, a good printed circuit board, product support, additional features, and to some extent, the brand name. When you buy a low-end 56K modem you are not likely get these features or regularly updated drivers.
In the 56K modem world, DRIVERS ARE EVERYTHING! This cannot be stressed enough. There are reports of many people who couldn't make a 56K connection, or a connection at all, until they upgraded their modem's drivers. Why is this? 56K modems and the V.90 standard are still a new technology. Driver upgrade improvements are made all the time for better connection rates, more stability, better compatibility, etc...
If you know what chipset your modem uses, you can try a initialization string to improve your connect rates. You should also be able to disable V.90 / 56-K Flex / X2 to connect using the V.34(28.8) protocol. This may be necessary in some cases. Click here to go to our 56K modem initialization page.
Most modems with the ESS, Cirrus, and PCtel chipsets are HSP "software" or "Windows" modems. HSP stands for Host Signal Processing. This refers to the modem using the host computersí processor in place of a dedicated chip on the modem to perform the Digital Signal Processing (DSP). Modems implementing one of these "software" chipsets use your CPU for some functions normally handled by the modem itself. A "hardware" modem will do all error correction, compression, data correction etc., on board since these functions are build into the modem itself. A HSP "Windows" modem or "software" modem will use your CPU to do these functions.
NOTE: Lucent and Rockwell chipset modems are also "Windows" modems, but they are not HSP modems and require much less CPU usage in order for the modem to operate correctly. A PCtel, ESS, or Cirrus chipset modem can require considerable CPU usage in order to operate correctly.
The following came straight from PCtel's web site:
"The following outlines the minimum computer system requirements for modems utilizing our technology.
Intel Pentium Based computer
In order to run a PC-TEL HSP based modem as 28.8Kbps or 33.6Kbps you should have a Pentium class 75MHz CPU with 256K cache and 8MB RAM. For systems running a Cyrix processor we recommend a 100MHz CPU or higher.
In order to run a PC-TEL HSP based 56K modem we recommend that you have a Pentium class 166MHz with 256K cache and 16MB RAM. For systems running a Cyrix processor we recommend a 200MHz CPU or higher.
486 Class Processors
We do not support 486 Class processors due to lack of performance capabilities."
As you can see, the system requirements for the HSP type chipset to work at all are rather high.  Due to the high system requirements, and general compatibility problems, we cannot recommend these types of modems at all!
For driver information for a modem using the PCtel chipset, please try: PCtel's "who made my modem" page.
For driver information for a modem using the ESS chipset, please try: ESS's driver update page.
NOTE: Cirrus recently announced they will no longer make modem chipsets. They have sold this portion of the company to Ambient Technologies. New modems manufactured by Ambient Technologies are DSP or "hardware" based modems so the problems associated with the older Cirrus HSP chipsets will not apply to these modems.
Since Cirrus no longer makes modem chipsets you will likely have problems finding drivers. You can try Windrivers.com. Select your modem manufacturer for more information.