I last covered modems in URPCs 18th edition. I'll quote some information from p. 1140:
Modems Without a UART (WinModems):
Modems without a UART chip, sometimes referred to as WinModems after the pioneering U.S. Robotics version, can save you money at purchase time but can cause problems with speed and operating-system compatibility later.
For users wanting an inexpensive internal modem, a modem that doesn’t use a traditional UART instead of a UART-equipped internal or external modem looks like a great deal, often costing less than $40, compared to $80 or more for a UART-equipped “hardware” modem. But, there is no free lunch for modem users. What can you lose with a modem that lacks a UART?
First, you need to realize that there are actually two types of UART-less modems: those that rely on Windows and the CPU for all operations (these modems are also called controllerless modems) and those that use a programmable digital signal processor (DSP) chip to replace the UART. Both types of modems use less power than traditional UART-based modems, making them better for use with notebook computers.
As you can see, the difference is that full hardware modems have (or use) a UART (Serial Port). An external modem
that connects to a serial port is a sure thing, hardware modems that are PCI cards
will incorporate both a serial port and a modem. Scott.