Description of Software Modems

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Traditionally, modems have performed the following basic functions:

• Receive and transmit signals.
• Convert signals from digital to analog or vice-versa. This is known as modulation or de-modulation of the signal and is performed by the Digital Signal Processor (DSP).
• Compress and Decompress data.
• Change a single stream of data into bits to be placed on the system bus. The UART chip is responsible for this function.

Some modems do not perform all of these functions by themselves. Instead, they rely on software (usually implemented as a protected-mode driver) to emulate the functionality of traditional modems. Because they are software driven, these modems only function on operating systems they are designed for. Modems designed for the Windows operating system are commonly called software modems (or winmodems).

The design of these modems differs greatly from one manufacturer or model to the next. For example, some are equipped with a DSP) and rely on software for other functions, while others may rely almost entirely on emulation.

The common factor for all software modems is a reliance on software emulation of the UART chip. This is the chip that the operating system communicates with to send or receive information across the serial or com ports. When the proper driver is loaded, calls to the UART chip are redirected to the emulation software. The program believes that it is communicating with the UART chip.

NOTE: If there is no MS-DOS driver loaded for your software modem, you are unable to use the modem in MS-DOS mode. An MS-DOS program running in Windows (also known as a virtual machine or VM) can use the modem if it is written to take advantage of the Windows protected-mode drivers.

It may be difficult to determine if you have a software modem since not all software modems are called software modems. To determine if you have a software modem, view your modem documentation or contact your hardware vendor.

NOTE: Echoing AT commands to a com port does not work with a software modem since MS-DOS expects to communicate with the UART chip directly.

Software modems may also be referred to as controller-less modems, Host Signal Processing (HSP) modems, or winmodems. Two popular chipsets for software modems are the RPI and the HSM chipsets.