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Description of Software Modems

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This article describes the differences between software modems andstandard 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) toemulate the functionality of traditional modems. Because they are softwaredriven, these modems only function on operating systems they are designedfor. Modems designed for the Windows operating system are commonly calledsoftware modems (or winmodems).

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

The common factor for all software modems is a reliance on softwareemulation of the UART chip. This is the chip that the operating systemcommunicates with to send or receive information across the serial or comports. When the proper driver is loaded, calls to the UART chip areredirected to the emulation software. The program believes that it iscommunicating with the UART chip.

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

It may be difficult to determine if you have a software modem since notall software modems are called software modems. To determine if you have asoftware modem, view your modem documentation or contact your hardwarevendor.

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

Software modems may also be referred to as controller-less modems, HostSignal Processing (HSP) modems, or winmodems. Two popular chipsets forsoftware modems are the RPI and the HSM chipsets.
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Article ID: 184068 - Last Review: 01/12/2015 16:00:31 - Revision: 3.0

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