Hi sridhar,
Please go through the attachment.
Regs,
sathish
Hi,
The following talks about Hardware and Software modems.
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Modem Taxonomy:
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A modem is a peripheral device that enables computers to communicate with
each other over conventional phone lines. The term modem stands for
Modulator/Demodulator. The purpose of a modem is to convert (modulate) the
digital signal that a computer understands into an analog signal that can be
carried over a phone line, and to re-convert (demodulate) the analog into a
digital signal at the other end. Demodulation consists of digitizing analog
waveforms using an A-to-D converter followed by the application of signal
processing algorithms. Modulation consists of a different set of signal
processing algorithms to produce a digitized waveform, which is sent through
a D-to-A converter.
Traditional modem communication standards assume that both ends of a data
connection are linked to the public switched telephony network (PSTN) by
analog lines. This limits the communication bandwidth to 33.6Kbps in each
direction. By assuming that one of the endpoints is connected digitally to
the PSTN (like most Internet Service Providers are), modern modems are able
to achieve speeds of up to 56Kbps downstream and 33.6Kbps upstream using the
V.90 protocol.
There are four functions that a modem provides:
1.An interface between analog phone lines and digital computer components
- A/D and D/A.
2.Signal modulations at different rates.
3.An Attention (AT) command set interpreter.
4.An asynchronous interface between the modem and the computer.
In addition to these functions, any modem card will provide buffering for
data flowing in both directions.
Modems can be classified into hardware-based modems (traditional modems) or
software-based modems, depending on where each of these functions are
executed.
Hardware-based Modems:
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Traditional modems implement all the modem functionality in hardware on the
modem card. Dedicated chips provide signal modulation and interpret the AT
command set. The card also provides A/D and D/A converters. On older modems,
the Universal Asynchronous Receiver/Transmitter (UART) chip implements the
asynchronous interface between the modem and the computer. Today, the PCI
bus interface often provides this functionality, replacing the UART chip.
Software-based Modems:
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Software-based analog modems use the host processor to perform some of the
modem functions traditionally performed on the modem card. Two types of
software-based modems have emerged:
1.Controllerless modems (also known as winmodems or linmodems), which
perform and interpret the standard attention (AT) commands on the main
processor. Signal modulation, A/D, and D/A are implemented by hardware on
the modem card.
2.Soft modems, which perform signal processing (as well as AT commands)
on the host CPU, unlike both regular and controllerless modems. Modem
data buffers may reside in host memory. Soft modems still have
hardware-based A/D and D/A converters.
Today, the software-based analog modems are very common on the new computer
systems, both for workstations and especially for laptops. Some of the
reasons for their success are low cost, low power consumption, and maximum
upgrade flexibility. Drawbacks are high CPU and memory usage
What Are Soft Modems?
The traditional modem has been a box or add-on card with a phone
connection on one end and a serial port on the other. It contains a
telephone line interface, codec, digital signal processor (DSP), and
control processor. The DSP converts transmitted data from the PC into
signals on the phone line and converts digitized line samples back
into demodulated bits (for example, V.90 and V.34). The control
processor implements AT command interpretation (V.250) and end-to-end
data processes such as error control (V.42) and data compression (V.42
bis).
In modems currently bundled with PCs, the serial port has disappeared
into a serial driver, while the hardware has moved onto an internal
card. The control processor functions are included in the serial driver
and the hardware retains the DSP. These modems are sometimes called
controllerless modems.
Soft modems take this migration one step further: they implement the DSP
functions in the serial driver. Typically, these drivers depend on Intel
MMX technology or similar extended instruction sets in the CPU.
What Are the Attractions of Soft Modems?
Cost:
----
PC manufacturers are looking to save money everywhere. CPUs
continue to increase in performance. Soft modems are an opportunity to
use that CPU performance to save hardware costs. (However, note that the
cost of modem DSP hardware is also falling anyway, subject to the same
Moore's law that drives down the cost of the CPU and memory.)
Flexibility:
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The fundamental algorithms of the modem are in a driver,
which can be modified as easily as any other software module. If a bug
is found, it is possible to create a fix and download it to end users
through their modems. When new modulation schemes come along, the new
code can be downloaded. For example, if a national Post, Telephone, and
Telegraph (PTT) starts demanding V.18 functionality (text phones), that
new function would also be easy to add.
Diagnostics capability:
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In the interests of improving the end-user experience with modems,
Microsoft and the industry are implementing a Modem Diagnostics
Specification, as well as code that uses it. Some of the elements in
this specification are difficult for traditional modems to provide
because the necessary information is buried in the DSP. In a soft modem,
that information is not hidden in a chip--it is accessible because it is
part of the software.
Examples for Controllerless modems: Linmodem and Winmodem.
What is a Winmodem?
Winmodems work only under Windows because they are not real modems. They are
essentially just digital signal processing (DSP) chips; all the modem
functions are accomplished in software that, in most cases, is available
only for Windows. This makes them cheaper than real modems.
What is a Linmodem?
A Linmodem is the Linux implementation of a "winmodem". These devices are
'less than' a modem in the sense that they depend on software to perform, to
a greater or lesser extent, the functions traditionally handled by modem
hardware. The rationale for this is, of course, that software is cheaper
than hardware, and can be upgraded/expanded/improved without the use of
screwdrivers (usually); however, for the modem to function at all, one
requires software that can run on one's preferred operating system.
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Thanks and regards,
sathish.j
