Modems




 

 
NetFAQs Home >> Windows >> Dial Up Connections

Modems: Advanced

Links: Advanced Overview | Soft vs. Hard | Init Strings | Glossary | Back to Modem Basics Page |

 

Advanced Modem Overview

Modem - A hardware device usually attached to a computer which allows the computer to communicate with another remote modem over a telephone line.  A modem does for computers what a telephone does for humans.  Just as a telephone set allows two humans to speak to one another over telephone wires, a modem allows two computers to speak to each other over telephone wires.  

Modems can be External or Internal, and Hardware-based or Software-based, or any combination of these four.

Modem Components - All modems, regardless of whether they are of the internal, external, or PC Card variety, include three key components in order to function:

1) Interface - connection between the modem and pc ("internal pci/isa" or "external serial"), 
                  - connection between the modem and the telephone line
2) Datapump - Main part of the modem. Converts analog to digital.
             - DSP (Digital Signal Processor) handles datapump on most modems.
             - HSP (Host Signal Processor) modems let the host's cpu handle the datapump.
3) Controller - Carries out commands sent to the modem.  
                   - Handles Flow Control between the computer and DSP.

Modem Types: 
External Modem - Connects to the computer through a serial interface on a COM port.

Internal Modem - Connects to the computer through a PCI/ISA slot.  Creates a virtual COM port.

Hardware Modem - The Datapump and Controller are both housed on the modem.
                        - Can be either an external or an internal modem.  

Software Modem - The Controller function is handled by the host computer's CPU.
                       - The Datapump typically still resides on the modem in a DSP (Digital Signal Processor).
                       - For some Soft Modems, the datapump is handled by the CPU, 
                            This is called HSP (Host Signal Processor)
                       - Almost all internal 56k modems are Software Modems.  External modems never are.

 

Software vs. Hardware Modems

Hardware Modems:

Traditional external modems connect to the computer through a serial port (COM port). These modems are never "WinModems", as everything required for the modem to function is contained within the modem itself.  There is a "Controller" which controls Error Correction, Compression, Flow Control, etc.  There is also "DSP" or "Datapump" which handles the main function of a modem:  analog-to-digital conversion.
    Interface - serial connection to computer, phone jack connection to wall
    Datapump - Handled inside the modem by a DSP (Digital Signal Processor).
    Controller - Handled inside the modem.

Traditional internal modems do not connect to the computer through a serial port, and so must emulate a standard COM port so that standard modem applications function.  An internal modem is known as a "Hardware" modem if it maintains all of the functions of an external modem.
    Interface - (1) pci/isa slot on motherboard, (2) phone jack connection to wall
    Datapump - Handled inside the modem by a DSP (Digital Signal Processor).
    Controller - Handled inside the modem.

External modems, V34 modems and laptop modems are 90% likely to be "Hardware" modems.
Internal modems are typically NOT "Hardware" modems.  
Common internal hardware controller based modems:
    Conexant/Rockwell ACF 
    Lucent/Agere Venus 
    Many USR models

Software Modems (WinModems):

Most PCI slot internal modems, however, are "Software" modems (a.k.a. - "WinModems"). "WinModem" is a widely used term used to refer to any modem that does not handle all of the functions of a traditional modem, but depends upon an operating-system driver in order to work.  There are actually 2 types of software modems: those that contain a DSP (digital signal processor), and those that don't (usually referred to as Soft or HSP modems). 

In the DSP variety, the datapump function (the main part of a modem) is performed by a dedicated digital signal processor or DSP built in to the modem.  This reduces the impact on the Host system resources (i.e. CPU, RAM and I/O channel consumption).  
    Interface - (1) pci/isa slot on motherboard, (2) phone jack connection to wall
    Datapump - Handled inside the modem by a DSP (Digital Signal Processor).
    Controller - Not handled by the modem.  Offloaded to the Computer's main processor.

In the HSP variety, the the datapump function (the main part of a modem) is offloaded to the main cpu of the host computer (thus called "Host Signal Processing" or HSP).  Without a built-in DSP on the modem, the Host Computer's CPU has to perform these functions.  This steals the valuable cycles that are intended for running applications and other tasks.  <http://www.k56flex.com/dsp.htm>
    Interface - serial connection to computer, phone jack connection to wall
    Datapump - Not handled by the modem.  Offloaded to the Computer's main processor.
    Controller - Not handled by the modem.  Offloaded to the Computer's main processor.

Comparison Table of Controller-based modem, controller-less modem and software modem.

 

Controller-based Modem

Controller-less Modem

Software Modem

Command Protocol

Micro-controller

Host-CPU

Host-CPU

Command Interpreter

Micro-controller

Host-CPU

Host-CPU

Virtual Comport

Micro-controller

Host-CPU

Host-CPU

Error Correction

Micro-controller

Host-CPU

Host-CPU

Data Compression

Micro-controller

Host-CPU

Host-CPU

De/Modulation

Data Pump

Data Pump

Host-CPU

A/D, D/A Converter

Codec(AFE)

Codec(AFE)

Codec(AFE)

Echo Cancellation

Codec/DSP

Codec/DSP

Codec(AFE)

External modems, V34 modems and laptop modems are 90% likely to NOT be "Software" modems.
Internal modems are most typically "Software" modems.  
Common Software Based (no hardware controller) modems
Software Modem w/DSP
:
    Win Modems
    Soft Modems
    HSP Modems
    AMR Modems
    many USB modems
    Rockwell/Conexant HCF, HSF, Soft56
    Lucent/Agere 'LT', AMR, Scorpio, Mars, Apollo
    Motorola
    PCTel
    BCM
    IntelHam

Common Pure Software Based (no hardware controller or DSP) modems
Host Signal Processor based (no hardware controller or DSP)
:
    Win Modems
    Soft Modems
    HSP Modems
    AMR Modems
    many USB modems
    Rockwell/Conexant HCF, HSF, Soft56
    Lucent/Agere 'LT', AMR, Scorpio, Mars, Apollo
    Motorola
    PCTel
    BCM
    IntelHam

The most common "win" modems are:
    US Robotics 56k Win Modem
    LT Win Modem
    SupraMax 56k
    HSP MicroModem
    PCTel
    Motorola SM56
    ESS
    Zoltrix
    Shark
    Any modem with HSP in the name.

 

Init Strings

Command Sets: http://www.modemhelp.org/sets.html

Before your modem software dials a phone number, it initializes your modem by sending it a series of commands (strings or AT commands).  Strings configure the modems' parameters for error correction, data compression, flow control, etc.  The screenshot below illustrates this:

 

After querying a modem, you see a series of commands and responses.  ATI3, for example, commands the modem to display the version of the driver it is using.  As you can see below, the driver is version 9.00-9K.

 

 

 

Even with these commands, however, the software may still be incompatible.  Therefore, additional parameters need to be set.  This is where initialization strings come into the picture.

 

Initialization strings, commonly referred to as init strings, are not the same for all modems.  What works for one type of modem may not work for another.  Init strings are built using a series of digits (letters and/or numbers).  Each digit or set of digits plays a significant role in the string.

 

For example, each digit in the init string AT+MS=V34 tells the modem to behave a certain way.

    AT    - calls the modem to attention.

    +MS  - selects modulation

    =V34 - is the modulation that disables the V.90 protocol and instructs the modem to connect as if it were a 33.6.

 

All commands begin with AT.  This tells the modem that additional parameters are coming.  Commands are accepted by a modem during Command Mode. Your modem is automatically in Command Mode until it dials a POP to establish a connection.

 

Note:  for multiple AT commands in the same command line, the commands are executed in the order received from the computer (DTE). Should execution of a command result in error, or a character not be recognized as a valid command, execution is terminated, the remainder of the command line is ignored, and the ERROR result code is issued. Otherwise, if all commands executed correctly, only the result code associated with the last command is issued; result codes for preceding commands are suppressed.

 

Important:  slowing down a modem can actually improve performance and speed.  For example, if your computer is speaking faster than your modem can understand, say at 115200, slowing it down to 57600 allows the modem to understand what is being said.  It doesn't have to ask the computer to repeat anything.  (Ever try communicating with an angry Puerto Rican woman?  Excuse me, could you repeat that?)  Init strings can do the same thing, they modify the behavior of the modem, slow it down, enable/disable functions.

 

To learn more about AT commands and to view specific AT commands by manufacturer, click here.

 

Glossary of Modem Terms

Chipset - A chipset is a group of microchips designed to work and sold as a unit in performing one or more related functions.

Interface-
DSP-
Controller-
HSP - Host Signal Processor

 

Modem - A hardware device usually attached to a computer which allows comm... A modem does for computers what a telephone does for humans.  Just as a telephone set allows two humans to speak to one another over telephone wires, a modem allows two computers to speak to each other over telephone wires.

Driver

Init String

Chipset Manufacturer

Modem Manufacturer

PC Manufacturer -

OEM (Original Equipment Manufacturer) -

Boxed Retail -

 

IKANO Communications 1998 - 2017

webmaster@netfaqs.com