Modem Concepts

Module NG30c

Contents

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Modem Background

What is a modem for?Modem: front view


When you want your computer to communicate over normal phone lines,you need a modem, because...
  • Computers use digital codes
  • Phones use analog voltages
  • Modems translate so computers can use the phone to communicate
If you connect to the Internet by TV cable, ISDN phone line, or DSL phoneline, your computer is sending digital codes, not analog voltages. Of course,you need a device to connect your computer to these lines, but technicallythey are not "modems" because they don't convert from digital to analogsignals. (In practice, many people call them "modems" anyway!)

What the word “modem” comes from

MOdulator-DEModulator = analog - digital

Overview of the communication process

(Figure from module NG02c.htm)
Overview of the communication process

Modem Technicalities

Setting up a Modem

Business end of modem, with wires close by.
  • Modem connecters and external components
    • Line to the phone: RJ-11 connector

      • RJ-11 plug: top sideRJ-11 plug: under side
    • Line to the computer: Serial line using RS-232

      • RS-232 plugs: 25-pin and 9-pin (in this case, both ends of the same cable)
  • At the phone end:

    • Plug phone line into modem, phone into modem 
  • At the computer end: about serial ports
    • Serial Connectors (Plugs)
    • RS-232 25-pin
    • RS-232 9-pin
    • DIN 9-pin (Macintosh only)

      • RS-232 25-pin with DIN-9 plug for Macintosh serial communication
  • Serial port capabilities
    • Older serial ports had a limit of 9600 baud
    • Newer ones have higher limits

Modem speeds: what they mean to you

  • Speed: the "baud rate"
    • Roughly = bits-per-second
    • Compression is often used, squeezing in more information per bit
    • Characters per second: divide by ten
  • Availability: Can you get a modem this speed; will it run on your lines?
  • Cost: approximate “street price”
  • What’s it good for: when is it worth using this speed?
 
Speed
Availability
Cost
What's it good for?
300, 
1200
Obsolete; this speed is available on fastermodems
* 
Primitive, noisy phone lines
2400
Obsolete; this speed is available on fastermodems
* 
Poor phone lines
9600
Obsolete; this speed is available on fastermodems
* 
Very old PCs
14,400
Obsolete; this speed is available on fastermodems
* 
Marginal service
28,800
Old, but still widely used
* 
OK service
33,000 
No longer available, but still used
* 
Good service
56,600 
Current
$30-250 
Best available on standard phone lines

What are those lights trying to tell you? 

If you have an external modem, you may be curious about what those littlelights mean. If you have problems, they may help you figure out what'sgoing on (or going off!). 

Modem front panel: close-up
 
Light
Abbreviation means:
Trying to tell you:
Power
Modem is on
AAAuto AnswerModem will answer phone
CDCarrier DetectModem detected a remote modem it can talk to
RDReceived DataModem got information from a remote modem
SDSend DataModem sent some information
TR or DTRData Terminal ReadyModem is on; or modem has been asked by thecomputer to get ready
CS or CTSClear to SendModem is ready to send information
OH
Off Hook 
Phone line is "live" (receiver off hook)
HS
High Speed 
Not low speed ;-)

Communication Parameters: what they mean

See also module NG34w, Troubleshooting Modems
  • Data bits: How many bits in a character unit (8 or 10)
  • Parity: Simple error-checking code – Even, Odd, or None
  • Stop bits: these signal the end of a character unit

Terminal Emulation

See module NG31c, Communications Software Conceptsto answer questions like... 
  • What is a terminal?
  • Why emulate one?
  • When would you need to use terminal emulation?
  • What are some common types of emulation?

Communication Software

See module NG31c, Communications Software Conceptsto answer questions like... 
  • What is communication software's job?
  • What types of communication software are available?
  • How do you configure communication software to connect to the Internet?

Making the Connection

 
Review module NG02c, What'sInvolved in Connectivity for details (including audio demonstration)of... 
  • Dialing the remote computer
  • Carrier detect
  • Speed negotiation
  • Turning off the speaker
  • Signalling the computer that a connection has been made
  • Doing your Stuff
  • Disconnecting

About this document...

Audience:

This is for people who want know what a modem is and a littleabout how to operate one.

Objectives:

When you successfully complete this lesson, you willbe able to...
  • Understand what a modem’s purpose is
  • Understand what "modem" means
  • Understand what is needed to connect an external modem to a typical computerand telephone
  • Understand what some typical modem display lights mean
  • Understand the typical baud rates used, and what their practical use is
  • Understand communications parameters: stop bits, parity bits, data bits
  • Understand what modem protocols are and what they mean to users
  • Understand the process needed to install and set up communications software
  • Understand the process needed to configure communications software to aspecific remote modem
  • Understand the use of common types of terminal emulation
  • Understand the process needed to establish a connection with a remote computer

Module :

This document is part of a modular instruction series in Computer InformationSystems. For more information, see the overviewor the list of modules in this series, NG: GeneralNetworking. 

Author:

Laurence J.Krieg

Institution:

Department of ComputerInformation Systems, WashtenawCommunity College
History:Original: 16 June 1995
Last modification: Monday, 05-Jun-2000 14:28:50EDT
Copyright:Copyright © 1999, Laurence J. Krieg.
Instructors: You may point to this file in your Web-basedmaterials.
Students: you may make a copy for your personal use.
All other uses: contact the author, LaurenceJ. Krieg for permission. Email krieg@ieee.org