Supporting Modems

1
Chapter 17

• Supporting Modems

2
You Will Learn

• How modems work and how to install them
• About software that modems use to communicate
  with the OS
• Guidelines for troubleshooting modems

3
Modems

• Devices used by PCs to communicate over phone
  lines
• Can be an external device connected to a USB or
  serial port
• Can be a modem card using either an ISA or PCI
  slot
• Can be a smaller and less expensive modem riser
  card

4
External Modem
5
Modem Card
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All About Modems

• Are both hardware and firmware
• MOdulation/DEModulation
• Convert digital data to analog, then back to
  digital
• Provide an RJ-11 connection
• Can be half-duplex or full-duplex

7
Modulation/Demodulation
8
RJ-11 Connection
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How Modems Are Rated

• By how they set communication rules (handshaking)
• By how fast they pass data over phone lines
  (measured in bits per second or bps)

10
Modem Speed

• 56 Kbps
• Most commonly used modem speed rating today
• V.92 standard
• Current standard for 56 Kbps transmission
• Maximum speed is often written into the
  manufacturers name for the modem

11
The Ceiling on Modem Speeds

• Analog phone lines were designed for transmission
  of human voice
• Affects ability to attain high transmission
  speeds for data
• Noise (line disturbance)

12
Modem Standards

• Written by the CCITT, now called the ITU
  (International Telecommunication Union)
• The more standards a modem supports, the better
  it can communicate with a variety of other modems

13
Modem Transmission Standards
14
Modem Transmission Standards (continued)
15
Modem Transmission Standards (continued)
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Data Compression

• Performed by firmware housed on the modem
• Follows one of three protocols (which also
  perform error correction)
• MNP-5
• CCITT V.42
• V.42bis

17
Error Correction

• Performed by firmware housed on the modem
• Breaks data up into small packets (frames)
• Sending modem performs calculations on a frame,
  then sums calculations into a checksum, which is
  attached to the data frame
• Receiving modem performs same calculations and
  runs a comparison of values before accepting
• Slows the process but guarantees almost 100
  accuracy

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Key Points about Handshaking

• Answering modem sends a guard tone so connection
  is not broken
• Answering modem establishes a carrier
• Equalization stage modem speed (line speed) is
  set between modems
• Modem speakers turn off methods of data
  transmission are agreed on

19
Serial Port (UART) Settings

• UART chip is responsible for communications over
  a serial port
• External modem is likely to use a serial port
• Internal modem has serial port (UART) logic on
  the modem card and must be assigned its own COM
  port that its UART controls
• Port settings control serial-port communication

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Port Settings
21
Port Settings (continued)
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Port and Modem Settings

• Communication between DTE and DCE (computer and
  modem) is controlled by port settings
• RS-232
• Digital
• Port speed

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Port and Modem Settings (continued)

• Communication between two DCE devices (two
  modems) is controlled by modem settings
• Phone line
• Analog
• Line speed

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Port and Modem Settings (continued)
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Modem Features

• Caller ID
• Call waiting
• Display readout
• Fax capabilities
• Flash ROM

• Plug and Play for Windows
• Voice/data capability
• Auto-answer
• Expansion slot or port type

26
Installing an Internal Modem

• Read documentation
• Determine available serial port
• Set jumpers or DIP switches on modem card
• Turn off computer remove case mount card
• Replace cover
• Plug phone line from house into line jack on modem

27
Setting Jumpers on an Internal Modem
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Installing an External Modem

• Connect high-quality cable to modem and to serial
  or USB port
• Plug electrical cord from modem into an 110V AC
  outlet
• Plug phone line from house into line jack on modem

29
Configuring a Modem Using Windows XP

• Windows Plug and Play will detect new hardware
  device
• Verify the OS configured the modem correctly
• Verify that modem speed is set to highest value
  in Maximum Port Speed text box
• To use Windows to make calls without using other
  software, create a dial-up Networking Connection

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Creating a Windows XP Dial-up Network Connection
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Configuring a Modem Using Windows 98

• Easier to use manufacturers drivers on CD
• When verifying modem properties
• Set modem speed at highest value in drop-down
  list
• Set port protocol at 8, No, and 1 (represents 8
  bits, no parity, 1 stop bit)
• Use hardware flow control

32
Creating a Windows 98 Dial-up Network Connection
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Ways to Test a Modem

• Use the HyperTerminal utility
• Use the OS to perform a diagnostic test
• AT commands

34
Testing a Modem with AT Commands
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Troubleshooting Guidelinesfor Modems

• What works and what doesnt work?
• What worked in the past but does not work now?
• What has changed since things last worked?
• Ask questions of users
• Try simple things first

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Potential Modem Problems

• Modem does not respond
• Modem says there is no dial tone, but you can
  hear it
• Modem dials, then says the other end is busy,
  even when you know it is not
• Sending modem and receiving modem take a long
  time to negotiate the connection

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Potential Modem Problems (continued)

• During a connection, it sounds as if handshaking
  starts all over again
• File transfers are slow
• Modem loses connection at odd times or is slow
• Modem drops the connection and gives NO CARRIER
  message

38
Potential Modem Problems (continued)

• When weather is bad, connection often disconnects
• When large files are downloaded, some data is
  lost
• Connection fails when large files are uploaded or
  downloaded
• Nothing but garbage across the connection

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Troubleshooting Modems
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Summary

• How modems work
• Installing and troubleshooting modems