Supporting Modems

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Chapter 10

• Supporting Modems

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You Will Learn

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

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All About 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, which can use either an ISA
  or PCI slot

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External Modem
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Modem Card
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Modem Manufacturers
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All About Modems

• Some motherboards have a small expansion slot
  called an audio/modem riser (AMR) or a
  communication and networking riser (CNR) slot
• Are both hardware and firmware
• MOdulation/DEModulation
• Convert digital data to analog, then back to
  digital
• Provide an RJ-11 connection
• Must be able to both receive and transmit data
  (full-duplex)

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AMR Slot
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Modulation/Demodulation
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RJ-11 Connection
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How Modems Are Rated

• Modem properties affect performance and
  compatibility with other modems
• Speed
• Protocols
• Data compression
• Error correction
• Getting started handshaking or training

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Modem Speeds

• Measured in bits per second (bps)
• 56.6 Kbps most common

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Viewing Modem Ratings
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The Ceiling on Modem Speeds

• Analog phone lines were designed for transmission
  of human voice affects ability to attain high
  transmission speeds for data
• Method used when an analog signal is converted to
  a digital signal
• Noise (line disturbance)

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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

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Modem Transmission Standards
continued
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Modem Transmission Standards
continued
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Modem Transmission Standards
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Data Compression on a Modem

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

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Error Correction on a Modem

• Performed by firmware housed on the modem
• Works by breaking data up into small packets
  (frames)
• Sending modem performs calculations on a frame,
  then sums calculations into a checksum

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Key Points aboutthe Handshaking Process

• Answering modem sends a guard tone so connection
  is not broken
• Carrier is established
• Equalization stage
• Modem speed (line speed) is set between modems
• Modem speakers turn off

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Serial Port Settings

• Serial ports that modems use are controlled by
  UART logic
• UART-to-UART communication is controlled by port
  settings
• Five protocols control these port settings

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Port Settings and Modem Settings

• Communication between DTE and DCE (computer and
  modem) is controlled by port settings
• Communication between two DCE devices (two
  modems) is controlled by modem settings

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Port Settings and Modem Settings
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Modem Features

• Caller ID
• Display readout
• Flash ROM
• Plug and Play for Windows
• Voice/data capability
• Auto-answer

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Installing an Internal Modem

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

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Setting Jumpers
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Installing an External Modem

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

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Configuring a Modem

• Start-Settings-Control Panel-Modem-Properties in
  Windows 98
• 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
• Create a Dial-Up Networking connection if desired
• Test the modem with HyperTerminal

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The AT Command Set

• Hayes language for modem control
• Each command line is prefaced with AT for
  ATtention
• A modem that uses this language stays in command
  mode any time that it is not connected to another
  modem

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Core AT Commands
continued
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Core AT Commands
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HyperTerminal Controlling a Modem Using AT Command
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Interchange Betweena Modem and a User
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Other Examplesof AT Commands
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Using AT Commands to Help Diagnose Modem Problems
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Troubleshooting Guidelinesfor Modems

• What works and what doesnt work?
• What has worked in the past that doesnt work
  now?
• What has changed since things last worked?

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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 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

• During 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 connection and gives NO CARRIER
  message

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Potential Modem Problems

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

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Troubleshooting Modems
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Chapter Summary

• How modems work
• Installing and troubleshooting modems