chapter 10 supporting modems. you will learn… how modems work and how to install them about...
TRANSCRIPT
Chapter 10
Supporting Modems
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
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
External Modem
Modem Card
Modem Manufacturers
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)
AMR Slot
Modulation/Demodulation
RJ-11 Connection
How Modems Are Rated• Modem properties affect performance and
compatibility with other modems Speed Protocols Data compression Error correction
• Getting started: handshaking or training
Modem Speeds• Measured in bits per second (bps)
• 56.6 Kbps most common
Viewing Modem Ratings
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)
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
Modem Transmission Standards
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Modem Transmission Standards
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Modem Transmission Standards
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
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
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
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
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
Port Settings and Modem Settings
Modem Features• Caller ID
• Display readout
• Flash ROM
• Plug and Play for Windows
• Voice/data capability
• Auto-answer
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
Setting Jumpers
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
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
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
Core AT CommandsCommand Description
AT Gets modem’s attention
*70 Disable call waiting
+++ Escape sequence
On Go Online
A/ Repeat last command
In Identification
Zn
&F
Reset
Factory default
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Core AT Commands
Command Description
A Answer the phone
Dn Dial
Hn Hang up
Mn Speaker control
Ln Loudness
Xn Response
HyperTerminal Controlling a Modem Using AT Command
Interchange Betweena Modem and a User
Other Examplesof AT Commands
Using AT Commands to Help Diagnose Modem Problems
Troubleshooting Guidelinesfor Modems
• What works and what doesn’t work?
• What has worked in the past that doesn’t work now?
• What has changed since things last worked?
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
Troubleshooting Modems
Chapter Summary• How modems work
• Installing and troubleshooting modems