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

continued…

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

continued…

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