17738_modem

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CONTENTS

All about modems.

Comparison of various types of modem.

Modem speed.

Modem standards.

Data compression and error correction.

Installation ,configuring & trouble shooting.

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All About Modems

Modem - device used by PC to

communicate over phone lines

External or embedded

Both hardware and firmware

Firmware stored in ROM chips containprotocol and instructions to format and

convert data for transport

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All About Modems (continued)

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PC data is communicated to the modem as

binary or digital data

Modem converts data to analog signal

(modulation) to travel over phone lines

Receiving modem converts analog data into

digital data (demodulates)

Data is converted from two states (0 and 1)

to waves that have an infinite number of

states

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Modems use different characteristics of

wave to correspond to the 0s and 1s of

digital data

Modem uses an RJ-11 connection to aphone wall outlet

Usually offers slot for telephone

Half-duplex transmit in one direction only

Full-duplex transmit in both directions at

once

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Comparison of Modems

Internal modem vs External modem

Standard vs Intelligent modems

Short haul vs Wireless modems

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Internal vs External modem

External Modem: This is a modem

separated from the system unit in the

computer case. It is connected to the serial

port of the computer by means of a cable.It is connected to the telephone wall jack

by another cable.

Internal Modem:An internal modem is a

circuit board (a modem card) that can be

added to the system unit of the computer. It

takes one of the expansion slots

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Standard vs. Intelligent Modems

Standard Modems:Modems used today arecalled standard modems. These modems are usuallyoperated by commands entered from amicrocomputer keyboard. Users control the functions

(dialing, etc.) of a modem through the keyboard. Mostmodems use a popular language developed by acompany "Hayes Microcomputer Products, Inc."

Intelligent Modems: Intelligent modems are alsocalled advanced modems. These modems canaccept new instructions and then respond to thecommands while transmitting data and information.These can be done by microprocessor chips andinternal read only memory (ROM) contained in themodem.

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Short-Haul and Wireless

Modems Short-Haul Modems: Short- haul modems are

devices that transmit signals down the cable through anycommon port.They are also called line drivers that cansend data for a distance of more than one mile. This type

of modem can be used within or across several buildingsin a company or a university campus.

Wireless Modems: Wireless modems transmit thedata signals through the air instead of by using a cable.They sometimes are called a radiofrequency modem. Thistype of modem is designed to work with cellular

technology, and wireless local area networks. Wirelessmodems are not yet perfected, but the technology israpidly improving.

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Cable modem

Fig .Motorola cable modem for broadband internet

access.

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Cable modem (contd.)

A cable modem is a unique type of modem that is

designed to modulate a data signal over cable

television infrastructure.

The term cable Internet access refers to the

delivery of Internet service over this

infrastructure.

Bandwidth of residential cable modem service

typically range from 3 Megabits per second

(Mbit/s) up to 15 Mbit/s or more.

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FAX MODEM

A fax modem in PC can send data to a

standard fax machine, which converts data

into hard copy form.

Incoming faxes received as image files are

saved to the PCs hard disk.

Fax modems provide certain advantages

over stand alone fax machines.

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ISDN MODEM

ISDN is an abbreviation for Integrated Services

Digital Network.

There is no such thing as a noisy or fuzzy line.

Always runs at 64kbps.

Call Setup is very fast (dialing, handshake and

authentication), usually under 2 seconds

(compared with 15 seconds at least for mostanalog modems).

Ability to get 2 calls at the same phone number

(one line is for voice, and the other line for your

internet connection).

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ISDNSYSTEM

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DSL MODEM

DSL Modems: DSL (Digital SubscriberLine) service is usually offered through thephone the company/ISP.

You can leave your Internet connection openand still use the phone line for voice calls.

The speed is much higher than a regular

modem (up to 1.5 Mbps ).

DSL doesn't necessarily require new wiring; itcan use the phone line you already have

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Modem Speeds

Modem speed partly determined by

transmission standard in use

Usually measured in bits per second (bps)

Baud rate is an older measurement ofnumber of times a signal changes in one

second

One signal equals one bit

Most common today is 56.6 Kbps

Use Control Panel to determine modem

rating

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The Ceiling on Modem Speeds

Speed is limited by various factors

Analog phone lines not designed for data

transmission

Digital phone lines have limits, too

Most signals are converted from analog to

digital and back again as they travel frombeginning to end

Noise on the line also affects performance

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Modem Standards

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Data Compression

Firmware housed on the modem can

perform error correction and compression

When data compression is performed on the

modem, it applies to all data transmitted

Software compression on the PC applies to

single-file transfer operations

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ErrorCorrection

Several transmission standards include

error correction

Modem may support more than one method

Method is selected during handshake

Error correction breaks data into small

packets, frames

Sending modem performs calculation on the

frame and sums into a checksum

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ErrorCorrection (continued)

Checksum is used to check or verify the

data later

Checksum is attached to the data frame Transmitted together

Receiving modem

Performs same calculation

Confirms answer against attached checksum

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Serial Port (UART) Settings

UART-to-UART (universal asynchronous

receiver-transmitter) communication

Serial port communication settings

Use device manager to see settings

Internal modem has its own UART on the

modem card to provide serial port logic

Internal modem must be assigned its own

COM port

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(continued)

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(continued)

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Modem Features

CallerID and call waiting

Display readout

Fax capabilities

Flash ROM

Plug and Play for Windows

Auto answer

Expansion slot or port type

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Installing and Configuring a

Modem Internal modem

Read documentation

Determine serial port availability

Set jumpers orDIP switches if necessary

Turn off computer and remove case

Find empty slot, remove the faceplate

Mound card firmly in slot, use screw tosecure

Replace cover, attach phone line and test

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Installing and Configuring a

Modem (continued) External Modem

Turn power off on computer

Use high quality cable and attach it to computerand modem

Plug electrical cord into power outlet and attach

phone line to modem

Turn on PC and look for system to recognize newhardware

Use Control Panel to verify configuration

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Testing a Modem

Use HyperTerminal by following onscreen

instructions

OS can perform diagnostic test

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Testing a Modem (continued)

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Troubleshooting Guidelines for

Modems If the modem does not respond:

Confirm all cabling

Confirm phone line from modem to wall jack is in

the line-in port

Verify wall jack by testing with telephone

Confirm configuration setting for dialing extra

character to reach outside line if necessary

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Modems (continued) If new installation and modem does not respond:

Read documentation

Confirm COM settings on modem and software

Look for other devices on the same COM

Internal modem: check DIP switches against OS Internal modem: use CMOS setup to disable serial

port to eliminate conflicts

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Modems (continued) Internal modem: confirm card is seated in slot

External modem: check cabling and test withknown-good cable

Using Device Manager, uninstall device, r-boot

and reconfigure drivers

Test with known good modem

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The modem says there is no

dial tone, even though you can

hear it

Check phone lines

Check if other equipment if present, i.e. faxmachine

Try ATO command

Straighten phone lines

Modem may be damaged by lightening

Test with known-good modem

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The modem dials and then says

that the other end is busy, even

when you know that it is not

International calls, try ATXO command first

Straighten phone lines and remove extra

equipment

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The sending modem and the

receiving modem take a very long

time to negotiate the connection

Noisy phone line

Remove extra equipment

Turn off data compression and try again

Turn off error correction and try again

Force slower speed

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The modem loses connection at

odd times or is slow Check communications software for speed

assigned. Look for error with modem speed and

port speeds Noisy phone line

Phone line too long

Disable call waiting using *70

Reinstall modem

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The modem drops the

connection and gives the NO

CARRIER message

Check other extensions on phone line

Disable call waiting

Remove extra equipment on line, straighten lines

Check modem setting; confirm Error control

required to connect is not checked

Try reducing speed

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You get nothing but garbage

across the connection Check port settings

Try 8 bits, no parity, one stop bit (8,No,1)

Slow down the port speed

Slow down the modem speed

Try different modulation type

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