1 contents all about modems. comparison of various types of modem. modem speed. modem standards....

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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 contain protocol and instructions to format and convert data for transport

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All About Modems (continued)

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All About Modems (continued)

• 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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All About Modems (continued)

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All About Modems (continued)• Modems use different characteristics of

wave to correspond to the 0s and 1s of digital data

• Modem uses an RJ-11 connection to a phone 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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All About Modems (continued)

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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 are called standard modems. These modems are usually operated by commands entered from a microcomputer keyboard. Users control the functions (dialing, etc.) of a modem through the keyboard. Most modems use a popular language developed by a company "Hayes Microcomputer Products, Inc."

• Intelligent Modems: Intelligent modems are also called advanced modems. These modems can accept new instructions and then respond to the commands while transmitting data and information. These can be done by microprocessor chips and internal read only memory (ROM) contained in the modem.

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Short-Haul and Wireless Modems

• Short-Haul Modems: Short- haul modems are devices that transmit signals down the cable through any common port.They are also called line drivers that can send data for a distance of more than one mile. This type of modem can be used within or across several buildings in a company or a university campus.

• Wireless Modems: Wireless modems transmit the data signals through the air instead of by using a cable. They sometimes are called a radiofrequency modem. This type of modem is designed to work with cellular technology, and wireless local area networks. Wireless modems are not yet perfected, but the technology is rapidly 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 PC’s 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 most analog 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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ISDN SYSTEM

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DSL MODEM

• DSL Modems: DSL (Digital Subscriber Line)  service is usually offered through  the phone the company/ISP.

• You can leave your Internet connection open and 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; it can 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 of

number 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 from beginning to end

• Noise on the line also affects performance

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Modem Standards

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Modem Standards (continued)

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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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Error Correction

• 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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Error Correction (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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Serial Port (UART) Settings (continued)

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Serial Port (UART) Settings (continued)

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Modem Features

• Caller ID 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 or DIP switches if necessary Turn off computer and remove case Find empty slot, remove the faceplate Mound card firmly in slot, use screw to

secure 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 computer

and modem Plug electrical cord into power outlet and attach

phone line to modem Turn on PC and look for system to recognize new

hardware 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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Troubleshooting Guidelines for 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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Troubleshooting Guidelines for Modems (continued)

Internal modem: confirm card is seated in slot

External modem: check cabling and test with known-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. fax

machine• 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