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

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

Comparison of Modems • Internal modem vs External modem • Standard vs Intelligent modems • Short haul vs Wireless modems

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

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.

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.

Cable modem Fig .Motorola cable modem for broadband internet access.

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.

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.

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).

ISDN SYSTEM

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

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

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

Modem Standards

Modem Standards (continued)

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

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

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

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

Serial Port (UART) Settings (continued)

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

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

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

Testing a Modem • Use HyperTerminal by following onscreen instructions • OS can perform diagnostic test

Testing a Modem (continued)

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

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

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

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

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

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

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

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

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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