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 continued…

Modem Transmission Standards continued…

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 about the 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 Commands CommandDescription ATGets modem’s attention *70Disable call waiting +++Escape sequence OnGo Online A/Repeat last command InIdentification Zn &F Reset Factory default continued…

Core AT Commands CommandDescription AAnswer the phone DnDial HnHang up MnSpeaker control LnLoudness XnResponse

HyperTerminal Controlling a Modem Using AT Command

Interchange Between a Modem and a User

Other Examples of AT Commands

Using AT Commands to Help Diagnose Modem Problems

Troubleshooting Guidelines for 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