Copyright © 2007 Heathkit Company, Inc. All Rights Reserved

PC Fundamentals

Presentation 45 Presentation 45 –– Modems Modems

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ObjectivesObjectives

At the end of this presentation, you will be able to:

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Explain three problems involved in sending data over ordinary telephone lines.

Explain the purpose of the Modem and the UART and briefly explain how each works.

Match the Modem speed to its “V dot” standard.

Recognize the “V dot” standards for Modem error correction and compression.

Match the Serial Port speed to its UART chip type.

List the de facto standards for the resources assigned to Serial Ports COM1 through COM4.

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Local Exchange

Local Exchange

AnalogAnalog

DigitalDigital

DigitalDigital

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Local Exchange

Local Exchange

AnalogAnalog

DigitalDigital

DigitalDigital

ModemModem ModemModem

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Modem-1

PC-1

Modem-2PC-2

Phone System

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CPU vs. Phone LineCPU vs. Phone Line Digital Signal High Speed Parallel Data

Analog Signal Low Speed Serial Data

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Analog

Digital

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9

The ModemThe Modem

Modulator / Demodulator

Converts the digital signals from the computer into the analog signals required by the telephone line.

Converts the analog signals from the telephone line into the digital signals required by the computer.

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Pulse Amplitude Modulation (PAM)

1 0 1 1 0 1

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11

Each Amplitude Level Represents Two Bits

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0001

10

11

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0º Phase Shift 90º Phase Shift

180º Phase Shift 270º Phase Shift

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13

BaudBaud

Baud - A single state change of the analog signal.

Baud Rate - The number of times per second that the analog signal changes state.

In early modems, one baud usually represented one bit.

In today’s modems, one baud represents more than one bit.

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Low Speed

14

High Speed

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10101110

ParallelParallel

10101110 SerialSerial

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The UARTThe UART

Universal Asynchronous Receiver/TransmitterThe heart of the Serial (COM) port.Converts the parallel data required by the CPU

into the serial data required by the serial port.Converts the serial data received at the serial

port into the parallel data required by the CPU.

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Making the CPU and Phone Line Making the CPU and Phone Line Compatible (Transmit).Compatible (Transmit).

CPUCPU

UARTUART ModemModem Phone Line

Phone Line

1. High Speed to Low Speed

2. Parallel to Serial

Digital to Analog

BufferMemoryBuffer

Memory

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Making the CPU and Phone Line Making the CPU and Phone Line Compatible (Receive).Compatible (Receive).

UARTUART ModemModem Phone Line

Phone Line

1. Serial to Parallel

2. Low Speed to High Speed

Analog to Digital

CPUCPU

BufferMemoryBuffer

Memory

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Two Types of ModemsTwo Types of Modems

Internal – ISA or PCI board that fits inside the computer.

External – Stand-alone box. Connects to one of the COM Ports.

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Internal ModemInternal Modem

CPUUART

FunctionModem Function

Phone Line

Internal Modem

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CPU UART External Modem

Phone Line

Motherboard

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CPU UART External Modem

Phone Line

Mother-board

I/OAdapter

Card

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Modulation StandardsModulation Standards

V.22 – 1200bps standard V.22bis – 2400bps standard V.32 – 9600bps standard V.32bis – 14,400 standard V.FC – 28,800 standard V.34 – 33,600 standard V.90 – 56kbps download, 33.6 kbps upload

standard V.92 – 56kbps download/upload standard

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V.42 Error-Correction StandardsV.42 Error-Correction Standards

Link Access Procedure for Modems (LAPM)

Microcom Networking Protocol – 4 (MNP-4)

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V.42bis Compression Standard.V.42bis Compression Standard.

Uses a compression technique to reduce the size of out going data.

Can compress data up to about 4 to 1. For example, a 33,600-bps Modem could

transfer data at a rates up to about 134,400 bps.

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The COM PortThe COM Port

Also called the Serial Port or RS-232 PortThe link between the computer and the Modem.The external Modem connects to the COM port

on the back of the computer via a serial cable.The internal Modem has a built-in COM port

that duplicates the function of the external COM port.

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9-Pin Serial Port25-Pin Serial Port

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Standard Modem COM Port Standard Modem COM Port SettingsSettings

COM Port IRQ I/O Address (Hex)

COM1 4 03F8

COM2 3 02F8

COM3 4 03E8

COM4 3 02E8

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CPU

UART ModemI/O

Memory

COM1

IRQ4

03F8

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CPU

UART ModemI/O

Memory

COM2

IRQ3

02F8

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134.4kbps

134.4kbps

33.6 kbps

33.6 kbps

33.6

kbp

s

33.6

kbp

s

134.4kbps

134.4kbps

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UART SpeedUART Speed

Determines the Serial Port speed. 16550 UART chip can reach speeds of 256

kbps.16440 UART chip can reach speeds of

115.2 kbps.8250 UART was supplied with the original

IBM XT machine.

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RJ-11RJ-11

RJ-11RJ-11

RJ-45RJ-45

InternalModem

InternalModem

NICNIC

Copyright © 2007 Heathkit Company, Inc. All Rights Reserved

PC Fundamentals

EndEnd