Modems

• Problem– Computer is digital– Telephone line is analog– Need translation device called a modem

DigitalSignal

Modem

AnalogSignal

Modems• Note:

– Device is digital;Transmission line is analog

– Cannot just say “modem translates between analog and digital” (For analog device and digital transmission line, need something else)

DigitalDevice

AnalogTransmission Line

Modulation

• A Modem is a Modulator and Demodulator

– Modulation is converting outgoing digital device signals into analog transmission line signals

– Demodulation is converting incoming analog transmission line signals into digital device signals

DigitalSignal

Modem

AnalogSignal

Modulation

• Example: Frequency Modulation– 1 is a high-frequency vibration– 0 is a low-frequency vibration

1

0

1

1

Modem Forms

• Internal Modem– On printed circuit board inside PC– Does not take up desktop space– Can be complex to install--must open up

computer

Modem Forms• External Modem

– Easy to install (just plug into serial port)– Takes up desk space– Needs wire to serial port (adds to wiring

clutter)– Needs electrical power

Modem Forms• PC Card Modems

– Fit in PC Card slots on notebooks– Easy to install– Do not take up desktop space– Expensive– Few desktop PCs have PC Card slots

Modem Connection for Internal Modem

Phone Line toTelephone

Phone Line toWall Jack

Modem Connection for External Modem

PhoneLine to

Telephone

Wall Power:usually uses

“brick” transformer

Serial Cableto Serial

Port

Phone Line toWall Jack