Modems

1
Modems

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

Analog Signal
Digital Signal
Modem
2
Modems

• Note
• Device is digitalTransmission line is analog
• Cannot just say modem translates between analog
  and digital (For analog device and digital
  transmission line, need something else)

Digital Device
Analog Transmission Line
3
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

Analog Signal
Digital Signal
Modem
4
Modulation

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

0
1
1
1
5
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

6
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

7
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

8
Modem Connection for Internal Modem
Phone Line to Wall Jack
Phone Line to Telephone
9
Modem Connection for External Modem
Serial Cable to Serial Port
Phone Line to Wall Jack
Phone Line to Telephone
Wall Power usually uses brick transformer