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Modem

What is it?

  • Short for modulator-demodulator.

  • A modem is a network device that both modulates and demodulates analog carrier signals (called sine waves) for encoding and decoding digital information for processing. 

  • Enables a computer to transfer data over the telephone or cable lines.

Modem

(Modulator+Demodulator)

The process of converting binary information into analog signals.

Process of converting analog signal back to binary information

Input To PC (RJ45)

Cable from ISP

Power supply

How does it work?

  • Software in computer activates modem, which listens for a dial tone on phone line.

  • Modem dials a specified number.

  • The modem at receiving end answers calls and sends out a signal.

  • The modem for sending end converts a digital signal to analog signals.

  • The modem at receiving end converts analog signals back to digital signals. 

Computer

Modulator

Demodulator

 1010

 1100

Modem

ISP

Internet

Types of Modems

In terms of physical size and shape,

Modem

External Modem

  • Connected to the PC through a cable, which is plugged into a serial port on the back of the system unit.

Internal Modem

  • Is a network device that is contained on an expansion board that plugs into the motherboard. 

  • Known as onboard modems.

Wireless Modem

  • Wireless modem consists of modem and antenna.

  • Known as radio frequency modem. 

In terms of Connection to modem,

Modem

DSL Modem

RJ11 cable

Call + Internet 

  • High-speed connection that allows the phone wires to transmit modulated digital content at high speed.

Co-axial

cable

Cable Modem

TV signal + Internet 

  • Faster than the regular telephone modems.

  • An external cable modem is connected to either ethernet card or USB port on the computer.

  • DSL again divided into 2 types according to its downstream and upstream.

  • ADSL (Asymmetric Digital Subscriber Line)

  • SDSL (Symmetric Digital Subscriber Line)

Features

  • Transmission speed.

  • Voice or data communication.

  • Fax capability. 

  • Auto-answering.

  • The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. 

  • Modems are generally classified by the maximum amount of data they can send in a given unit of time, 

  • Usually in bits per second, eg. 32 Mbps

  • or bytes per second, eg. 6 MBps

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