Modem

1. Short for MODulator/DEModulator, the first Modem known as the Dataphone, which was first released by AT&T in 1960. It later became more common for home users when Dennis Hayes and Dale Heatherington released the 80-103A Modem in 1977.

A Modem is a hardware device that allows a computer to send and receive information over telephone lines by converting digital data into an analog signal used on phone lines. In the picture below, is an example of an internal expansion card Modem. Click the image to get a description about each of the components found on the card.

Internal modem

Modems are referred to as an asynchronous device, meaning that the device transmits data in an intermittent stream of small packets. Once received, the receiving system then takes the data in the packets and reassembles it into a form the computer can use.

Stop
1 bit
Data
8 bits
Start
1 bit
Stop
1 bit
Data
8 bits
Start
1 bit
Packet
10 bits
Packet
10 bits

The above chart represents how an asynchronous transmission would be transmitted over a phone line. In asynchronous communication, one byte (eight bits) is transferred within one packet, which is equivalent to one character. However, for the computer to receive this information, each packet must contain a Start and a Stop bit; therefore, the complete packet would be ten bits. In the above chart is a transmission of the word HI, which is equivalent to two bytes (16 bits).

Dial-up Modem MP3 audio fileIf you did not grow up on a dial-up Modem or you are nostalgic, click the example MP3 link to the left, to hear a Modem connecting to the Internet. In this audio file, you hear the Modem dialing a phone number and then communicating with the other Modem over the phone line. The squealing noise heard after the phone number is the Modem establishing a connection (handshaking). Once the connection is established the Modem goes silent.

Types of computer Modems

Below are the four versions of a computer Modem found in computers.

Onboard Modem is a Modem that is built onto the computer motherboard. These Modems cannot be removed, but can be disabled through a Jumper or CMOS Setup.

Internal Modem connects to a PCI slot inside a newer desktop computer or ISA slot on an older computer. The Internal Modem shown at the beginning of this document is an example of a PCI Modem.

External computer modemExternal Modem is located within a box and is hooked up externally to the computer, usually the Serial Ports or USB port. The picture is an example of an external USRobotics Modem.

Removable Modem that is used with older laptops PCMCIA slot and is removed when you need the PCMCIA slot for another device, but are not planning on using the Modem.

Related pages

2. A Modem or Broadband Modem is also used to refer to the hardware device that connects a computer or router to a broadband network. For example, a Cable Modem and DSL Modem are two examples of these types of Modems.

Also see: Cable modem, Connection, Dial-up, External modem, Internet terms, Modem terms, Modulate, RJ-11, RTS, RXD