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What Is A Modem?

What Is A Modem

Date First Published: 3rd March 2022

Topic: Computer Networking

Subtopic: Network Hardware

Article Type: Computer Terms & Definitions

Difficulty: Medium

Difficulty Level: 4/10

Learn more about what a modem is in this article.

Short for a modulator-demodulator, a modem is a device that allows devices, such as smartphones, tablets, computers, routers, and switches to connect to the internet. It modulates (converts) an analogue carrier signal from a telephone or cable wire to digital data, which include 1s and 0s that a computer is able to recognise. In addition, it converts digital data from another device into an analogue signal that can be transferred over standard telephone lines. This device is pronounced 'mow-dem'.

Modems both modulate and demodulate analogue carrier signals, also known as sine waves for the purpose of encoding and decoding information for processing. Modulation and demodulation are performed by a modem at the same time and therefore, the term modem is a combination of the words 'modulate' and 'demodulate'.

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Modems are both considered input and output devices because they both send (upload/input) and receive (download/output) data.

How Do Modems Work?

Modems enable a computer to send and receive information over telephone lines. Modems work by 'modulating' (converting) digital data to an analogue audio signal and then transferring it over a telephone line. In a similar way to when an analogue signal is received, the modem demodulates it (converts it back into a digital signal).

Types Of Modems

  • Internal modem - A modem that connects to an expansion slot of a computer. Internal modems connect to a socket on the motherboard inside a desktop or laptop computer. These normally come preinstalled within the computer.
  • External modem - A modem that externally connects to a computer through a USB or serial port. It can be used when a computer is unable to fit an internal modem inside.
  • Removable modem - Modems that are used with the PCMCIA slot of older laptops and can be added or removed when necessary.

Speed Of Modems

The speed of modems is measured in B/s (bits per second) and Kb/s (kilobits per second). This is the speed that modems can send and receive data. Normally, modern dial-up modems have a speed of 56 Kb/s. The line quality that the modem communicates over has an effect on the speed. The fastest speed that is used in today’s dial-up modem is 56 Kbit/s, which is equal to 56,000 B/s.

Earlier speeds (bauds) of modems included 110 baud, 300 baud, 1200 baud, 2400 baud, 4800 baud, 9600 baud, 14.4 Kbit/s, 28.8 Kbit/s, and 33.6 Kb/s. Baud is a unit that is used for measuring the speed of data transfer, equal to the number of times a signal changes state in the transmission medium per second in a digitally modulated signal.

Difference Between A Modem and A Router

It is commonly believed that a modem and a router, perform the same functions, but they do not. A modem connects directly to the internet, decodes the signal from the ISP, does not set up a LAN, and is not responsible for Wi-Fi. A modem is a box that allows networks to connect to the wider internet. It will connect the source of the internet from a home to an ISP. The modem is connected to the router, which has the role of allowing all wired and wireless devices in a network to use that internet connection at once. It routes data to other devices, forms a LAN, does not decode the signal from an ISP, and a modem is required to connect it to the ISP.

If only one device is required to connect to the internet, a device can directly connect to a modem. Modems usually have one Ethernet port that a single device could connect to in order to get internet access. However, most networks have more than one device, so this method is impractical and is why routers are used. Routers separate an ISP network into different subnets.

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There are some modems with integrated routers that perform both functions of a modem and a router and can be rented from an ISP.


Modems have been around since 1960. The first commercial modem called the Bell 103 was released by AT&T, an American telecommunications company at this time period. It included full-duplex transmission, frequency-shift keying and a speed of 300 B/s. These modems were intended to hold the receiver of a telephone in a cradle with wired connections running from the cradle to the computer.

When bulletin board systems became more popular, this meant that users were in need of faster speeds for transferring larger files and images. Market demand led to the development of the V2.2 or Bell 212 modem at 1200 baud. This new modem enabled more characters and introduced a simple data compression that worked well with text but had poor results with images.

The first PC modem, known as the 80-103A was created by Dale Heatherington in 1977. This modem offered the necessary features and connected directly to the phone. The success of the first PC modem caused DC Hayes Associates, later known as Hayes Microcomputer Products to start. A range of landmark technologies, such as the Smartmodem and Hayes Command Set were developed.

The Smartmodem was special as it had the ability to convert from data to command mode using a sequence known as the Hayes Command Set, which included a specific "guard time" in order to prevent the data being sent from confusing the modem.

Although it has been greatly improved since its introduction to handling faster speeds and better technology, the Hayes Command Set has remained one of the most popular.

In addition, faster rates were introduced. The transfer rates were increased, new data compression standards were developed, and V.42 was released, the Auto Reliable Protocol, shortly after 2400 baud modems became popular on V.22 BIS.

The speed was much faster, and the communications were significantly more error-free and better compressed. In the 1990s, the 4800 baud modem was replaced by the 9600 baud modem, which was introduced in 1991. The other used single sideband communication, which allowed modems to make use of two channels instead of one on the phone line.


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