How does a Modem work ?

How does a Modem work ?

A modem, short for Modulator/Demodulator, is an electronic device used to access the Internet. Modems have been first introduced in 1960s and since then have been continuously evolving both in terms of functionalities and performances.

Why do we need a Modem ?

A modem is typically used to send and receive data over a phone or cable line. To do that, a Modem modulates the data to be transmitted in carrier waves signals and demodulates the incoming carrier waves signals to extract the received data. Wireless Modems convert the data into and from radio or infrared signals. Modems are generally stand alone external devices but can also be internal or built in components of computers, Fax machines …

modem modulator demodulator

Types of Modems

Different types of Modems exist:

Dial-Up Modem

A Dial-Up Modem uses the oldest technology for internet connection over the phone lines. This requires no additional infrastructure other than the telephone network to make and answer the calls and establish the connection with the ISP servers. Also and since it can only transmit and receive signals that are within the frequency range of voice communication, we can use the phone line either to access the internet or to make calls not both simultaneously.

dial up Modem
Dial Up Modem.

Data transmission rate for a dial-up connection increased from approximately 300 bit/s at the beginning of the 1960s to reach the maximum theoretical transfer speed of 56 kbit/s around the end of the 1990s.

Dial Up Internet Connection
Dial Up Internet Connection.

At the beginning of the year 2000, Broadband Internet access via cable, digital subscriber line (DSL), satellite and later on fiber optics started replacing dial-up access in many parts of the world. However, dial-up connection is sometime still the only choice available for rural or remote areas, where broadband installations are not prevalent due to low population density and high infrastructure cost.

DSL (Digital Subscriber Line) Modem

Also called DSL transceiver or ATU-R, it provides a broadband Internet access. A DSL transceiver connects either directly to the customer’s equipment via USB or 10 base-T Ethernet port or through a router. In the other side of the connection, the DSL service provider has a DSL Access Multiplexer (DSLAM) to receive customer connections.

dsl modem
1und1 Homeserver DSL Modem.

DSL is a family of technologies used to transmit digital data over telephone lines. DSL exploits the extra bandwidth capacity, or range of frequencies, to carry information on the phone line without disturbing the line’s ability to carry conversations. This allows a simultaneous use of a phone line for internet connection and voice calls.

The quality of a DSL connection is greatly affected by the distance to the nearest DSL provider’s central office (where the DSLAM equipment is located). The farther away you get from the central office, the weaker the signal becomes.

DSL Internet Connection
DSL Internet Connection.


The most commonly used DSL technology for internet access is asymmetric DSL (ADSL). In ADSL, the data throughput in the upload direction (the direction to the service provider) is lower than in download direction, hence the designation of asymmetric service. ADSL can theoretically provide up to 8 Megabits per second (Mbps) Download speed and up to 640 kilobits per second (Kbps) Upload speed. However, these speeds are practically never fully reached because these depends on the connection conditions and distance.

ADSL isn’t the only type of DSL, and it’s not the only way to get high-speed Internet access. Other technologies like VDSL (Very high bit-rate DSL), SDSL (Symmetric DSL) or Uni-DSL (Universal DSL) address the inherent distance limitation problem and some can offer up to 100 Mbps and 12 Mbps download and upload speeds respectively.

Cable Modem

Like a DSL Modem, a Cable Modem provides high speed broadband internet access. But unlike a DSL modem that uses phone lines to provide internet access, a cable modem uses the Cable TV installation for that.

Netgear Cable Modem
Netgear Cable Modem.

A Cable Modem provides bi-directional data communication via a radio frequency channel typically on a 6 MHz frequency slice (same frequency slice reserved for every television channel signal) of the overall bandwidth offered on a cable installation.

In some systems, coaxial cable is the only medium used for distributing signals. In other systems, fiber-optic cable goes from the cable company to different neighborhoods or areas. Then the fiber is terminated and the signals move onto coaxial cable for distribution to individual houses.

cable internet connection
Cable Internet Connection.

The use of CableTV installation to provide internet access to households started in the 1990s where cable providers started offering Internet services in addition to a TV channels package. To access Internet over Cable, two types of equipment are required: a cable modem on the customer end and a cable modem termination system (CMTS) at the cable provider’s end.

A cable modem is able to separate or tune the Internet data from normal CableTV stream. Then, modulates/demodulates the signal and finally, converts it to an Analog/Digital signal to be sent or received respectively. A cable modem termination system (CMTS) provides many of the same functions a DSLAM provides in a DSL system. This includes managing the traffic coming in from a group of customers on a single channel and routing it to an Internet service provider (ISP). Cable download speeds range anywhere from 1 Mbps to 1,000 Mbps (1 Gbps). The national average is around 100 Mbps. Upload speeds aren’t quite the same, though, usually ranging from 1 Mbps to 50 Mbps.



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