What Is a Modem and Why You Need One to Get Internet

What is a modem?

A modem is a device that sits between two networks—your local area network (LAN) and your internet provider’s network. It serves as your local shipping and receiving station.

So what does it do, exactly? We get into the nitty-gritty of it all if you want the hard technical details. But if we use the sending and receiving station analogy, the modem receives luggage from a delivery truck that crosses a long bridge between the modem and your provider. It then repackages your luggage and sends it along a superfast subway connected to your home network’s Grand Central Station—the router.

The term “modem” is short for Modulator and Demodulator, which is what a modem does. On a more technical level, modulation superimposes a digital wave onto an analog radio frequency (RF) carrier wave to send data to and from your provider’s internet network. Demodulation works in reverse—it separates the RF carrier wave from the digital data your home network can use.

If your eyes just glossed over, we get it. Just remember you can’t access cable, DSL, or satellite internet without a modem or gateway (a router/modem combo).

Why do you need a modem?

You need a modem to send and receive data from two different networking roads: your internet provider’s connection and your home network’s connection.

Here’s the deal: internet data is already formatted for home network use on your ISP’s end, but there are no Ethernet cables between you and your internet provider. So providers use a variety of other methods to send data, which aren’t Ethernet-compatible. Fiber is also Ethernet-ready, but it uses lines with glass fibers instead of copper wires, requiring the use of an ONT.

It’s all one big networking mess if you think about it. The internet backbone is all fiber, and your internet provider’s core network is probably fiber, too. The differences generally come from the last mile connection to your home.

Regardless, translation needs to happen to get Ethernet-ready data across these three different road types. Cable and DSL internet use RF carrier waves to send and receive data across copper wires. In both cases, providers superimpose a digital wave of data onto these carrier waves to travel through its network—like a raft of data flowing down a river of electricity. The modem must lift the digital raft from the electrical waters to extract the passenger ones and zeroes and send them on a subway train to your router.

Fiber internet doesn’t use carrier waves at all. Instead, it uses laser or LED pulses. For instance, a single pulse translates into a “one,” and no pulse equates to a “zero”—the two basic units of data. Moreover, these light pulses can reach 60 miles before they begin to degrade.

Ethernet uses a similar pulse method, but it relies on electrical pulses (signals) rather than light to send data between two points. This pulse method is one of the reasons why fiber is better than cable, DSL, and satellite because there’s no adding and subtracting data from radio waves—just a Morse code-like deciphering of pulses.

That all said, a modem sends the extracted data through an Ethernet cable that’s received by the router. In turn, the router sends the data through other Ethernet connections as electrical impulses. For Wi-Fi, the router performs its own modulation and demodulation.

Does a modem require a router?

A modem doesn’t require a router. Technically, it just needs an internet connection and an Ethernet connection to a device—that could be a router, computer, game console, or some other Ethernet-ready device.

In contrast, a router absolutely needs a modem, as it can’t understand the internet without its translator. A router is more like an airport with a built-in subway station. It receives a package through the modem’s subway connection and reroutes that package through another subway line to your wired device. If the destination is wireless, the router repackages the data and sends it on an airplane.

However, a router can function as a network manager without the modem. Check out our explanation about how a router works for more information.

What else does a modem do?

The typical standalone modem only translates. But a wireless gateway combines a modem with a router, so it’s both a translator and a network manager.

FAQ about modems

What is Data Over Cable Service Interface Specification (DOCSIS)?

DOCSIS is a standard developed by CableLabs and other companies that allows cable TV providers to send and receive internet data over unused TV channels.

Here are the different DOCSIS versions released over the years:

As the chart shows, you need a cable modem based on DOCSIS 3.0 or newer to get a gigabit connection.⁹

What is a data channel?

A data channel is a set amount of frequency used to upload or download data to and from a cable internet provider’s CMTS.

Generally, cable modems support more downstream channels than upstream ones. The more downstream channels a modem supports, the more megabits you can download each second.

For example, a modem listing 4×4 means it has an equal number of downstream and upstream channels. A 32×8 modem has 32 downstream channels but just 8 upstream ones.

The DOCSIS version also applies in terms of speed. A 32×8 modem based on DOCSIS 3.0 has a maximum download speed of 1,400 Mbps while DOCSIS 3.1 enables 10,000 Mbps. Check out our guide on the best modems for gigabit internet if you want to replace your provider’s unit.

Can you use any modem with your internet connection?

No, you cannot use any modem. As described above, you can’t use a cable modem with a DSL connection, nor a DSL modem with a fiber connection. Your provider typically supplies the correct device for the internet type you have.

You can purchase a modem if you want to avoid monthly fees, but it must be authorized by your internet provider. Spectrum, for example, provides a list of compatible modems that work with its three cable internet plans.⁸ We list the best cable modems for Xfinity, Spectrum, Cox, and more.

Glossary

Cable modem termination system (CMTS)

A CMTS serves as the middleman between the internet and the customer’s modem. It handles multiple modems simultaneously and is responsible for assigning public IP addresses to any device connected to the modem’s Ethernet port.

On the network side, the CMTS includes Ethernet ports that connect to the provider’s routers. On the client-facing side, the CMTS uses RF connectors to send modulated carrier waves through a hybrid fiber coax cable. These waves are distributed across the service area’s network until it reaches the client’s modem.⁴

Digital subscriber line access multiplexer (DSLAM)

A DSLAM sits between all DSL modems in a service area and the internet provider’s network. It converts upstream electrical signals into bitstreams and merges them all together into a single connection—downstream is the reverse of the process. DSLAM units typically reside at distribution points or local telephone operators.⁷

Intermediate frequency

Intermediate frequencies sit between a baseband frequency and a carrier frequency. These frequencies allow the modem to support various satellite internet providers without the need for hardware that supports every possible carrier frequency.²

Manchester Signal Encoding

This format defines how binary ones and zeros are represented in an electrical form.6 The device driver creates a unit of data (frame) which is then converted into an electrical signal by the networking circuitry using this format.

Media access control (MAC)

MAC handles how data moves between two points on a network. It assigns an address to each device so they can communicate with each other. It also controls the hardware responsible for communication on a network, whether it’s wired, wireless, or optical.

Optical network unit (ONU)

While an optical network terminal (ONT) serves as the end-user’s “modem” within the home or office, the optical network unit resides outside on a utility pole or in a standalone cabinet. It’s connected directly to the PON (see below) and converts light pulses into electrical signals. It’s used in fiber-to-the-curb services and typically connects to a splitter that feeds internet connectivity through a coaxial or telephone cable to multiple homes and businesses.

Optical line terminal (OLT)

An optical line terminal is a device located in the central office (or headend) and serves as the starting point in a PON (see below). It formats data received from the provider’s core network to match the framing and wavelengths used in the connected PON. It also combines all the data received from the ONTs connected to the PON and sends it along the provider’s core network.³

Passive optical network (PON)

Passive optical network is the term used to describe a fiber network infrastructure installed within a neighborhood. It supports speeds faster than 1,000 Mbps (1 Gbps) and uses a point-to-multipoint design to feed fiber internet to multiple homes and offices. The layout usually consists of an OLT, a feeder cable, distribution cables, optical splitters, drop cables, a distribution hub, and ONTs. The design is passive because there are no electrical devices between the OLT at the central office and the subscriber’s ONT.

Quadrature amplitude modulation (QAM)

QAM is an efficient use of data transmission over limited bandwidth. It essentially sequences data transmissions so all the data is not received at once, consuming the entire available bandwidth. Here are a few examples:

• 8QAM — 3 bits per symbol, or 8 different states
• 16QAM — 4 bits per symbol, or 16 different states.
• 64QAM — 6 bits per symbol, or 64 different states
• 256QAM — 8 bits per symbol, or 256 different states

A symbol is a type of waveform that represents individual bits. For example, data moves across copper wires by applying voltage pulses. Each high pulse represents a binary one, and each low pulse (or no pulse) translates to zero. These high and low pulses are symbols.

Physical layer (PHY)

The physical layer is the first (and bottom) layer in the Open Systems Interconnection (OSI) model for computer networking, and it deals with physical connections.⁵

How does a modem work—the technical explanation

In short, a modem receives a signal from your internet provider’s connection and extracts data from that signal. The modem then sends the data through its Ethernet port as electrical pulses.

Technically, fiber doesn’t use a modem but instead relies on an ONT that interprets light signals as ones and zeroes. But we included it here anyway because it still translates an internet signal into data your devices can use.

If you really want to know the technical details of how a modem works, you’ve come to the right section. We divide the explanation into four parts and give you the step-by-step process each device uses to retrieve data from your internet provider and send it to your devices.

Jump to: How a cable modem works | How a DSL modem works | How a fiber ONT works | How a satellite modem works

How a cable modem works

Here’s a short list of what happens after your provider’s signal enters your home:

Step 1: The modem’s tuner receives your internet provider’s modulated analog signal from the coaxial cable and moves it to the demodulator.

Step 2: The demodulator separates the digital wave from the carrier wave using quadrature amplitude modulation (QAM).

Step 3: The demodulator sends the digital wave to the analog-to-digital converter (ADC).

Step 4: The ADC reverts the digital wave into a bitstream.

Step 5: The error correction module checks that the conversion is correct.

Step 6: The media access control (MAC) and processor separate the bitstream from digital TV data, if needed.

Step 7: The modem’s PHY transceiver uses Manchester encoding to convert the bitstream into electrical pulses.

Step 8: The modem transmits these pulses over an Ethernet cable by applying specific voltages to the copper wires.

When you reverse the steps, the modem eventually modulates the carrier wave using your data wave and delivers them to your provider’s CMTS. In turn, the CMTS demodulates and routes your data to the fiber-based internet. Fiber uses LED and laser pulses to send data, not carrier waves.

How a DSL modem works

DSL modems typically plug into a filter that separates voice from data. Here’s a short list of what happens:

Step 1: Your provider’s DSL access multiplexer (DSLAM) creates 247 separate channels, each 4 kHz wide with an analog carrier signal.

Step 2: The DSLAM superimposes a digital waveform onto each carrier wave and sends it to the subscriber.

Step 3: The subscriber’s low-pass filter receives and separates the DSL frequencies (8 kHz to 1 MHz) from the voice frequencies.

Step 4: The filter passes the DSL signals to the modem (or transceiver).

Step 5: The modem’s RJ11 socket receives the modulated DSL signals and passes them on to the demodulator.

Step 6: The demodulator separates the digital wave from the carrier wave using quadrature amplitude modulation.

Step 7: The demodulator sends the digital wave to the analog-to-digital (ADC) converter.

Step 8: The ADC reverts the digital wave into a bitstream.

Step 9: The media access control and PHY transceiver uses Manchester encoding to convert the bitstream into electrical pulses.

Step 10: The modem transmits these pulses over an Ethernet cable by applying specific voltages to the copper wires.

How an optical network terminal (ONT) works

Fiber internet to the home does not use the traditional modem but instead relies on a termination point and an optical network terminal. Here’s how an ONT works:

Step 1: The ONT receives LED or laser pulses through its fiber-optic connection to the terminal point.

Step 2: The media access control and processor separate the bitstream from all other data.

Step 3: The PHY transceiver uses Manchester encoding to convert the bitstream into electrical pulses.

Step 4: The ONT transmits these pulses over an Ethernet cable by applying specific voltages to the copper wires. To transmit data back to the internet provider, the ONT turns the diode on and off, signaling ones and zeroes.

Keep in mind that ONT devices primarily reside in fiber-to-the-home setups. Other fiber internet installations—like fiber-to-the-curb—rely on copper cabling to fill the “last mile.” Hence, the cable or DSL modem scenario applies up until the street cabinet housing the area’s optical network unit or the passive optical network’s (PON) optical line terminal (OLT).

How a satellite modem works

Depending on the installation, a satellite modem connects to your satellite dish using one or two coaxial cables. Here’s what happens when the modem receives data from a satellite dish:

Step 1: The low-noise block downconverter (LNB) on your satellite dish sends the signal across a coaxial cable to your modem.

Step 2: The analog tract inside the modem converts all received signals to an intermediate frequency (IF) signal for use by the demodulator. It then passes the signal through an anti-aliasing filter (AAF) and is sampled.

Step 3: The demodulator separates the digital signal from the analog IF-based analog carrier wave.

Step 4: The analog-to-digital converter reverts the digital wave into a bitstream.

Step 5: The forward error correction (FEC) decoder removes artificial redundancy from the bitstream. This redundancy helped reduce errors during the transmission.

Step 6: The differential encoder removes the modulation that prevented the ones and zeroes from swapping places during transmission.

Step 7: The descrambler puts all the ones and zeroes back into a logical order. Scrambling data optimizes energy dispersal only.

Step 8: The demultiplexer splits one multiplexed bitstream into multiple streams if needed.

Step 9: The media access control and PHY transceiver uses Manchester encoding to convert the bitstream(s) into electrical impulses.

Step 10: The modem transmits these impulses over an Ethernet cable by applying specific voltages to the copper wires. The transmitter portion of a satellite modem works in reverse.

Some satellite modems do not have a modulator. Instead, they rely on a DSL connection or the local public switched telephone network (PSTN) to upload data.

Related topics:

• Improve Your Wi-Fi Speed in 10 Simple Steps
• The Consumers Guide to Internet Speed
• Best Cable Modems for Xfinity, Spectrum, Cox, and More

Sources

1. Dick McClue, ”Overview of Satellite Communications,” Accessed September 7, 2021.
2. All About Circuits, ”The Benefits of an Intermediate Frequency in RF Systems,” Accessed September 7, 2021.
3. Router-Switch, “What’s the OLT/Optical Line Terminal?” Jan 24, 2018. Accessed September 8, 2021.
4. Nanopdf, “Introduction to the Cisco CMTS,” Accessed September 8, 2021.
5. OSI-Model, “Physical Layer,” Accessed September 9, 2021.
6. LiveAction, “Manchester Signal Transition,” Accessed September 9, 2021.
7. Verse Technology, “What Is a DSLAM?” December 30, 2015. Accessed September 9, 2021.
8. Spectrum, “Authorized Modems to Use on the Spectrum Network,” May 27, 2021. Accessed October 5, 2021.
9. CableLabs, “The Evolution of DOCSIS,” Accessed October 5, 2021.