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How Does an ADSL Modem Work?

written by: Daniel Barros•edited by: Rebecca Scudder•updated: 3/4/2010

Been thinking about how it is that your Internet access actually works? We've got the answer inside if you're using DSL to access this site right now.

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    Asymmetrical Wonder

    If most people knew the amount of physics, engineering, and chemistry it takes for a simple laptop to receive a website like Facebook and then update their status, they would faint. In an instant, you’re combining all of the three sciences to achieve something our ancestors studying electricity and the elements couldn’t have even dreamed of.

    Now, that being said, let’s take an up-close-and-personal look at the ADSL modem that is a part of the process that makes the above scenario possible. Since we’ve already covered how a Wi-Fi signal gets from your computer to your modem, we’ll start after the signal has left your computer.

    Once you’ve typed the name of the website and clicked “Go”, your computer’s Wi-Fi radio antenna sends out a signal to the wireless router requesting the specific website you typed in. This signal reaches the router, which then sends the signal out to the ADSL modem.

    As its name suggests, the Asymmetric Digital Subscriber Line (ADSL) modem is a modem that uses your existing telephone line for Internet, but in a very clever way. To understand this, we’ll first need to take a look at how a telephone works.

    A telephone works by passing your voice through a switch that operates at frequencies under the 4000 Hz (or 4 KHz) that the human ear can actually listen to. Now, the copper lines that the telephone company uses to give your house telephone access have the ability to do what is known as multi-plexing or muxing. When you use muxing with the telephone line, you can essentially pass several different frequencies without interfering with one another inside the same copper wire due to the fact that at the much higher frequencies of DSL, you won’t ever get any interference with voice calls on the telephone.

    At around 25 KHz, you’ll find the upstream signal to the Internet. At around 240 KHz all the way to 2 MHz, you’ll find your download signal to the Internet. This is why the modem is asymmetrical – because it provides you with a much larger capacity for download than it provides you with uploading. This is ideal for the home user because they’ll be downloading from the Internet much more than they will be uploading anything other than small photos or statuses.

    Once the Wi-Fi signal from your computer reaches the modem, it is capable of using that large frequency range to give your entire household access to the Internet. The speed is only limited by the tech employed by the ISP – the more advanced modems use a larger range of frequencies while also separating the channels into individual streams. What this means is that each stream, or each computer’s signal gets routed to a small cut portion of the bandwidth, which is spaced out in 4 KHz bands. Essentially, with this division system, the modem is treated like several (read: 250 or so) different modems – which means that when one channel goes down, you’re ready to go on another channel.

    It’s time to wrap this up neatly by talking about the process as a whole. Your computer sends out a radio Wi-Fi signal, which is then interpreted by the router, which sends out a request to the modem to use a portion of the bandwidth. The modem then takes that request and uses the higher frequency range of the copper telephone wire to send out the signal to a central routing node which then forward your request to the website’s server (or server farm). Those servers then accept the request and send back the information necessary to display the web page on the screen.