How Dial-up Modems Work

When you connect to the Internet with a dial-up connection, you are using a dial-up modem. This computer device is responsible for establishing and maintaining the Internet connection, as well as translate analog signals for your computer.

The first thing that takes place when you click connect is that you are prompting your modem to dial an access number on your telephone line. It's just like dialing a number on a telephone. It dials the number, then you hear the squealing and screeching noises. This is the sound of your modem contacting your ISP's remote computer.

Once they are in communication, your modem sends test packets of information to determine what speed will be functional. Your modem must be able to communicate clearly with the ISP modem and the connection speed will depend on how quickly that can be accurately done. For example, if the modem sends packets too quickly, the content of the message could be become jumbled or lost. If this happens, the packet must be resent, and time is lost. This means that sending information at the fastest possible speed of 56K would actually take longer than whatever speed is eventually established. Most connection speed end up being between 20 and 50 kbps.

Another factor that affects connection speeds between modems over a phone line is the amount of static or line noise, like humming, is on the phone line. These noises can interfere with the signals being sent and received on the modem. If there is too much noise entirely, the modem with give an error saying a connection could not be established. This is why ISP's often refer their customers to contact their phone providers, because a phone line with too much static must be repaired. Humans can hear one another over static or noise more easily when communicating, but to a modem, static is overbearing to analog signals.

Once a connection speed is determined and a connection is established, the modem will make sure your username and password is verified by the ISP's modem, and gives the message "You are now connected..." etc.

The next task for the modem is presenting information from the Internet or email server. This is done by uploading and downloading. When you click on a link, or update your status on Facebook, or send an email, you are submitting information to or requesting information from the Internet. These are all types of uploading. Your modem takes these uploading commands and translates them to analog signals, different frequencies that can be sent over the phone line.

When a webpage loads, or you open an attachment or download software, you are receiving information from the Internet. All of these types of actions is downloading. Everything that is received from the Internet is sent through your ISP's modem to your modem over the analog phone line again. This information too has to be translated to analog signals. When your modem receives this information, it translates the information back to digital data. You computer interprets the data, and in turn, it become the pictures, sounds, and text that make up webpages, emails, streaming music and video clips.