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Stages of DSL connection-making

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This page explains how a DSL modem connects to the internet.

Start up

After powering up, a DSL modem typically goes through these stages:

  1. Self-testing
  2. DSL sync
  3. Authentication
  4. In-service


Upon power-up, a modem typically performs tests to detect proper operation of its hardware and to detect problems in its software and data. This stage usually takes only a few seconds.

DSL sync

The first stage of connecting to the internet is to obtain a 'link level' DSL connection to the Central Office or DSLAM. Once established, the modem will use this connection to carry internet traffic.

Obtaining and maintaining a good DSL connection to the Central Office or DSLAM is crucial to good performance. To maximize performance, the modem tries to sense the quality of your phone line and adapt to conditions. The most basic requirement is obtaining DSL synchronization (detecting and locking onto the DSL signal).

Things that can go wrong at this stage, in order of most likely first, include:

  • no connection to the phone line
  • electrical noise from other devices on your phone line
  • low power signal caused by being far from the Central Office or DSLAM
  • electrical noise on the line from sources outside your location
  • problems at the Central Office or DSLAM.


Once a DSL connection has been made with the Central Office or DSLAM, it can be used by the modem to talk with the NCF authentication server. This requires your modem to identify itself with a login and password (for security and billing purposes). Your modem can do the authentication for you but of course it must have been programmed with the correct information.

The most common problems at this stage include:

  • not having correct login and password information programmed into your modem
  • temporary network problems that prevent the authentication server from confirming the log-in information that your modem provides to it.


Finally, when the DSL connection is up, your modem has been authenticated and granted access, it tells your computer that it is ready. Your computer can then send and receive information from the internet. Requests go from your computer to your modem (via ethernet cable or wirelessly), then from your modem to the network (via your phone line), then over the network to whichever web site computer you specified.

Connecting to your modem

Your modem also looks for connections to your computer(s) via its ethernet ports or, if equipped and enabled, via wireless. The establishment of these links goes through similar stages as described above for DSL. Ethernet connections are usually quite reliable and trouble-free, but wireless connections can suffer from interference with other wireless signals. There are only eleven channels allocated for wireless use in Canada and in most urban locations, especially in apartment buildings or townhouse complexes, there can be twenty or more networks within range of your computer, which can make interference hard to avoid. For more information see About wireless.

Once you have a 'link level' connection to your modem, you can use a web browser to communicate with the computer inside the modem, just as if it were a web site on the internet.