DSL Network Guide Chapter 1: Introduction

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Welcome to the world of DSL. This tutorial is designed for anyone who is looking to better understand DSL technology. Hopefully anyone from a first time computer user to a total computer geek can get something out of this tutorial and apply it to better understand how DSL works.
DSL runs over your phone lines using a different frequency than dial-up. You could think of the phone lines like the receiver on your stereo system. There are many frequencies available on the phone lines but the phone lines only take advantage on a small part of that bandwidth. DSL takes advantage of the unused frequencies. Because dial-up was not created with the idea of the other frequencies being used by any other device the dial-up often creates "noise" on those frequencies and nobody cared to correct this. Because of that all phones on the phone line DSL uses need a filter on them. This prevents both the telephones from disturbing the DSL and also from the DSL disturbing the voice frequencies. Without filters you may hear screeching and computer noises on your phones. You can also experience intermittent or no synch on the DSL modem or slow speeds. You may also have to power cycle your modem frequently.
Your phone lines, whether or not you have DSL, go to a local B-Box once it leaves your house. That B-Box eventually hooks up to a CO (Central Office.) Your phone lines end at the CO. When you place a call the CO determines where the CO of the other person is. It establishes a connection with that CO which then goes to that persons B-Box and finally their telephones. The CO is also where all the equipment for DSL is located at, unless your DSL is RT based (see Chapter 11: Remote Terminals.)
Once the CO has split the DSL signal off of your phone lines it sends it to the CO of your ISP. Up to this point all the equipment is owned and maintained by your local telco (telephone company) but from here it's in the hands of your ISP. At the ISP CO your username and password is verified, your speed is limited to what you purchased, an IP address is assigned, and other ISP specific features are taken care of.
Once the ISP CO has taken care of its duties your connection is sent off to the regular internet that everyone else uses. Congratulations you are now online.
This tutorial covers a general over view of how DSL works. Each ISP is different and there may be variations on how this works depending on whom you purchase your phone service and your DSL from. Even one ISP can have different setups in different areas.

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