How the ADSL Connection Works
Introduction
Contents
ADSL (Asymmetric Digital Subscriber Line) is one of the most popular technologies for Internet connection, using the infrastructure of your local telephone company. There are several types of DSL (a.k.a. xDSL) connections, such as VDSL, with ADSL being the most popular one in most countries. Its name comes from the fact that its download speed is higher than its upload speed, hence classified as asymmetrical. A symmetrical connection is one where the download and the upload speeds are the same.
The ADSL connection makes use of the Point-to-Point Protocol (PPP), more specifically PPPoE (PPP over Ethernet) or PPPoA (PPP over AAL5), depending on the telephone company’s infrastructure.
There are several ADSL standards. In the table below, we show the most common ones, which are based on the same copper wiring used by the analog telephone system, allowing ADSL technology to be deployed without replacing the existing telephone wiring.
| Name | Standard | Maximum Download | Maximum Upload | Channels |
| ADSL | G.992.1 | 8 Mbps | 1.4 Mbps | 256 |
| ADSL2 | G.992.3 | 12 Mbps | 1.4 Mbps | 256 |
| ADSL2+ | G.992.5 | 24 Mbps | 1.4 Mbps | 512 |
| ADSL2+ Annex M | G.992.5 Annex M | 24 Mbps | 2.8 Mbps | 512 |
Note: The exact number of guard (i.e., reserved) channels varies according to the phone company. Because of that, the maximum theoretical speeds vary according to the bibliography, depending on the number of channels actually available and used in the calculations.
On the next page, we will explain in detail how the ADSL technology works and the difference between the existing standards.