Digital Subscriber Line (DSL) is used as an alternative to ISDN for home users. There are a number of types of DSL connections, but the most important ones include the following:
- ADSL
- HDSL
- VDSL
- SDSL
Asymmetric Digital Subscriber Line (ADSL) is the most common form of DSL connection that functions over standard telephone lines. The reason it is called asymmetric is that it offers unequal download and upload throughput, with the download rate being higher than the upload rate. A standard ADSL connection usually offers a maximum of 24Mbps download throughput and a maximum of 3.5Mbps upload throughput over a distance of up to 3 km.
With ADSL the customer connects to a Digital Subscriber Line Access Multiplexer (DSLAM) located at the service provider. DSLAM is a DSL concentrator device that aggregates connections from multiple users.
| Note: One of the issues with ADSL is the limited distance a subscriber can be from a DSLAM. |
High Bitrate DSL (HDSL) and Very High Bitrate DSL (VDSL) are other DSL technologies used on a large scale that offer increased throughput when compared to ADSL. VDSL can operate at rates of up to 100Mbps.
Symmetric DSL (SDSL) offers the same download and upload throughput, but it was never standardised or used on a large scale.