Ubuntu also supports the use of a digital subscriber line (DSL) service. Although it refers to the different types of DSL available as xDSL, that name includes ADSL, IDSL, SDSL, and other flavors of DSL service; they can all be configured using the Internet Connection Wizard. DSL service generally provides 256Kbps to 24Mbps transfer speeds and transmits data over copper telephone lines from a central office to individual subscriber sites (such as your home). Many DSL services provide asymmetric speeds with download speeds greater than upload speeds.
DSL service is an "always-on" type of Internet service, although you can turn off the connection under Ubuntu using the Network Device Control. An always-on connection exposes your computer to malicious abuse from crackers who trawl the Internet attempting to gain access to other computer systems. In addition to the capability to turn off such connections, Ubuntu provides a firewall to keep crackers out; you configured a simple firewall during the original installation. The firewall can also be configured from the Security Level Configuration tool found in the System Settings menu selection as Security Level.
A DSL connection requires that you have an ethernet network interface card (sometimes a USB interface that is not easily supported in Linux) in your computer or notebook. Many users also configure a gateway, firewall, or other computer with at least two network interface cards in order to share a connection with a LAN. We looked at the hardware and protocol issues earlier on in this chapter. Advanced configuration of a firewall or router, other than what was addressed during your initial installation of Ubuntu, is beyond the scope of this book.
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