The reason that many people talk about DSL not being "true always-on" is due to the practices of some service providers in regard to consumer ADSL; it is not an attribute of DSL per se.
Many ISPs model their DSL services on dial-up networking using a protocol called PPPoE (point-to-point protocol over Ethernet). Each time the DSL device connects (your PC or DSL router), it authenticates to a RADIUS server just like a dial up connection to the Internet. In some cases the ISPs log off sessions that have been idle for a specified period of time, hence, the reference to "true always-on".
There are several reasons ISPs adopted this approach.
- They already had the authentication, accounting and billing systems in place making this approach quite attractive from an operational perspective.
- Many ISPs are concerned about running out of IP addresses. Each time you connect, they give you a new address from a limited pool. By disconnecting idle users, they can conserve addresses.
- The approach also helps curve bandwidth usage. Always-on connections with static addresses can be used to support Web, e-mail and FTP servers at your house. Many ISPs don't want people using the connection for inbound services to their homes, just outbound Web surfing and e-mail access.
This was first published in January 2003