What does it mean to be "true always on"? Does that mean that ADSL is not really constantly on?
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.
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