When an ADSL modem has booted and makes a connection, what information is sent across the wire and what is received? Does this differ if the ADSL modem has been assigned a static IP address or a dynamic IP address?
What I've figured out so far:
Most ADSL connections use either PPP over Ethernet over ATM ("PPPoE" mode), or DHCP over Ethernet over ATM ("DHCP" mode).
The former, also called PPPoE, uses the same PPP protocol as used by dial-ups – you authenticate with a username/password and get assigned an address. All packets going to the Internet are tunneled inside this PPP connection, so there's a slight overhead.
In the second case, your router contacts the ISP's DHCP server directly and (probably) authenticates you based on the physical phone line, and further data packets are sent directly. The DHCP protocol is the same as used in your home network.
The process is the same for both static and dynamic addresses – the DHCP/PPP server is simply told to reserve a specific address for you, instead of picking any available one.
PPP, IP, DHCP and everything else normally goes over Ethernet – but in this case, the Ethernet frames are further encapsulated in ATM cells and are sent over a statically-configured ATM "virtual circuit". (In case of PPPoA, there is no Ethernet framing involved – PPP data is sent over ATM directly.)
Finally, everything is converted into electrical signals and sent over the phone line.
...at least, that's how it seems to be working here. I may be horribly wrong, for I have never worked at an ISP and just like messing with networks.