The following describes how to set up PPP over ATM, a.k.a, PPPoA. Currently, the only hardware supported is the Alcatel Speedtouch USB ADSL modem
PPPoA is supplied as a port in FreeBSD because the firmware is not distributable under Alcatel's license agreement.
To install the port, simply use the ports collection to install the net/pppoa port and follow the instructions provided there.