AccessConcentrator is the device to which a PPPoE client connects. When using PPPoE to an ADSL modem, the ADSL modem is the AccessConcentrator. Not to be confused with the RemoteAccessNode? (RAN), which terminates the PPP layer, not the PPPoE layer.
Interestingly, ADSL/PPPoE modems (those that use PPPoE to talk to the subscriber end) in New Zealand, use PPPoA to talk to the RemoteAccessNode?.