technical reports about the xDSL architecture but it's not so easy to find a good description of the global architecture. Those are ASCII-art protocol stack I inferred from those documents. What's in there may be wrong, feel free to correct me.
You can find relevant diagrams in TR-025 (figure 4, for end-to-end ATM network; figure 5 for L2TP; figure 6 for PPP termination at the access node), in TR-101 (figure 4 fot the U interface; figure 7 for the V interface), TR0-59 (figure 11).
The link between the client and the RAS is a point-to-point link. The PPP link-layer protocol is used. It provides:
multiplexing of multiple network protocols (IPv6, IPv4, IPX, etc);
A PPP session is first established between the user router (usually this will be the user modem-router/box) to the LAC.
This session is transported over Ethernet in the local network and then over ATM: the xDSL modem change the encapsulation of the PPP session from PPPoE to PPPoA. Alternatively, the modem could encapsulate the Ethernet frames over ATM(PPPoEoA).
On new deployments, it is recommended to use Ethernet instead of ATM for the aggregation.
The modem and the router are usually combined in a single modem-router (the “box”).
The modem can be connected to the router with USB. In this case, what is used? ATM-over-USB? Ethernet over USB (CDC or RNDIS)?
RFC2684 defines two encapsulation methods over AAL5.
IPsec can be used to secure the L2TP session.
Ethernet is supposed to be used instead of ATM for aggregation in new deployments.
Instead of converting from PPPoE to PPPoA, the modem can encapsulate the Ethernet frame over ATM behaving like a simple bridge. In this, case we have PPP over Ethernet over ATM (PPPoEoA). This is usually called PPPoE in the DSL context.
Interworking functions can be used at the AN in order to transition between the ATM layer (user side) and the Ethernet layer (network side).
SLIP is an old protocol, similar to PPP, for encapsulation of IP over a serial line.
QinQ at the V interface for tagging the different users: this way it is possible to provide L2 VPN with VLAN support.
Instead of extending the PPP, the BNC can terminate it and transport the IP packets directly.
The modem and the router are often merged in a modem-router:
Instead of converting between PPPoE to PPPoA, the modem can encapsulate PPPoE over ATM (PPPoEoA). The modem can be seen as an Ethernet bridge. This solution is often called PPPoE because ATM was previously always used for aggregation.
In this example, the aggregation network is Ethernet based. This is recommended for new deployments. PPPoE (without ATM) is used instead of PPPoA. As before, the modem can be seen as an Ethernet switch.
Ethernet aggregation (no ATM) without PPP.
Details of the interfaces can be found in TR-059 page 9.
The S interface;
The T interface: interface between the user premises and the DSL modem/RG. The RG may initiate the PPP session itself or transport PPPoE sessions initiated from the local network.
The U interface: interface between the B-NT (xDSL modem) and the Access Node (DSLAM);
The V interface
The A10 interface: interface between the Regional Broadband Network and the NSP/ASP/ISP POP.
The U interface is the interface between the B-NT (the xDSL modem) and the Access Node (DSLAM):
TR-043 compares the different ATM-based solutions.
RFC2684 defined two methods of protocol multiplexing over AAL5:
VC multiplexing: a different ATM VC is used for each encapsulated protocol;
LLC encapsulation: LLC headers are used which includes SAP fields (both SSAP and DSAP) for protocol multiplexing. An intermediate NLPID field (for routed PDU in NLPID format) or a SNAP frame can be used.
[Client] [BRAS] [LNS] | | [RADIUS Proxy]| [RADIUS] | | | | | | | | | | | | | | | I] Initial challenge | | | | | |<------| | | | CHAP Challenge |------>| | | | CHAP Success | |------>|------------>| RADIUS Access-Request | |<------|<------------| RADIUS Access-Accept | | | | | Tunnel-Type=L2TP | | | | | Tunnel-Medium-Type=IPv4 | | | | | Tunnel-Server-Endpoint=lns.example.com | | | | | Tunnel-Password=potato |<------| | | | CHAP Success | | | | | | | | | | | | | | | II] Tunnel establishment | | | | | | |-------------->| | L2TP Start-Control-Connection-Request | |<--------------| | L2TP Start-Control-Connection-Reply | |-------------->| | L2TP Start-Control-Connection-Connected | |<--------------| | L2TP Zero-Length Body Ack. | | | | | | | | | | III] Call establishment | | | | | | |-------------->| | L2TP Incoming-Call-Request | |<--------------| | L2TP Incoming-Call-Reply | |-------------->| | L2TP Incoming-Call-Connected | |<--------------| | L2TP Zero-Length Body Ack. | | | | | | | | | | | | | | | IV] New challenge | | | | | |<----------------------| | CHAP Challenge |---------------------->| | CHAP Response | | | |---->| RADIUS Access-Request | | | |<----| RADIUS Access-Accept |<----------------------| | CHAP Success | | | | |
IPv6 over PPP, RFC5072
PPP Authentication Protocols, RFC1334
Multiprotocol over AAL5, RFC2684
Broadband Services Architecture for Access to Legacy Data Networks over ADSL, Issue 1, TR-012
Recommendation of PPP over ATM at the U interface for ATM end-to-end network.
Core Network Architecture Recommendations for Access to Legacy Data Networkds over ADSL, TR-025
Protocols at the U interface for Accessing Data Networks using ATM/DSL, TR-043
Describes PPPoEoA, IPoEoA, IPoA. A this time ATM was always used so PPPoEoA was named PPPoE and IPoEoA was named IPoE.
DSL Evolution - Architecture Requirements for the Support of QoS-Enabled IP Services, TR-059
Ethernet based aggregation, TR-101 issue 2
IPv6 for PPP Broadband Access, TR-187
Suggests at least /60 IPv6 delegated prefix for home network and recommends /56. Suggests up to /48 for large organisations.
In this page “PPPoE” is PPP over Ethernet over ATM (PPPoEoA).