The broadband protocol stacks

The Broadband Forum as a lot of 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).

Classic POTS access with PPP

[IP  ][IP ]
[    ][  |V.90][V.90|   ][   ]
Client     Modem            Modem       RAS

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);

  • configuration of the network protocol parameters (IP address, netmask, IPv6 delegated prefix, etc.) using dedicated PAP, CHAP, EAP, etc.),

  • compression, encryption;

  • etc.

Basic example: PPPoA

                 PPPoE                    PPPoA                  L2TP

[IP ][IP       ][IP  ]
            [PPP  ][PPP ]
            [PPPoE][PPPoE|PPPoA  ][PPPoA  |L2TP][L2TP]
                            [RFC2684][RFC2684|UDP ][UDP ]
                            [AAL5   ][AAL5   |IP  ][IP  ]
[Eth][Eth|Eth  ][Eth  |ATM    ][ATM     ][ATM    |    ] … [    ]
[PHY][PHY|PHY  ][PHY  |xDSL   ][xDSL|PHY][PHY    |    ] … [    ]
      S             T                 U            V               A10
         Router       B-NT/xDSL modem   AN/DSLAM       LAC/BRAS     …   LNS
PPPoE to PPPoA to L2TP


  • 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.

  • Based on authentication information supplied by the user router, the LAC will choose the suitable ISP, authenticate the user of this ISP RADIUS server. If the authentication succeeds, the RADIUS server will ask the LAC to extend the PPP session over L2TP/UDP/IP to the ISP LNS.


  • 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:

                        PPPoA                   L2TP

[IP ][IP         ][IP  ]
            [PPP    ][PPP ]
            [PPPoA  ][PPPoA  |L2TP][L2TP]
            [RFC2684][RFC2684|UDP ][UDP ]
            [AAL5   ][AAL5   |IP  ][IP  ]
[Eth][Eth|ATM    ][ATM     ][ATM    |    ] … [    ]
[PHY][PHY|xDSL   ][xDSL|PHY][PHY    |    ] … [    ]
      T               U            V               A10
        Modem-Router    AN/DSLAM       LAC/BRAS    …   LNS


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.

                               PPPoE                             L2TP

[IP ][IP       ][IP  ]
            [PPP  ][PPP ]
            [PPPoE][PPPoE  |L2TP][L2TP]
[Eth][Eth|Eth  ][Eth          ][Eth    |UDP ] … [UDP ]
                      [     |AAL5   ][AAL5   |IP  ] … [IP  ]
                      [     |ATM    ][ATM     ][ATM    |    ] … |    ]
[PHY][PHY|PHY  ][PHY  |xDSL   ][xDSL|PHY][PHY    |    ] … [    ]
      S             T                 U            V               A10
         Router       B-NT/xDSL modem   AN/DSLAM       LAC/BRAS    …  LNS
PPPoE to PPPoEoA to L2TP

Ethernet 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.

                             PPPoE                                L2TP

[IP ][IP       ][IP  ]
            [PPP  ][PPP ]
            [PPPoE][PPPoE  |L2TP][L2TP]
                                                      [       |UDP ][UDP ]
                                             [QinQ][QinQ   |IP  ][IP  ]
[Eth][Eth|Eth  ][Eth          ][Eth      ][Eth    |    ] … [    ]
[PHY][PHY|PHY  ][PHY  |xDSL   ][xDSL|PHY ][PHY    |    ] … [    ]
      S             T                 U             V               A10
         Router       B-NT/xDSL modem   AN/DSLAM        LAC/BRAS    …  LNS


Ethernet aggregation (no ATM) without PPP.

[IP ][IP       ][IP        ][IP  ]
                                             [QinQ][QinQ |    ] … [    ]
[Eth][Eth|Eth  ][Eth          ][Eth      ][Eth  |    ] … [    ]
[PHY][PHY|PHY  ][PHY  |xDSL   ][xDSL|PHY ][PHY  |    ] … [    ]
      S             T                 U             V             A10
         Router        B-NT              AN             BNG        …  
                       (xDSL modem)      (DSLAM)


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 T interface

        [ IP    ]
        [ PPP   ]
[ IP  ] [ PPPoE ]
[ Eth ] [ Eth   ]
[ PHY ] [ PHY   ]

The U Interface

The U interface is the interface between the B-NT (the xDSL modem) and the Access Node (DSLAM):

          [IP     ]
          [PPP    ]
[IP     ] [PPPoE  ]           [IP     ]
[Eth    ] [Eth    ] [IP     ] [PPP    ]         [IP   ]
[RFC2684] [RFC2684] [RFC2684] [RFC2684]         [PPP  ]
[AAL5   ] [AAL5   ] [AAL5   ] [AAL5   ] [IP   ] [PPPoE]
[ATM    ] [ATM    ] [ATM    ] [ATM    ] [Eth  ] [Eth  ] [Eth]
[PHY    ] [PHY    ] [PHY    ] [PHY    ] [PHY  ] [PHY  ] [PHY]

 IPoEoA    PPPoEoA   IPoA      PPPoA     IPoE    PPPoE   Eth
 a         b         c         d         e       f       g
Protocol stacks fot the U interface

TR-043 compares the different ATM-based solutions.


  • When talking about DSL, “PPPoE” was often used over ATM. In this context, PPPoE is often used for PPP over Ethernet over ATM (PPPoEoA, option b). Sometimes, the name “PPPoEoE” is used for PPP over Ethernet without ATM (option f).

The V Interface

                              [IP  ]
[IP     ] [PPP    ]           [PPP ]
[802.1ad] [802.1ad] [802.1ad] [AAL5]
[Eth    ] [Eth    ] [Eth    ] [ATM ]
[PHY    ] [PHY    ] [PHY    ] [PHY ]
 IPoE      PPPoE
 a         b         c
Protocol stacks fot the V interface

More stuff

Multiplexing over AAL5

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.

                 [            ]
[    ]           [LLC         ]
[AAL5]           [AAL5        ]
[ATM ]           [ATM         ]
VC Multiplexing  LLC Encapsulation
Encapsulation over AAL5

L2TP and RADIUS message exchange

[Client] [BRAS]          [LNS]
   |       | [RADIUS Proxy]|  [RADIUS]
   |       |       |       |     |
   |       |       |       |     |
   |       |       |       |     |  I] Initial challenge
   |       |       |       |     |
   ||       |       |     |  CHAP Success
   |       |------>|------------>|  RADIUS Access-Request
   |       ||     |  L2TP Start-Control-Connection-Request
   |       ||     |  L2TP Start-Control-Connection-Connected
   |       ||     |  L2TP Incoming-Call-Request
   |       ||     |  L2TP Incoming-Call-Connected
   |       ||     |  CHAP Response
   |       |       |       |---->|  RADIUS Access-Request
   |       |       |       |


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.