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The broadband protocol stacks

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The Broadband Forum as a lot of technical reports about the xDSL architecture but it is not so easy to find a good description of the global architecture. Those are ASCII-art protocol stack I inferred from those documents. What is 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 (Point to Point) termination at the access node), in TR-101 (figure 4 for the U interface; figure 7 for the V interface), TR0-59 (figure 11).

Table of content

Classic POTS access with PPP

[IP    ]<-------------------------------------->[IP    ]
[PPP   ]<-------------------------------------->[PPP   ]
[RS-232]<->[RS-232|V.92]<------>[V.92|RS-232]<->[RS-232]
Client      Modem                  Modem       RAS
Protocol stack for POTS access with PPP

The link between the client and the Remote Access Server (RAS) is a point-to-point link. The PPP link-layer protocol is used. It provides:

Basic example: PPPoA

                 PPPoE                    PPPoA                  L2TP
             <-------------> <------------------------------> <---------->

[IP ]<->[IP       ]<------------------------------------------------->[IP  ]
            [PPP  ]<---------------------------------/PPP         \-->[PPP ]
            [PPPoE]<->[PPPoE|(NLPID)]<-------------->[(NLIPD)|L2TP]<->[L2TP]
                            [(LLC  )]<-------------->[(LLC)  |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

Explanations:

Variations

Modem-router

The modem and the router are often merged in a modem-router:

                        PPPoA                   L2TP
            <-------------------------------><---------->

[IP ]<->[IP         ]<------------------------------->[IP  ]
            [PPP    ]<---------------/PPP         \-->[PPP ]
            [(NLPID)]<-------------->[(NLIPD)|L2TP]<->[L2TP]
            [(LLC)  ]<-------------->[(LLC)  |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
Modem-router

PPPoEoA

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         \-->[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         \-->[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
PPPoE to L2TP

IPoE

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)
IPoE

Interfaces

Details of the interfaces can be found in TR-059 page 9.

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    ]
[(SNAP) ] [(SNAP) ] [(NLPID)] [(NLPID)]         [IP   ]
[(LLC)  ] [(LLC)  ] [(LLC)  ] [(LLC)  ]         [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 for the U interface

TR-043 compares the different ATM-based solutions.

Notes:

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 for the V interface

More stuff

Multiplexing over AAL5

RFC2684 defines two methods of protocol multiplexing over AAL5:

                 [.... / ...  ]
                 [SNAP / NLPID]
[    ]           [LLC         ]
[AAL5]           [AAL5        ]
[ATM ]           [ATM         ]
VC Multiplexing  LLC Encapsulation
Encapsulation over AAL5

L2TP and RADIUS message exchange

Sequence diagram
Sequence diagram showing L2TP and RADIUS interaction
ASCII art diagram
[Client] [BRAS]          [LNS]
   |       | [RADIUS Proxy]|  [RADIUS]
   |       |       |       |     |
   |       |       |       |     |
   |       |       |       |     |  I] Initial challenge
   |       |       |       |     |
   |<------|       |       |     |  CHAP Challenge
   |------>|       |       |     |  CHAP Response
   |       |------>|------------>|  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
   |       |       |       |     |

References