may be selected over others depending upon the service
being delivered, and the architectures of the various networks involved in delivering the service to
the end user.

Notice:
The DSL Forum is a non-profit corporation organized to create guidelines for DSL network
system development and deployment. This Technical Report is a draft, and has not been
approved by members of the Forum. Even if approved, this document is not binding on the DSL
Forum, any of its members, or any developer or service provider involved in DSL. This document
is subject to change, but only with the approval of membership of the Forum. Protocols at the U Interface
DSL Forum TR-043
for Accessing Data Networks using ATM/DSL




2
Table of Contents
1

STATEMENT OF THE PROJECT.
3

1.1

Scope
3

1.2

Motivation 3

2

END-TO-END SERVICE INTEROPERABILITY MODEL OVER DSL SYSTEMS

3

2.1

Specific Reference model
3

2.2

Customer
Premises 4

2.3

Access Network
4

2.4

Regional Broadband Network
5

2.5

Service Provider Networks
5

3

END-TO-END INTEROPERABLE DSL NETWORK ARCHITECTURE 5

3.1

Core Network/Regional Broadband Network
5

3.2

Protocol Stacks at the U Reference Point
5

3.2.1

Common
Features 6

3.2.2

PPP over ATM (PPP/AAL5/ATM/DSL)
10

3.2.3

IP over Ethernet (IP/Ethernet/AAL5/ATM/DSL)
11

3.2.4

PPP over Ethernet (PPP/PPPoE/Ethernet/AAL5/ATM/DSL)
12

3.2.5

IP/AAL5/ATM/DSL 13

4

REFERENCES 15

5

GLOSSARY 15

APPENDIX A

VOLUNTARY TUNNELLING MECHANISMS
17

A.1

L2TP (L2TP/IP/any stack from 3.2)
17

A.2

IPsec (IPsec/IP/any stack from 3.2)
17

APPENDIX B

COMPARISON TABLE
18

APPENDIX C

PROTOCOL EFFICIENCY
19


Protocols at the U Interface
DSL Forum TR-043
for Accessing Data Networks using ATM/DSL




3
1
Statement of the project.
1.1 Scope
The focus of this document is to document common current and expected (in the near-term) user plane
protocols that are transported over ATM over DSL, independent of transmission layer line code, at what is
commonly known as the U reference point for the application of access to data networks. This series
of recommendations will be dependent upon existing standards, soon-to-be standards, and informational
RFCs.
This document reflects the DSL Forums consensus on best current practices for deployments of
ATM/DSL. Current refers to the date of this documents publication.
The doc deals specifically with connection attributes at L2/2.5, not service attributes.
This document will augment information contained in TRs that make reference to and are based on TR-
12. Documents that were based on or reference the TR-12 architecture are TR-18, TR-25, and TR-32.
1.2 Motivation
TR-012 named PPPoA as the DSL Forums standard for use at the U interface. This document was
written to update the information presented in TR-012, because of changes in the industry over the
intervening years. Since publication of TR-012, many service providers have deployed DSL using PPPoE,
IP over 2684-encapsulated Ethernet, and IP over AAL5. Part of the reason for this is that there are
benefits unique to these solutions that are not provided by PPPoA.
Because these protocols are in wide use, and are used for good reasons, it became necessary for the DSL
Forum to expand its list of standard protocols. The new set of protocols needed to be documented, to
provide input to Testing and Interoperability, and other DSL Forum working groups. These groups need
input as to what protocols to include in their testing, provisioning, and other scenarios.
2
End-to-end Service Interoperability model over DSL systems
2.1
Specific Reference model
Service
Provider
Customer
Premises
Regional
Broadband
Network
Access
Node
A10
A11
S
T
U
V
NOTE: V, U, S and T correspond to ITU practice
A10 and A11 are borrowed from DAVIC as there are no ITU equivalents
Network
Termination (B-NT)
Note:
Expanded in TR-017
[14]
Customer
Premises
Network

Figure 1. Architectural reference model Protocols at the U Interface
DSL Forum TR-043
for Accessing Data Networks using ATM/DSL




4
The end-to-end DSL-based network architecture for convenience can be decomposed into the following
subnetworks: the customer premises network, the access network, the regional broadband network and the
service provider networks. They are shown in Figure 2.

Customer Premises
Network
Access
Network
Service Provider
Network
Regional Broadband
Network
ISP
ROC
Content
Provider
Regional
Broadband
Network
(ATM core
network)
Local
loop
ADSL
modem
TE
Home
CO
ATM Access
Switch *
Access
node
* This may or may not
be located in the CO
ADSL
modem
TE
SOHO
Internet
Corporate
Network

Figure 2. Example of an end-to-end DSL-based Broadband Network Architecture
2.2 Customer
Premises
The customer premises include residences, home offices and small business offices. Each will contain one
or more terminal equipment devices (such as PCs, workstations, set-tops, etc.) possibly interconnected by
a customer premises network. The DSL modem on the customer premises is called the B-NT
(Broadband Network Termination).
2.3 Access
Network
The DSL access network encompasses the DSL modems at customer premises and the access multiplexer
system at the CO connected via the local loop. The DSL Forum TR-017 [2] addresses the layer 2 protocols
and specifically describes the implementation of ATM transport over DSL links. TR-017 identifies and
defines the functional blocks of ATM-based DSL access network which are formally referred to as
Broadband Network Termination (B-NT) for DSL modem and Access Node (AN) for access multiplexer
system. The access node is frequently referred to as the DSLAM (DSL Access Multiplexer). TR-017
also addresses the control and management planes related to supporting ATM in user plane. It includes
the ATM PVC support, signaling for SVC support and operations and maintenance functionality to
support ATM over DSL. When the backbone network is ATM, the access node is connected to an ATM
access switch. The DSL access node and ATM access switch may or may not be co-located. The function
of the access node and access switch is to:
n provide physical port concentration
n provide bandwidth concentration in the form of statistical multiplexing of non-CBR traffic classes
n to possibly provide logical port concentration when a service interworking function is co-located in
the access network
n support the ability to offer differentiated services in the network. Protocols at the U Interface
DSL Forum TR-043
for Accessing Data Networks using ATM/DSL




5
2.4
Regional Broadband Network
A regional broadband network, interconnects the central offices in a geographical area. The function of
the regional broadband network is a combination of transport and possibly switching.
2.5
Service Provider Networks
The service provider networks include the ISP POPs, content provider networks, corporate networks and
regional operation center (ROC). An ISP POP is for connecting to the Internet and provides ISP services
such as e-mail and Web hosting. A content provider network consists of a server farm for distributing
content. The corporate networks may be connected to the regional broadband network to allow remote
access from a home (telecommuting) or from branch offices. The ROC is operated by the access network
operator to manage the entire access network, and possibly to provide value-added services.
3
End-to-end Interoperable DSL Network Architecture
3.1
Core Network/Regional Broadband Network
Requirements for various configurations of the Core Network can be found in TR-025
[
4
]
. One thing that
all the supported network configurations have in common is that they make use of ATM over DSL at the
U reference point, identified in [2], going into the access node. The protocols listed below do not change
any of the base assumptions or core technologies listed in that document. However, enhancements can be
made to that document to better describe the handling of the additional protocol stacks listed below.
3.2
Protocol Stacks at the U Reference Point
This section details various protocol stacks that may be used at the U reference point, to support access to
data networks and meet the requirements outlined above. All rely on AAL5 over ATM over DSL.
A number of the stacks make use of PPP (Point-to-Point Protocol). Once ATM layer connectivity is
established between the customer premises and the service provider network, the session setup and release
phases at the link level and network level can be established using PPP.
Several also encapsulate Ethernet over AAL5, allowing the Ethernet used within the end-customer
premises to be bridged on to the access network.
See Appendix B for a table directly comparing the protocols listed in this section.
Figure 3 shows the protocol stacks that are documented in this section. Protocols at the U Interface
DSL Forum TR-043
for Accessing Data Networks using ATM/DSL




6

DSL
ATM
AAL5
LLC/VCMux
Eth
PPPoE
PPPoE
DSL
ATM
AAL5
LLC/VCMux
Eth
IP/Eth
(commonly referred to
simply as 1483)
DSL
ATM
AAL5
LLC/VCMux
PPPoA
PPPoA
DSL
ATM
AAL5
LLC/VCMux
IP/AAL5
PPP
IP
PPP
IP
DSL
ATM
AAL5
LLC/VCMux
L2TP
L2TPoA
PPP

Figure 3: Protocol Stacks at the "U" Interface
3.2.1 Common
Features
The following protocols are used in more than one of the recommended stacks. They are split out here, so
that pros and cons and other attributes of the stacks that are due to the use of these common protocols can
be identified once, and not