Residential Broadband Architectural Framework
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Technical Committee
Residential Broadband
Architectural Framework
AF-RBB-0099.000
July, 1998
af-rbb-0099.000 RBB Architectural Framework
July 1998
© 1998 by The ATM Forum. The ATM Forum hereby grants its members the limited right
to reproduce in whole, but not in part, this specification for its members internal use only and
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reserved. Except as expressly stated in this notice, no part of this document may be
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and retrieval system, without the prior written permission of The ATM Forum.
The information in this publication is believed to be accurate as of its publication date. Such
information is subject to change without notice and The ATM Forum is not responsible for
any errors. The ATM Forum does not assume any responsibility to update or correct any
information in this publication. Notwithstanding anything to the contrary, neither The ATM
Forum nor the publisher make any representation or warranty, expressed or implied,
concerning the completeness, accuracy, or applicability of any information contained in this
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NOTE: The user's attention is called to the possibility that implementation of the ATM
interoperability specification contained herein may require use of an invention covered by
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Table of Contents
1. Introduction...................................................................................5
1.1 Reference Architecture 5
1.2 Reference Elements 6
1.2.1 Core ATM Network .................................................................................... 6
1.2.2 ATM Access Network ................................................................................ 6
1.2.3 Access Network Termination................................................................... 7
1.2.4 Home ATM Network .................................................................................. 8
1.2.5 ATM End System ....................................................................................... 8
1.3 Reference Interfaces 8
1.3.1 Access Network Interface......................................................................... 8
1.3.2 UNIW, UNIX and UNIH Interfaces .............................................................. 9
2. Services......................................................................................1 0
2.1 Connection Configurations 10
2.2 Connection Types 10
2.3 Establishment of Connections 10
2.4 Traffic Management Considerations 10
3. ATM Access Networks.................................................................1 1
3.1 ATM over Hybrid Fiber Coax Reference Architecture 11
3.2 ATM Passive Optical Network Based Access Networks 12
3.2.1 ATM Passive Optical Network for FTTH...............................................13
3.2.2 ATM Passive Optical Network for FTTC/Cab......................................15
3.3 Asymmetric Digital Subscriber Line (ADSL) 17
3.3.1 Transport of ATM over ADSL.................................................................18
3.4 Very High Speed Digital Subscriber Line (VDSL) 19
4. Home ATM Network (HAN)..........................................................2 2
4.1 Infrastructure 22
4.1.1 Infrastructure Topology...........................................................................23
4.1.2 Patching.....................................................................................................23
4.1.3 Cables........................................................................................................24
4.1.4 Connectors................................................................................................24
4.1.5 Timing References...................................................................................25
5. Signaling ....................................................................................2 6
5.1 Access Network Functionality 26
5.2 Taxonomy of System Scenarios 27
5.2.1 Scenario 1.................................................................................................27
5.2.2 Scenario 2.................................................................................................27
5.2.3 Scenario 3.................................................................................................28
5.2.4 Scenario 4:................................................................................................29
5.2.5 Scenario 5:................................................................................................30
5.3 Signaling at the UNIX, UNIW and UNIH 30
5.4 Signaling at the ANI 30
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5.4.1 VB5 Interface ............................................................................................30
5.4.2 The ATM Inter-Network Interface ..........................................................31
5.4.3 The User-Network Interface...................................................................31
6. References..................................................................................3 2
6.1 Normative References 32
6.2 Informative References 32
7. Glossary......................................................................................3 3
Appendix A Example Home ATM Network Implementations ...........3 5
A.1 Home Network Fabric 35
A.1.2 Higher level switching ............................................................................36
A.1.3 Interworking Unit......................................................................................36
A.1.4 Example Home Network Fabric............................................................36
A.2 Example Configurations Permitting Multiple Attachment 36
A.2.1 Minimum Network Configuration..........................................................37
A.2.2 Adding Additional Ports..........................................................................38
A.3 Adding Additional Functionality 38
A.3.1 Downstream demultiplexing..................................................................38
A.3.2 Intra-home communications..................................................................38
A.4 Non ATM Home Networks 39
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1. Introduction
The ATM Forum Residential Broadband (RBB) Architectural Framework defines ATM
access systems and home networks.
It describes the Access-Network Interface (ANI) and the User-Network Interface (UNI).
This specification references, where appropriate, other standards and specifications.
1.1 Reference Architecture
The Residential Broadband reference architecture identifies the RBB interfaces in the
context of different access and home networks.
The RBB generic reference architecture consists of five elements:
1. Core ATM Network
2. ATM Access Network
3. Access Network Termination
4. Home ATM Network
5. ATM end system
Core ATM ATM Access Access Network Home ATM ATM End
Network Network Termination Network System
ANI UNIW UNIX UNIH
Figure 1-1 RBB Reference Architecture
One or more of the elements in the reference architecture may be null in some scenarios;
therefore, the interfaces at one or more of these reference points may be the same.
There may be more than one interface specification for each of these reference points.
Notes:
1. There are no UB, TB or SB reference points shown in Figure 1-1 as there is no direct
mapping at these reference points to the interfaces identified that apply in all cases.
In particular, the S and T reference points used in the ITU-T Recommendation I.310
have specific significance to the signaling system and definitions of the functional
elements that are not appropriate to the RBB environment. Therefore, the exact
interpretation at these reference points will depend upon the local network
architecture and regulatory environment.
The reference configurations in this section and subsequent sections show abstract
functional groupings, which generally correspond to real devices. Real devices may
comprise one abstract functional grouping, more than one abstract functional grouping or a
portion of an abstract functional grouping. In the last case, the interface between the
devices or subsystems that comprise the functional grouping are not the subject of this
specification but may be the subject of other standards or specifications.
The reference configurations also show interfaces which are the subject of this
specification, whether by inclusion or by reference to other standards or specifications.
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When two or more functional groupings are present in a real device, the interface between
them need not be exposed, even if it is the subject of this specification.
1.2 Reference Elements
1.2.1 Core ATM Network
The Core ATM network (Figure 1-2) is comprised of the following functional groupings:
1. A network of one or more ATM switches
2. Servers (which provide network-specific functions)
3. Network management
Core ATM Network
ANI
Network Management
ATM Switches Servers
ANI
Figure 1-2 Functional Decomposition of the Core ATM Network
1.2.2 ATM Access Network
The ATM Access Network (Figure 1-3) is comprised of the following functional
groupings:
1. ATM Digital Terminal (ADT)
2. Access Distribution Network
ADT is a generic term used throughout this specification. Individual Access Network
technologies have their own nomenclature corresponding to this function.
ANI UNIW
ATM Access
Network
ATM Digital Access Distribution
Terminal Network
ANI UNIW
Figure 1-3 Functional Decomposition of the ATM Access Network
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1.2.3 Access Network Termination
The Access Network termination (NT) is a functional grouping that connects the ATM
Access Network to the home ATM network. UNIW is the interface at the Access Network
side of the NT. UNIX is the interface at the home side of the NT.
The function of the NT is dependent upon the Access Network and home network
technologies. The NT may be either passive or active. A passive NT is defined such that
the interfaces at the UNIX and UNIX are identical at all layers. It may contain only passive
components (e.g. electrical protectors) or components that are active in the electrical or
optical domain (e.g. filters or amplifiers) but does not contain components that are active in
the digital domain (i.e., it does not contain modulation/demodulation or higher layer
functions).
An active NT may contain PMD layer functions in the digital domain, such as
modulation/demodulation and media conversion. It may also contain functions at the TC
and MAC layers. The physical device which contains the NT may also contain other
functions (e.g. a home distribution device).
Note: Whether or not the NT is provided as part of the Access Network, and thus
whether the customer interface is at UNIW or UNIX, is not determined by this
specification.
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1.2.4 Home ATM Network
The Home ATM Network (HAN) connects the Access Network Termination and the ATM
End System(s). Realizations of the HAN may range from a simple transparent-pass-
through passive network to a complete local network with switching functions.
The HAN is comprised of the following functional groupings (Figure 1-4):
1. Home Distribution Device.
2. Home Distribution Network.
UNIX UNIH
Home ATM Network
Home
Distribution Home Distribution
Device Network
UNIX UNIH
Figure 1-4 Functional Decomposition of the Home ATM Network
The Home Distribution Device performs switching and/or concentration of ATM virtual
connections between the UNIX and devices connected to the home ATM network at UNIH
(including support for ATM virtual connections between such devices.. It may contain
PHY, MAC or ATM layer functionality and may also contain signaling. The Home
Distribution Device is optional and need not be present in all Home ATM Networks. Some
of its functions could be realized together with the Network Termination in a single device.
The Home Distribution Network transports ATM traffic to and from the ATM End System
and may be implemented with a single point to point link, with a star configuration or with
a shared media tree and branch topology.
Suitable Home ATM Networks are described in this document, and a residential specific
PHY is defined in [1]. Other Home ATM Network implementations are not precluded.
1.2.5 ATM End System
The ATM End System contains functions above the ATM layer, possibly including end-
user applications.
1.3 Reference Interfaces
1.3.1 Access Network Interface
The Access Network Interface (ANI) is the interface between the Access Network and the
Core ATM network. It is independent of any specific Access Network technology. Section
5 provides a description of ANI interfaces.
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1.3.2 UNI W , UNIX and UNIH Interfaces
The UNI W, UNIX and UNIH interfaces are specific to the Access Network technology,
Access Network termination, Home Network and ATM End System. These interfaces
support a cell-based UNI, or optionally a frame-based UNI [2], for ATM transport
between these elements.
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2. Services
The services and capabilities specified for ATM Residential Broadband networks are the
same as those specified for other ATM networks. Network operators may elect to offer,
and equipment in the home may elect to use if offered, all or a subset of these services and
capabilities. It is intended that seamless interoperation be possible between ATM End
Systems attached to RBB networks and ATM End Systems attached to other ATM
networks.
2.1 Connection Configurations
Point-to-point and unidirectional point-to-multipoint connections are specified for RBB
networks, as in the UNI 3.1 specification. Additional connection types are being studied in
the ATM Forum and the ITU-T and may be specified in the future for RBB networks.
2.2 Connection Types
Virtual Path Connections and Virtual Channel Connections are specified for RBB networks
as in the UNI 3.1 specification.
2.3 Establishment of Connections
Permanent virtual connections are specified for RBB networks as in the UNI 3.1
specification.
Switched virtual connections are specified for RBB networks, as in the UNI 3.1
specification. Mechanisms and capabilities for SVCs are described in Section 5.
2.4 Traffic Management Considerations
The CBR, rt-VBR, nrt-VBR, ABR and UBR service categories, their associated traffic
contract parameters and QoS parameters are specified for RBB networks, as in the TM 4.0
specification. The PHY layers of some RBB Access Network technologies have
characteristics (e.g., asymmetrical bandwidth, shared media, dynamic rate) which are
atypical of PHY layers used in non-residential environments. The effect of these
characteristics on ATM traffic management is being studied. They may also affect
Connection Admission Control policies and the contribution of the Access Network to end-
to-end QoS in ways that are network specific.
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3. ATM Access Networks
This section describes several ATM Access Networks which have been considered by the
ATM Forum. This section is not meant to provide an exhaustive list of RBB access
technologies.
The following Access Network descriptions include only that portion of the network which
deals with the transport of ATM services. The carriage of non-ATM services by these
networks is beyond the scope of this specification.
3.1 ATM over Hybrid Fiber Coax Reference Architecture
Figure 3-1 shows the Reference Architecture for ATM over Hybrid Fiber coax (HFC) cable
television (CATV) transmission systems with a passive NT. In an HFC transmission
system, modulated digital signals are frequency division multiplexed onto the optical and
coaxial physical medium along with analog television signals. Service is delivered to the
subscriber at a coaxial CATV interface. Equipment at a central location (which is called a
headend), broadcasts signals on the medium in the downstream direction (i.e., toward the
subscribers). Any signal can be received (but not necessarily understood) by subscriber
equipment by tuning to the corresponding frequency division multiplexed (FDM) channel.
In the upstream direction, the physical medium is shared among subscriber equipment
(which are called stations), which transmit signals that are received only by the headend. A
Media Access Control (MAC) layer protocol arbitrates access by stations to the upstream
medium. For an overview of CATV systems in North America refer to the Cablelabs
website [14]; similar networks are used in other countries and regions, although system
parameters and engineering practices vary.
RF Splitter
ATM
End
NT System
Station
ANI UNIHFC
Adaptor ATM
End
Station System
Headend HFC Distribution
Controller Network
Home
Distribution
UNIHFC Device See Text
NT Station
See Section 4
Core ATM ATM Access Access Network Home ATM ATM End
Network Network Termination Network System
ANI UNIW UNIX UNIH
Figure 3-1 ATM Over HFC Passive NT Reference Architecture
The Headend Controller (HC) provides the necessary functionality to support ATM over
the shared HFC media. It includes ATM switching and/or concentration, signaling, MAC
layer functions, TC functions and upstream and downstream PMD functions.
The HFC distribution network is the fiber and coax distribution plant, including elements
such as diplexers, fiber nodes, amplifiers and directional couplers. Non-ATM services will
ordinarily share the HFC plant, but are not the subject of this specification.
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The NT function in CATV networks is present only for reasons of safety and signal
integrity. It consists of grounding facilities and possibly other electrical protection, splitters
or directional couplers.
The Station is the entity in the home that incorporates necessary support for ATM transport
over the CATV media. It contains PMD, TC and MAC layer functions. The station is a
component of other entities in the RBB reference model.
Two cases are identified for the NT and the Home ATM network (HAN):
In the passive home distribution scenario (the term ‘passive NT scenario’ is also used, but
is inaccurate for CATV distribution), the HAN is structured as a branching tree of coaxial
cable, with RF splitters at the branching points. Station functions are present either in the
ATM End System or in an external adapter device. There may be more than one ATM End
System or adapter device in the home, each containing a Station.
In the active distribution scenario the home distribution device contains a Station, this HAN
is described in Section 4.
The IEEE 802.14 is developing a standard for ATM transport over HFC Access Networks.
The standard is to include upstream and downstream physical layers and a MAC protocol.
The MAC protocol ensures that the services of Section 2 of this specification can be
provided, despite the shared media nature of the HFC Access Network. It is intended that
an addendum to this specification will reference the IEEE 802.14 standard when it is
completed.
3.2 ATM Passive Optical Network Based Access Networks
This section describes ATM optical networks and access architectures based on this type of
system. Figure 3-2 shows possible local access architectures, ranging from FTTH (Fiber to
the Home), through FTTB/C(Fiber to the Building/Curb) to FTTCab (Fiber to the
Cabinet). The OAN (Optical Access Network), which can be configured in a variety of
ways such as ring, point-to-multipoint, or point-to-point, is common to all architectures
shown is Figure 3-2. The FTTC and FTTCab cases use metallic media as described in
Section 3.3 and Section 3.4.
FTTH O
OAN N
T
O O FTTB/C
L OAN copper N
N
T U T
O FTTCab
OAN N
N
copper
U T
Access Home
Network Network
Figure 3-2 Network Architecture
The ATM Passive Optical Network (ATM-PON) is one of these OAN configurations.
Passive optical splitters enable the PON's capacity to be shared across several
terminal/ONUs (Optical Network Unit) over multiple subscribers.
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Because of the shared media, functions in a PON-based Access Network are required to
provide privacy and security. In addition, a medium access control (MAC) is required to
arbitrate access to the medium in the upstream direction.
The ITU-T SG 15 is developing a recommendation, G.983, for ATM PONs. It is intended
that an addendum to this specification will reference the ITU-T recommendation when it
has been approved.
3.2.1 ATM Passive Optical Network for FTTH
Figure 3-3 shows the Reference Architecture for ATM over a FTTH architecture using a
PON. The Optical Line Termination (OLT) provides the necessary functionality to support
ATM over the shared PON media. The OLT will generally contain the following functions:
1. ATM layer cross-connection or switching.
2. MAC layer functions, including downstream addressing and upstream
transmission control.
3. PHY layer functions, including E/O (Electrical/Optical) and O/E
(Optical/Electrical) conversion.
4. Access network security.
5. Interface to the Core ATM network.
The PON distribution network is comprised of the fiber distribution plant with a passive
optical splitter function.
The PON network termination provides the necessary functionality to support ATM over
the shared PON media. The NT will generally contain the following functions:
1. ATM layer multiplexing.
2. MAC layer functions, including downstream addressing, upstream transmission
control.
3. PHY layer functions, including E/O and O/E conversion.
4. Access network security.
5. Interfaces to the HAN.
Core ATM ATM Access Access Network Home ATM ATM End
Network Network Termination Network System
ANI UNIW UNIX UNIH
ANI NT
Optical Line
Terminal UNIW UNIX
OLT NT
Passive Optical
Splitter Combiner
Figure 3-3 ATM over FTTH using PON Reference Architecture
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3.2.1.1 Functional Aspects of Network Elements
3.2.1.1.1 The Optical Line Termination (OLT)
The OLT consists of three parts; the Access Node Interface Line Terminal (ANI-LT), PON
Line Terminal (PON-LT) and Virtual Path and Virtual Channel Cross Connect
(managed/dynamic) (VP/VC-CC) for VP grooming.
1. ANI-LT
The Access Node Interface Line Terminal (ANI-LT) connects the OLT to the core ATM
network or an ATM node.
2. VP/VC-XC
The Virtual Path and Virtual Channel Cross Connect (VP/VC-CC) provides connections
between the ANI-LT and the PON-LT.
3. PON-LT
The PON Line Terminal handles the opto-electronic conversion process, inserts ATM cells
into the downstream PON payload and extracts ATM cells from the upstream PON
payload. Downstream frame timing is performed by the PON-LT.
3.2.1.1.2 The Optical Network Termination (NT)
The PON NT is active and connects the Access Network delivery mechanism from the in-
house distribution. The NT consists of three parts; PON Line Termination (PON-LT),
Virtual Path Multiplexer (VP-MUX) for VP multiplexing, and the User Network Interface
Line Terminal (UNI-LT).
1. PON-LT
The PON Line Termination (PON-LT) handles the opto-electronic conversion process. The
PON-LT extracts ATM cells from the downstream PON payload and inserts ATM
cells into the upstream PON payload based on synchronization acquired from the
downstream frame timing.
2. VP-MUX
The Virtual Path Multiplexer (VP-MUX) multiplexes UNI-LTs to PON-LT. Only valid
ATM cells can be passed through the VP-MUX, so many VPs can share the
assigned upstream bandwidths effectively.
3. UNI-LT
The User Network Interface Line Terminal (UNI-LT) interfaces over UNIx to a Home
ATM Network (HAN).
3.2.1.2 ATM Transport Protocol Model
Figure 3-4 illustrates an ATM transport protocol model aligned with the reference
architecture shown in Figure 3-3.
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OLT NT
ATM ATM
Access Access
PHY PHY
PHYPON PHYPON
ANI UNIPON UNI
PON MAC & PHY
Figure 3-4 ATM Transport Protocol Model
In this architecture, the ATM transport protocols at a UNIPON consists of Physical,
Access, and ATM layers. This architecture is only intended to address the transport of
ATM user data and not to address the control or management of ATM (hence there are no
management or signaling protocols shown).
3.2.2 ATM Passive Optical Network for FTTC/Cab
Figure 3-5 shows the Reference Architecture for ATM over a FTTC/Cab architecture using
PON and xDSL systems. The Optical Line Termination (OLT) provides the necessary
functionality to support ATM over the shared PON. The OLT will generally contain the
following functions:
1. ATM layer cross connection and switching.
2. MAC layer functions, including downstream addressing, upstream transmission
control.
3. PHY layer functions, including E/O and O/E conversion.
4. Access network security.
5. Interface to the Core ATM network.
The PON distribution network is comprised of the fiber distribution plant with a passive
optical splitter. Non-ATM services may be carried over the PON network; the means for
doing so are not the subject of this specification. Any of the Passive or Active NT
Scenarios may apply.
The Optical Network Unit (ONU) provides the necessary functionality to support ATM
over the shared PON media. The ONU will generally contain the following functions:
1. ATM layer multiplexing.
2. MAC layer functions, including downstream addressing, upstream transmission
control.
3. PHY layer functions, including E/O and O/E conversion.
4. Access network security.
5. Interface to the xDSL transmission system.
The xDSL ATM Interface Unit (xDSL-AIU) provides the network termination function,
i.e. conversion from the xDSL transmission system to the interface to the end system. It
contains the following functions:
1. xDSL Transceiver Unit - Remote Terminal end (xTU-R).
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2. Mux/Demux.
3. ATM layer functions
4. Interface to the HAN.
Core ATM ATM Access Access Network Home ATM ATM End
Network Network Termination Network System
ANI UNIW UNIX UNIH
UNIW
Optical Line
Terminal UNIW Optical ATM
ANI OLT xTU-C
Network
Passive Optical Unit xTU-C xTU-R PHY
Splitter Combiner UNIxDSL UNIX
xDSL-AIU
Figure 3-5 ATM over FTTC/Cab using PON and xDSL Reference Architecture
3.2.2.1 Functional Aspects of Network Elements
3.2.2.1.1 The Optical Line Termination (OLT)
The OLT consists of three parts; the Access Node Interface Line Terminal (ANI-LT),
Virtual Path and optionally Virtual Channel Cross Connect (VP-XC) for VP/VC grooming
and the PON Line Termination (PON-LT).
1. ANI-LT
The Access Node Interface Line Termination (ANI-LT) connects the OLT to the core ATM
network or an ATM node.
2. VP/VC-XC
The Virtual Path and Virtual Channel Cross Connect (VP/VC-CC) provides connections
between the ANI-LT and the PON-LT.
3. PON-LT
The PON Line Terminal handles the opto-electronic conversion process, inserts ATM cells
into the downstream PON payload and extracts ATM cells from the upstream PON
payload. Downstream frame timing is performed by the PON-LT.
3.2.2.1.2 Optical Network Unit and xTU-C
The ONU connects the PON Access Network delivery mechanism from the xDSL
transmission system. The ONU consists of three parts; PON Line Terminal (PON-LT) and
Virtual Path/Virtual Channel Multiplexer (VP-MUX) for VP multiplexing and multiple
xDSL Transceiver Unit - Central office (xTU-C) line cards.
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1. PON-LT
The PON Line Terminal (PON-LT) handles the opto-electronic conversion process. The
PON-LT extracts ATM cells from the downstream PON payload and inserts ATM
cells into the upstream PON payload based on synchronization acquired from the
downstream frame timing.
2. VP-MUX
The Virtual Path Multiplexer (VP-MUX) multiplexes xTU-Cs (xDSL Transceiver Unit-
Central Office) to PON-LT. Only valid ATM cells can be passed through the VP-
MUX, so many VPs can share the assigned upstream bandwidths effectively.
3. xTU-C
For details of the xDSL Transceiver Unit - Central Office see sections 3.4 and 3.5.
3.2.2.1.3 xDSL-AIU
For details of the xDSL ATM Interface Unit see sections 3.4 and 3.5.
3.2.2.2 ATM Transport Protocol Model
Figure 3-6 illustrates an ATM transport protocol model aligned with the reference
architecture shown in Figure 3-5.
OLT ONU NT
ATM ATM ATM
Access Access
PHY PHY PHY PHY
PHYPON PHYPON xDSL xDSL
ANI UNIPON UNIxDSL UNI
PON MAC & PHY xDSL PHY
Figure 3-6 PON Transport Protocol Mapping
For details of an example short-range FTTC PMD Layer specification refer to the DAVIC
specification [3].
3.3 Asymmetric Digital Subscriber Line (ADSL)
Asymmetric Digital Subscriber Line (ADSL) is a transmission system which supports high
bit rates over existing metallic twisted pair Access Networks. ADSL provides a high bit rate
downstream channel (towards the ATM End System) and a lower bit rate upstream channel
(from the ATM End System towards the network). ADSL may support bit rates up to 6
Mb/s in the downstream direction and up to 640 Kb/s in the upstream direction, depending
on loop length.
Figure 3-7 compares the RBB reference architecture with the ADSL functional model
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Core ATM ATM Access Access Network Home ATM ATM End
Network Network Termination Network System
ANI UNIW UNIX UNIH
ATM ATU-C
Switch/ ATM
Concen- ATU-C
V trator
ATU-C ADSL-R PHY
U ADSL-NT T
ADSL-ADT
Figure 3-7 Mapping of ADSL (Active NT) Functional Model on the RBB
Reference Architecture
The ADSL-ADT may contain the following functions:
1. Concentrator and/or switch
2. ADSL Transceiver Unit-Central Office (ATU-C)
3. ATM layer functions
4. Interface to the Core ATM network
5. POTS splitter to separate the POTS and ADSL channels
The ADSL Transceiver Unit-Central Office end (ATU-C) provides the necessary
functionality to support transmission over point-to-point metallic twisted pair media.
Typically, it includes the following functions:
1. PMD layer functions, such as, upstream demodulation, downstream
modulation, Scrambling, FEC and interleaving
2. TC layer functions, e.g., OAM, cell delineation
3. POTS splitter functions
The ADSL-AIU typically consists of the following functions:
1. ADSL Transceiver Unit - Remote Terminal end (ATU-R)
2. Mux/Demux
3. ATM layer functions
4. Interface to the HAN
The ATU-R is the inverse of the ATU-C. It may include:
1. PMD layer functions, such as downstream demodulation, upstream modulation,
Scrambling, FEC and interleaving
2. TC layer functions, e.g., OAM, cell delineation
3. POTS splitter functions
3.3.1 Transport of ATM over ADSL.
ADSL transport has three unique characteristics which affect operation of the ATM layer.
These are:
1. asymmetry and channel capacity
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RBB Architectural Framework af-rbb-0099.000
July 1998
2. forward error correction and interleaving
3. dynamic rate adaptation and rate repartitioning.
The asymmetry ratio and channel capacity is a function of the link attenuation and noise
environment, and is normally established at power up by the selection of one of a number
of possible rates (with a granularity of 32 kb/s). A mechanism needs to exist so the capacity
selected is known for the purpose of CAC. Channeling the total capacity into a number of
independent sub-channels is possible, but not required for ATM where the same
functionality can be achieved by VC based multiplexing, except where dual latency
(discussed below) needs to be supported.
In order to achieve an acceptable BER, FEC with interleaving is often required. This
spreads the effect of impulsive noise interference over many widely separated bits which
allows error correcting codes to be effective. However the time scale of the noise events is
such that interleaving delays of ~20 ms can result. Such a delay may not be acceptable for
all applications, in particular those which are more concerned with delay than error rate.
Therefore the concept of dual latency is supported which allows both interleaved and non-
interleaved channels.
The prevailing noise environment will change on both a short and a long time scale. It is
possible to allow for the worst case noise situation at start up, but this will then not be
using the full link capacity. It is possible to dynamically adapt the bit-rate after start-up to
maximize the instantaneous throughput, but this offers many challenges for maintaining an
agreed QOS on existing calls, and CAC.
The basic mechanisms for the transport of ATM over ADSL have been specified by the
ADSL Forum [4]. This covers basic architecture, channelization, TC issues and Physical
Layer OAM, but the more complex matters relating to dual latency and dynamic rate
adaptation are not specified. Technical Subcommittee T1E1 is developing Issue 2 of ANSI
T1.413 which incorporates relevant work of the ADSL Forum. It is intended that an
addendum to this specification will reference the T1.413 Issue II or the equivalent ITU-T
recommendation when approved.
3.4 Very High Speed Digital Subscriber Line (VDSL)
Very high speed Digital Subscriber Line (VDSL) is a transmission system which aims to
support very high bit rates over at least parts of the existing copper Access Network. It is
very similar to ADSL in concept, but has the following distinctions. The maximum
downstream bit rate is higher but the reach is significantly less. Both asymmetrical and
symmetrical VDSL systems are under consideration.
VDSL will use more spectrum than ADSL. From the transmission point of view, the main
concerns are RF egress given this increased bandwidth, and spectral compatibility with
ADSL.
From the ATM point of view the issues are very similar to those of ADSL i.e. channel
capacity, dual latency and dynamic rate adaptation. There are however two additional
considerations. VDSL’s enhanced capacity enables it to offer more applications, and VDSL
represents a potential migration path from ADSL. Therefore, the definition of the ATM
HAN must take into account the distribution of VDSL delivered services from the point of
view of bit-rate, multiple services, multiple QOS, and having many attached devices.
VDSL standardization is at a less advanced state than that of ADSL. Bodies known to be
active in this area include ANSI T1E1.4, ETSI (TM3 and TM6), the GX FSAN initiative
and the ITU-T SG15 WP4.
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af-rbb-0099.000 RBB Architectural Framework
July 1998
Figure 3-8 compares the RBB reference architecture with the VDSL functional model
Core ATM ATM Access Access Network Home ATM ATM End
Network Network Termination Network System
ANI UNIW UNIX UNIH
VDSL-LT
Access
Node ATM
VDSL-LT
V
VDSL-R PHY
VDSL-LT
U VDSL-NT T
VDSL-ADT
FTTx (x=C/Cab/B/E
Figure 3-8 Mapping of VDSL (Active NT case) Functional Model on the RBB
Reference Architecture
The VDSL-ADT (VDSL ATM Digital Terminal) may contain the following functions:
1. VDSL Transceiver Units - Central Office (VTU-C).
2. POTS splitter to segregate the POTS and VDSL channels.
3. Concentrator and/or switch.
4. Mux/Demux.
5. Interface to some kind of an Optical Access Network (PON or point-to-point
links), in the case of FTTC/Cab/B architectures.
6. Interface to the Core ATM Network, in the case in which the ADT is located at
the Central Office (FTTE - Fiber To The Exchange).
The VDSL Transceiver Unit - Central Office VTU-C provides the necessary functionality to
support transmission over the point-to-point metallic twisted pair media. Typically, it
includes the following functions:
1. PMD layer functions, such as, upstream demodulation, downstream
modulation, Scrambling, FEC and interleaving.
2. TC layer functions, such as OAM and cell delineation.
3. POTS splitter functions
The VDSL distribution network consists of a metallic twisted pair plant.
The VDSL-AIU may contain the following functions:
1. VDSL Transceiver Unit - Remote terminal (VTU-R).
2. Mux/Demux.
3. ATM layer functions.
4. Interface(s) to the Home ATM network.
The VDSL Transceiver Unit - Remote terminal (VTU-R) is the inverse of the VTU-C.
Typically, it includes the following functions:
1. PMD layer functions, such as upstream modulation, downstream demodulation,
scrambling, FEC and interleaving.
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