ETSI ETR 242 ed.1 (1995-11)

SLOVENSKI STANDARD
01-januar-2005
Signalizacijski protokoli in komutacija (SPS) – Standardizacija pri zagotavljanju
odprtosti omrežij (ONP) za dostop do krajevne zanke
Signalling Protocols and Switching (SPS); Open Network Provision (ONP)
standardization for access to the local loop
Ta slovenski standard je istoveten z: ETR 242 Edition 1
ICS:
33.040.30 Komutacijski in signalizacijski Switching and signalling
sistem systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

ETSI ETR 242
TECHNICAL November 1995
REPORT
Source: ETSI TC-SPS Reference: DTR/SPS-03037
ICS: 33.080
ONP, AN, LE, local loop, V interface, V5 interface
Key words:
Signalling Protocols and Switching (SPS);
Open Network Provision (ONP) standardization
for access to the local loop
ETSI
European Telecommunications Standards Institute
ETSI Secretariat
F-06921 Sophia Antipolis CEDEX - FRANCE
Postal address:
650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE
Office address:
c=fr, a=atlas, p=etsi, s=secretariat - secretariat@etsi.fr
X.400: Internet:
Tel.: +33 92 94 42 00 - Fax: +33 93 65 47 16
*
Copyright Notification: No part may be reproduced except as authorized by written permission. The copyright and the
foregoing restriction extend to reproduction in all media.
© European Telecommunications Standards Institute 1995. All rights reserved.

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ETR 242: November 1995
Whilst every care has been taken in the preparation and publication of this document, errors in content,
typographical or otherwise, may occur. If you have comments concerning its accuracy, please write to
"ETSI Editing and Committee Support Dept." at the address shown on the title page.

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ETR 242: November 1995
Contents
Foreword .5
1 Scope .7
2 References.7
3 Definitions and abbreviations .9
3.1 Definitions .9
3.2 Abbreviations .11
4 Standardization requirements .11
4.1 Introduction .11
4.2 Liberalization of infrastructure.11
4.3 Standardization requirements at the loop - core boundary.12
5 Status of access network and V5 standards work .13
5.1 V5-related documents.13
5.2 Other AN-related documents.14
6 Architecture and modelling.15
6.1 The general network architecture and modelling.15
6.2 The architecture and modelling of the access connection element.17
6.3 The network selection function in the ISDN.20
6.4 Discussion of the term "core network".22
7 Analysis of V5 and associated specifications.22
7.1 Introduction .22
7.1.1 Scope of V5 specifications .22
7.1.2 Multiple V5 interface applications.23
7.1.3 Architecture aspects.23
7.1.4 Control principles.24
7.1.4.1 General requirements and assumptions .24
7.1.4.2 Control of ISDN user port for the PL capability.24
7.1.5 Provisioning strategy and requirements .24
7.1.6 Protocol requirements for PSTN and ISDN.25
7.2 Access network services not covered by the V5 standards.25
7.3 Consideration of CEC BC-T-309 requirements .25
7.3.1 Separate AN and switched-service providers .25
7.3.2 Access to more than one switched-service provider.25
7.3.3 User selection of switched-service provider (permanent) .26
7.3.4 User selection of switched-service provider (semi-permanent) .26
7.3.5 User selection of switched-service provider (call-by-call).26
8 Support of leased lines.27
8.1 Semi-permanent leased line connections.27
8.2 Permanent leased line connections.27
8.3 Permanent line capability for permanent leased line connections.27
9 Management .27
9.1 Access network management architecture.27
9.2 Management interface capabilities .28
9.3 Management associated with user selection of switched-service provider .28
9.4 Management associated with leased lines .29

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ETR 242: November 1995
10 Additional specifications required . 29
10.1 V5 PSTN signal mappings . 29
10.1.2 The need for V5 PSTN mappings. 29
10.1.3 The current approach to the production of V5 PSTN mappings. 29
10.1.4 Standardization of the V5 PSTN mappings . 30
10.2 Management . 30
11 Conclusion. 30
Annex A: Private Branch eXchange (PBX) selection of service provider . 31
History. 32

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ETR 242: November 1995
Foreword
This ETSI Technical Report (ETR) was produced by the Signalling Protocols and Switching Technical
Committee of the European Telecommunications Standard Institute (ETSI).
This work was initiated in response to a Commission of the European Communities (CEC) mandate
(BC-T-309) on ONP standardization for access to the local loop.

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ETR 242: November 1995
Blank page
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ETR 242: November 1995
1 Scope
This ETSI Technical Report (ETR) is the response to BC-T-309 [1] "Standardization Mandate Forwarded
to CEN/CENELEC/ETSI in the Field of Information Technology and Telecommunications" entitled "ONP
Standardization for Access to the Local Loop" issued by the Commission of the European Communities
(CEC). The ETR has been prepared by an ETSI Task Group, set up by ETSI Technical Committee SPS
(Signalling Protocols and Switching) which is responsible for the V5 interface specifications.
The first step of the mandated work is the development of this ETR covering:
a) the standardization requirements for interfaces between an Access Network (AN) and a core
network taking into account the regulatory situation after 1998, where these interfaces could serve
as a boundary of responsibility between an organization providing the local access network and an
organization providing a telecommunication service;
b) an analysis of the V5 series interface specifications as a basis for these standards;
c) identification of additional specifications required to extend and to complement the above interface
specifications, in particular in relation to the selection on a call-by-call, semi-permanent and
permanent basis of core network or service provider;
d) an analysis of the Q3 specifications as a basis for the network management standards to
complement the above interface specifications.
Clause 4 outlines the standardization requirements, in response to item a).
Clause 5 outlines the status of standardization work in both ETSI and ITU-T, concerned with V5 interfaces
and related matters which are relevant to BC-T-309 [1].
Clause 6 provides an introduction to network architecture and modelling concepts, drawn from ITU-T
Recommendations, which are essential to ONP consideration of access to the local loop.
Clause 7 provides an analysis of the V5 and related standards and identifies the extent to which they meet
the needs of BC-T-309 [1] and the manner in which they do so. Clauses 7 and 8 address item b).
Clause 8 discusses the particular considerations which apply to leased lines.
Clause 9 discusses the management capabilities which are necessary to support the needs of
BC-T-309 [1]. It covers item d).
Clause 10 identifies the additional standardization which is desirable in order to properly meet the needs
of BC-T-309 [1]. It therefore covers item c).
Clause 11 summarizes the material in this ETR, so as to provide easy access to the main conclusions of
the study.
Annex A discusses a supplementary approach to service provider access, which may be appropriate in
some situations for Private Branch eXchange (PBX) users.
2 References
This ETR incorporates by dated and undated reference, provisions from other publications. These
references are cited at the appropriate places in the text and the publications are listed hereafter. For
dated references, subsequent amendments to or revisions of any of these publications apply to this ETR
only when incorporated in it by amendment or revision. For undated references the latest edition of the
publication referred to applies.
[1] CEC BC-T-309 (23-02-1994): "Standardization mandate forwarded to
CEN/CENELEC/ETSI in the field of information technology: ONP
standardization for access to the local loop".

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ETR 242: November 1995
[2] ETS 300 011: "Integrated Services Digital Network (ISDN); Primary rate user-
network interface; Layer 1 specification and test principles" (based on ITU-T
Recommendation I.431).
[3] ETS 300 012: "Integrated Services Digital Network (ISDN); Basic user-network
interface; Layer 1 specification and test principles" (based on ITU-T
Recommendation I.430).
[4] ETS 300 166 (1993): "Transmission and Multiplexing (TM); Physical and
electrical characteristics of hierarchical digital interfaces for equipment using the
2 048 kbit/s-based plesiochronous or synchronous digital hierarchies" (based on
CCITT Recommendation G.703).
[5] ETS 300 167 (1993): "Transmission and Multiplexing (TM); Functional
characteristics of 2 048 kbit/s interfaces" (based on CCITT Recommendations
G.704 and G.706).
[6] ETS 300 233: "Integrated Services Digital Network (ISDN); Access digital
section for ISDN primary rate" (based on ITU-T Recommendation G.962).
[7] ETS 300 297: "Integrated Services Digital Network (ISDN); Access digital
section for ISDN basic rate" (based on ITU-T Recommendation G.960).
[8] ETS 300 324-1: "Signalling Protocols and Switching (SPS), V interfaces at the
digital Local Exchange (LE); V5.1 interface for the support of Access Network
(AN); Part 1: V5.1 interface specification" (also ITU-T Recommendation G.964).
[9] ETS 300 347-1: "Signalling Protocols and Switching (SPS), V interfaces at the
digital Local Exchange (LE); V5.2 interface for the support of Access Network
(AN); Part 1: V5.2 interface specification" (also ITU-T Recommendation G.965).
[10] ETS 300 376-1 (1994): "Signalling Protocols and Switching (SPS); Q3 interface
at the Access Network (AN) for configuration management of V5 interfaces and
associated user ports; Part 1: Q3 interface specification" (also draft ITU-T
Recommendation Q.57CM).
[11] ETS 300 377-1 (1994): "Signalling Protocols and Switching (SPS); Q3 interface
at the Local Exchange (LE) for configuration management of V5 interfaces and
associated customer profiles; Part 1: Q3 interface specification" (also draft
ITU-T Recommendation Q.57CM).
[12] ETS 300 378-1 (1994): "Signalling Protocols and Switching (SPS); Q3 interface
at the Access Network (AN) for fault and performance management of V5
interfaces and associated user ports; Part 1: Q3 interface specification" (also
draft ITU-T Recommendation Q.57FPM).
[13] ETS 300 379-1 (1995): "Signalling Protocols and Switching (SPS); Q3 interface
at the Local Exchange (LE) for fault and performance management of V5
interfaces and associated customer profiles; Part 1: Q3 interface specification"
(also draft ITU-T Recommendation Q.57FPM).
[14] CCITT Recommendation G.703 (1991): "Physical/electrical interfaces of
hierarchical digital interfaces" (see also ETS 300 166 [4]).
[15] CCITT Recommendation G.704 (1991): "Synchronous frame structures used at
primary and secondary hierarchical levels" (see also ETS 300 167 [5]).
[16] CCITT Recommendation G.705 (1988): "Characteristics required to terminate
digital links on a digital exchange".

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ETR 242: November 1995
[17] ITU-T Recommendation I.112 (1993): "Vocabulary of terms for ISDNs".
[18] CCITT Recommendation I.324 (1991): "ISDN network architecture".
[19] CCITT Recommendation I.340 (1988): "ISDN connection types".
[20] ITU-T Recommendation I.411 (1993): "ISDN user-network interfaces -
Reference configurations".
[21] ITU-T Recommendation I.414 (1993): "Overview of Recommendations on
layer 1 for ISDN and B-ISDN customer accesses".
[22] CCITT Recommendation I.511 (1988): "ISDN-to-ISDN layer 1 internetwork
interface".
[23] CCITT Recommendation I.601 (1988): "General maintenance principles of ISDN
subscriber access and subscriber installation".
[24] CCITT Recommendation M.3602 (1992): "Application of maintenance principles
to ISDN subscriber installations".
[25] CCITT Recommendation M.3603 (1992): "Application of maintenance principles
to ISDN basic rate access".
[26] CCITT Recommendation M.3604 (1992): "Application of maintenance principles
to ISDN primary rate access".
[27] CCITT Recommendation Q.511 (1988): "Exchange interfaces towards other
exchanges".
[28] ITU-T Recommendation Q.512 (1995): "Digital exchange interfaces for
subscriber access".
[29] ITU-T Recommendation Q.521 (1993): "Digital exchange functions".
3 Definitions and abbreviations
3.1 Definitions
For the purposes of this ETR, the following definitions apply, together with those given in standards
referenced:
Access Network (AN): A system implemented between the Local Exchange (LE) and users, replacing
part or the whole of the local line distribution network. The functions associated with the V5 interface(s) of
an AN can be configured and operated flexibly via a management Q interface. An AN may consist of
multiplexing, cross connect and transmission functions. The V5 interface standard is independent of the
transmission media used inside the AN. An AN may support services which are outside the scope of the
V5 standards.
Local Exchange (LE): An exchange on which user lines are terminated via an AN. The functions
associated with the V5 interface(s) on a LE can be configured and operated flexibly via a management Q
interface. An LE may also directly terminate user lines but these are outside the scope of the V5
standards.
V5 interface: A general term for the group of V interfaces for connection of ANs to the LE, i.e. V5.1 and
V5.2 interface.
Q3 interface: A TMN interface between a Network Element (NE), such as an AN or LE, and the
Operations System (OS) responsible for the management of that NE.

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ETR 242: November 1995
service provider: An organization which provides a telecommunications service. In this ETR, it refers
particularly to the organization which provides a switched telephony and/or ISDN service (and therefore
operates the local exchange) or a leased line service.
AN provider: An organization which operates an access network and therefore provides user access to
one or more service providers.
transit network provider: An organization which provides a switched service, particularly long distance
service, between other switched-service providers.
provisioned: A parameter is said to be provisioned if the Q interface has the capability to verify and
change it. Such a parameter may have a default value and/or may be altered by a local interface.
user selection of switched-service provider (permanent): The case where selection of
switched-service provider is by pre-arrangement between the user and the AN and service providers and
is set up by management action.
user selection of switched-service provider (semi-permanent): The case where selection of
switched-service provider is set up:
a) at times pre-arranged between the user and the AN and service providers, with switch-over
by automatic management action, e.g. different service providers during the working day and
at night, or
b) under user control via a user-network management interface.
user selection of switched-service provider (call-by-call): The case where selection of
switched-service provider is part of the call set-up procedure.
transit network selection: The case where selection of transit network provider is part of the call set-up
procedure.
user selection of leased-line service provider (permanent): The case where selection of leased-line
service provider is by pre-arrangement between the user and the AN and service providers and is set up
by management action.
semi-permanent leased line: A permanently established connection between two User-Network
Interfaces (UNIs) routed through the switched digital network.
permanent leased line: A permanently established connection provided between two leased-line UNIs
routed through the transmission network bypassing network nodes of the switched digital network. The
UNIs may be analogue or digital. Permanent leased line services, provided by an access network, bypass
the local exchange and have no effect on the V5 interface.
Permanent Line (PL): A permanently established connection provided between two ISDN UNIs routed
through the transmission network bypassing network nodes of the switched digital network. The PL
reduces the access capability at the UNI for switched services.
user access: The means by which a user is connected to a telecommunication network in order to use
the service and/or facilities of that network.
access connection element (subscriber access): The equipment providing the concatenation of
functional groups between and including the Exchange Termination (ET) and the Network Termination
type 1 (NT1).
bearer channel: A 64 kbit/s time slot in the V5.1 or V5.2 interface allocated for a B-channel of an ISDN
user port or a PCM encoded 64 kbit/s channel from a PSTN user port.
control: Control is concerned with status and control of user ports; V5.1 or V5.2 interface layer 1 and
layer 2 establishment and other common procedures.

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ETR 242: November 1995
3.2 Abbreviations
For the purposes of this ETR, the following abbreviations apply:
AN Access Network
BA Basic Access
CE Connection Element
CEC Commission of the European Communities
CRF Connection Related Function
DN Directory Number
ET Exchange Termination
IN Intelligent Network
ISDN Integrated Services Digital Network
ISUP ISDN User Part
LE Local Exchange
LT Line Termination
NE Network Element
NT1 Network Termination type 1
ONP Open Network Provision
OS Operations System
PBX Private Branch eXchange
PCM Pulse Code Modulation
PL Permanent Line
PRA Primary Rate Access
PSTN Public Switched Telephone Network
TMN Telecommunication Management Network
UNI User-Network Interface
4 Standardization requirements
4.1 Introduction
Open Network Provision (ONP) is the Community policy aimed at harmonizing conditions for open and
efficient access to, and use of, Public Telecommunications Networks and Public Telecommunications
Services.
This clause investigates the first item of BC-T-309 [1] (see item a) in clause 1), i.e. the standardization
requirements for interfaces between an AN and a core network. It is assumed that the V5 series interface
specifications (i.e. ETS 300 324-1 [8] and ETS 300 347-1 [9]) serve as a basis for these standards.
4.2 Liberalization of infrastructure
With the liberalization of the infrastructure the AN and the core network could be operated by different
providers. Figure 1 shows the connection of a user to the network.
To allow interconnection of different providers of the entities shown above the interfaces between them
needs to be open and standardized. Additionally every entity needs to support user selection of the
connected network providers in a flexible manner.
Three different interfaces can be identified in figure 1.
a) Network Termination Point (User - Network Interface):
In a competitive environment, the user may choose to connect his equipment to different AN providers.
This interface would be at the coincident S and T reference point or T reference point in the narrowband
Integrated Services Digital Network (ISDN) case, for which standards are available. However, there is no
equivalent European standard for the Public Switched Telephone Network (PSTN), the physical interface
and protocol being national matters. This impacts the situation at the loop-core boundary.

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ETR 242: November 1995
Local Exchange /
Network
Loop / Core
Transit Network
Termination
Boundary
Boundary
Point
Manage- Manage-
Manage-
ment ment
ment
Access Local
Local
Access Trunk
Local
Network Exchange
Exchange
Network Exchange
Exchange
user
other
entities
Transit Network
Core Network
Local Loop
Figure 1: Connecting a user to the network
b) Loop - Core boundary:
Since the infrastructure is going to be liberalized, this interface needs to be based on standards. ANs and
core networks could be offered by different providers. Every AN provider could offer a certain range of
connections to core networks and the user could choose the AN provider he/she prefers. There have to be
means for the user to tell the AN, which core network is to be used. This selection needs to be as easy as
possible for the user.
The interface could be at the V reference point for ISDN. To support PSTN services at the V reference
point, the mappings of PSTN User-Network Interface (UNI) signalling protocols onto the V reference point
PSTN protocol have to be standardized, as discussed in clause 10.
NOTE: If no agreement on a minimum set of PSTN functionality can be achieved on a
European basis, these protocol mappings will have to be provided by the national
regulatory bodies.
c) Local Exchange - Transit Network boundary:
In some European countries, the user can choose the long distance carrier on a per-call basis by dialling a
certain code. The interface between LE and transit network could be the Signalling System No. 7 ISDN
User Part (ISUP).
4.3 Standardization requirements at the loop - core boundary
a) Open interface:
The standardization requirements depend on the services to be covered.
As this ETR will show, the V5 standards can be used as a basis for this interface for ISDN and PSTN.
Clause 7 investigates the possibilities of the V5 standards. The V5.1 standard, ETS 300 324-1 [8]
supports PSTN and ISDN Basic Access (BA). The V5.2 standard, ETS 300 347-1 [9] also supports ISDN
Primary Rate Access (PRA).
Since there is no common standard for PSTN, the V5 interface can only provide a tool kit of protocol
elements to be used to convey the information about the static and dynamic electrical conditions at the
user port to the LE and vice versa. If the V5 interface should serve as an open interface for PSTN, this
protocol mapping needs to be standardized. This covers not only the use of the protocol elements on the
V5 interface but requires also the exact specification of the static and dynamic electrical behaviour at the
UNI and their mapping onto V5 messages. Clause 10 expands on this standardization requirement.

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ETR 242: November 1995
Broadband services, however, are not yet covered by any standard. ETSI work items for broadband
V5-type interfaces exist (DE/SPS-03046-1 and DE/SPS-03047-1) and work is progressing.
Connections to dedicated data networks could be seen as leased lines. Leased lines are considered in
clause 8.
b) Selection of core network:
Call-by-call selection of the core network requires call related functions in the AN. As clause 6 will show,
this is in contradiction with the V5 architecture. Since it is assumed that the V5 interface should serve as a
basis for the loop/core boundary this kind of selection is considered to be impracticable.
Semi-permanent and permanent selection of the service provider is supported via management functions.
All entities in figure 1 are associated with management systems. Telecommunication Management
Network (TMN) based Q3 interface specifications exist for V5 related AN and LE configuration
management and fault and performance management (ETS 300 376-1 [10], ETS 300 377-1 [11],
ETS 300 378-1 [12] and ETS 300 379-1 [13], see also clause 9).
In order to select the core network, the user could be given access to the management system of the
access provider. No standards exist in this area.
Every provider manages a certain amount of user related data. Selection of core networks requires
co-ordinated actions in management systems of different providers. No standards exist in this area.
5 Status of access network and V5 standards work
This clause lists the European standards, ETRs and related ITU-T Recommendations existing or in the
process of being drafted, both V5 related and for ANs in general.
5.1 V5-related documents
Reference Subject Status
(November 1995)
ETS 300 324-1 [8] V5.1 interface specification published 1994
(also ITU-T Rec. G.964)
ETS 300 324-2 Protocol Implementation Conformance Statement published 1994
(PICS) proforma specification for the V5.1 interface
ETS 300 324-3, 5 and 7 Test Suite Structure and Test Purposes (TSS&TP) for published 1995
the V5.1 interface
ETS 300 324-4, 6 and 8 Abstract Test Suites (ATS) for the V5.1 interface on Public Enquiry 89
ETS 300 324-9 Physical layer test specification for the V5.1 interface prepared for Vote
ETS 300 347-1 [9] V5.2 interface specification published 1994
(also ITU-T Rec. G.965)
ETS 300 347-2 PICS proforma specification for the V5.2 interface published 1994
ETS 300 347-3, 5 and 7 TSS&TP for the V5.2 interface prepared for Vote
ETS 300 347-4, 6 and 8 ATS for the V5.2 interface on Public Enquiry 89
ETS 300 347-9 Physical layer test specification for the V5.2 interface prepared for Vote
ETR 150 V5 interface PSTN mappings published 1994

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ETR 242: November 1995
Reference Subject Status
(November 1995)
ETS 300 376-1 [10] Q3 interface at the AN for the configuration published 1994
(also draft ITU-T Rec. management of V5 interfaces; Q3 interface specification
Q.57CM)
ETS 300 376-2 Q3 interface at the AN for the configuration published 1995
management of V5 interfaces; Managed Object
Conformance Statement (MOCS) proforma specification
ETS 300 377-1 [11] Q3 interface at the LE for the configuration management published 1994
(also draft ITU-T Rec. of V5 interfaces; Q3 interface specification
Q.57CM)
ETS 300 377-2 Q3 interface at the LE for the configuration management published 1995
of V5 interfaces; MOCS proforma specification
ETS 300 378-1 [12] Q3 interface at the AN for the fault and performance published 1995
(also draft ITU-T Rec. management of V5 interfaces; Q3 interface specification
Q.57FPM)
ETS 300 378-2 Q3 interface at the AN for the fault and performance drafting
management of V5 interfaces; MOCS proforma
specification
ETS 300 379-1 [13] Q3 interface at the LE for the fault and performance published 1995
(also draft ITU-T Rec. management of V5 interfaces; Q3 interface specification
Q.57FPM)
ETS 300 379-2 Q3 interface at the LE for the fault and performance drafting
management of V5 interfaces; MOCS proforma
specification
DE/SPS-03024 Reference points and interfaces for digital exchanges drafting in progress
(based on ITU-T Rec.
Q.511, Q.512 and Q.513)
DE/SPS-03027 LE and AN performance design; Requirements for call drafting in progress
handling and bearer connection management
DTR/SPS-03040 Identification of the applicability of existing protocol for TC SPS approval
specifications for a VB interface in an access
arrangement with ANs
5.2 Other AN-related documents
Reference Subject Status
(November 1995)
ETS 300 463 Requirements of passive Optical Access Networks prepared for Vote
(OANs) to provide services up to 2 Mbit/s bearer
capacity
DE/TM-02209 Operation and maintenance of OANs work item created
DTR/TM-02226 Operations and maintenance of OANs; Ensembles drafting in progress
applicable to OANs
DTR/TM-03024 General functional architectures for ANs drafting in progress
draft ITU-T Rec. G.9XX Framework Recommendation on access networks; drafting in progress
Architecture and functions, access types, management
and service node aspects
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ETR 242: November 1995
6 Architecture and modelling
Clause 4 provides a summary of BC-T-309 [1] with the identified architecture as extracted from the
mandate. There are, however, some inconsistencies of this access concept shown in figure 1 and the
internationally specified network architecture for ISDN, which has been taken as the basis for the definition
of the functions and procedures for the V5 interfaces. Although there is no such architecture defined for
PSTN in ITU-T, the ISDN architecture can generally be used for PSTN as well because the concept of the
architecture based on connections is the same for both ISDN and PSTN and both types of accesses are
often supported by the same LE.
6.1 The general network architecture and modelling
Figure 2 shows the general ISDN reference configuration as defined in CCITT Recommendation
I.324 [18], figure 2/I.324. It shows the relevant reference points for the reference configuration, for easy
comparison with other figures of this ETR.
ST S
T
Customer Customer
Public ISDN
TE network network TE
Overall ISDN connection
NOTE 1: The functional grouping customer network is described in ITU-T Recommendation I.411 [20].
NOTE 2: In the case where the customer network is null (i.e. null NT2), then the ISDN connection type
can be considered to end at the S/T reference point.
NOTE 3: Other configurations are possible where the call is asymmetrical or terminates in or involves
Higher Layer Functions (HLF).
NOTE 4: The terms "customer network" and "Public ISDN connection type" do not presuppose a
particular regulatory situation in any country and are used purely for technical reasons. The
connection type concept is defined in CCITT Recommendation I.340 [19].
Figure 2: General ISDN reference configuration
Figure 3 gives the reference configuration of public ISDN connection type taken from CCITT
Recommendation I.324 [18], figure 3/I.324. This figure is useful to identify, on the basis of the defined
connection type, the individual Connection Elements (CEs) forming the connection, the "access CE" and
the "transit CE". This may help to clarify, on the basis of the defined reference configuration in ITU-T, the
separation of the functions and the support of expected functional capabilities of the relevant network
elements (e.g. AN, LE).
International
National National
CRF
Local  transit  transit Local
T
T
CRF  CRF  CRF CRF
IRP IRP IRP IRP
Access CE National transit CE
International CE National transit CE Access CE
IRP Internal Reference Point
CRF Connection Related Functions
CE Connection Element
Figure 3: Reference configuration of public ISDN connection type

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ETR 242: November 1995
The following definition of the access connection element is taken from CCITT Recommendation
I.324 [18], subclause 4.2.1.1:
The access connection element is bounded by the T reference point at the customer end and the
reference point which marks the transition from the access signalling system to the common
channel signalling system on the network side.
The model for the access connection element in the case of 64 kbit/s circuit switched is shown in
Figure 4/I.324. Depending on the national situations and on the type of access, a number of
different possibilities are available for this element, in particular with regard to the use of multiplexer
(Mpx) or Remote Switching Units (RSU).
Figure 4/I.324 of CCITT Recommendation I.324 [18] is reproduced in figure 4 for information. The
definition and figure 4 show clearly that the access connection element is not bounded at the V reference
point. This conclusion is based on ITU-T Recommendation Q.521 [29], defining the exchange functions,
(refer to clause 4 of ITU-T Recommendation Q.521 [29]) and the definition of the connection related
function in CCITT Recommendation I.324 [18], subclause 4.2.2.1:
The connection related function includes all aspects involved in establishing and controlling the
connections within the particular connection element. This includes functions such as exchange
terminations, switching, control, network management, operation and maintenance. The specific
capabilities of each CRF are not specified in the general reference model: this is done in the
reference configuration for each group of connection types.
Access link CRF
T
a) V a)
Mpx
NT1 RSU S
Common channel
C
signalling
Reference point
network
a)   May not exist
NT1 Network termination 1
S 64 kbit/s circuit switch
C Signalling handling and exchange control functions
Mpx (Remote) multiplexer
RSU Remote switching unit and/or concentrator
CRF Connection related function
Figure 4: Access connection element model
Figure 5 is reproduced from figure 1 of ITU-T Recommendation Q.521 [29] and shows that in the Q-series
of Recommendations there is no reference point defined (as mentioned in CCITT Recommendation
I.324 [18]) between the functions belonging to the access connection element and the transit connection
element. A possible location could be the switching matrix but this is just a functional point because the
terminating part of the access connection element and the originating part of the transit connection
element is functionally part of and therefore implemented in the LE. This view is fully covered by the
definition of the term reference point in CCITT Recommendation I.324 [18], subclause 1.2:
Reference points are the conceptual points at the conjunction of two functional groupings. In a
particular example, a reference point may correspond to a physical interface between pieces of
equipment, or in other examples there may not be any physical interface corresponding to the
reference point. Interfaces will not be defined by CCITT for an ISDN unless the corresponding
reference points have been already specified.

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ETR 242: November 1995
This general principle is also applicable to any other architecture and modelling concept. Nevertheless it is
evident that the Exchange Termination (ET) function at the left hand side of figure 5 is part of the access
connection element defined in CCITT Recommendation I.324 [18].
A or B
V
Connection functions
ET Switching functions
ET
Connection /resources
handling functions
Service processing functions
Signalling functions
Control functions
Operation and maintenance functions
Figure 5: Local exchange functional model
The ET function itself is defined in ITU-T Recommendation I.112 [17] as follows:
The functional group containing at least the layer 2 and layer 3 network-side functions of the I.420
interface at the T reference point.
NOTE 1: This may not be true if concentrators or intelligent equipment are located in the local
line distribution network.
NOTE 2: The ET is not the switching function. The extent to which the ET supports call control
processing and management is not defined.
6.2 The architecture and modelling of the access connection element
In the definition of the access connection element in CCITT Recommendation I.324 [18] there is the
indication that there may be different physical realizations possible. This is shown in figure 6 which is
based on ITU-T Recommendation I.414 [21] and ITU-T Recommendation Q.512 [28].
From figure 6 one can easily identify that, depending on the implementation of the access connection
element, more than one V reference point may exist between the T reference point and the ET function.
Five basic configuration types can be distinguished:
a) the passive access type;
b) the direct access type;
c) the remotely connected direct access type;
d) the remote access type; and
e) the remotely connected remote access type.
The passive access type uses only the transmission medium provided in the access connection element
between the UNI and the ET function in the LE. One example is the analogue PSTN access using a
copper pair from the customer premises to the LE.
The direct access type connects the UNI to the ET function in the LE through its individual access digital
section using a transmission system. There is functionally an individual V-reference point for any customer
between the ET function and the access digital section. Examples are the ISDN basic access with its
associated V1-reference point and the ISDN primary rate access with its associated V3-reference point.

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ETR 242: November 1995
Z
Analogue customer access
ET
TE
2-wire line
Z
ISDN customer access
Rec. I.601
T
V
Customer equipment
Access connection element
Rec. M.3602 Recs. M.3603 and M.3604
Line
ET
DIRECT ACCESS
TE1 NT1 LT
V1
ETS 300 012 (I.430) Basic access digital section
V1
ETS 300 297 (G.960)
S
ET
TE1 NT2 NT1 LT
V3
R
ETS 300 011 (I.431) Primary rate digital section
V3
ETS 300 233 (G.962)
TE2 TA
Local application of multiplex
ETS 300 012 (I.430)
in Access Network
n
ETS 300 011 (I.431)
ET
REMOTE ACCESS
M
S
V5.1
V5.1
V1
TE1 NT2 NT1 LT
ETS 300 012 (I.430)
Basic access digital section
ETS 300 297 (G.960)
Digital access link
Z
Digital section
TE
Remote application
V5.2
of concentration in
Access Network
ET
n
C LT LT
S
V5.2
V1 or V3
V5.2
TE1 NT2 NT1 LT
ETS 300 012 (I.430)
Basic or primary rate digital section
ETS 300 011 (I.431)
ETS 300 297 (G.960) ETS 300 233 (G.962)
Digital access link
V3
Remote application
of concentrator
TE1 NT1 LT
ETS 300 011 (I.431)
V2
Primary rate access
ET
digital section n
LT LT
C
V2
V2
ETS 300 233 (G.962)
V1
Digital section
TE1
NT1 LT
ETS 300 012 (I.430)
Basic access digital section
ETS 300 297 (G.960)
Digital access link
Figure 6: Overview of PSTN and ISDN customer access configurations

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ETR 242: November 1995
The remotely connected direct access type connects the UNI to the ET function in the LE through its
individual access digital section using a transmission system. But for the provision of the remote access a
further digital section is incorporated between the V-reference point at the network side of the access
digital section and the V-reference point at the LE. This additional digital section shall not change the
structure and the content of the information at the V-reference point at the network side of the access
digital section, i.e. it is considered transparent to the information. In this access type there exists the
relevant V-reference point at the network side of the access digital section and an associated V'-reference
point at the LE.
The remote access type uses the concatenation approach of V-reference points which is based on a
functional hierarchy of reference points. This approach allocates the V-reference points to either:
a) the Access Individual Class of reference points; or
b) the Access Integrating Class of reference points.
The Access Integrating Class of reference points is defined as that providing a signal structure which is a
multiplex of several accesses of the same or of different access types. The V2 and V5 reference points
belong to this class. In this case, functionally both, the customer individual access V-reference point and
the access integrating reference point, are present in the access connection element.
The remotely connected remote access type is identical to the remote access type but the AN is remotely
connected to the LE by a transparent digital link as defined in clause 4 of both V5 interface standards.
The functional split between the AN and the LE is identical in all five configuration types with regard to call
handling. The functions from the T reference point up to the V reference point associated with the LE is
limited to the transport of the access bearer channels and information, which may include flexible
allocation of bearer channels under control of the LE. The peer point for the call control protocol is the ET
layer 3 and this function is allocated to the LE.
Local Line Distribution Network
Access Network (AN) Local Exchange
PSTN Access
V5.2
ET ET
TE LC
L1/2/3 V5.2
Z
ISDN-BA V1
T
Note 1
ET
TE NT1 LT
L1/2*
V1
ISDN-BA Note 1
T
ET
TE
NT1 LT
L1/2*
V3
ISDN-PRA
T
Note 1
ET
LT
TE NT1
L1/2*
Q
LE
ISDN-PRA T Note 1
V3
ET
TE
NT
LT
L1/2*
Analogue Leased Line
Note 1
TE
ET
LC
L1
Digital Leased Line
Note 1
Permanent Leased
ET
TE
LC
Line Service
L1
Network
Q
AN
NOTE 1: The selection of channels and the service allocation is part of the provisioning.
NOTE 2: The
...