Internet Standards/CD2.mdf/ccitt/1992/i/i327.asc

Text File | 1993-08-21 | 17KB | 606 lines

IMPORT R:\\ART\\WMF\\ITU.WMF \* mergeformat

INTERNATIONAL  TELECOMMUNICATION  UNION







CCITT    I.327

THE  INTERNATIONAL

TELEGRAPH  AND  TELEPHONE

CONSULTATIVE  COMMITTEE









INTEGRATED  SERVICES

DIGITAL  NETWORK  (ISDN)

OVERALL  NETWORK  ASPECTS

AND  FUNCTIONS,

ISDN  USER-NETWORK  INTERFACES





B-ISDN  FUNCTIONAL  ARCHITECTURE









Recommendation  I.327





IMPORT R:\\ART\\WMF\\CCITTRUF.WMF \* mergeformat            Geneva, 1991

















































































Printed in Switzerland





FOREWORD

    The CCITT (the International Telegraph and Telephone Consultative
Committee) is the permanent organ of the International Telecommunication
Union (ITU). CCITT is responsible for studying technical, operating and tar-
iff questions and issuing Recommendations on them with a view to standard-
izing telecommunications on a worldwide basis.

    The Plenary Assembly of CCITT which meets every four years, establishes
the topics for study and approves Recommendations prepared by its Study
Groups. The approval of Recommendations by the members of CCITT
between Plenary Assemblies is covered by the procedure laid down in
CCITT Resolution No. 2 (Melbourne, 1988).

    Recommendation I.327 was prepared by Study Group XVIII and
was approved under the Resolution No. 2 procedure on the 5th of April
1991.





___________________





CCITT  NOTES

1)    In this Recommendation, the expression _Administration_ is used for con-
ciseness to indicate both a telecommunication Administration and a recog-
nized private operating agency.

2)    A list of abbreviations used in this Recommendation can be found in
Annex B.

















πITU1991

All rights reserved. No part of this publication may be reproduced or utilized
in any form or by any means, electronic or mechanical, including photocopy-
ing and microfilm, without permission in writing from the ITU.

    Preamble to B-ISDN Recommendations

    In 1990, CCITT SG XVIII approved a first set of Recommendations
on B-ISDN. These are:

I.113 ∙ Vocabulary of terms for broadband aspects of ISDN

I.121 ∙ Broadband aspects of ISDN

I.150 ∙ B-ISDN asynchronous transfer mode functional characteristics

I.211 ∙ B-ISDN service aspects

I.311 ∙ B-ISDN general network aspects

I.321 ∙ B-ISDN Protocol Reference Model and its application

I.327 ∙ B-ISDN functional architecture

I.361 ∙ B-ISDN ATM Layer specification

I.362 ∙ B-ISDN ATM Adaptation Layer (AAL) functional description

I.363 ∙ B-ISDN ATM Adaptation Layer (AAL) specification

I.413 ∙ B-ISDN user-network interface

I.432 ∙ B-ISDN user-network interface ∙ Physical Layer specification

I.610 ∙ Operation and maintenance principles of B-ISDN access

    These Recommendations address general B-ISDN aspects as well as
specific service- and network-oriented issues, the fundamental characteris-
tics of

the asynchronous transfer mode (ATM), a first set of relevant ATM oriented
parameters and their application at the user-network interface as well as
impact on operation and maintenance of the B-ISDN access. They are an
integral part of the well established I-Series Recommendations. The set of
Recommendations are intended to serve as a consolidated basis for ongoing
work relative to B-ISDN both within CCITT and in other organizations.
They may also be used as a first basis towards the development of network
elements.

    CCITT will continue to further develop and complete these Recom-
mendations in areas where there are unresolved issues and develop addi-
tional Recommendations on B-ISDN in the I-Series and other series in the
future.

PAGE BLANCHE

Recommendation I.327

Recommendation I.327

B-ISDN  FUNCTIONAL  ARCHITECTURE

1    Introduction

    The general functional architecture model for the ISDN is described in
RecommendationI.324. The concepts and associated definitions adopted in
RecommendationI.324 also apply to the B-ISDN, i.e.reference configura-
tions, functional group, reference points.

    The objective of this Recommendation is to provide a basic functional archi-
tecture of the B-ISDN to complement RecommendationI.324. The model is
not intended to require or exclude any specific implementation of the B-
ISDN but to provide a guide for the specification of B-ISDN capabilities.

    Recommendation I.310 describes the functions of an ISDN. These
functions are by their nature static (i.e.time-independent). The relative dis-
tribution and allocation of these functions is the subject of the architecture of
the ISDN and is described in this Recommendation. The dynamic aspects of
these functions are modelled in RecommendationI.310 as executive pro-
cesses.

    Therefore, the key components in this architecture model are: the
functions which are contained in the B-ISDN, where they are located and the
relative topology for their distribution in the B-ISDN.

2    General architecture of the B-ISDN

    In B-ISDN implementations some of the B-ISDN functions will be imple-
mented within the same network elements, whereas other specific B-ISDN
functions will be dedicated to specialized network elements. Various differ-
ent B-ISDN implementations are likely to be realized depending on national
conditions.

    A basic component of the B-ISDN is a network for asynchronous
transfer mode (ATM) switching of both constant bit rate (CBR) and variable
bit rate (VBR) end-to-end connections. These connections will support
64kbit/s based ISDN services.

3    Architectural aspects of the B-ISDN

    The basic architectural model defined in Recommendation I.324 is comple-
mented as shown in Figure1/I.327. This shows the main information transfer
and signalling capabilities of the B-ISDN.

    The architecture of the B-ISDN includes low Layer capabilities and
high Layer capabilities. These capabilities support services within the B-
ISDN and within other networks by means of interworking B-ISDN with
those other networks.

3.1    Low Layer capabilities

    From the functional capabilities of the B-ISDN, as shown in Figure1/I.327,
the information transfer capabilities require further description.

    Broadband information transfer is provided by ATM at the B-ISDN user-net-
work interface (UNI) and at switching entities inside the network.

FIGURE 1/I.327 = 12,5 cm = 489



    ATM is a specific packet oriented transfer mode using an asynchro-
nous time division multiplexing technique. The multiplexed information
flow is organized in fixed size blocks, called cells. A cell consists of an
information field and a header: the primary role of the header is to identify
cells belonging to the same virtual channel connection. Cells are assigned on
demand, depending on the source activity and the available resources. Cell
sequence integrity on a virtual channel connection is preserved by the ATM
Layer.

    ATM is a connection oriented technique. A connection within the
ATM Layer consists of one or more links, each of which is assigned an iden-
tifier. These identifiers remain unchanged for the duration of the connection.
It should be noted that signalling information for a given connection is con-
veyed using a separate identifier.

    Although ATM is a connection oriented technique, it does offer a
flexible transfer capability common to all services, including connectionless
services. Examples of mechanisms supporting connectionless data services
are illustrated in AnnexA.

    The switching and transmission capabilities, as described in Recommenda-
tion I.324, are also applicable in B-ISDN. The support of 64kbit/s based
ISDN services by a network based on ATM needs further study.

    3.2    High Layer capabilities

    Normally, the high Layer functional capabilities are involved only in the ter-
minal equipment. However, for the support of some services, provision of
high layer functions could be made via special nodes in the B-ISDN belong-
ing to the public network or to centres operated by other organizations and
accessed via B-ISDN user-network or network-node interfaces (NNIs).

4    Location of functions in the B-ISDN

4.1    Overall

    In considering a B-ISDN call (i.e. an instance of a telecommunication ser-
vice) two major functional areas are involved:

i)    the customer equipment (TE and optional customer network),

ii)    the public B-ISDN.

    In the case where the customer network is a B-ISPBX based network
providing the same B-ISDN connection type as the public B-ISDN, then the
overall B-ISDN connection ends at the SB reference point as shown in
Figure2/I.327.

    Note1 ∙ In the case where the customer network is null then the B-
ISDN connection type can be considered to end at the coincidentSB, TB ref-
erence point.

    Note2 ∙ Other configurations are possible where the call is asym-
metrical, or terminates in or involves HLFs.

    Note3 ∙ The terms _B-ISPBX/private B-ISDN_ and _public B-
ISDN_ do not presuppose a particular regulatory situation in any country and
are used purely for technical reasons.

FIGURE 2/I.327 = 6,5 cm = 254



4.2    Partitioning of the overall B-ISDN connection

    The partitioning of functions within the B-ISDN connection type is done by
using connection elements, basic connection components and reference
points as defined in RecommendationI.324.

    4.2.1    Connection elements

    The first level of partitioning of the overall B-ISDN connection type is the
connection element (CE). The partitioning is based on the identification of
reference points between connection elements.

    Figure 3/I.327 identifies five CEs for a mixed private/public B-ISDN overall
connection type: the private access CE, the private transit CE, the public
access CE, the public national transit CE and the public international transit
CE.

FIGURE 3/I.327 = 9,5 cm = 371



4.2.2    Functional groups in the B-ISDN connection elements

    In B-ISDN, the virtual path connection is introduced for routing groups of
virtual channels in the network. Therefore two levels of connection handling
will exist in the B-ISDN. These levels must be represented by two different
switching blocks in the connection elements, one switching according to the
virtual path identifier (VPI) and another switching according to the virtual
channel identifier (VCI). Each of these switching blocks is under the control
of its respective control block.

    A general connection element model in B-ISDN is thus described
using five functional blocks: a switching block for VPI, the SVPI; a control
block for VPI, the CVPI; a switching block for VCI, the SVCI, a control
block for VCI, the CVCI, and an interconnection link (see Figure4/I.327).
The link block incorporates all the functions implementing the physical
layer. Different links may be identified, e.g.access links and transit links.

    In a particular reference configuration for connection types, the con-
nection elements can be realized using a subset of the five functional blocks,
for example to represent a connection in the network where only VPI han-
dling is implemented.

FIGURE 4/I.327 = 5,5 cm = 215



4.2.3    B-ISDN connection element generic description

    The generic B-ISDN connection element is shown in Figure 5/I.327. It repre-
sents the logical interrelationship between the functional blocks supporting
the B-ISDN connections and the means to control the connections. The
B-ISDN connection is supported by the links and the switching blocks SVPI
and SVCI. The connections are controlled by the control blocks CVPI and
CVCI. These control blocks logically interface to the user-network signal-
ling system on the user side of an access connection element and with the
internodal signalling network. For the control of semi-permanent connec-
tions, the control blocks also interface to the network management function.
These management interface definitions will be the subject of further stud-
ies.

FIGURE 5/I.327 = 11 cm = 430



4.3    Functional architecture models for the B-ISDN

    Appendix I gives examples of functional architecture models using the prin-
ciples established in RecommendationI.324. These principles are basically
those of Reference Points and Functional Groups identified in Figure8/
I.324.



ANNEX  A

(to Recommendation I.327)

Support of connectionless data services in a B-ISDN

    Recommendation I.211 identifies the connectionless data service aspects of
B-ISDN. Connectionless data services are supported in the B-ISDN using
ATM connections between functional groups able to handle connectionless
messages. These functional groups may be outside the B-ISDN or may pro-
vide a B-ISDN service.

    Two mechanisms for supporting connectionless data services are described
in RecommendationI.211, _ 2.7:

1)    indirectly via a B-ISDN Connection Oriented service (Case A),

2)    directly via a B-ISDN Connectionless Data service (Case B).

    The respective functional architecture model for those two methods
are represented in Figures A-1/I.327 andA-2/I.327. The direct provision of a
B-ISDN Connectionless Data service and the protocol specification are for
further study.

    In order to access a Connectionless Data service, a connection has to
be established between the user and the Connectionless service function
(CLSF). This connection can be:

∙    a semi-permanent Virtual Path Connection. All the VC connections
in this VP connection are dedicated to the Connectionless Data service;

∙    a switched or semi-permanent VC connection.

    CLSF terminates connectionless protocol and routes cells to a desti-
nation user according to routing information included in user cells.

FIGURE A-1/I.327 = 11, 5 cm = 449 (2 fig. sur la mΩme page)



FIGURE A-2/I.327  = 12,5 cm = 489 (sur la mΩme page que fig. prec.)

APPENDIX  1

(to Recommendation I.327)

Examples for functional architecture models for B-ISDN



    Functional architecture models aim to identify various possible
physical arrangements for the realization of the network when interconnect-
ing equipments. Depending on the national situations and on the type of
access, a number of different functional architecture models exist for access-
ing the B-ISDN:

∙    a star structure where customers have direct individual links to the local
exchange (LE) (see FigureI-1/I.327);

∙    a multistar structure with a remote unit (RU) between the customer
and the main exchange. This is a two stage local network, each stage being
star structured (see Figure I-2/I.327);

∙    a multistar structure, tree-shaped for distributive communication
between the local exchange and the remote unit (see FigureI-3/I.327).

    Other functional architecture models, such as Metropolitan Area
Networks, and access technologies such as Passive Optical Network, are for
further study.

    Note∙The passive optical network concept consists of a shared
medium based on a tree topology that allows the connection of several cus-
tomers to the local exchange using the same medium.

    The Metropolitan Area Network logical concept is based on a dis-
tributed (not centralized) local CRF. Customers have access to the network
using a shared medium based on different topologies.



FIGURE I-1/I.327 = 10,5 cm = 410 (sur mΩme page)



FIGURE I-2/I.327 = 11 cm = 430

FIGURE I-3/I.327 (page pleine avec les remarques)

ANNEX  B

(to Recommendation I.327)

Alphabetical list of abbreviations used in this Recommendation





ACE        Access connection element

AL        Access link

B-NT1        Network termination 1 for B-ISDN

B-ISPBX        Private branch exchange for B-ISDN

CBR        Constant bit rate

CE        Connection element

CLSF        Connectionless service function

CRF        Connection related function

DPL        Primary link for distribution services

IPL        Primary link for interactive services

IRP        Internal reference point

LE        Local exchange

LFC        Local function capabilities

LT        Line termination

NNI        Network-node interface

PLK        Primary link

RU        Remote unit

SP        Service provider

SPL        Service provider link

TCE        Transit connection element

TCRF        Transit connection related function

TE        Terminal equipment

VBR        Variable bit rate

VCI        Virtual channel identifier

VPI        Virtual path identifier