SIST ETS 300 138-1 E1:2005

Integrated Services Digital Network (ISDN); Closed User Group (CUG) supplementary service; Digital Subscriber Signalling System No. one (DSS1) protocol; Part 1: Protocol specification

ISDN User-Network Layer 3 Signalling Protocols for Supplementary Services

Digitalno omrežje z integriranimi storitvami (ISDN) - Dopolnilna storitev: zaprta skupina uporabnikov (CUG) - Protokol digitalne naročniške signalizacije št. 1 (DSS1) - 1. del: Specifikacija protokola

General Information

• Status: Published
• Publication Date: 30-Jun-2005
• ICS: 33.080 - Integrated Services Digital Network (ISDN)
• Technical Committee: SPN - Services and Protocols for Networks
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 01-Jul-2005
• Due Date: 01-Jul-2005
• Completion Date: 01-Jul-2005
• Ref Project: ETSI ETS 300 138-1 ed.1 (1992-05) - Integrated Services Digital Network (ISDN); Closed User Group (CUG) supplementary service; Digital Subscriber Signalling System No. one (DSS1) protocol; Part 1: Protocol specification

Standards Content (Sample)

SLOVENSKI STANDARD
SIST ETS 300 138-1 E1:2005
01-julij-2005
'LJLWDOQRRPUHåMH]LQWHJULUDQLPLVWRULWYDPL,6'1'RSROQLOQDVWRULWHY]DSUWD
VNXSLQDXSRUDEQLNRY&8*3URWRNROGLJLWDOQHQDURþQLãNHVLJQDOL]DFLMHãW
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Integrated Services Digital Network (ISDN); Closed User Group (CUG) supplementary
service; Digital Subscriber Signalling System No. one (DSS1) protocol; Part 1: Protocol
specification
Ta slovenski standard je istoveten z: ETS 300 138-1 Edition 1
ICS:
33.080 Digitalno omrežje z Integrated Services Digital
integriranimi storitvami Network (ISDN)
(ISDN)
SIST ETS 300 138-1 E1:2005 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST ETS 300 138-1 E1:2005

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SIST ETS 300 138-1 E1:2005
EUROPEAN ETS 300 138-1
TELECOMMUNICATION May 1992
STANDARD
Source: ETSI TC-SPS Reference: T/S 46-33H
ICS: 33.080
ISDN, supplementary service
Key words:
Integrated Services Digital Network (ISDN);
Closed User Group (CUG) supplementary service;
Digital Subscriber Signalling System No. one (DSS1) protocol;
Part 1: Protocol specification
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 - [e-mail]
X.400: Internet:
Tel.: [phone] - Fax: [phone]
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 1992. All rights reserved.
New presentation - see History box

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ETS 300 138-1: May 1992
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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Contents
Foreword.5
1 Scope .7
2 Normative references .7
3 Definitions.8
4 Symbols and abbreviations.9
5 Description .9
6 Operational requirements.9
6.1 Provision and withdrawal .9
6.2 Requirements on the originating network side.10
6.3 Requirements on the destination network side .10
7 Coding requirements.11
7.1 ASN.1 description of coding requirements .11
7.2 Coding of the Cause information element .12
8 State definitions .12
9 Signalling procedures at the coincident S and T reference point .13
9.1 Activation, deactivation and registration.13
9.2 Invocation and operation.13
9.2.1 Call originating from a user with the CUG supplementary service (explicit
request). 13
9.2.1.1 Normal operation.13
9.2.1.2 Exceptional procedures.13
9.2.2 Call originating from a user with the CUG supplementary service (default
request). 14
9.2.2.1 Normal operation.14
9.2.2.2 Exceptional procedures.14
9.2.3 Call originating from a user without the CUG supplementary service.15
9.2.3.1 Normal operation.15
9.2.3.2 Exceptional procedures.15
9.2.4 Call terminating at a user with the CUG supplementary service.15
9.2.4.1 Normal operation.15
9.2.4.2 Exceptional procedures.16
9.2.5 CUG checks at the originating and destination network.16
10 Procedures for interworking with private ISDNs .19
11 Interactions with other networks .19
12 Interactions with other supplementary services .19
13 Parameter values (timers).19
14 Dynamic description (SDL).19
14.1 The closed user group process .19
14.2 Relation to basic call control. 20

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Annex A (informative): Signalling flows .25
Annex B (informative): Diagrammatic description of coding requirements .28
B.1 CUG call invoke component (typical example).28
B.2 CUG call return error component .29
History .30

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ETS 300 138-1: May 1992
Foreword
This European Telecommunication Standard (ETS) has been produced by the Signalling Protocols and
Switching (SPS) Technical Committee of the European Telecommunications Standards Institute (ETSI).
This ETS is part 1 of a multi-part standard covering the Digital Subscriber Signalling System No. one
(DSS1) protocol specification for the Integrated Services Digital Network (ISDN) Closed User Group
(CUG) supplementary service, as described below:
Part 1: "Protocol specification";
Part 2: "Protocol Implementation Conformance Statement (PICS) proforma specification";
Part 3: "Test Suite Structure and Test Purposes (TSS&TP) specification for the user";
Part 4: "Abstract Test Suite (ATS) and partial Protocol Implementation eXtra Information for Testing
(PIXIT) proforma specification for the user";
Part 5: "TSS&TP specification for the network";
Part 6: "ATS and partial PIXIT proforma specification for the network".
In accordance with CCITT Recommendation I.130, the following three level structure is used to describe
the supplementary telecommunications services as provided by European public telecommunications
operators under the pan-European Integrated Services Digital Network (ISDN):
- Stage 1: is an overall service description, from the user's stand-point;
- Stage 2: identifies the functional capabilities and information flows needed to support the service
described in stage 1; and
- Stage 3: defines the signalling system protocols and switching functions needed to implement the
service described in stage 1.
This ETS details the stage 3 aspects (signalling system protocols and switching functions) needed to
support the Closed User Group (CUG) supplementary service. The stage 1 and stage 2 aspects are
detailed in ETS 300 136 (1992) and ETS 300 137 (1992), respectively.
This reprint includes all previous Corrigenda as shown in the History box at the last page.

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1 Scope
This first part of ETS 300 138 specifies the stage three of the Closed User Group (CUG) supplementary
service for the pan-European Integrated Services Digital Network (ISDN) as provided by European public
telecommunications operators at the T reference point or coincident S and T reference point (as defined in
CCITT Recommendation I.411 [1]) by means of the Digital Subscriber Signalling System No. one (DSS1).
Stage three identifies the protocol procedures and switching functions needed to support a
telecommunications service (see CCITT Recommendation I.130 [2]).
In addition this standard specifies the protocol requirements at the T reference point where the service is
provided to the user via a private ISDN.
This standard does not specify the additional protocol requirements where the service is provided to the
user via a telecommunications network that is not an ISDN.
The CUG supplementary service enables users to form groups, to and from which access is restricted. A
specific user may be a member of one or more closed user groups. Members of a specific closed user
group can communicate among themselves but not, in general, with users outside the group.
The CUG supplementary service is applicable to all telecommunication services.
Further parts of this standard specify the method of testing required to identify conformance to this
standard.
This standard is applicable to equipment, supporting the CUG supplementary service, to be attached at
either side of a T reference point or coincident S and T reference points when used as an access to the
public ISDN.
2 Normative references
This ETS incorporates by dated and undated reference, provisions from other publications. These
normative 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 ETS only when incorporated in it by amendment or revision. For undated references the latest
edition of the publication referred to applies.
[1] CCITT Recommendation I.411 (1988): "ISDN user-network interfaces -
Reference configurations".
[2] CCITT Recommendation I.130 (1988): "Method for the characterisation of
telecommunications services supported by an ISDN and network capabilities of
an ISDN".
[3] CCITT Recommendation I.112 (1988): "Vocabulary of terms for ISDNs".
[4] CCITT Recommendation I.210 (1988): Principles of telecommunication services
supported by an ISDN and the means used to describe them".
[5] CCITT Recommendation E.164 (1988): "Numbering plan for the ISDN era".
[6] ETS 300 136 (1992): "Integrated Services Digital Network (ISDN); Closed User
Group (CUG) supplementary service; Service description".
[7] ETS 300 195-1: "Integrated Services Digital Network (ISDN); Supplementary
service interactions; Digital Subscriber Signalling System No. one (DSS1)
protocol; Part 1: Protocol specification".
[8] CCITT Recommendation X.208 (1988): "Open Systems Interconnection (OSI);
Model and Notation: Service definition: Specification of Abstract Syntax Notation
One (ASN.1)".

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[9] CCITT Recommendation X.209 (1988): "Open Systems Interconnection (OSI);
Model and Notation: Service definition: Specification of basic encoding rules for
Abstract Syntax Notation One (ASN.1)".
[10] ETS 300 102-1 (1990): "Integrated Services Digital Network (ISDN); User-
network interface layer 3; Specifications for basic call control".
[11] ETS 300 102-2 (1990): "Integrated Services Digital Network (ISDN); User-
network interface layer 3; Specifications for basic call control; Specification
Description Language (SDL) diagrams".
[12] ETS 300 196-1: "Integrated Services Digital Network (ISDN); Generic functional
protocol for the support of supplementary services; Digital Subscriber Signalling
System No. one (DSS1) protocol; Part 1: Protocol specification".
[13] CCITT Recommendation X.219 (1988): "Remote Operations: Notation and
Service Definition".
[14] CCITT Recommendation Z.100 (1988): "Functional Specification and Description
Language (SDL)".
3 Definitions
For the purpose of this standard, the following definitions apply:
Basic telecommunication service: a bearer service or teleservice. The terms "bearer service" and
"teleservice" are defined in CCITT Recommendation I.112 [3], § 2.2, definitions 202 and 203.
CUG call: see ETS 300 136 [6], Clause 3.
CUG index: the closed user group index is a parameter used by the calling user to select a particular
closed user group when originating a call. The index is also used by the network to indicate to the called
user the closed user group from which an incoming call has originated. This index has only local
significance, i.e. the index used by the calling user is, in general, different from the index used by the called
user to identify the same closed user group.
CUG interlock code: this is a means of identifying closed user group membership within the network. At
the calling side, if a closed user group match exists, the CUG index identifying a closed user group maps to
the closed user group interlock code for that closed user group. If a closed user group match exists at the
called side the closed user group interlock code identifying a closed user group maps to the CUG index
representing that closed user group. Closed user group interlock code is not an access concept, but is
used for clarity during the descriptions of signalling procedures and flows.
Default number: an ISDN number registered within the public ISDN following prior agreement between the
third user and the public ISDN.
Incoming access: see ETS 300 136 [6], Clause 3.
Incoming calls barred within a closed user group: see ETS 300 136 [6], Clause 3.
Integrated Services Digital Network (ISDN): see CCITT Recommendation I.112 [3], § 2.3, definition
308.
ISDN number: a number conforming to the numbering plan and structure specified in CCITT
Recommendation E.164 [5].
Network: the DSS1 protocol entity at the network side of the user-network interface.
Outgoing access: see ETS 300 136 [6], Clause 3.

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Outgoing calls barred within a closed user group: see ETS 300 136 [6], Clause 3.
Preferential CUG: a closed user group user subscribing to preferential closed user group nominates a
CUG index which the network uses as a default to identify the required closed user group in the absence of
any closed user group information in the outgoing call request. A preferential closed user group applies to
an ISDN number (or to an ISDN number/service - see subclause 6.1) and not to a specific closed user
group.
Service; Telecommunications service: see CCITT Recommendation I.112 [3], § 2.2, definition 201.
Supplementary service: see CCITT Recommendation I.210 [4], § 2.4.
User: the DSS1 protocol entity at the user side of the user-network interface.
4 Symbols and abbreviations
CES Connection Endpoint Suffix
CUG Closed User Group
DSS1 Digital Subscriber Signalling System No. one
IA Incoming Access
ICB Incoming Calls Barred within a closed user group
ISDN Integrated Services Digital Network
OA Outgoing Access
OCB Outgoing Calls Barred within a closed user group
5 Description
Essentially normal call establishment procedures shall apply but, additionally, to provide the CUG
supplementary service, the network shall analyse the call request from the calling user in conjunction with
the closed user group attributes associated with both the calling and called users (as identified by their
ISDN numbers). As a result of this analysis the call can either fail for CUG supplementary service reasons
or be allowed to proceed.
The network provider may define the maximum number of closed user groups of which a user can be a
member.
Since the fundamental purpose of the CUG supplementary service is to prevent certain connections the
network shall strictly control interactions with some other supplementary services to protect closed user
group integrity.
6 Operational requirements
6.1 Provision and withdrawal
The provision of the CUG supplementary service to a new member and also the assignment of the various
CUG supplementary service options to a new or existing member, shall require a prior arrangement
between the member and the network provider.
The CUG supplementary service shall be provided on a subscription basis. As a network provider option,
the CUG supplementary service may be offered with subscription options.

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The options can be divided into two groups:
a) the options shown in table 1 shall apply per ISDN number. The option values may be
assigned individually for each basic service, or set of basic services, available at the
ISDN number with the CUG supplementary service.
b) the option shown in table 2 shall apply per closed user group provided at the ISDN
number with the CUG supplementary service.
Table 1: Options available per ISDN number
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‡Option (NOTE)        ‡ Values            ‡
ˆ˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜¯˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜·
‡1) Preferential CUG     ‡ Nominated CUG index,     ‡
‡               ‡ or none designated.      ‡
ˆ˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜¯˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜·
‡2) Outgoing access      ‡ Allowed, or          ‡
‡               ‡ not allowed.         ‡
ˆ˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜¯˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜·
‡3) Incoming access      ‡ Allowed, or          ‡
‡               ‡ not allowed.         ‡
ˆ˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜`˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜·
‡NOTE: If, for a user with the CUG supplementary service, a  ‡
‡basic service, or set of basic services, is not included in ‡
‡at least one closed user group, then:            ‡
‡- preferential CUG shall have the "none designated" option  ‡
‡ value;                           ‡
‡- outgoing access shall have the "allowed" option value if  ‡
‡ normal outgoing calls using that basic service, or set of ‡
‡ basic services, are required;               ‡
‡- incoming access shall have the "allowed" option value if  ‡
‡ incoming calls using that basic service, or set of basic  ‡
‡ services, are required.                  ‡
�˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜�
Table 2: Options available per closed user group
�˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜´˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜¿
‡Option            ‡ Values            ‡
ˆ˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜¯˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜·
‡1) Barring within the     ‡ None,            ‡
‡  closed user group     ‡ incoming calls, or      ‡
‡               ‡ outgoing calls.       ‡
�˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜`˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜�
The options assigned to a closed user group member shall be stored in the network.
Withdrawal of the CUG supplementary service shall be as a result of network provider action either at the
request of a particular member, or for administrative reasons.
6.2 Requirements on the originating network side
For correct interactions with certain other supplementary services, the originating network side shall store,
for the duration of the call, details of whether a normal or a CUG call was requested in the information sent
to the destination network side. The CUG interlock code (if any) of the call request to the destination
network side shall also be retained. However, if the network knows that such interactions are not possible
(e.g. the user has only the CUG supplementary service) then the information may be discarded.
6.3 Requirements on the destination network side
For correct interactions with certain other supplementary services, the destination network side shall store,
for the duration of the call, details of whether a normal or a CUG call request was passed to the called
user. The CUG interlock code (if any) of the call request shall also be retained. However, if the network
knows that such interactions are not possible (e.g. the user has only the CUG supplementary service) then
the information may be discarded.

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7 Coding requirements
7.1 ASN.1 description of coding requirements
Table 3 provides an Abstract Syntax Notation one (ASN.1) description of the coding of the Facility
information element components necessary to support this service in accordance with CCITT
Recommendations X.208 [8] and X.209 [9] and uses the OPERATION and ERROR macro as defined in
figure 4/X.219 of CCITT Recommendation X.219 [13].
Table 3
Closed-User-Group-Service-Operations
                 {ccitt identified-organisation etsi (0) 138 operations-and-errors (1)}
DEFINITION ::=
BEGIN
EXPORTS        CUGCall, InvalidOrUnregisteredCUGIndex.
           RequestedBasicServiceViolatesCUGConstraints,
           OutgoingCallsBarredWithinCUG, IncomingCallsBarredWithinCUG,
           UserNotMemeberOfCUG,
           InconsistencyInDesignatedFacilityAndSubscriberClass
IMPORTS        OPERATION,
           ERROR
           FROM Remote-Operation-Notation
               {joint-iso-ccitt remote-operations(4) notation(0)},
           notSubscribed, basicServiceNotProvided
           FROM General-Errors
               {ccitt identified-organisation etsi (0) 196 general-errors};
CUGcall ::=      OPERATION
            -- in Facility information element. Invoked from calling user to
            -- originating network side. Also from destination network side
            -- to called user
           ARGUMENT SEQUENCE {
               OARequested DEFAULT FALSE,
               CUGIndex OPTIONAL}
            -- in SETUP message
           ERRORS {
               invalidOrUnregisteredCUGIndex,
               requestedBasicServiceViolatesCUGConstraints,
               outgoingCallsBarredWithinCUG,
               incomingCallsBarredWithinCUG,
               userNotMemberOfCUG,
               basicServiceNotProvided,
               incosistenceyInDesignatedFacilityAndSubscriberClass,
               notSubscribed}
            -- in clearing message to calling user. Also to destination
            -- network side.

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Table 3 (concluded)
InvalidOrUnregisteredCUGIndex ::= ERROR
RequestedBasicServiceViolatesCUGConstraints ::= ERROR
OutgoingCallsBarredWithinCUG ::= ERROR
IncomingCallsBarredWithinCUG ::= ERROR
UserNotMemberOfCUG ::= ERROR
InconsistenceyInDesignatedFacilityAndSubscriberClass ::= ERROR
OARequested ::=    [1] IMPLICIT BOOLEAN
CUGIndex  ::=    [2] IMPLICIT INTEGER (0.32767)
cUGCall CUGCall ::= 2
invalidOrUnregisteredCUGIndex InvalidOrUnregisteredCUGIndex ::= 16
requestedBasicServiceViolatesCUGConstraints RequestedBasicServiceViolatesCUGConstraints ::= 17
outgoingCallsBarredWithinCUG OutgoingCallsBarredWithinCUG ::= 18
incomingCallsBarredWithinCUG IncomingCallsBarredWithinCUG ::= 19
userNotMemberOfCUG UserNotMemberOfCUG ::= 20
inconsistencyInDesignatedFacilityAndSubscriberClass InconsistencyInDesignatedFacilityAndSubscriberClass
::= 21
END -- of Closed-User-Group-Service-Operations
7.2 Coding of the Cause information element
The cause value for use in the Cause information element in certain CUG service related circumstances (as
described in Clause 9 and Clause 11) is defined in table 4.
Table 4
�˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜´˜˜˜˜˜˜˜˜´˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜´˜˜˜˜˜˜˜˜˜˜˜˜˜¿
‡ Cause value  ‡    ‡             ‡       ‡
ˆ˜˜˜˜˜˜˜´˜˜˜˜˜˜˜˜· Cause ‡             ‡       ‡
‡ Class ‡ Value ‡ number ‡     Cause      ‡ Diagnostics ‡
‡ bits ‡ bits  ‡    ‡             ‡       ‡
‡ 7 6 5 ‡ 4 3 2 1‡    ‡             ‡       ‡
ˆ˜˜˜˜˜˜˜¯˜˜˜˜˜˜˜˜¯˜˜˜˜˜˜˜˜¯˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜¯˜˜˜˜˜˜˜˜˜˜˜˜˜·
‡    ‡    ‡    ‡             ‡       ‡
‡ 1 0 1 ‡ 0 1 1 1‡  87  ‡ User not a member of CUG ‡   -   ‡
‡    ‡    ‡    ‡             ‡       ‡
�˜˜˜˜˜˜˜`˜˜˜˜˜˜˜˜`˜˜˜˜˜˜˜˜`˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜`˜˜˜˜˜˜˜˜˜˜˜˜˜�
8 State definitions
No specifically defined ETS 300 102-1 [10] protocol control states shall be required for the CUG
supplementary service.

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To facilitate understanding of the service the following closed user group process states are used in the
dynamic description (SDL):
- CUG idle;
- outgoing CUG;
- incoming CUG.
These states are specified for the purpose of the protocol definition; these states need not be provided in
an implementation.
9 Signalling procedures at the coincident S and T reference point
9.1 Activation, deactivation and registration
Not applicable.
9.2 Invocation and operation
The CUG supplementary service shall be invoked by:
- a call originating from a CUG supplementary service user. The user may explicitly
request the CUG supplementary service, but in the absence of an explicit request the
CUG supplementary service default procedures shall be automatically applied;
- a call terminating at a CUG supplementary service user.
9.2.1 Call originating from a user with the CUG supplementary service (explicit request)
9.2.1.1 Normal operation
To request explicitly the CUG supplementary service the calling user shall include in the outgoing SETUP
message a Facility information element containing a cUGCall invoke component. If no Calling party number
information element is included by the calling user in the SETUP message, the default number stored in the
originating network shall be used for the assignment of the closed user group.
The network shall perform internal checks appropriate to the originating network based on the contents of
the cUGCall invoke component and the closed user group attributes of the calling user. The outcomes of
these checks are defined in table 5 (including notes).
NOTE: the network may respond to the SETUP message with a SETUP ACKNOWLEDGE or
CALL PROCEEDING message or the call may be cleared for some reason unrelated
to the CUG supplementary service before the checks are completed.
If the result of the checks relevant to the originating network side allows the call to proceed then the
destination network shall perform further internal checks based on the closed user group attributes (if any)
of the called user. The outcomes of these checks are defined in table 6 (including notes).
If the call is successfully offered to the called user, then basic call control procedures shall apply at the
calling user's interface.
9.2.1.2 Exceptional procedures
If, as a result of the checks relevant to either the originating or destination network, the network cannot
allow the call to proceed for a CUG supplementary service related reason, then the network shall fail the
call attempt and include in the first clearing message returned to the calling user (before the alerting
phase) a Facility information element containing a cUGCall return error component with the appropriate
indication as defined by tables 5 and 6 (including notes).

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