ETSI ETS 300 261 ed.1 (1993-11)

SLOVENSKI STANDARD
SIST ETS 300 261 E1:2005
01-maj-2005
Zasebno telekomunikacijsko omrežje (PTN) – Medcentralni signalizacijski protokol
- Dopolnilna storitev: predaja klica
Private Telecommunication Network (PTN); Inter-exchange signalling protocol; Call
transfer supplementary service
Ta slovenski standard je istoveten z: ETS 300 261 Edition 1
ICS:
33.040.35 Telefonska omrežja Telephone networks
SIST ETS 300 261 E1:2005 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST ETS 300 261 E1:2005
EUROPEAN ETS 300 261
TELECOMMUNICATION November 1993
STANDARD
Source: ETSI TC-ECMA Reference: DE/ECMA-00047
ICS: 33.080
PTN, ECMA-178, QSIG-CT
Key words:
Private Telecommunication Network (PTN);
Inter-exchange signalling protocol
Call transfer supplementary service
ETSI
European Telecommunications Standards Institute
ETSI Secretariat
Postal address: F-06921 Sophia Antipolis CEDEX - FRANCE
Office address: 650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE
X.400: c=fr, a=atlas, p=etsi, s=secretariat - Internet: secretariat@etsi.fr
Tel.: +33 92 94 42 00 - Fax: +33 93 65 47 16
No part may be reproduced except as authorized by written permission. The copyright and the
Copyright Notification:
foregoing restriction extend to reproduction in all media.
© European Telecommunications Standards Institute 1993. All rights reserved.
New presentation - see History box

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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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Table of contents
Foreword 7
1Scope 9
2 Conformance 9
3 References 9
4 Definitions 10
4.1 External definitions 10
4.2 End PTNX 10
4.3 Primary PTNX 10
4.4 Redirection Number 10
4.5 Secondary PTNX 10
4.6 Transferring PTNX 11
5 List of acronyms 11
6 Signalling protocol for the support of SS-CT 11
6.1 SS-CT description 11
6.2 SS-CT operational requirements 11
6.2.1 Provision/Withdrawal 11
6.2.2 Requirements on a Transferring PTNX 11
6.2.3 Requirements on a Primary PTNX 11
6.2.4 Requirements on a Secondary PTNX 11
6.2.5 Requirements on a Transit PTNX 12
6.3 SS-CT coding requirements 12
6.3.1 Operations 12
6.3.2 Information elements 16
6.3.2.1 Facility information element 16
6.3.2.2 Information elements embedded in the Facility information
element 16
6.3.2.3 Other information elements 16
6.3.3 Messages 16
6.4 SS-CT state definitions 17
6.4.1 States at a Transferring PTNX 17
6.4.1.1 CT-Idle 17
6.4.1.2 CT-Await-Answer-From-UserC 17
6.4.1.3 CT-Await-Identify-Response 17
6.4.1.4 CT-Await-Initiate-Response 17
6.4.2 States at a Primary PTNX 17
6.4.2.1 CT-Idle 17
6.4.2.2 CT-Await-Setup-Response 17
6.4.2.3 CT-Await-Connect 17
6.4.3 States at a Secondary PTNX 17
6.4.3.1 CT-Idle 17
6.4.3.2 CT-Await-Setup 17
6.5 SS-CT signalling procedures for invocation and operation 17
6.5.1 Actions at a Transferring PTNX 18
6.5.1.1 Normal procedures for transfer by join 18
6.5.1.2 Exceptional procedures for transfer by join 19
6.5.1.3 Normal procedures for transfer by rerouting 19

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6.5.1.4 Exceptional procedures for transfer by rerouting 19
6.5.2 Actions at a Primary PTNX 20
6.5.2.1 Normal procedures for transfer by join 20
6.5.2.2 Exceptional procedures for transfer by join 20
6.5.2.3 Normal procedures for transfer by rerouting 20
6.5.2.4 Exceptional procedures for transfer by rerouting 21
6.5.3 Actions at a Secondary PTNX 22
6.5.3.1 Normal procedures for transfer by join 22
6.5.3.2 Exceptional procedures for transfer by join 22
6.5.3.3 Normal procedures for transfer by rerouting 22
6.5.3.4 Exceptional procedures for transfer by rerouting 23
6.5.4 Actions at a Transit PTNX 24
6.5.5 Subsequent actions at a Primary and a Secondary PTNX 24
6.6 SS-CT impact of interworking with public ISDNs 24
6.6.1 Actions at a Gateway PTNX 24
6.6.1.1 Impact of interworking if User A is in the PTN 24
6.6.1.2 Impact of interworking if a PTN User is transferred by the
public ISDN 24
6.6.2 Actions at other types of PTNX 25
6.7 SS-CT impact of interworking with non-ISDNs 25
6.7.1 Actions at a Gateway PTNX 25
6.7.1.1 Transfer within the PTN 25
6.7.1.2 Transfer within the non-ISDN 25
6.7.1.3 Co-operation with a non-ISDN in providing transfer by rerouting 26
6.7.2 Actions at other types of PTNX 26
6.8 SS-CT Parameter Values (Timers) 26
6.8.1 Timer T1 26
6.8.2 Timer T2 26
6.8.3 Timer T3 27
6.8.4 Timer T4 27
Annex A (normative): Protocol Implementation Conformance Statement (PICS) proforma 28
A.1 Introduction 28
A.2 Instructions for completing the PICS proforma 28
A.2.1 General structure of the PICS proforma 28
A.2.2 Additional information 29
A.2.3 Exception information 29
A.3 PICS proforma for ETS 300 261 30
A.3.1 Implementation identification 30
A.3.2 Protocol summary 30
A.3.3 General 30
A.3.4 Procedures for SS-CT-Join 31
A.3.5 Additional procedures for SS-CT-Rerouting 32
A.3.6 Coding 33
A.3.7 Timers 33
Annex B (informative): Imported ASN.1 definitions relating to numbers 34
Annex C (informative): Examples of message sequences 37
C.1 Example message sequence for normal operations of call transfer by join, both calls active 38

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C.2 Example message sequence for call transfer by join, one call alerting 39
C.3 Example message sequence for normal operation of call transfer by rerouting 40
C.4 Example message sequence for normal operation of call transfer by rerouting, one call alerting 42
Annex D (informative): Specification and Description Language (SDL): Representation of procedures 44
D.1 SDL Representation of SS-CT at a Transferring PTNX 44
D.2 SDL Representation of SS-CT at a Primary PTNX 47
D.3 SDL Representation of SS-CT at a Secondary PTNX 51
History 53

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Foreword
This European Telecommunication Standard (ETS) has been produced by the European Computer Manufacturers
Association (ECMA) on behalf of its members and those of the European Telecommunications Standards Institute
(ETSI).
This ETS is one of a series of standards defining services and signalling protocols applicable to Private
Telecommunication Networks (PTNs) incorporating one or more interconnected nodes. The series uses the ISDN
concepts as developed by CCITT and is also within the framework of standards for open systems interconnection as
defined by ISO.
This ETS specifies the signalling protocol for use at the Q-reference point in support of the Call Transfer
supplementary service (CT).
The ETS is based upon the practical experience of ECMA member companies and the results of their active and
continuous participation in the work of ISO, CCITT, ETSI and other international and national standardisation bodies.
It represents a pragmatic and widely based consensus.
This ETS was produced by ECMA using the ECMA guidelines for the production of standards and using the ECMA
stylesheet. In order to avoid undue delays in the voting process for this ETS it has been agreed that this ETS will not be
converted to the ETSI stylesheet.

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1Scope
This ETS specifies the signalling protocol for the support of the Call Transfer supplementary service (SS-CT)
at the Q reference point between Private Telecommunication Network Exchanges (PTNXs) connected together
within a Private Telecommunication Network (PTN).
SS-CT is a supplementary service which enables a User to transform two of that User's calls (at least one of
which must be answered) into a new call between the two other users of these two calls.
The Q reference point is defined in ENV 41004.
Service specifications are produced in three stages and according to the method specified in ENV 41005. This
ETS contains the stage 3 specification for the Q reference point and satisfies the requirements identified by the
stage 1 and stage 2 specifications in ETS 300 260.
The signalling protocol for SS-CT operates on top of the signalling protocol for basic circuit switched call
control, as specified in ETS 300 172, and uses certain aspects of the generic procedures for the control of
supplementary services specified in ETS 300 239.
The impact on the protocol of interactions between the Call Transfer service and other supplementary services
is outside the scope of this ETS.
This ETS is applicable to PTNXs which can interconnect to form a PTN.
2 Conformance
In order to conform to this ETS, a PTNX shall satisfy the requirements identified in the Protocol
Implementation Conformance Statement (PICS) proforma in annex A.
3 References
ENV 41004 Reference configuration for connectivity relations of private telecommunication
network exchanges (1989).
ENV 41005 Method for the specification of basic and supplementary services of private
telecommunication networks (1989).
ENV 41007 Definition of terms in private telecommunication networks (1989).
ETS 300 171 Private Telecommunication Network (PTN); Specification, functional models and
information flows, Control aspects of circuit mode basic services (1992).
ETS 300 172 Private Telecommunication Network (PTN); Inter-exchange signalling protocol,
Circuit mode basic services (1992).
ETS 300 196 ISDN - Generic Functional Protocol for the Support of Supplementary Services -
DSS1 Protocol.
ETS 300 238 Private Telecommunication Network (PTN); Signalling between private
telecommunication exchanges, Protocol for the support of name identification
supplementary services (1993).
ETS 300 239 Private Telecommunication Network (PTN); Signalling between private
telecommunication exchanges, Generic functional protocol for the support of
supplementary services (1993).
ETS 300 260 Private Telecommunication Networks (PTN); Specification, functional models and
information flows, Call transfer supplementary service (1993).

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CCITT Recommendation I.112 Vocabulary of terms for ISDNs (1988).
CCITT Recommendation I.210 Principles of telecommunication services supported by an ISDN and the
means to describe them (1988).
CCITT Recommendation Z.100 Specification and description language (1988).
4 Definitions
For the purpose of this ETS, the following definitions apply.
4.1 External definitions
This ETS uses the following terms defined in other documents:
- Alerting (ETS 300 260);
- Answered (ETS 300 260);
- Application Protocol Data Unit (APDU) (ETS 300 239);
- Basic Service (CCITT Recommendation I.210);
- Gateway PTNX (ETS 300 172);
- Interpretation APDU (ETS 300 239);
- Network Facility Extension (NFE) (ETS 300 239);
- Originating PTNX (ETS 300 239);
- Primary Call (ETS 300 260);
- Private (ENV 41007);
- Private Telecommunication Network Exchange (PTNX) (ENV 41007);
- Public ISDN (ENV 41007);
- Secondary Call (ETS 300 260);
- Signalling (CCITT Recommendation I.112);
- Supplementary Service (CCITT Recommendation I.210);
- Supplementary Service Control Entity (ETS 300 239);
- Telecommunication Network (ENV 41007);
- Terminal (ENV 41007);
- Terminating PTNX (ETS 300 239);
- Transfer by join (ETS 300 260);
- Transfer by rerouting (ETS 300 260);
- Transit PTNX (ETS 300 239);
- User (ETS 300 171);
- User A (ETS 300 260);
- User B (ETS 300 260);
- User C (ETS 300 260).
4.2 End PTNX
Within the context of a call, a PTNX which is not acting as a Transit PTNX, i.e. an Originating PTNX, a
Terminating PTNX, or a Gateway PTNX.
4.3 Primary PTNX
The End PTNX which is on the end of the Primary Call nearest to User B.
4.4 Redirection Number
The number of a transferred User, as provided to the PTNX of the other transferred User.
4.5 Secondary PTNX
The End PTNX which is on the end of the Secondary Call nearest to User C.

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4.6 Transferring PTNX
The End PTNX which initiates the call transfer procedures on behalf of User A.
5 List of acronyms
APDU Application Protocol Data Unit
ASN.1 Abstract Syntax Notation no. 1
ISDN Integrated Services Digital Network
NFE Network Facility Extension
PICS Protocol Implementation Conformance Statement
PTN Private Telecommunication Network
PTNX Private Telecommunication Network Exchange
SDL Specification and Description Language
SS-CT Supplementary Service Call Transfer
TE Terminal Equipment
6 Signalling protocol for the support of SS-CT
6.1 SS-CT description
Call Transfer (CT) is a supplementary service which enables a user to transform two of that user's calls (at
least one of which must be answered) into a new call between the two other users in the two calls.
This supplementary service is applicable to basic services defined in ETS 300 171.
Call transfer can be achieved by using one of two methods; transfer by join and transfer by rerouting.
Support of transfer by join is mandatory. Support of transfer by rerouting is an option, which, if not
supported by all PTNXs involved in the operation of call transfer, allows fall back to using transfer by join.
NOTE 1
When an active call has been transferred to an alerting call, the supervision during the alerting phase and
the possible procedures to be followed in case the alerting call remains unanswered are outside the scope of
this ETS.
6.2 SS-CT operational requirements
6.2.1 Provision/Withdrawal
Provision and withdrawal shall be in accordance with 6.2.1 of ETS 300 260.
6.2.2 Requirements on a Transferring PTNX
The basic call procedures specified in ETS 300 172 shall be supported. Generic procedures for the call-
related control of supplementary services, as specified in ETS 300 239 for an End PTNX, shall apply.
6.2.3 Requirements on a Primary PTNX
The basic call procedures specified in ETS 300 172 shall be supported.
Generic procedures for the call-related control of supplementary services, as specified in ETS 300 239 for
an End PTNX, shall apply.
6.2.4 Requirements on a Secondary PTNX
The basic call procedures specified in ETS 300 172 shall be supported.
Generic procedures for the call-related control of supplementary services, as specified in ETS 300 239 for
an End PTNX, shall apply.

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6.2.5 Requirements on a Transit PTNX
The basic call procedures specified in ETS 300 172 shall be supported.
Generic procedures for the call-related control of supplementary services, as specified in ETS 300 239 for
a Transit PTNX, shall apply.
For SS-CT the requirements are limited to the passing on of Facility information elements for which the
destination, as indicated in the NFE, is not the Transit PTNX.
6.3 SS-CT coding requirements
6.3.1 Operations
The following operations, defined in Abstract Syntax Notation number 1 (ASN.1) in table 1 shall apply.
Table 1 - Operations in support of SS-CT
Call-Transfer-Operations
{ccitt(0) identified-organization(3) etsi(0)
qsig-call-transfer(261) call-transfer-operations (0)}
DEFINITIONS EXPLICIT TAGS ::=
BEGIN
IMPORTS OPERATION,ERROR FROM Remote-Operation-Notation
{joint-iso-ccitt(2) remote-operations(4) notation(0) }
Extension FROM Manufacturer-specific-service-extension-definition
{ccitt(0) identified-organization(3) etsi(0)
qsig-generic-procedures (239) msi-definition(0) }
Name FROM Name-Operations { ccitt(0) identified-organization (3)
etsi(0) qsig-name (238) name-operations (0) }
notAvailable, invalidCallState, supplementaryServiceInteractionNotAllowed,
FROM General-Errors {ccitt(0) identified-organization(3)
etsi (0) 196 general-errors (2)}
PresentedAddressScreened, PresentedNumberScreened, PartyNumber,
PartySubaddress FROM Addressing-Data-Elements { ccitt(0)
identified-organization(3) etsi(0) 196 addressing-data-elements (6)}
-- Note. The definitions of PresentedAddressScreened,
-- PresentedNumberScreened, PartyNumber, and PartySubaddress are
-- reproduced in annex B
QSIGInformationElement FROM Generic-parameters-definition {
ccitt(0) identified-organization(3) etsi(0) qsig-generic-procedures
(239) qsig-generic-parameters (6) };
ptn OBJECTIDENTIFIER ::= {iso(1) identified-organization(3) icd-ecma(0012)
private-isdn-signalling-domain(09)}
CallTransferIdentify ::= OPERATION
ARGUMENT DummyArg
RESULT CTIdentifyRes
ERRORS { notAvailable,
invalidCallState,
supplementaryServiceInteractionNotAllowed,
unspecified}

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CallTransferAbandon ::= OPERATION
ARGUMENT DummyArg
CallTransferInitiate ::= OPERATION
ARGUMENT CTInitiateArg
RESULT DummyRes
ERRORS { notAvailable,
invalidCallState,
invalidReroutingNumber,
unrecognizedCallIdentity,
establishmentFailure,
supplementaryServiceInteractionNotAllowed,
unspecified }
CallTransferSetup ::= OPERATION
ARGUMENT CTSetupArg
RESULT DummyRes
ERRORS { notAvailable,
invalidCallState,
invalidReroutingNumber,
unrecognizedCallIdentity,
unspecified }
CallTransferActive ::= OPERATION
ARGUMENT CTActiveArg
CallTransferComplete ::= OPERATION
ARGUMENT CTCompleteArg
CallTransferUpdate ::= OPERATION
ARGUMENT CTUpdateArg
SubaddressTransfer ::= OPERATION
ARGUMENT SubaddressTransferArg
DummyArg ::= CHOICE {
NULL,
[1] IMPLICIT Extension,
[2] IMPLICIT SEQUENCE OF Extension
}
DummyRes ::= CHOICE {
NULL,
[1] IMPLICIT Extension,
[2] IMPLICIT SEQUENCE OF Extension
}
CTIdentifyRes ::= SEQUENCE {
callIdentity CallIdentity,
reroutingNumber PartyNumber,
resultExtension CHOICE {
[6] IMPLICIT Extension,
[7] IMPLICIT SEQUENCE OF Extension
} OPTIONAL
}

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CTInitiateArg ::= SEQUENCE {
callIdentity CallIdentity,
reroutingNumber PartyNumber,
argumentExtension CHOICE {
[6] IMPLICIT Extension,
[7] IMPLICIT SEQUENCE OF Extension
} OPTIONAL
}
CTSetupArg ::= SEQUENCE {
callIdentity CallIdentity,
argumentExtension CHOICE {
[0] IMPLICIT Extension,
[1] IMPLICIT SEQUENCE OF Extension
} OPTIONAL
}
CTActiveArg ::= SEQUENCE {
connectedAddress PresentedAddressScreened,
basicCallInfoElements QSIGInformationElement OPTIONAL,
-- ETS 300 172 information elements Party category and
-- Progress indicator are conveyed
connectedName Name OPTIONAL,
argumentExtension CHOICE {
[9] IMPLICIT Extension,
[10] IMPLICIT SEQUENCE OF Extension
} OPTIONAL
}
CTCompleteArg ::= SEQUENCE {
endDesignation EndDesignation,
redirectionNumber PresentedNumberScreened,
basicCallInfoElements QSIGInformationElement OPTIONAL,
-- ETS 300 172 information elements Party category and
-- Progress indicator are conveyed
redirectionName Name OPTIONAL,
callStatus CallStatus DEFAULT answered,
argumentExtension CHOICE {
[9] IMPLICIT Extension,
[10] IMPLICIT SEQUENCE OF Extension
} OPTIONAL
}

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CTUpdateArg ::= SEQUENCE {
redirectionNumber PresentedNumberScreened,
redirectionName Name OPTIONAL,
basicCallInfoElements QSIGInformationElement OPTIONAL,
-- ETS 300 172 information elements Party category and
-- Progress indicator are conveyed
argumentExtension CHOICE {
[9] IMPLICIT Extension,
[10] IMPLICIT SEQUENCE OF Extension
} OPTIONAL
}
SubaddressTransferArg ::= SEQUENCE {
redirectionSubaddress PartySubaddress,
argumentExtension CHOICE {
[0] IMPLICIT Extension,
[1] IMPLICIT SEQUENCE OF Extension
} OPTIONAL
}
CallStatus ::= ENUMERATED {
answered(0),
alerting(1) }
CallIdentity ::= NumericString (SIZE (1.4))
EndDesignation ::= ENUMERATED {
primaryEnd(0),
secondaryEnd(1) }
Unspecified ERROR PARAMETER Extension
unspecified Unspecified ::= {ptn 1008}
callTransferIdentify CallTransferIdentify ::= {ptn ct-identify(7) }
callTransferAbandon CallTransferAbandon ::= {ptn ct-abandon(8) }
callTransferInitiative CallTransferInitiative ::= {ptn ct-initiate(9) }
callTransferSetup CallTransferSetup ::= {ptn ct-setup(10) }
callTransferActive CallTransferActive ::= {ptn ct-active(11) }
callTransferComplete CallTransferComplete ::= {ptn ct-complete(12) }
callTransferUpdate CallTransferUpdate ::= {ptn ct-update(13) }
subaddressTransfer SubaddressTransfer ::= {ptn subaddress-transfer(14) }
invalidReroutingNumber ERROR ::= {ptn 1004 }
-- used when establishment of the new connection fails because
-- the reroutingNumber is not a valid PTN address
unrecognizedCallIdentity ERROR ::= {ptn 1005 }
-- used when establishment of the new connection fails because it
-- could not be associated with a SS-CT entity at the Secondary PTNX
establishmentFailure ERROR ::= {ptn 1006 }
-- used when establishment of the new connection fails and
-- no other error applies
END -- of Call-Transfer-Operations

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6.3.2 Information elements
6.3.2.1 Facility information element
APDUs of the operations defined in 6.3.1 shall be coded in the Facility information element in
accordance with ETS 300 239.
When conveying the invoke APDU of the operations defined in 6.3.1, the destinationEntity data
element of the NFE shall contain value endPTNX.
When conveying the invoke APDU of operations callTransferAbandon, callTransferComplete,
callTransferActive, callTransferUpdate or subaddressTransfer, the Interpretation APDU shall contain
value discardAnyUnrecognisedInvokePdu.
When conveying the invoke APDU of operation callTransferSetup, the Interpretation APDU shall
contain value clearCallIfAnyInvokePduNotRecognised.
When conveying the invoke APDU of operation callTransferIdentify or callTransferInitiate, the
Interpretation APDU shall be omitted.
6.3.2.2 Information elements embedded in the Facility information element
APDUs of the operations defined in 6.3.1 may contain information elements defined in and coded
according to ETS 300 172. These shall be embedded in data elements of type QSIGInformationElement
as specified in 11.3.3.4 of ETS 300 239.
In data element basicCallInfoElements, which is of type QSIGInformationElement, the embedded
contents shall be coded as Party category and/or Progress indicator information elements specified in
ETS 300 172.
6.3.2.3 Other information elements
The following information elements used during the establishment of the new connection (transfer by
rerouting) shall be coded as specified in ETS 300 172:
- Bearer capability;
- Called party number;
-Cause;
- Sending complete;
- Transit counter.
6.3.3 Messages
Except for cases where a basic call message is to be conveyed at the same time, the Facility information
element shall be conveyed in a FACILITY message as specified in ETS 300 239.
The following messages used during the establishment of the new connection and release of the old
connections (in case of transfer by rerouting) shall be as specified in ETS 300 172:
- CALL PROCEEDING;
- CONNECT;
- CONNECT ACKNOWLEDGE;
- DISCONNECT;
- PROGRESS;
- RELEASE;
- RELEASE COMPLETE;
- SETUP.

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6.4 SS-CT state definitions
6.4.1 States at a Transferring PTNX
The procedures at the Transferring PTNX are written in terms of the following conceptual states existing
within the SS-CT control entity in that PTNX in association with a particular Call Transfer request from
User A.
6.4.1.1 CT-Idle
SS-CT is not operating.
6.4.1.2 CT-Await-Answer-From-UserC
A callTransferComplete invoke APDU with callStatus having value alerting has been sent to the
Primary PTNX. This state may be used during transfer by join.
6.4.1.3 CT-Await-Identify-Response
A callTransferIdentify invoke APDU has been sent to the Secondary PTNX. This state is used during
transfer by rerouting.
6.4.1.4 CT-Await-Initiate-Response
A callTransferInitiate invoke APDU has been sent to the Primary PTNX. This state is used during
transfer by rerouting.
6.4.2 States at a Primary PTNX
The procedures at the Primary PTNX are written in terms of the following conceptual states existing
within the SS-CT control entity in that PTNX in association with the primary call, i.e. a particular call of
User B.
6.4.2.1 CT-Idle
SS-CT is not operating.
6.4.2.2 CT-Await-Setup-Response
A callTransferSetup invoke APDU has been sent to the Secondary PTNX. This state is used during
transfer by rerouting.
6.4.2.3 CT-Await-Connect
The primary call has been transferred to an alerting secondary User, and the primary User has been
notified. A CONNECT message indicating answering by the secondary User is awaited.
6.4.3 States at a Secondary PTNX
The procedures at the Secondary PTNX are written in terms of the following conceptual states existing
within the SS-CT control entity in that PTNX in association with a particular call of User C.
6.4.3.1 CT-Idle
SS-CT is not operating.
6.4.3.2 CT-Await-Setup
A callTransferIdentify return result APDU has been sent to the Transferring PTNX. This state is used
during transfer by rerouting.
6.5 SS-CT signalling procedures for invocation and operation
References in this clause to protocol control states refer to basic call protocol control states defined in ETS
300 172.

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NOTE 2
The specification in this section is based on each of the End PTNXs being a different PTNX, but this section
is also applicable to scenarios where two of the three PTNXs are the same. In those scenarios some of the
signalling procedures and message flows described in this section are internal to the PTNX implementation
and therefore outside the scope of this ETS.
Annex C contains some examples of message sequences.
6.5.1 Actions at a Transferring PTNX
Call Transfer procedures shall be initiated on a request from User A specifying the two calls in which
User A is involved to be acted upon. The Transferring PTNX shall check that one of the two calls is in
protocol control state Active and is therefore a valid primary call, and that the other call is in protocol
control state Active or Call Delivered and is therefore a secondary call.
If User C is a User in a non-ISDN, additional states are valid for the secondary call as specified in 6.7.2.
NOTE 3
Additional checks carried out by the Transferring PTNX, e.g. to satisfy the requirements of ETS 300 260,
are outside the scope of this ETS.
NOTE 4
The SDL representation of procedures at a Transferring PTNX is shown in D.1 of annex D.
After validation of the request for call transfer, the Transferring PTNX shall determine which variant of
call transfer is to be attempted: join or rerouting.
NOTE 5
This depends on the capabilities of the Transferring PTNX, the known network topology, and on the
known capabilities of the Primary and Secondary PTNXs in the current call contexts.
If call transfer by rerouting procedures are to be attempted 6.5.1.3 and 6.5.1.4 shall apply, otherwise call
transfer by join procedures specified in 6.5.1.1 and 6.5.1.2 shall apply.
On successful completion of call transfer (either by join or by rerouting), the Transferring PTNX shall
release User A from the two calls and, depending on the procedures at the access, indicate acceptance to
User A.
On failure of call transfer, e.g. because of an invalid request or because of failure of transfer by rerouting,
the Transferring PTNX shall either retain the two calls at User A and indicate rejection to User A or take
implementation dependent action if the calls have been released already from User A.
6.5.1.1 Normal procedures for transfer by join
The Transferring PTNX shall join the B-channels of the primary and secondary calls and send a
callTransferComplete invoke APDU in a FACILITY message to both the Primary and Secondary PTNX
using the call references of the primary and secondary call respectively. Within the argument,
endDesignation shall be included to give a distinctive designation to each end of the new call. If the
secondary call was not in protocol control state Active when transferred, the Transferring PTNX shall
include callStatus with value alerting in the argument of the invoke sent to the Primary PTNX. In
addition other information may be indicated if available: redirectionNumber and redirectionName to
identify the other User in the transferred call, and basicCallInfoElements carrying the category of the
redirected User and/or progress indications encountered during setup of the other call.
If the secondary call is not in protocol control state Active at the time of initiation of the transfer, the
Transferring PTNX shall enter state CT-Await-Answer-From-UserC in which it shall continue to
intercept the signalling connections associated with the former primary and secondary calls.

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SIST ETS 300 261 E1:2005
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ETS 300 261:1993
In state CT-Await-Answer-From-UserC the Transferring PTNX shall convey all received
callTransferUpdate and subaddressTransfer invoke APDUs from the Primary PTNX to the Secondary
PTNX and vice-versa. On receipt in state CT-Await-Answer-From-UserC of any ETS 300 172 message
...