ISO/IEC 21991:2002
(Main)Information technology — Telecommunications and information exchange between systems — Corporate Telecommunication Networks — Signalling interworking between QSIG and H.323 — Call completion supplementary services
Information technology — Telecommunications and information exchange between systems — Corporate Telecommunication Networks — Signalling interworking between QSIG and H.323 — Call completion supplementary services
ISO/IEC 21991:2002 specifies signalling interworking between "QSIG" and "H.323" in support of call completion supplementary services within a Corporate telecommunication Network (CN). "QSIG" is a signalling protocol that operates at the Q reference point between Private Integrated services Network eXchanges (PINX) within a Private Integrated Services Network (PISN). The Q reference point is defined in ISO/IEC 11579-1. A PISN provides circuit-switched basic services and supplementary services to its users. QSIG is specified in other Standards, in particular ISO/IEC 11572 (call control in support of basic services), ISO/IEC 11582 (generic functional protocol for the support of supplementary services) and a number of standards specifying individual supplementary services. ISO/IEC 13870 specifies the QSIG protocol in support of call completion services. "H.323" is a set of signalling protocols for the support of voice or multimedia communication within a packet network, in particular a packet network that uses the Internet Protocol (IP) as its network layer protocol (IP network). H.323 signalling protocols operate between endpoints in an IP network, either indirectly via one or more gatekeepers, or directly. An endpoint can be a terminal or a gateway to another network. H.323 is an "umbrella" recommendation referring to various ITU-T recommendations, in particular Recommendations H.225.0 and H.245 (basic communication capabilities) and Recommendation H.450.1 (generic functional protocol for the support of supplementary services). Recommendation H.450.9 specifies the H.323 protocol in support of call completion services. NOTE H.450.9 applies only to the 1998 version of H.323 (also known as H.323 version 2) and to later versions. In both ISO/IEC 13870 (QSIG) and ITU-Recommendation H.450.9 (H.323), the call completion supplementary services are Completion of Calls to Busy Subscribers (SS-CCBS) and Completion of Calls on No Reply (SS-CCNR). These supplementary services apply after a call establishment attempt has failed because the called user was busy or not available, and provide means to re-establish the call when the called user becomes available. Interworking between QSIG and H.323 permits a call originating at a user of a PISN to terminate at a user of a private IP network, or a call originating at a user of a private IP network to terminate at a user of a PISN. In such a scenario, ISO/IEC 21991:2002 allows the completion of calls when the called user becomes available after having been busy (SS-CCBS), or having not answered the original call (SS-CCNR). Interworking between a PISN employing QSIG and a public IP network employing H.323 is outside the scope of ISO/IEC 21991:2002. However, the functionality specified in ISO/IEC 21991:2002 is in principle applicable to such a scenario when deployed in conjunction with other relevant functionality (e.g., number translation, security functions, etc.). ISO/IEC 21991:2002 is applicable to any interworking unit that can act as a gateway between a PISN employing QSIG and a private IP network employing H.323.
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INTERNATIONAL ISO/IEC
STANDARD 21991
First edition
2002-06-15
Information technology —
Telecommunications and information
exchange between systems — Corporate
Telecommunication Networks — Signalling
interworking between QSIG and H.323 —
Call completion supplementary services
Technologies de l'information — Télécommunications et échange
d'information entre systèmes — Réseaux de télécommunications de
corps — Travail de signalisation entre QSIG et H.323 — Compléments de
service d'achèvement d'appel
Reference number
ISO/IEC 21991:2002(E)
©
ISO/IEC 2002
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ISO/IEC 21991:2002(E)
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ISO/IEC 21991:2002(E)
Contents Page
Foreword v
Introduction vi
1 Scope 1
2 Conformance 1
3 Normative references 1
4 Definitions 2
4.1 External definitions 2
4.2 Other definitions 2
4.2.1 Entity A 2
4.2.2 Entity B 2
4.2.3 Gateway 2
4.2.4 IP network 2
4.2.5 Leg A 2
4.2.6 Scenario A1 2
4.2.7 Scenario A2 2
5 Acronyms 3
6 Service architecture 3
6.1 Service architecture for invocation and operation 3
6.1.1 ISO/IEC 13870 service architecture 3
6.1.2 H.450.9 service architecture 4
6.1.3 Scenarios for interworking 4
6.1.4 Selection of the same gateway for all phases 4
6.2 Options 4
7 Protocol interworking – General requirements 5
8 Protocol interworking – Messages and APDUs 5
8.1 Signalling phase 1 - invocation of call completion 5
8.1.1 Scenario A1 5
8.1.2 Scenario A2 6
8.2 Signalling phase 2 – user B available notification 6
8.2.1 Scenario A1 7
8.2.2 Scenario A2 8
8.3 Signalling phase 3 – CC call establishment 9
8.3.1 Scenario A1 9
8.3.2 Scenario A2 9
8.4 Signalling phase 4 – cancellation of SS-CC 9
8.4.1 Scenario A1 10
8.4.2 Scenario A2 10
9 Protocol interworking – content of APDUs 10
9.1 APDU content mapping from QSIG to H.323 11
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ISO/IEC 21991:2002(E)
9.1.1 ccbsRequest/ccnrRequest invoke APDU mapping 11
9.1.2 ccbsRequest/ccnrRequest return result APDU mapping 11
9.1.3 ccCancel/ccExecPossible invoke APDU mapping 11
9.2 APDU content mapping from H.323 to QSIG 12
9.2.1 ccbsRequest/ccnrRequest invoke APDU mapping 12
9.2.2 ccbsRequest/ccnrRequest return result APDU mapping 12
9.2.3 ccCancel/ccExecPossible invoke APDU mapping 12
Annexes
A - Implementation Conformance Statement (ICS) proforma 13
B - Message flow examples 20
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ISO/IEC 21991:2002(E)
Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission) form the
specialized system for worldwide standardization. National bodies that are members of ISO or IEC participate in the
development of International Standards through technical committees established by the respective organization to deal with
particular fields of technical activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other
international organizations, governmental and non-governmental, in liaison with ISO and IEC, also take part in the work. In the
field of information technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
The main task of the joint technical committee is to prepare International Standards. Draft International Standards adopted by
the joint technical committee are circulated to national bodies for voting. Publication as an International Standard requires
approval by at least 75 % of the national bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of patent rights.
ISO and IEC shall not be held responsible for identifying any or all such patent rights.
ISO/IEC 21991 was prepared by ECMA (as ECMA-326) and was adopted, under a special “fast-track procedure”, by Joint
Technical Committee ISO/IEC JTC 1, Information technology, in parallel with its approval by national bodies of ISO and IEC.
Annex A forms a normative part of this International Standard. Annex B is for information only.
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ISO/IEC 21991:2002(E)
Introduction
This International Standard is one of a series of Standards defining the interworking of services and signalling protocols
deployed in Corporate telecommunication Networks (CNs). The series uses telecommunication concepts as developed by
ITU-T and conforms to the framework of International Standards on Open Systems Interconnection as defined by ISO/IEC.
This International Standard defines the signalling protocol interworking for call completion supplementary services between a
Private Integrated Services Network (PISN) and a packet-based private telecommunication network based on the Internet
Protocol (IP). It is further assumed that the protocol for the PISN part is that defined for the Q reference point (QSIG) and that
the protocols for the IP-based network are based on ITU-T Recommendation H.323.
This International Standard is based upon the practical experience of ECMA member companies and the results of their active
and continuous participation in the work of ISO/IEC JTC 1, ITU-T, ETSI and other international and national standardization
bodies. It represents a pragmatic and widely based consensus.
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INTERNATIONAL STANDARD ISO/IEC 21991:2002(E)
Information technology — Telecommunications and information exchange between
systems — Corporate Telecommunication Networks — Signalling interworking
between QSIG and H.323 — Call completion supplementary services
1 Scope
This International Standard specifies signalling interworking between “QSIG” and “H.323” in support of call completion
supplementary services within a Corporate telecommunication Network (CN).
“QSIG” is a signalling protocol that operates at the Q reference point between Private Integrated services Network eXchanges
(PINX) within a Private Integrated Services Network (PISN). The Q reference point is defined in ISO/IEC 11579-1. A PISN
provides circuit-switched basic services and supplementary services to its users. QSIG is specified in other Standards, in
particular ISO/IEC 11572 (call control in support of basic services), ISO/IEC 11582 (generic functional protocol for the
support of supplementary services) and a number of standards specifying individual supplementary services. ISO/IEC 13870
specifies the QSIG protocol in support of call completion services.
“H.323” is a set of signalling protocols for the support of voice or multimedia communication within a packet network, in
particular a packet network that uses the Internet Protocol (IP) as its network layer protocol (IP network). H.323 signalling
protocols operate between endpoints in an IP network, either indirectly via one or more gatekeepers, or directly. An endpoint
can be a terminal or a gateway to another network. H.323 is an “umbrella” recommendation referring to various ITU-T
recommendations, in particular Recommendations H.225.0 and H.245 (basic communication capabilities) and
Recommendation H.450.1 (generic functional protocol for the support of supplementary services). Recommendation H.450.9
specifies the H.323 protocol in support of call completion services.
NOTE - H.450.9 applies only to the 1998 version of H.323 (also known as H.323 version 2) and to later versions.
In both ISO/IEC 13870 (QSIG) and ITU-Recommendation H.450.9 (H.323), the call completion supplementary services are
Completion of Calls to Busy Subscribers (SS-CCBS) and Completion of Calls on No Reply (SS-CCNR). These supplementary
services apply after a call establishment attempt has failed because the called user was busy or not available, and provide
means to re-establish the call when the called user becomes available.
Interworking between QSIG and H.323 permits a call originating at a user of a PISN to terminate at a user of a private IP
network, or a call originating at a user of a private IP network to terminate at a user of a PISN. In such a scenario, this
International Standard allows the completion of calls when the called user becomes available after having been busy (SS-
CCBS), or having not answered the original call (SS-CCNR).
Interworking between a PISN employing QSIG and a public IP network employing H.323 is outside the scope of this
International Standard. However, the functionality specified in this International Standard is in principle applicable to such a
scenario when deployed in conjunction with other relevant functionality (e.g., number translation, security functions, etc.).
This International Standard is applicable to any interworking unit that can act as a gateway between a PISN employing QSIG
and a private IP network employing H.323.
2 Conformance
In order to conform to this International Standard, a gateway shall satisfy the requirements identified in the Implementation
Conformance Statement (ICS) proforma in annex A.
3 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of this
International Standard. For dated references, subsequent amendments to, or revisions of, any of these publications do not
apply. However, parties to agreements based on this International Standard are encouraged to investigate the possibility of
applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the
normative document referred to applies. Members of ISO and IEC maintain registers of currently valid International Standards.
ISO/IEC 11572:2000, Information technology — Telecommunications and information exchange between systems — Private
Integrated Services Network — Circuit mode bearer services — Inter-exchange signalling procedures and protocol
ISO/IEC 11579-1:1994, Information technology — Telecommunications and information exchange between systems — Private
integrated services network — Part 1: Reference configuration for PISN Exchanges (PINX)
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ISO/IEC 21991:2002(E)
ISO/IEC 11582:1995, Information technology — Telecommunications and information exchange between systems — Private
Integrated Services Network — Generic functional protocol for the support of supplementary services — Inter-exchange
signalling procedures and protocol
ISO/IEC 13870:2001, Information technology — Telecommunications and information exchange between systems — Private
Integrated Services Network — Inter-exchange signalling protocol — Call completion supplementary services
ISO/IEC 21409:2001, Information technology — Telecommunications and information exchange between systems — Corporate
telecommunication networks — Signalling interworking between QSIG and H.323 — Generic functional protocol for the
support of supplementary services
ITU-T Rec. H.225.0, Call signalling protocols and media stream packetization for packet-based multimedia communication
systems
ITU-T Rec. H.245, Control protocol for multimedia communication
ITU-T Rec. H.323, Packet-based multimedia communications systems
ITU-T Rec. H.450.1:1998, Generic functional protocol for the support of supplementary services in H.323
ITU-T Rec. H.450.9:2000, Call Completion Supplementary Services for H.323
4 Definitions
For the purposes of this International Standard, the following definitions apply.
4.1 External definitions
This International Standard uses the following terms defined in other documents:
– Call (ISO/IEC 21409)
– Corporate telecommunication Network (CN) (ISO/IEC 21409)
– Endpoint (ITU-T Rec. H.323)
– Gatekeeper (ITU-T Rec. H.323)
– Private Integrated Services Network (PISN) (ISO/IEC 21409)
– Private Integrated services Network eXchange (PINX) (ISO/IEC 11579-1)
Additionally the definitions in ISO/IEC 13870 and ITU-T Recommendation H.450.9 apply as appropriate.
4.2 Other definitions
4.2.1 Entity A
Signalling entity at the PINX or H.323 endpoint serving the calling user (user A).
4.2.2 Entity B
Signalling entity at the PINX or H.323 endpoint serving the called user (user B).
4.2.3 Gateway
A gateway as defined in H.323 specifically for the purpose of interworking with a network employing QSIG.
4.2.4 IP network
A network, unless otherwise stated a CN, offering connectionless packet-mode services based on the Internet Protocol (IP) as
the network layer protocol.
4.2.5 Leg A
Call segment that lies between entity A and entity B.
4.2.6 Scenario A1
Interworking arrangement in which entity A (PINX A) is in the PISN and entity B is in the IP network.
4.2.7 Scenario A2
Interworking arrangement in which entity A (endpoint A) is in the IP network and entity B is in the PISN.
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ISO/IEC 21991:2002(E)
5 Acronyms
APDU Application Protocol Data Unit
CN Corporate telecommunication Network
ICS Implementation Conformance Statement
IP Internet Protocol
PINX Private Integrated services Network eXchange
PISN Private Integrated Services Network
SS-CC Supplementary Service Call Completion (either SS-CCBS or SS-CCNR)
SS-CCBS Supplementary Service Completion of Calls to Busy Subscribers
SS-CCNR Supplementary Service Completion of Calls on No Reply
6 Service architecture
6.1 Service architecture for invocation and operation
6.1.1 ISO/IEC 13870 service architecture
The QSIG protocol for call completion invocation and operation is based around two signalling entities or PINX types:
• entity A – the PINX serving the calling user (user A);
• entity B – the PINX serving the called user (user B).
Where a user is in another network, the role of entity A or entity B is performed by the other network, the gateway or the two
in combination. However, from the QSIG point of view the role is performed by the gateway, which acts as a gateway PINX.
This can be represented diagrammatically as shown in figure 1.
Leg A
Entity A
Entity B
Figure 1 — Call completion architecture
From this it can be seen that there is only one segment or “leg” as far as the QSIG protocol is concerned (regardless of any
transit entities through which the signalling may pass).
However, an instance of SS-CC consists of several consecutive phases, each phase being a separate instance of call related or
call independent signalling. In other words, leg A in figure 1 represents several calls or call independent signalling connections
between entities A and B, which all belong to a single instance of SS-CC but may take different routes through the network.
The typical course of action for an instance of SS-CC is:
• Prerequisite: unsuccessful call attempt because of busy or absent user B;
• invocation of SS-CC, using a call independent signalling connection (signalling phase 1);
• monitoring of user B; there is no (network) signalling involved during this phase;
• user B available notification, using call independent signalling, and suspension/resumption of SS-CC if user A is busy at
this stage (signalling phase 2);
• recall, resulting in an automatically initiated basic call from user A to user B (signalling phase 3);
• cancellation of SS-CC, by the network (e.g. after timeout), or by user A or user B at any time (signalling phase 4).
NOTE - Interworking is only concerned with the phases that involve (network) signalling.
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ISO/IEC 21991:2002(E)
6.1.2 H.450.9 service architecture
The architecture shown above for QSIG applies also to H.450.9, except that PINXs are replaced by H.323 entities as follows:
• entity A – the calling endpoint;
• entity B – the called endpoint.
Either entity can alternatively be located at a gatekeeper or proxy (as defined in H.450.9) acting on behalf of the respective
endpoint, the signalling between endpoint and gatekeeper/proxy being outside the scope of H.450.9. In this case the
gatekeeper/proxy performs the role of entity A or B, respectively. However, from the H.450.9 point of view the role is
performed by the endpoint.
Where a user is in another network, the role of entity A or entity B is performed by the other network, the gateway or the two
in combination. However, from the H.450.9 point of view the role is performed by the gateway.
6.1.3 Scenarios for interworking
The architectures for ISO/IEC 13870 and H.450.9 are very similar.
This means that the same architecture is applicable to the inter-networking situation between an IP network and a PISN, where
one user involved is served by the IP network and the other is served by the PISN.
For the point of interworking, two scenarios arise, depending on which side of the interworking point the PISN lies:
• Scenario A1: Entity A (PINX A) in PISN, entity B (endpoint B) in IP network;
• Scenario A2: Entity A (endpoint A) in IP network, entity B (PINX B) in PISN.
The point of interworking will be implemented in a gateway, which acts as both an H.323 endpoint from the point of view of
the IP network and a gateway PINX from the point of view of the PISN.
6.1.4 Selection of the same gateway for all phases
Since the different phases of an SS-CC instance in general use separate signalling paths, messages belonging to different
phases may pass through different gateways. In this situation it does not make sense for a gateway to maintain SS-CC related
states on either side, unless it can be assured that all signalling for a particular SS-CC instance passes through the same
gateway. This is the case if there is only one gateway available for connecting the particular pair of users A and B at any time,
but usually not if the load is shared dynamically between several gateways.
One possible way to always choose the same gateway is by means of addressing. If the gateways themselves are addressable
entities with their own alias addresses / PISN numbers – in contrast to being only indirectly addressed via the alias addresses /
PISN numbers of users reachable through the gateway – a specific gateway can be selected:
• on the IP side of the gateway, by using both elements destinationAddress (=gateway address) and remoteExtensionAddress
(=PISN user address) within the relevant address fields of the H.450.9 APDUs or the Setup message;
• on the PISN side of the gateway, by using a (temporary) gateway number, selected (by the gateway) to represent the
remote user in this instance of SS-CC at this gateway. The gateway then has to map between the temporarily assigned
number and the pair of elements destinationAddress / remoteExtensionAddress.
However, the interworking procedures specified in this International Standard do not by themselves require the use of the same
gateway for all signalling related to a particular instance of SS-CC.
NOTE - It will be required if connection retention and connection release methods have to be interworked (see 6.2 and 8.1 for more
information).
6.2 Options
SS-CC can be – and has been – implemented in different ways. Both QSIG and H.323 offer several options, mainly for ease of
interworking with existing implementations in other networks.
The following options of SS-CC can be selected by implementations both in QSIG and in H.323:
• Service retention: If a CC call attempt fails because user B is busy again, SS-CC may either re-enter the monitoring phase
(service retention) or be cancelled (service non-retention).
• Connection retain/release method: A call independent signalling connection may or may not be maintained between
entities A and B during monitoring phases, i.e. phases without signalling.
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For both options negotiation is possible, and both can be interworked between QSIG and H.323.
The following option is only available in QSIG:
• Path reservation/non-reservation: A network path is established for the CC call before informing user A (path
reservation), or user A is informed first and the CC call is then established as a basic call upon acceptance by user A (path
non-reservation).
H.323 supports only path non-reservation, therefore path reservation cannot be directly interworked.
7 Protocol interworking – General requirements
Protocol interworking between H.323 and QSIG for call completion supplementary services shall be in accordance with
ISO/IEC 21409, as modified by the requirements of clause 8.
When transmitting an APDU in one protocol as a result of receiving the corresponding APDU in the other protocol, the
mapping of elements in the received APDU to corresponding elements in the transmitted APDU shall be in accordance with
ISO/IEC 21409, where applicable. Optional elements of one protocol that have no corresponding element in the other protocol
shall be discarded if received.
8 Protocol interworking – Messages and APDUs
In the rules specified below for the different scenarios, the following apply:
1. If the required action is to transmit a QSIG or H.323 FACILITY message but the call state does not permit a FACILITY
message to be sent at that time, the action to be taken is an implementation matter.
2. If the required action is to include an APDU in a transmitted QSIG or H.323 message conditional upon that message being
transmitted and that message is not to be transmitted (owing to basic call interworking considerations), the action to be
taken is an implementation matter.
3. If the required action is dependent on the call independent signalling connection extending or being able to be extended
into the other network and this cannot be achieved, the action to be taken is an implementation matter.
8.1 Signalling phase 1 - invocation of call completion
The invocation procedure of call completion uses a call independent signalling connection, which may be retained for
signalling phase 2 onwards (see below) or released at the end of signalling phase 1.
The interworking rules imply that the signalling connection is either retained on both sides or released on both sides of the
gateway. Interworking scenarios where the connection is released on one side and retained on the other are not provided for.
NOTE 1 - Element retain-sig-connection in the ccbsRequest / ccnrRequest invoke APDU will be passed on by the gateway if present and
allows entity A to ask for a specific method. Entity B will honour this request unless further interworking requires a different method. If the
element is not present entity B will choose the method.
NOTE 2 - A possible reason for "mixed" scenarios is a concatenation of networks some of which are neither H.323 nor QSIG. An
implementation may support such scenarios in a proprietary way. In this case the side without signalling connection must be able to locate
the gateway initially used (which has retained the connection on the other side). 6.1.4 shows how this could be achieved.
Service retention is negotiated end-to-end between entities A and B.
8.1.1 Scenario A1
A gateway that supports scenario A1 shall behave in accordance with the rules of table 1, by carrying out the required action
when a given condition occurs. Each condition applies to the receipt of a QSIG message from entity A, or receipt of an H.323
message from entity B.
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ISO/IEC 21991:2002(E)
Table 1 — Message and APDU handling requirements for signalling phase 1, scenario A1
Rule Condition Required action
1.1 Receipt of a QSIG SETUP message for a call If the call independent signalling connection can be
independent signalling connection, containing a extended into the IP network, transmit an H.323 SETUP
QSIG ccbsRequest or ccnrRequest invoke APDU. message for a call independent signalling connection,
containing an H.323 ccbsRequest or ccnrRequest invoke
APDU.
1.2 Receipt of an H.323 CONNECT message containing If a QSIG CONNECT message is to be transmitted,
an H.323 ccbsRequest or ccnrRequest return result include in the QSIG CONNECT message a QSIG
APDU. ccbsRequest or ccnrRequest return result APDU.
1.3 Receipt of an H.323 RELEASE COMPLETE Transmit a QSIG RELEASE message containing a QSIG
message containing an H.323 ccbsRequest or ccbsRequest or ccnrRequest return result APDU if the
ccnrRequest return result APDU. QSIG call state permits.
1.4 Receipt of an H.323 RELEASE COMPLETE Transmit a QSIG RELEASE message containing a QSIG
message containing an H.323 ccbsRequest or ccbsRequest or ccnrRequest return error APDU if the
ccnrRequest return error APDU. QSIG call state permits.
8.1.2 Scenario A2
A gateway that supports scenario A2 shall behave in accordance with the rules of table 2, by carrying out the required action
when a given condition occurs. Each condition applies to the receipt of an H.323 message from entity A, or receipt of a QSIG
message from entity B.
Table 2 — Message and APDU handling requirements for signalling phase 1, scenario A2
Rule Condition Required action
1.1 Receipt of an H.323 SETUP message for a call If the call independent signalling connection can be
independent signalling connection, containing an extended into the PISN, transmit a QSIG SETUP message
H.323 ccbsRequest or ccnrRequest invoke APDU. for a call independent signalling connection, containing a
QSIG ccbsRequest or ccnrRequest invoke APDU.
1.2 Receipt of a QSIG CONNECT message containing a If an H.323 CONNECT message is to be transmitted,
QSIG ccbsRequest or ccnrRequest return result include in the H.323 CONNECT message an H.323
APDU. ccbsRequest or ccnrRequest return result APDU.
1.3 Receipt of a QSIG RELEASE message containing a Transmit an H.323 RELEASE COMPLETE message
QSIG ccbsRequest or ccnrRequest return result containing an H.323 ccbsRequest or ccnrRequest return
APDU. result APDU if the H.323 call state permits.
1.4 Receipt of a QSIG RELEASE message containing a Transmit an H.323 RELEASE COMPLETE message
QSIG ccbsRequest or ccnrRequest return error containing an H.323 ccbsRequest or ccnrRequest return
APDU. error APDU if the H.323 call state permits.
8.2 Signalling phase 2 – user B available notification
The procedures of this phase use either the retained call independent signalling connection or a new one if the previous one
was released after signalling phase 1.
If user A is busy at this stage, SS-CC will be suspended and resumed when user A becomes free, which will result in
monitoring the status of user B again, and signalling phase 2 will eventually be re-entered. However, in scenario A2 there is a
slight danger of incompatible protocol states at entities A and B caused by one specific combination of options where QSIG
and H.323 deviate. The procedures below specify a method to resolve this conflict with a high probability of success.
If user A is not busy the service will proceed from signalling phase 2 to signalling phase 3 (or signalling phase 4 in case of
failure).
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8.2.1 Scenario A1
A gateway that supports scenario A1 shall behave in accordance with the rules of table 3, by carrying out the required action
when a given condition occurs. Each condition applies to the receipt of a QSIG message from entity A, or receipt of an H.323
message from entity B.
Table 3 — Message and APDU handling requirements for signalling phase 2, scenario A1
Rule Condition Required action
2.1 Receipt of an H.323 SETUP message for a call If the call independent signalling connection can be
independent signalling connection, containing an extended into the PISN, transmit a QSIG SETUP message
H.323 ccExecPossible invoke APDU. containing a QSIG ccExecPossible invoke APDU.
2.2 Receipt of an H.323 FACILITY message containing Transmit a QSIG FACILITY message containing a QSIG
an H.323
...
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