ISO/IEC 11574:2000
(Main)Information technology — Telecommunications and information exchange between systems — Private Integrated Services Network — Circuit-mode 64 kbit/s bearer services — Service description, functional capabilities and information flows
Information technology — Telecommunications and information exchange between systems — Private Integrated Services Network — Circuit-mode 64 kbit/s bearer services — Service description, functional capabilities and information flows
This International Standard specifies the service description and control aspects, including functional capabilities and information flows, of standardised circuit-mode bearer services which may be supported by a Private Integrated Services Network (PISN). This International Standard includes the following basic services: _ Circuit-mode 64 kbit/s unrestricted 8 kHz structured bearer service category; _ Circuit-mode 64 kbit/s 8 kHz structured bearer service category usable for speech information transfer; _ Circuit-mode 64 kbit/s 8 kHz structured bearer service category usable for 3,1 kHz audio information transfer. A PISN shall support at least one of the above three bearer services to conform with this International Standard. The scope of this International Standard does not include: _ the negotiation of service at call establishment time, _ the change of service during a call, and _ unidirectional services. This International Standard includes optional procedures for the provision of functions equivalent to the following public ISDN supplementary services: Subaddress and Multiple Subscriber Number. NOTE 1 - Supplementary services and other bearer services which can be used in conjunction with 64 kbit/s circuit switched bearer services specified in this International Standard are dealt with in other standards. NOTE 2 - Service specifications are based on information concerning the corresponding public ISDN service available at the time of publication of this International Standard. NOTE 3 - ITU-T treat Subaddressing and Multiple Subscriber Number as supplementary services. NOTE 4 - The use of the Direct Dial In supplementary service of a public ISDN for calls incoming to a PISN from a public ISDN is regarded as part of the basic services in a PISN. NOTE 5 - The use of the Calling Line Identification Presentation and Connected Line Identification Presentation supplementary services of a public ISDN for obtaining the Originating Number or the Connected Number of a call from or to a public ISDN is regarded as part of the basic services in a PISN. NOTE 6 - The provision (either explicitly or implicitly) by the user to the network, of its own number (Originating Number or Connected Number), and the provision of an Originating Number or a Connected Number by a PISN to another network is a part of the basic services in a PISN and not a part of the Calling Line Identification Presentation and Connected Line Identification Presentation supplementary services. Those supplementary services are concerned only with the presentation of the number from the network to the served PISN user.
Technologies de l'information — Télécommunications et échange d'information entre systèmes — Réseau privé à intégration de services — Services porteurs sur 64 kbit/s en mode circuit — Description du service, aptitudes fonctionnelles et flux d'informations
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INTERNATIONAL ISO/IEC
STANDARD 11574
Second edition
2000-03-15
Information technology —
Telecommunications and information
exchange between systems — Private
Integrated Services Network —
Circuit-mode 64 kbit/s bearer services —
Service description, functional
capabilities and information flows
Technologies de l'information — Télécommunications et échange
d'information entre systèmes — Réseau privé à intégration de
services — Services porteurs sur 64 kbit/s en mode circuit —
Description du service, aptitudes fonctionnelles et flux d'informations
Reference number
ISO/IEC 11574:2000(E)
©
ISO/IEC 2000
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ISO/IEC 11574:2000(E)
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ISO/IEC 11574:2000(E)
Contents Page
Foreword vi
Introduction vii
Section 1: General 1
1 Scope 1
2 Normative references 1
3 Terms and definitions 2
4 Symbols and abbreviations 2
5 Provision of services by a PISN 3
5.1 Bearer services 3
5.2 Teleservices 3
5.3 Control and signalling 4
5.4 Interworking considerations 4
5.5 Service model 4
Section 2: Service Description (stage 1 description) 5
6 Circuit-mode 64 kbit/s unrestricted 8 kHz structured bearer service category 5
6.1 Definition 5
6.2 Description 5
6.3 Procedures 6
6.4 Network capability for charging 6
6.5 Interworking considerations 6
6.5.1 Interworking with a public ISDN and certain other digital networks 6
6.5.2 Interworking with networks supporting only a restricted digital information transfer capability 6
6.5.3 Interworking with analogue networks 6
6.6 Static Description: Service Attributes 6
7 Circuit-mode 64 kbit/s 8 kHz structured bearer service category usable for speech information transfer 7
7.1 Definition 7
7.2 Description 7
7.3 Procedures 7
7.4 Network capability for charging 7
7.5 Interworking considerations 7
7.5.1 Interworking with a public ISDN and certain other digital networks 7
7.5.2 Interworking with analogue networks 7
7.5.3 Encoding law conversion 7
7.6 Static Description: Service Attributes 8
8 Circuit-mode 64 kbit/s 8 kHz structured bearer service category usable for 3,1 kHz audio information
transfer 8
8.1 Definition 8
8.2 Description 8
8.3 Procedures 9
8.4 Network capability for charging 9
8.5 Interworking considerations 9
8.5.1 Interworking with a public ISDN and certain other digital networks 9
8.5.2 Interworking with analogue networks 9
8.5.3 Encoding law conversion 9
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ISO/IEC 11574:2000(E)
8.6 Static Description: Service Attributes 9
9 Common procedures for services within a PISN 10
9.1 Provision of services 10
9.2 Normal procedures 10
9.2.1 Call establishment at the calling PISN user 10
9.2.2 Call establishment at the called PISN user 11
9.2.3 Terminating the service (call release) 11
9.3 Exceptional procedures / unsuccessful outcome 12
10 Interworking 13
10.1 General Interworking considerations 13
10.1.1 Incoming calls 13
10.1.2 Outgoing calls 13
10.1.3 PISN transit calls 13
10.2 Interworking with public-ISDN 13
10.2.1 Receipt of service request from a public ISDN 13
10.2.2 Sending a service request to a public ISDN 14
10.2.3 Receipt of a service response from public ISDN 14
10.2.4 Sending service response to public ISDN 14
11 Dynamic Description 14
Section 3: Functional capabilities and information flows (stage 2 description) 18
12 Functional model 18
12.1 Functional model description 18
12.2 Description of the functional entities 18
12.2.1 Call Control Agent functional entity 18
12.2.2 Call Control functional entity 19
13 Definition of information flows 20
13.1 Conventions used within the description of information flows 20
13.1.1 Convention for the description of mandatory or optional information 20
13.1.2 Convention for the naming of information flows 21
13.2 SETUP 22
13.3 REPORT 25
13.4 CHANNEL_ACKNOWLEDGE 26
13.5 CHANNEL_CONNECT 26
13.6 DISCONNECT 26
13.7 RELEASE 27
13.8 INFORMATION 27
13.9 SETUP_REJECT 27
13.10 PROCEEDING 28
14 Information flow sequences 28
14.1 Functional entity actions 28
14.1.1 Originating CCA functional entity 28
14.1.2 Originating CC functional entity 29
14.1.3 Transit CC functional entity 30
14.1.4 Destination CC functional entity 31
14.1.5 Destination CCA functional entity 32
14.1.6 Incoming gateway CC functional entity 33
14.1.7 Outgoing gateway CC functional entity 34
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14.2 Normal call establishment 36
14.3 Normal call establishment with digit-by-digit sending and automatic answer 37
14.4 Unsuccessful calls with the provision of tones and announcements 38
14.5 Unsuccessful calls without the provision of tones and announcements 39
14.6 Incoming interworking with a non-ISDN 40
14.7 Outgoing interworking with a non-ISDN 41
14.8 Outgoing interworking with digit-by-digit sending 42
14.9 Basic call clearing 43
14.10 Incoming interworking with a public ISDN 44
14.11 Outgoing interworking with a public ISDN 45
15 SDL diagrams for functional entities 46
15.1 Originating CCA functional entity SDL diagrams 46
15.1.1 Originating CCA states used in SDL diagrams 46
15.1.2 Originating CCA SDL diagrams 47
15.2 Originating CC functional entity SDL diagrams 51
15.2.1 Originating CC states used in SDL diagrams 51
15.2.2 Originating CC SDL diagrams 52
15.3 Transit CC functional entity SDL diagrams 59
15.3.1 Transit CC states used in SDL diagrams 59
15.3.2 Transit CC SDL diagrams 60
15.4 Destination CC functional entity SDL diagrams 65
15.4.1 Destination CC states used in SDL diagrams 65
15.4.2 Destination CC SDL diagrams 66
15.5 Destination CCA functional entity SDL diagrams 73
15.5.1 Destination CCA states used in SDL diagrams 73
15.5.2 Destination CCA SDL diagrams 73
16 Allocation of functional entities to physical entities 77
Annex A (normative) Service attributes 78
Annex B (normative) Teleservices 79
Annex C (informative) Bibliography 81
Annex D (informative) Errors in ISO/IEC 11574 1st edition 82
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ISO/IEC 11574:2000(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
In the field of information technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1. 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.
International Standard ISO/IEC 11574 was prepared by ECMA (as ECMA-142) 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.
This second edition cancels and replaces the first edition (ISO/IEC 11574:1994), which has been technically revised.
Annexes A and B form a normative part of this International Standard. Annexes C and D are for information only.
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ISO/IEC 11574:2000(E)
Introduction
This International Standard is one of a series of standards defining services and signalling protocols applicable to Private
Integrated Services Networks (PISNs). The series uses ISDN concepts as developed by ITU-T and conforms to the framework
of International Standards for Open Systems Interconnection as defined by ISO/IEC.
This International Standard contains specifications of basic services.
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 JTC1, 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 11574:2000(E)
Information technology — Telecommunications and information exchange between
systems — Private Integrated Services Network — Circuit-mode 64 kbit/s bearer
services — Service description, functional capabilities and information flows
Section 1: General
1Scope
This International Standard specifies the service description and control aspects, including functional capabilities and
information flows, of standardised circuit-mode bearer services which may be supported by a Private Integrated Services
Network (PISN).
This International Standard includes the following basic services:
� Circuit-mode 64 kbit/s unrestricted 8 kHz structured bearer service category;
� Circuit-mode 64 kbit/s 8 kHz structured bearer service category usable for speech information transfer;
� Circuit-mode 64 kbit/s 8 kHz structured bearer service category usable for 3,1 kHz audio information transfer.
A PISN shall support at least one of the above three bearer services to conform with this International Standard.
The scope of this International Standard does not include:
� the negotiation of service at call establishment time,
� the change of service during a call, and
� unidirectional services.
This International Standard includes optional procedures for the provision of functions equivalent to the following public ISDN
supplementary services: Subaddress and Multiple Subscriber Number.
NOTE 1 - Supplementary services and other bearer services which can be used in conjunction with 64 kbit/s circuit switched bearer services
specified in this International Standard are dealt with in other standards.
NOTE 2 - Service specifications are based on information concerning the corresponding public ISDN service available at the time of
publication of this International Standard.
NOTE 3 - ITU-T treat Subaddressing and Multiple Subscriber Number as supplementary services.
NOTE 4 - The use of the Direct Dial In supplementary service of a public ISDN for calls incoming to a PISN from a public ISDN is regarded
as part of the basic services in a PISN.
NOTE 5 - The use of the Calling Line Identification Presentation and Connected Line Identification Presentation supplementary services of a
public ISDN for obtaining the Originating Number or the Connected Number of a call from or to a public ISDN is regarded as part of the
basic services in a PISN.
NOTE 6 - The provision (either explicitly or implicitly) by the user to the network, of its own number (Originating Number or Connected
Number), and the provision of an Originating Number or a Connected Number by a PISN to another network is a part of the basic services in
a PISN and not a part of the Calling Line Identification Presentation and Connected Line Identification Presentation supplementary services.
Those supplementary services are concerned only with the presentation of the number from the network to the served PISN user.
2 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 11571:1998, Information technology — Telecommunications and information exchange between systems — Private
Integrated Services Network — Addressing.
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 11574:2000(E)
ITU-T Rec. G.711:1988, Pulse code modulation (PCM) of voice frequencies.
ITU-T Rec. I.112:1993, Vocabulary of terms for ISDNs.
ITU-T Rec. I.140:1993, Attribute technique for the characterization of telecommunications services supported by an ISDN and
network capabilities of an ISDN.
ITU-T Rec. I.210:1993, Principles of telecommunications services supported by an ISDN and the means to describe them.
ITU-T Rec. I.231:1988, Circuit-mode bearer service categories.
ITU-T Rec. I.251.1:1992, Number identification supplementary services — Direct Dialling-In.
ITU-T Rec. I.251.3:1992, Number identification supplementary services — Calling Line Identification Presentation.
ITU-T Rec. I.251.5:1995, Number identification supplementary services — Connected Line Identification Presentation (COLP).
ITU-T Rec. I.520:1993, General arrangements for network interworking between ISDNs.
ITU-T Rec. X.31:1995, Support of packet-mode terminal equipment by an ISDN.
3 Terms and definitions
For the purposes of this International Standard, the following terms and definitions apply. For other terms used in this
International Standard, the definitions in ISO/IEC 11579-1 and ITU-T Rec. I.112 apply.
3.1 call : The instance of the use of a service.
3.2 intervening network (IVN) : The generic term for any real type of network which is employed for the provision of inter-
PINX connections.
3.3 mixed public/private ISDN : An overall ISDN which consists of any concatenation of public/private networks.
NOTE 7 - Services are transparent to the users across public and private network components of a mixed public/private network.
3.4 network call control entity : The collection of network functions concerned with the control of services, as opposed to
functions concerned with the transfer of user information.
3.5 Private Integrated Services Network (PISN) : A private network providing services to a specific set of users.
NOTE 8 - Contrary to a Public ISDN which provides services to the general public.
NOTE 9 - The term PISN covers more than a private ISDN.
3.6 Private Integrated Services Network Exchange (PINX) : A PISN nodal entity which provides automatic connection
handling functions used for the provision of telecommunication services. A nodal entity may consist of one or more nodes.
3.7 PISN user : An entity which uses telecommunication services offered by a PISN, and which therefore directly or indirectly
uses the services of the Network Layer.
3.8 service [Telecommunication services] : That which is offered by a PISN operator and/or owner to its customers in order
to satisfy a specific telecommunication requirement.
Unless otherwise stated, the term “service” shall mean “bearer [telecommunication] service".
3.9 user : An entity which uses telecommunication services offered by a network, and which therefore directly or indirectly
uses the services of the Network Layer.
4 Symbols and abbreviations
CC Clearing Cause
CC [FE] Call Control generic functional entity
CCA Call Control Agent generic functional entity
cfm | c confirmation
CH Call History
CI Channel Identifier
CN Connected Number
CS Connected Subaddress
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CT Connection Type
DN Destination Number
DS Destination Subaddress
FE Functional Entity
HLC High Layer Compatibility
ind | i indication
ISDN Integrated Services Digital Network
ISO International Organisation for Standardisation
LLC Low Layer Compatibility
NC Number complete indication
ON Originating Number
OS Originating Subaddress
OSI Open Systems Interconnection
PINX Private Integrated Services Network Exchange
PISN Private Integrated Services Network
PSTN Public Switched Telephone Network
Rec. (ITU-T) Recommendation
req | rq request
resp | rs response
RT Report Type
SDL Specification and Description Language
TE Terminal Equipment
5 Provision of services by a PISN
Basic services within a PISN consist of bearer services and teleservices. A bearer service is defined only up to a certain layer,
in any case no higher than Layer 3. The definition of a teleservice also encompasses the higher layers up to Layer 7 (although
some of the layers can be empty or not specified, as with for example, Telephony).
The basic services defined in this International Standard correspond to the 64 kbit/s circuit-mode basic services defined in
ITU-T Recommendation I.231.
5.1 Bearer services
PISN circuit-mode bearer services provide a means of transferring information between users at the physical layer level.
Service attributes above Layer 3 are not defined. Consequently, the provision of bearer services involves only low layer
functions. A bearer service can support a variety of high layer protocols.
A circuit-mode bearer service provides an end to end connection (at the physical layer) for the conveyance of user information.
Each switching point intervenes only at the physical layer. This gives a constant bit rate and fixed delays which are very close
to the inherent delays of the transmission media.
5.2 Teleservices
The provision of a teleservice involves high layer functions, generally using the underlying low layer capabilities of a bearer
service. A PISN can support a teleservice by supporting a bearer service having the same capabilities as those required by the
teleservice and by satisfying any special control requirements of the teleservice. The provision of high layer functions in
support of a teleservice is not a necessary part of a PISN and is beyond the scope of this International Standard.
When requesting a teleservice from a PISN, the user has to explicitly indicate the bearer capabilities required in the same way
as when a bearer service is requested. In addition, an indication of the teleservice required is provided by the PISN user,
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ISO/IEC 11574:2000(E)
primarily for passing the indication through the network to the called PISN user in order to allow compatibility checking. A
PISN can optionally make use of this information for purposes such as barring certain teleservices to certain PISN users, or for
the provision or activation of supplementary services on a per teleservice basis, e.g., call forwarding. Any use of this
information by a PISN is outside the scope of, but is not precluded by, this International Standard.
Annex B provides guidelines for, and additional information about, teleservices.
5.3 Control and signalling
In order for information transfer to take place, an information connection must exist between the PISN users concerned. A
demand service involves the establishment and release of information connections according to the demands of users. From the
point of view of users, calls have to be established and released, and this involves call control functions. Call control requires
knowledge of the properties of the user information to be transferred in order to provide appropriate capabilities.
In general, more than one network element (e.g., PINX, terminal) is involved in a call, and therefore call control is distributed.
Consequently call control information needs to be conveyed between network elements. The conveyance of this information is
a function of signalling.
PISN services use message based signalling information, which means that signalling information is carried over a dedicated
logical connection, separate from the connection established for conveying user information.
NOTE 10 - The possible use of the signalling connection also to provide user-to-user information transfer is the function of the User-to-User
Signalling supplementary service, and is outside the scope of this International Standard.
5.4 Interworking considerations
In general, interworking between a PISN bearer service and a bearer service provided by another network requires
interworking functions, both for information transfer and for signalling.
When interworking with the same service in a public ISDN, the interworking function for information transfer is null.
However, interworking has an impact on signalling.
5.5 Service model
This International Standard uses the following service model in order to specify services.
The PISN provides bearer capabilities between end users for the support of the bearer service requested by the user to support
applications. The PISN user controls the bearer capabilities through the control plane. Coordination between the bearer
capabilities and the control plane is maintained by each PINX involved in the connection.
The user terminal interfaces are identified by an address, which in a PISN is defined by a PISN number or a concatenation of a
number and a Subaddress.
The control plane processes address information along with other parameters as necessary to effect the necessary routeing. This
International Standard views control functions as services being provided by a Network Call Control entity, which are
accessible through control service access points. Coordination functions use the services of the Network Call Control entity
when coordinating call control with the transfer of user information, thereby providing bearer capability to PISN users. Unless
explicitly stated the terms “network” and “Network Call Control Entity” are used interchangeably. See figure 1.
User User
Network Call
SAP
SAP
Control Entity
SAP = Service Access Point
Figure 1 – Service model
The primitives used across Network Call Control service access points are as follows.
� SETUP_request / indication / response / confirmation; used for call establishment.
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ISO/IEC 11574:2000(E)
� RELEASE_request / indication / response / confirmation; used for call rejection and release.
� REPORT_request / indication; used for reporting to the calling user:
– that the call is proceeding,
– that the called PISN user is being alerted,
– the presence of in-band tones or announcements, and
– of interworking situations.
� INFORMATION_request; used for providing additional destination addressing information not provided with the
SETUP_request.
In the Stage 1 description, the control aspects of services are specified in terms of the primitives listed above at the Network
Call Control service access points. The entire Network Call Control is treated as a single entity.
In the Stage 2 description, the internal behaviour of Network Call Control is specified by breaking it down into a number of
Functional Entities (FE) and specifying the information flows between them. The result is a model of the form shown in figure
2. The particular model used for the basic call is specified in section 3 of this International Standard. Other models based on
this generic model are used for specifying supplementary services. Supplementary services are not specified in this
International Standard.
User User
Network Call
SAP SAP
Control Entity
FE FE FE FE
SAP = Service Access Point
Figure 2 – Generic model for Stage 2
Section 2: Service Description (stage 1 description)
6 Circuit-mode 64 kbit/s unrestricted 8 kHz structured bearer service category
6.1 Definition
This bearer service category provides information transfer at 64 kbit/s without alteration between PISN users. The service can
support various PISN user applications. Examples include:
� speech (see Note 11);
� 3,1 kHz audio (see Note 11);
� multiple subrate information streams multiplexed into 64 kbit/s by the PISN user;
� transparent access to a public or private X.25 network (ITU-T Rec. X.31 case A for access to a public X.25 network).
NOTE 11 - Whilst speech and 3,1 kHz audio have been given as applications for this bearer service, the PISN user should ensure that a
compatible encoding scheme is in operation. In any case, no network provision can be made for the control of such items as echo and loss, as
the network is unaware of the application in use. Furthermore, the quality of service attribute value for information transfer delay indicates
the suitability of a particular version of this bearer service for speech communication.
6.2 Description
This circuit-mode bearer service category allows:
� two PISN users to communicate in a point to point configuration via the network using 64 kbit/s digital signals, in both
directions continuously and simultaneously for the duration of a call;
� in conjunction with a conference call supplementary service (the procedures of which are outside the scope of this
International Standard), three or more PISN users to communicate in a multi-point configuration.
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ISO/IEC 11574:2000(E)
Once the information channel connection has been established according to the procedures described in clauses 9 to 11, it is
available for the transmission of 64 kbit/s digital signals in both directions continuously and simultaneously, without alteration
by the network. The network shall place no restrictions on the content of the digital signals.
6.3 Procedures
These are common to all services in this International Standard and are given in clause 9.
6.4 Network capability for charging
This is outside the scope of this International Standard.
6.5 Interworking considerations
6.5.1 Interworking with a public ISDN and certain other digital networks
Services in this category are able to interwork with the same services in a public ISDN. The interworking function for user
information transfer is null.
6.5.2 Interworking with networks supporting only a restricted digital information transfer capability
During an interim period, some other networks may only support restricted 64 kbit/s digital information transfer capability, i.e.,
information transfer capability solely restricted by the requirement that the all-zero octet is not allowed. Interworking can be
achieved according to the rules given in Appendix I of ITU-T Rec. I.520 (the PISN being treated as an ISDN with unrestricted
64 kbit/s capability). The PISN shall assume that the interworking functions are provided in the other network. The PISN need
not be affected by this interworking, other than by conveying the appropriate signalling indication to and from the user.
6.5.3 Interworking with analogue networks
The PISN may support calls between data terminals and an analogue network. In this case the following procedures apply.
1
A V–series terminal shall be connected to the PISN via a terminal adaptor. The PISN shall provide an interworking function
(including a modem) for calls to or from a user of an analogue network (e.g., PSTNs or private analogue networks). To effect a
connection, the PISN should use a 64 kbit/s unrestricted connection between its user and the interworking function, and a
3,1 kHz audio connection (or equivalent) from the interworking unit to the analogue network.
NOTE 12 - If additional information concerning layer 1 protocols is available, the PISN may provide the interworking function.
In general, when a call originates in an analogue network, the analogue network is unable to indicate to the PISN the service
required. If this is the case, the called PISN user is offered a 3,1 kHz audio bearer service with an indication of such
interworking.
NOTE 13 - If at the called PISN user there is a terminal adaptor which is unable to accept an incoming 3,1 kHz audio call but is able to
accept an incoming 64 kbit/s unrestricted call, the introduction of an interworking function in the PISN can be achieved only if there is
service negotiation between
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