SIST EN 300 392-10-11 V1.3.1:2006
(Main)Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D); Part 10: Supplementary services stage 1; Sub-part 11: Call Waiting (CW)
- Abstract
The present document defines the stage 1 description of the call waiting supplementary service (SS-CW) for the Terrestrial Trunked Radio (TETRA) as provided by European operators. The stage 1 description is an overall service description from the user point of view but does not deal with the details of the human interface itself (see ITU-T Recommendation I.130 [1]). SS-CW permits a called user to acknowledge an incoming individual call while he is already busy. Subsequently that user shall have the choice to accept, reject or ignore that incoming call. The present document specifies the service description of the supplementary service and the procedures to be expected with successful and unsuccessful outcomes. In addition the present document specifies the interactions with other TETRA supplementary services and inter-working considerations. Charging principles are outside the scope of the present document.
- Status
- Published
- Publication Date
- 31-Mar-2006
- Current Stage
- 6060 - National Implementation/Publication (Adopted Project)
- Start Date
- 01-Apr-2006
- Due Date
- 01-Apr-2006
- Completion Date
- 01-Apr-2006
- Mandate
- T-172-1A
SIST EN 300 392-10-11 V1.3.1:2006 is the Slovenian adoption of the ETSI European Standard for Terrestrial Trunked Radio (TETRA) Voice plus Data (V+D), Part 10, Sub-part 11, covering the Call Waiting (CW) supplementary service. It specifies the stage 1 service description from the user point of view, including how a busy called user can be offered an incoming individual call and then accept, reject, or ignore it. The document is aimed at TETRA service providers, network designers, and terminal implementers who need to understand the service behaviour and its interactions with other supplementary services.
What does SIST EN 300 392-10-11 V1.3.1:2006 specify?
SIST EN 300 392-10-11 V1.3.1:2006 specifies the stage 1 description of SS-CW for TETRA as provided by European operators. It focuses on service behaviour, not on the detailed human interface, and it excludes charging principles.
The document is organized into these main parts:
- Clause 1 - Scope
- Clause 2 - References
- Clause 3 - Definitions and abbreviations
- Clause 4 - SS-CW stage 1 specification
- Annex A - Bibliography
- Annex B - Change Requests
| Annex | What it covers |
|---|---|
| Annex A | Bibliography |
| Annex B | Change Requests incorporated into this version |
What are the key requirements of SIST EN 300 392-10-11 V1.3.1:2006?
SIST EN 300 392-10-11 V1.3.1:2006 describes how SS-CW works when a user is already busy, how the network offers waiting calls, and what happens when the call is accepted, rejected, ignored, or cleared. The practical effect is that implementers must support the service flow, timers, and interactions consistently with TETRA call control.
Service applicability and subscription
Clause 4.1.2 says SS-CW applies to all circuit mode individual teleservices and bearer services. Clause 4.2.1 says provision can be per subscriber or as a network-wide option for all individual subscribers. In practice, operators decide whether to enable it per user or as a general service policy.
Activation, invocation, and waiting-call handling
Clause 4.2.2.1 says the service is activated by the provider on provision and deactivated on withdrawal, and the served user should also be able to activate or deactivate it after provision. Clause 4.2.2.2 allows the network to offer a new individual call to a busy user, subject to the network limit for waiting calls. Clause 4.2.2.3 says the served user invokes SS-CW by sending a positive response within the same time window used for a basic call response. In practice, the terminal has to keep the offered call available long enough for the user to decide.
Limits, timers, and response choices
Clause 4.2.2.2 allows more than one waiting call, but the maximum number is a network option and one is recommended. Clause 4.2.4 defines timer T2 as the time limit for a valid SS-CW invocation and notes that 30 seconds may be suitable, while the actual value is network dependent. The user can accept, reject, or ignore the waiting call, and if T2 expires the network clears the call.
Exceptional cases
Clause 4.2.3 covers failures such as activation when the service is not subscribed, invocation when the waiting-call limit is reached, and cases where the network cannot support the location change procedure. In practice, these clauses tell the network when to reject the request and which disconnect cause to return to the calling user.
Interactions with other supplementary services
Clause 4.3 states which services do not interact with SS-CW and which ones take precedence or interact under specific conditions. Notable cases include:
- SS-CFU takes precedence over SS-CW.
- SS-CFB may take precedence, or interact in limited cases.
- SS-CFNRy interacts through the earlier expiry of its own timer or T2.
- HOLD can be used to free resources before accepting a waiting call.
- PPC can itself become a waiting call, even if the waiting-call limit would otherwise be exceeded.
For implementers, this is where service coexistence decisions are made.
Interworking
Clause 4.4 covers interworking between TETRA networks and with external networks. The key point is that notification of SS-CW invocation and the related disconnect causes must be passed correctly across network boundaries. If the serving user moves to another network and SS-CW migration is not supported, the waiting calls are cleared and the users are informed.
What terms does SIST EN 300 392-10-11 V1.3.1:2006 define?
SIST EN 300 392-10-11 V1.3.1:2006 defines several terms used to describe the call waiting procedure.
- Busy - the state in which no new call can be offered to the served user; in this document it applies to individual calls.
- Served user - the user who may invoke SS-CW while busy.
- User A - the party already in an individual call with the served user.
- User B - the served user who has activated SS-CW and may invoke it.
- User C - the user who originates the incoming individual call that becomes a waiting call.
- Timer T2 - the waiting time allowed for the served user to accept or clear the offered call.
- Network Determined User Busy (NDUB) - the case where the network determines that the user is busy and cannot offer a new call.
Who uses SIST EN 300 392-10-11 V1.3.1:2006?
SIST EN 300 392-10-11 V1.3.1:2006 is used by TETRA network operators, service designers, radio terminal vendors, and system integrators. It is relevant when defining subscriber service profiles, implementing busy-state call handling, testing supplementary-service behaviour, or checking interworking with other TETRA services and external networks.
What changed in SIST EN 300 392-10-11 V1.3.1:2006 from the previous edition?
SIST EN 300 392-10-11 V1.3.1:2006 is the January 2004 publication of the document following Edition 2 and the earlier V1.3.0 version. Annex B states that change requests were incorporated for SS-CF interactions, timer definitions, and the busy definition.
Which standards are used with SIST EN 300 392-10-11 V1.3.1:2006?
SIST EN 300 392-10-11 V1.3.1:2006 cites these references:
- ITU-T Recommendation I.130 - service characterization method for telecommunication services and ISDN network capabilities
- ITU-T Recommendation Z.100 - Specification and Description Language (SDL), used for the overall SDL figures
- ETSI EN 300 392-2 - TETRA Air Interface (AI), used for basic call procedures and timers
- ETSI EN 300 392-9 - General requirements for supplementary services, used for shared definitions and service framework
What does the SIST EN 300 392-10-11 V1.3.1:2006 document contain?
SIST EN 300 392-10-11 V1.3.1:2006 contains a service description, procedural text, definitions, abbreviations, interaction rules, and interworking rules for SS-CW. It also includes two overall SDL figures that model invocation, operation, and activation. Annex B provides a table of incorporated change requests, which is useful for tracking how the published version was updated. The document is written for design and implementation work rather than for test procedures or charging rules.
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Frequently Asked Questions
SIST EN 300 392-10-11 V1.3.1:2006 is a standard published by the Slovenian Institute for Standardization (SIST). Its full title is "Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D); Part 10: Supplementary services stage 1; Sub-part 11: Call Waiting (CW)". This standard covers: The present document defines the stage 1 description of the call waiting supplementary service (SS-CW) for the Terrestrial Trunked Radio (TETRA) as provided by European operators. The stage 1 description is an overall service description from the user point of view but does not deal with the details of the human interface itself (see ITU-T Recommendation I.130 [1]). SS-CW permits a called user to acknowledge an incoming individual call while he is already busy. Subsequently that user shall have the choice to accept, reject or ignore that incoming call. The present document specifies the service description of the supplementary service and the procedures to be expected with successful and unsuccessful outcomes. In addition the present document specifies the interactions with other TETRA supplementary services and inter-working considerations. Charging principles are outside the scope of the present document.
The present document defines the stage 1 description of the call waiting supplementary service (SS-CW) for the Terrestrial Trunked Radio (TETRA) as provided by European operators. The stage 1 description is an overall service description from the user point of view but does not deal with the details of the human interface itself (see ITU-T Recommendation I.130 [1]). SS-CW permits a called user to acknowledge an incoming individual call while he is already busy. Subsequently that user shall have the choice to accept, reject or ignore that incoming call. The present document specifies the service description of the supplementary service and the procedures to be expected with successful and unsuccessful outcomes. In addition the present document specifies the interactions with other TETRA supplementary services and inter-working considerations. Charging principles are outside the scope of the present document.
SIST EN 300 392-10-11 V1.3.1:2006 is classified under the following ICS (International Classification for Standards) categories: 33.070.10 - Terrestrial Trunked Radio (TETRA). The ICS classification helps identify the subject area and facilitates finding related standards.
SIST EN 300 392-10-11 V1.3.1:2006 is associated with the following European legislation: Standardization Mandates: T-172-1A. When a standard is cited in the Official Journal of the European Union, products manufactured in conformity with it benefit from a presumption of conformity with the essential requirements of the corresponding EU directive or regulation.
SIST EN 300 392-10-11 V1.3.1:2006 is available in PDF format for immediate download after purchase. The document can be added to your cart and obtained through the secure checkout process. Digital delivery ensures instant access to the complete standard document.
Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D); Part 10: Supplementary services stage 1; Sub-part 11: Call Waiting (CW)33.070.10Prizemni snopovni radio (TETRA)Terrestrial Trunked Radio (TETRA)ICS:Ta slovenski standard je istoveten z:EN 300 392-10-11 Version 1.3.1SIST EN 300 392-10-11 V1.3.1:2006en01-april-2006SIST EN 300 392-10-11 V1.3.1:2006SLOVENSKI
STANDARD
ETSI ETSI EN 300 392-10-11 V1.3.1 (2004-01) 2
Reference REN/TETRA-03094 Keywords CW, data, radio, speech, stage 1, supplementary service, TETRA ETSI 650 Route des Lucioles F-06921 Sophia Antipolis Cedex - FRANCE
Tel.: +33 4 92 94 42 00
Fax: +33 4 93 65 47 16
Siret N° 348 623 562 00017 - NAF 742 C Association à but non lucratif enregistrée à la Sous-Préfecture de Grasse (06) N° 7803/88
Important notice Individual copies of the present document can be downloaded from: http://www.etsi.org The present document may be made available in more than one electronic version or in print. In any case of existing or perceived difference in contents between such versions, the reference version is the Portable Document Format (PDF). In case of dispute, the reference shall be the printing on ETSI printers of the PDF version kept on a specific network drive within ETSI Secretariat. Users of the present document should be aware that the document may be subject to revision or change of status. Information on the current status of this and other ETSI documents is available at http://portal.etsi.org/tb/status/status.asp If you find errors in the present document, send your comment to: editor@etsi.org 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 2004. All rights reserved.
DECTTM, PLUGTESTSTM and UMTSTM are Trade Marks of ETSI registered for the benefit of its Members. TIPHONTM and the TIPHON logo are Trade Marks currently being registered by ETSI for the benefit of its Members. 3GPPTM is a Trade Mark of ETSI registered for the benefit of its Members and of the 3GPP Organizational Partners. SIST EN 300 392-10-11 V1.3.1:2006
ETSI ETSI EN 300 392-10-11 V1.3.1 (2004-01) 3
Contents Intellectual Property Rights.5 Foreword.5 1 Scope.7 2 References.7 3 Definitions and abbreviations.7 3.1 Definitions.7 3.2 Abbreviations.8 3.2.1 General abbreviations.8 3.2.2 Supplementary service abbreviations.8 4 SS-CW stage 1 specification.9 4.1 Description.9 4.1.1 General description.9 4.1.2 Qualifications on applicability to telecommunication services.9 4.2 Procedures.9 4.2.1 Provision/withdrawal.9 4.2.2 Normal procedures.9 4.2.2.1 Activation, deactivation, definition, registration, interrogation and cancellation.9 4.2.2.1.1 Activation/Deactivation.9 4.2.2.1.2 Definition.9 4.2.2.1.3 Registration.9 4.2.2.1.4 Interrogation.9 4.2.2.1.5 Cancellation.10 4.2.2.2 Invocation.10 4.2.2.3 Operation.10 4.2.3 Exceptional procedures.11 4.2.3.1 Activation, deactivation, definition, registration, interrogation and cancellation.11 4.2.3.1.1 Activation/Deactivation.11 4.2.3.1.2 Definition.11 4.2.3.1.3 Registration.11 4.2.3.1.4 Interrogation.11 4.2.3.1.5 Cancellation.11 4.2.3.2 Invocation and operation.11 4.2.3.2.1 Invocation.11 4.2.3.2.2 Operation.12 4.2.4 Protocol timer.13 4.3 Interactions with other supplementary services.13 4.3.1 Calling Line Identification Presentation.13 4.3.2 Calling/Connected Line Identification Restriction.13 4.3.3 Connected Line Identification Presentation.13 4.3.4 Call Report.13 4.3.5 Talking Party Identification.13 4.3.6 Call Forwarding Unconditional (SS-CFU).13 4.3.7 Call Forwarding On Busy (SS-CFB).14 4.3.8 Call Forwarding on No Reply (SS-CFNRy).14 4.3.9 Call Forwarding on Not Reachable (SS-CFNRc).14 4.3.10 List Search Call (LSC).15 4.3.11 Call Authorized by Dispatcher (CAD).15 4.3.12 Short Number Addressing (SNA).15 4.3.13 Area Selection (AS).15 4.3.14 Access Priority (AP).15 4.3.15 Priority Call (PC).15 4.3.16 Call Hold (HOLD).15 4.3.17 Call Completion to Busy Subscriber (CCBS).16 4.3.18 Late Entry (LE).16 SIST EN 300 392-10-11 V1.3.1:2006
ETSI ETSI EN 300 392-10-11 V1.3.1 (2004-01) 4
4.3.19 Transfer of Control (TC).16 4.3.20 Pre-emptive Priority Call (PPC).16 4.3.21 Include Call (IC).16 4.3.22 Advice of Charge (AC).16 4.3.23 Barring of Outgoing Calls (BOC).16 4.3.24 Barring of Incoming Calls (BIC).16 4.3.25 Discreet Listening (DL).17 4.3.26 Ambience Listening (AL).17 4.3.27 Dynamic Group Number Assignment (DGNA).17 4.3.28 Call Completion on No Reply (CCNR).17 4.3.29 Call Retention (CRT).17 4.4 Inter-working considerations.17 4.4.1 Inter-working between different TETRA networks.17 4.4.2 Inter-working with external networks.18 4.5 Overall SDL.18 Annex A (informative): Bibliography.22 Annex B (informative): Change Requests.23 History.24
ETSI ETSI EN 300 392-10-11 V1.3.1 (2004-01) 5
Intellectual Property Rights IPRs essential or potentially essential to the present document may have been declared to ETSI. The information pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the ETSI Web server (http://webapp.etsi.org/IPR/home.asp). Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee can be given as to the existence of other IPRs not referenced in ETSI SR 000 314 (or the updates on the ETSI Web server) which are, or may be, or may become, essential to the present document. Foreword This European Standard (Telecommunications series) has been produced by ETSI Project Terrestrial Trunked Radio (TETRA). The present document is part 10, sub-part 11 of a multi-part deliverable covering the Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D), as identified below: EN 300 392-1: "General network design"; EN 300 392-2: "Air Interface (AI)"; EN 300 392-3: "Interworking at the Inter-System Interface (ISI)"; ETS 300 392-4: "Gateways basic operation"; EN 300 392-5: "Peripheral Equipment Interface (PEI)"; EN 300 392-7: "Security"; EN 300 392-9: "General requirements for supplementary services"; EN 300 392-10: "Supplementary services stage 1"; EN 300 392-10-1: "Call Identification (CI)"; ETS 300 392-10-2: "Call report"; ETS 300 392-10-3: "Talking Party Identification (TPI)"; EN 300 392-10-4: "Call Forwarding (CF)"; ETS 300 392-10-5: "List Search Call (LSC)"; EN 300 392-10-6: "Call Authorized by Dispatcher (CAD)"; ETS 300 392-10-7: "Short number addressing"; EN 300 392-10-8: "Area Selection (AS)"; ETS 300 392-10-9: "Access priority"; EN 300 392-10-10: "Priority Call (PC)"; EN 300 392-10-11: "Call Waiting (CW)"; EN 300 392-10-12: "Call Hold (HOLD)"; ETS 300 392-10-13: "Call completion to busy subscriber"; EN 300 392-10-14: "Late Entry (LE)"; SIST EN 300 392-10-11 V1.3.1:2006
ETSI ETSI EN 300 392-10-11 V1.3.1 (2004-01) 6
ETS 300 392-10-15: "Transfer of control"; ETS 300 392-10-16: "Pre-emptive Priority Call (PC)"; EN 300 392-10-17: "Include Call (IC)"; EN 300 392-10-18: "Barring of Outgoing Calls (BOC)"; EN 300 392-10-19: "Barring of Incoming Calls (BIC)"; ETS 300 392-10-20: "Discreet Listening (DL)"; EN 300 392-10-21: "Ambience Listening (AL)"; EN 300 392-10-22: "Dynamic Group Number Assignment (DGNA)"; ETS 300 392-10-23: "Call completion on no reply"; ETS 300 392-10-24: "Call Retention (CRT)"; EN 300 392-11: "Supplementary services stage 2"; EN 300 392-12: "Supplementary services stage 3"; ETS 300 392-13: "SDL model of the Air Interface (AI)"; ETS 300 392-14: "Protocol Implementation Conformance Statement (PICS) proforma specification"; TS 100 392-15: "TETRA frequency bands, duplex spacing and channel numbering"; TS 100 392-16: "Network Performance Metrics"; TS 100 392-17: "TETRA V+D and DMO Release 1.1 specifications".
National transposition dates Date of adoption of this EN: 19 December 2003 Date of latest announcement of this EN (doa): 31 March 2004 Date of latest publication of new National Standard or endorsement of this EN (dop/e):
30 September 2004 Date of withdrawal of any conflicting National Standard (dow): 30 September 2004
ETSI ETSI EN 300 392-10-11 V1.3.1 (2004-01) 7
1 Scope The present document defines the stage 1 description of the call waiting supplementary service (SS-CW) for the Terrestrial Trunked Radio (TETRA) as provided by European operators. The stage 1 description is an overall service description from the user point of view but does not deal with the details of the human interface itself (see ITU-T Recommendation I.130 [1]). SS-CW permits a called user to acknowledge an incoming individual call while he is already busy. Subsequently that user shall have the choice to accept, reject or ignore that incoming call. The present document specifies the service description of the supplementary service and the procedures to be expected with successful and unsuccessful outcomes. In addition the present document specifies the interactions with other TETRA supplementary services and inter-working considerations. Charging principles are outside the scope of the present document. 2 References The following documents contain provisions which, through reference in this text, constitute provisions of the present document. • References are either specific (identified by date of publication and/or edition number or version number) or non-specific. • For a specific reference, subsequent revisions do not apply. • For a non-specific reference, the latest version applies. Referenced documents which are not found to be publicly available in the expected location might be found at http://docbox.etsi.org/Reference. [1] ITU-T Recommendation I.130: "Method for the characterization of telecommunication services supported by an ISDN and network capabilities of an ISDN". [2] ITU-T Recommendation Z.100: "Specification and Description Language (SDL)". [3] ETSI EN 300 392-2: "Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D); Part 2: Air Interface (AI)". [4] ETSI EN 300 392-9: "Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D);
Part 9: General requirements for supplementary services". 3 Definitions and abbreviations 3.1 Definitions For the purposes of the present document, the terms and definitions given in EN 300 392-9 [4] and the following apply: busy: served user state in which no more a new call can be offered to him NOTE 1: In the present document busy is only applicable for individual calls. NOTE 2: If the user MS supports concurrent calls, then busy is defined independently for each basic service. served user: user who may invoke the supplementary service (i.e. acknowledge an incoming individual while he is busy, instead of clearing it) SIST EN 300 392-10-11 V1.3.1:2006
ETSI ETSI EN 300 392-10-11 V1.3.1 (2004-01) 8
timer T2: measures the waiting time for the offered call from user C to be either accepted or cleared by the served user B after SS-CW has been invoked user A: user engaged in an individual call with the served user (this call can be in any state) NOTE: If the served user is engaged in a group call when he receives an incoming individual call, then user A may be any of the other participants in that group call. user B: served user who has activated SS-CW and may invoke it user C: user who has originated an individual call to the served user while that served user is busy (i.e. the served user may invoke the call waiting supplementary service for the call from user C). 3.2 Abbreviations 3.2.1 General abbreviations For the purposes of the present document, the following general abbreviations apply: ISDN Integrated Services Digital Network MS Mobile Station NDUB Network Determined User Busy SDL (Functional) Specification and Description Language SS Supplementary Service NOTE: The abbreviation SS is only used when referring to a specific supplementary service. 3.2.2 Supplementary service abbreviations For the purposes of the present document, the following supplementary service abbreviations apply: AC Advice of Charghe AL Ambience Listening AP Access Priority AS Area Selection BIC Barring of Incoming Calls BOC Barring of Outgoing Calls CAD Call Authorized by Dispatcher CCBS Call Completion to Busy Subscriber CCNR Call Completion on No Reply CFB Call Forwarding on Busy CFNRc Call Forwarding on Not Reachable CFNRy Call Forwarding on No Reply CFU Call Forwarding Unconditional CRT Call Retention CW Call Waiting DGNA Dynamic Group Number Assignment DL Discreet Listening HOLD Call Hold IC Include Call LSC List Search Call PC Priority Call PPC Pre-emptive Priority Call SNA Short Authorized by Dispatcher TC Transfert of Control SIST EN 300 392-10-11 V1.3.1:2006
ETSI ETSI EN 300 392-10-11 V1.3.1 (2004-01) 9
4 SS-CW stage 1 specification 4.1 Description 4.1.1 General description SS-CW permits a called user to acknowledge an incoming individual call while he is already busy. That call shall then be qualified as a waiting call. Subsequently that user shall have the choice to accept, reject or ignore the waiting call. 4.1.2 Qualifications on applicability to telecommunication services This supplementary service shall be applicable to all circuit mode individual teleservices and bearer services. 4.2 Procedures 4.2.1 Provision/withdrawal The provision of SS-CW shall be on a per individual subscriber basis, or as a network option general for all (TETRA) individual subscribers. The corresponding subscriptions shall be valid for all basic services for which SS-CW applies (see clause 4.1.2). The supplementary service shall be withdrawn by the service provider: - for administrative purposes; or - at the request of the served user, if individually provided. No specific information shall then be given to the subscribers by the network. 4.2.2 Normal procedures 4.2.2.1 Activation, deactivation, definition, registration, interrogation and cancellation 4.2.2.1.1 Activation/Deactivation The CW supplementary service shall be activated by the service provider upon provision. The CW supplementary service shall be deactivated by the service provider upon withdrawal. The served user should also be able to activate/deactivate the service after provision. 4.2.2.1.2 Definition Not applicable. 4.2.2.1.3 Registration Not applicable. 4.2.2.1.4 Interrogation Not applicable. SIST EN 300 392-10-11 V1.3.1:2006
ETSI ETSI EN 300 392-10-11 V1.3.1 (2004-01) 10 4.2.2.1.5 Cancellation Not applicable. 4.2.2.2 Invocation If SS-CW has been activated for the served user, when a new individual call is addressed to that user and the network has determined that such user is already engaged in a call (i.e. that user is then in the busy state whether he participates in an individual call or in a group call), the network shall offer that call to him unless it has already reached the maximum number of such additional calls. NOTE 1: In ISDN, the case just addressed (where the network cannot present a new additional call when the called user is busy) is one of those corresponding to the situation defined as "Network Determined User Busy" (NDUB). When the served user is offered such incoming individual call (from user C) while being busy, he shall be able to invoke SS-CW. He shall do so in sending a positive response to the offered call within the same predefined period of time as for sending a basic call response. The same shall apply if the network offers a new incoming individual call to the served user without knowing that he is in the busy state. NOTE 2: The served user may be in the busy state without the network knowing it when the served user participates in a group call. The served user may be able to invoke SS-CW for more than one incoming individual call. However the maximum number of waiting calls at any one time per busy subscriber shall be a network option, and it is recommended that that maximum number be one. 4.2.2.3 Operation When the called user has invoked SS-CW for a call (incoming individual): - if the current value of the basic call timer T302 (see clause 14.6 of EN 300 392-2 [3]) in the served user MS is expected to be less than T2, then it should be set to a value larger than T2; - the network shall apply the individual basic call procedure for informing user C that: - the basic call hook selection is on/hook off signalling; - the called user has been offered the call. - in addition the network shall notify user C that SS-CW has been invoked for his call. If the served user is participating in an individual call, with user A, he should free resources (i.e. especially air interface resources i
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