1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 1 TELECOMMUNICATION (03/93)STANDARDIZATION SECTOROF ITU$)4!,G0G035“3#2)“%2G0G03).!,).G0G03934%-G0G0.O G0G0 34!%G0G0 G0G0$%3#2)04)/.G0G0 or set-up, retrieve, or connect toanother call. One B-channel shall be kept available for the user as long as the user
2、hasi) one or more calls on hold with reservation; andii) is not currently connected to any other call.That is, the network shall not reserve more than one B-channel for a user, regardless of how a user is defined (e.g. asidentified by a CEI).When the served user wishes to re-establish communications
3、 with a held call, the retrieve function is requested. Thesuccess of the retrieve function depends upon whether or not a B-channel was reserved, and whether a B-channel iscurrently available to the served user.2.2.2 Specific terminologyFigure 2-1 below shows a model of the Functional Entities (FEs)
4、that may be involved in holding a call. The modelpresented is consistent with Stage 2 work that defines FEs and scenarios for information flows between FEs.2 Recommendation Q.953 (03/93)User A - - - - Service provider A Network B - - - - User B1) 2) 3) 4)The model in Figure 2-1 is used as follows:1)
5、 User A This is an ISDN TE that is connected to an ISDN service provider A. User A is a terminal wherea party invokes the Call Hold service.2) Service provider A Service provider A is the ISDN connection between user A and either an NT2 or apublic ISDN which is capable of supporting ISDN Call Hold s
6、upplementary service requests from user A.3) Network B The “network” connection for user B. Network B may or may not be an ISDN network.4) User B This is a terminal involved in a call with user A who is to be held. If user A initiated a call touser B, then the call must be in at least the Outgoing C
7、all Proceeding state (U3) before user A cansuccessfully invoke the Call Hold supplementary service.User B does not have to be in an ISDN network for user A to be able to invoke the ISDN Call Holdsupplementary service. This Recommendation describes the optional notification information that may bedel
8、ivered to user B when user B is also an ISDN TE.Service provider A may be the same as network B if user A and user B are both served by service provider A. The“networks” shown in Figure 2-1 may be either public or private networks or combinations thereof.The processing of operations of the Call Hold
9、 supplementary service is local between user A and service provider A.Network B and user B are relevant in the above model only for the optional notification to user B that the A-B call haseither been placed on hold or been retrieved. It is thus not necessary for either network B or user B to suppor
10、t the CallHold supplementary service: it is only necessary that service provider A support the Call Hold supplementary serviceand for user A to have subscribed to the service.2.2.3 Qualifications on the applicability to telecommunication servicesThe Call Hold supplementary service may be supplied wi
11、th the services as identified in 2.3.2.2.4 State definitionsThe call states, as defined in Recommendation Q.931, shall be utilized in the operation of Call Hold supplementaryservice, as appropriate.Additionally, the six auxiliary states for this service shall be utilized as identified in 6/Q.932 as
12、applicable.Table 2-1 shows the states which shall be used on the user side and network side for the HOLD supplementary service.These states are specified for the purpose of the protocol definition; the states need not be provided in animplementation.2.3 Operation requirements2.3.1 Provision/withdraw
13、al2.3.1.1 Network optionAs a network option, the Call Hold supplementary service shall be valid after transmitting the call proceeding message.FIGURE 2-1/Q.953Call hold modelRecommendation Q.953 (03/93) 3TABLE 2-1/Q.953States for the HOLD supplementary serviceFor the operation of the Call Hold suppl
14、ementary service, the network shall1) reserve a B-channel when the user subscribes to the reservation option;2) when a B-channel has been reserved, always keep a B-channel available fori) the retrieval of a held call;ii) use with an incoming call; oriii) use with an outgoing call, if the user has su
15、bscribed to the reservation option;3) reserve one B-channel for a user when there are one or more calls on hold; and4) optionally provide remote notification of call holds and retrievals by either of the notification options asdescribed in 2.5.2.1.2.3.1.2 Terminal subscriptionA user, as identified b
16、y a terminal, has the choice of the following option for the Call Hold supplementary service:1) Reservation option, meaning reservation of a B-channel when a call is placed on hold (yes/no).2.3.2 Requirements on the originating network sideNot applicable.2.3.3 Requirements at the terminating network
17、 sideNot applicable.2.4 Coding requirementsThe hold and retrieve family of messages that will be used for the invocation and control of the Call Hold supplementaryservice, as defined in 7.1/Q.932, follows:1) HOLD2) HOLD ACKNOWLEDGE3) HOLD REJECT4) RETRIEVE5) RETRIEVE ACKNOWLEDGE6) RETRIEVE REJECTThe
18、 codepoints of the notification information element, as defined in Recommendation Q.932, shall be utilized in theCall Hold supplementary service operation, as appropriate.User A statesHOLD Idle The HOLD supplementary service has not been activated.HOLD Hold Requested The call hold part of the HOLD s
19、upplementary service has beenrequested by the user.HOLD Call Held The HOLD supplementary service has been activated.HOLD Retrieve Requested The call retrieve part of the HOLD supplementary service hasbeen requested by the user.Network statesHOLD Idle The HOLD supplementary service has not been activ
20、ated.HOLD Call Held The HOLD supplementary service has been activated.4 Recommendation Q.953 (03/93)2.5 Signalling requirements2.5.1 Activation/deactivation/registrationActivation of the Call Hold supplementary service shall be by subscription. Deactivation of the service shall be bywithdrawal.2.5.2
21、 Invocation and operation2.5.2.1 Procedures at the originating interfaceThe originating local exchange is the exchange where the service is controlled. The originating local exchange for thisdescription is not necessarily the originating exchange for the basic call.2.5.2.1.1 Call Hold normal operati
22、onA call may be placed on hold by the calling user at any time after the call has been answered or additionally as a serviceprovider option:1) after alerting has commenced; or2) after the calling user has provided all of the information necessary for processing the call.The calling user may invoke t
23、he Call Hold supplementary service in states U3 (after receiving CALL PROCEEDING),U4 (after receiving ALERTING), or U10 (after receiving CONNECT).User A initiates the Call Hold supplementary service by requesting the hold function according to 6.2.2/Q.932 utilizingthe call reference indicating the c
24、all for which the Call Hold supplementary service applies.Upon receipt of the HOLD message, service provider A shall check whether:1) user A is in a valid call state; and2) the call requested to be held is a circuit mode call; and3) user A is a valid subscriber of the Call Hold supplementary service
25、.The order in which these checks are performed is not specified.When the B-channel reservation user subscription option applies, the reservation function defined in 6.4/Q.932 shall beused.When the call is held before alerting, the notification shall be delayed by network A, at least until the indica
26、tion ofalerting has been received from the remote user.If service provider A determines that access to the requested hold service is authorized and available, service provider Ashall acknowledge the hold function according to 6.2/Q.932.When a call is held before answer, network A has the option of s
27、ending the notification towards network B immediatelyor delaying the sending of the notification towards network B until after answer. When a call is held before answer andnetwork A sends the notification towards network B, network B has the option of delivering the notification to terminalsthat hav
28、e responded with ALERTING or delivering the notification after answer. If user B is on an ISDN interface, thenotification shall be delivered to user Bs interface as specified by the notification procedures in 9.2/Q.932, containing aNotification indicator information element indicating “Remote HOLD”.
29、2.5.2.1.1.1 Holding a call before the A-B call is active2.5.2.1.1.1.1 Holding a call after receiving a CALL PROCEEDING message (by calling user A) (U3)It is assumed that user A has initiated a call to a user B; has completed dialling (i.e. the provision of call informationusing either enblock or ove
30、rlap sending methods); has received a CALL PROCEEDING message from service providerA and has entered the Outgoing Call Proceeding state (U3).User A may send a HOLD message to service provider A indicating the call reference value of the call to user B. Aftersending this message, user A enters the Ho
31、ld Request auxiliary state.Recommendation Q.953 (03/93) 5If all conditions for holding a call are met, service provider A shall return to user A a HOLD ACKNOWLEDGEmessage indicating the call reference value of the call to user B. After transmitting the message, service provider A shallensure that th
32、e B-channel is deactivated (the B-channel may have been activated when service provider A has deliveredthe CALL PROCEEDING message and indicated interworking in a Progress indicator information element). Serviceprovider A may provide hold notification as described in 2.5.2.1.1.2.5.2.1.1.1.2 Holding
33、a call after receiving an ALERTING messageIt is assumed that user A has initiated a call to user B; has completed dialling; and has either received a CALLPROCEEDING message followed by an ALERTING message, or the first message returned to user A by serviceprovider A was an ALERTING message. User A h
34、as entered the Call Delivered state (U4).User A may send a HOLD message to service provider A indicating the call reference value of the call to user B. Aftersending this message, user A enters the Hold Request auxiliary state.Service provider A shall accept this hold request if all conditions for h
35、olding a call are met. Service provider A shallreturn to user A a HOLD ACKNOWLEDGE message again indicating the call reference value of the call to user B.After transmitting the message, service provider A shall ensure that the B-channel is deactivated (the B-channel mayhave been activated when serv
36、ice provider A delivered a CALL PROCEEDING or the ALERTING message andindicated interworking in a Progress indicator information element), and shall enter the Call Held auxiliary state.Service provider A may provide hold notification as described in 2.5.2.1.1.Upon receiving the HOLD ACKNOWLEDGE mess
37、age from service provider A, user A shall enter the Call Heldauxiliary state and, if previously connected shall disconnect from the B-channel allocated to the A-B call.2.5.2.1.1.2 Holding a call once the A-B call is activeUser A may send a HOLD message to service provider A indicating the call refer
38、ence value of the call to user B. Aftersending this message, user A enters the Hold Request auxiliary state.Service provider A shall accept this hold request if all conditions for holding a call are met. Service provider A shallreturn to user A a HOLD ACKNOWLEDGE message indicating the call referenc
39、e value of the call to user B. Aftertransmitting the message, service provider A shall ensure that the B-channel is deactivated, and shall enter the Call Holdauxiliary state.Service provider A may provide hold notification as described in 2.5.2.1.1.2.5.2.1.1.3 Clearing a held callA held call may be
40、cleared by either user A, user B, or by service provider A (when the A-B call is not completed to auser B). The held A-B call may be in the Outgoing Call Proceeding (U3), Call Delivered (U4), or Active (U10) statewhen the call is cleared. When the A-B call is cleared, service provider A shall contin
41、ue to reserve a channel for user Auntil there are no more held calls with reservation associated with user A.2.5.2.1.1.3.1 Clearing a held call by user AThe held call may be cleared by user A by generally following the procedures of 5.3.3/Q.931. User A may send aDISCONNECT message to service provide
42、r A, start timer T305 and enter the Disconnect Request state (the call remainsin the Call Held auxiliary state).Upon receipt of the RELEASE message from service provider A, user A shall cancel timer T305, send a RELEASECOMPLETE message, release the call reference, return to the Null state and enter
43、the HOLD Idle auxiliary state. TheB-channel shall not be released because it was deactivated when the Call Held auxiliary state was entered.2.5.2.1.1.3.2 Clearing a held call by service provider AThe held call may be cleared by service provider A as defined by procedures in 6.2/Q.932 and 5.3.4/Q.931
44、.Service provider A may initiate clearing of a held call by sending the RELEASE message as defined in 5.3/Q.931.6 Recommendation Q.953 (03/93)2.5.2.1.1.4 Operations available with call(s) on holdOnce user A places a call on hold, the operations that user A may perform are affected by whether or not
45、channelreservation was in effect.Call(s) held with channel reservationAssume that user A subscribes to the Call Hold supplementary service with channel reservation, and places one or morecalls on hold. A B-channel will be reserved for user A as long as there is at least one call on hold. When all re
46、servationsare cleared, all channels become available for use by either the service provider or any terminal at user As interface.The reserved B-channel may be used by user A to make an outgoing call; accept an incoming call; or retrieve a held call.The notification of an incoming call (i.e. the deli
47、very of a SETUP message) is affected by whether or not any calls areheld, for when there is a held call, there are channels that may be “inactive, but not free”. This affects the operation ofthe Call Waiting supplementary service (see 2.6.1).Note that all of the above operations may be performed at
48、any time after a call has been placed on hold. This includesholding a call before it has reached the Active state.Calls held without channel reservationWhen user A does not have channel reservation, user A may use an available B-channel to make an outgoing call; accept an incoming call; or retrieve
49、a held call.Since no B-channel has been reserved for user A, access to a B-channel to perform any of the above operations would bethe same as when no calls are held.The notification of an incoming call (i.e. the delivery of the SETUP message), is not affected by whether or not any callsare held since in this case there are no channels that are “inactive, but not free”.Assume that user A subscribes to the Call Hold supplementary service without channel reservation, and places more thanone call on hold. User A may attempt to retrieve a held call on a selected B-c
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