11.3.1 Call establishment

3GPP43.068Release 17Stage 2TSVoice Group Call Service (VGCS)

A voice group call can be established by either a service subscriber or by a dispatcher.

11.3.1.1 Service subscriber call establishment

11.3.1.1.1 Initial stage

The initial signalling from the calling service subscriber informs the network that a voice group call is required and details the group ID; it may specify originator-to-dispatcher information. No information relative to the group call area is given by the calling service subscriber. As an option the calling service subscriber may add a prefix to the group ID, which is taken into acount by the network when selecting the area.

The network shall perform a number of checks in order to determine how to handle the call:

– check of the ability of the subscriber to establish the call;

– check whether the call can be initiated from the cell;

– check of the existence of an on-going call of the same group call reference.

The originating MSC shall check the VLR records for the ability of the subscriber to start the call. If the service subscriber has no subscription for the voice group call service with the indicated group ID, the call shall be released. In addition, the VLR shall return barring and identity presentation restriction checks to the MSC.

In a RANflex configuration the originating MSC shall then derive the group call serving MSC address from the originating location area. If the group call serving MSC is different from the originating MSC (or in a RANflex configuration with group call redundancy if the originating MSC does not belong to the group call serving MSC redundancy pool) then the group call serving MSC is interrogated by means of the MAP service SEND_GROUP_CALL_INFO. The interrogation request contains the originating Cell Id and the Group Id. It also contains the initiating service subscriber’s IMSI, the actual talker priority, and subscribed additional information.
If the group call serving MSC is a relay MSC for the group call, then its GCR shall derive the anchor MSC address and the group call reference from the originating Cell Id and Group Id, and return it to the VMSC. If the group call serving MSC is the anchor MSC for the group call, then its GCR shall derive the group call reference from the originating Cell Id and Group Id, and return it to the VMSC.

Without RANflex configuration or if in a RANflex configuration with or without group call redundancy the originating MSC is the group call serving MSC (derived from the originating location area) it shall then request information from the GCR by giving the group ID and the originating cell ID as defined in subclause 9.2.

If the group ID has a length of 8 decimal digits, the operator can define for this group ID only one group call and one group call area within his network. If the length of the group ID is less than 8 decimal digits, the operator can define more than one group call using the same group ID, but different group call areas. Because of the possibility of overlapping group call areas, each call requires a unique reference, assigned by the GCR related to the originating MSC or the group call serving MSC (in a RANflex configuration with or without group call redundancy). The group call reference shall be composed of the group ID and a group call area ID (see clause 9).

If the length of the group ID is less than 8 decimal digits, the GCR first derives the group call area ID from the group ID and the originating cell ID. If no group call area ID is related to the group ID and originating cell ID, the call shall be released. If a group call area ID is related to the group ID and originating cell ID, the GCR shall transfer the corresponding group call attributes to the MSC.

In a network supporting prefixes together with group IDs, the operator can define more than one group call area for a specific combination of group ID and originating cell ID. In such a network, if the calling service subscriber has added a prefix to the group ID, the GCR shall derive the group call area ID from the prefix. The last digit of the group call area ID shall match the prefix. If the prefix is not provided or does not match the last digit of the group call area IDs to be considered, the network shall select a default value for the prefix and derive the group call area ID from this default prefix. The default value for the prefix is dependent on the network of the originating MSC.

NOTE 1: Typically, the group call area ID derived from the default prefix value will select a complete group call membership, not a subset, and the group call area will include the group call areas derived from the other prefix values.

NOTE 2: A mobile station not supporting the use of prefixes with group IDs can only set up voice group calls to the group defined by the default prefix.

If the group ID has a length of 8 decimal digits, the group call reference is equal to the group ID. The GCR shall check whether the call was initiated within the group call area stored in the GCR. If not, the call shall be released; otherwise, the GCR shall transfer the corresponding group call attributes to the MSC.

When the GCR has transfered the group call attributes to the MSC, the call shall be considered as on-going by the GCR, unless the GCR’s associated MSC acts as group call relay MSC in a RANflex configuration with or without group call redundancy.

The GCR of the relay MSC and in a RANflex configuration with or without group call redundancy, if the group call serving MSC of the initiating service subscriber’s current LAC is different from the VMSC, the GCR of this group call serving MSC shall store the initiating service subscriber’s IMSI, talker priority, additional information and originating cell id for later processing.

If the originating MSC is not the group call anchor MSC for the voice group call as indicated in the GCR, then the voice group call request shall be passed to the group call anchor MSC; in that case, if the initiation of the voice group call had specified originator-to-dispatcher information and processing of originator-to-dispatcher information is supported by the MSC, the originator-to-dispatcher information is transformed by the originating MSC into UUS1 and sent to the anchor MSC.

When passing the call request to the anchor MSC the originating MSC shall include the VGCS prefix plus group call reference in the calling party number of the IAM. If in a RANflex configuration the originating MSC is not the group call serving MSC, it shall in addition include the address of the group call serving MSC in the generic number parameter of the IAM, with the number qualifier indicator set to "additional calling party number". Without RANflex configuration or if in a RANflex configuration with or without group call redundancy the originating MSC is the group call serving MSC, it shall additionally include the address of the originating relay MSC in the generic number parameter of the IAM, with the number qualifier indicator set to "additional calling party number", when passing the call request to the anchor MSC.

If the group call reference is composed of Group ID and group call area ID, it is possible that two service subscribers or a service subscriber and a dispatcher or two dispatchers may attempt to establish a call using the same group ID and corresponding to the same group call area ID. If the two voice group calls are established with the same group ID but for different group call areas then separate voice group calls shall be established. If the group call areas overlap, it is up to receiving mobile station to determine which call to participate in. If more than one call is made to identical group ID and group call area, the network shall reject all but one of the call attempts.

If the group ID has a length of 8 decimal digits, if more than one call attempt is made to the same group ID, the network shall reject all but one of the call attempts.

A service subscriber which is entitled by his subscription to establish voice group calls while roaming shall only be able to use supra-PLMN group IDs as defined in subclause 9.1 in case of roaming. In case of roaming, the mobile station shall only react on notifications for supra-PLMN group IDs.

If the GCR receives a new interrogation related to a group call reference where the call is indicated as on-going in the GCR, the GCR shall provide the on-going status together with the group call reference and – in a RANflex configuration with group call redundancy – together with the address of the MSC within the redundancy pool where the group call is ongoing back to the MSC. The MSC shall then

a) in a configuration without RANflex;

b) in a RANflex configuration; and

c) in a RANflex configuration with group call redundancy, if the group call is ongoing at the requesting MSC:

release the call with a cause user busy in case of a service subscriber originated call request. The mobile station of the service subscriber shall then look for notifications of the respective group ID on the NCH and join the voice group call.

In case of a dispatcher originated voice group call request, the MSC shall join the dispatcher to the conference bridge of the voice group call, or

d) in a RANflex configuration with group call redundancy, if the group call is ongoing at another MSCwithin the redundanc pool:

forward the original request to the MSC within the redundancy pool where the group call is ongoing.

Authentication of the calling service subscriber can be performed by the network as for normal calls.

11.3.1.1.2 Establishment of the transmission means

A voice group call channel shall be established in all the cells throughout the identified group call area using physical channels selected by the BSCs as appropriate. The downlink channels shall be established without any return signalling from mobile stations. Whilst the downlink channel is being established, the MSC shall form a conference bridge containing the appropriate channels to all BTSs in the group call area or in case of A-interface link sharing to all BSCs in the group call area. The MSC is responsible for adding dispatchers to the conference bridge.

Alternatively, the network may establish voice group call channels in a cell on demand, i.e. if mobile stations respond to the notifications as defined in subclause 4.2.2.1 (see "uplink reply procedure"). As a network option the applicability of the uplink reply procedure for a voice group call can be controlled by the GCR of the anchor MSC. If the anchor MSC supports this feature, it shall allow all affected BSCs to establish voice group call channels on demand only if the GCR indicates that the uplink reply procedure is applicable. If the relay MSC supports this feature, it shall allow all affected BSCs to establish voice group call channels on demand only if the anchor MSC indicates during the establishment of the voice group call that the uplink reply procedure is applicable.

In parallel, a dedicated suitable traffic channel may be allocated to the calling service subscriber if not already the case.

If the "talker channel parameter" is used and indicates that the network shall always establish and maintain a dedicated channel for the talking service subscriber, a dedicated suitable traffic channel shall be allocated to the calling service subscriber. If there are not sufficient radio resources in the originating cell to allocate a dedicated channel and a group call channel, the group call establishment fails and the voice group shall be released.

The call will be considered established provided that at least the downlink channel in the originating cell, in the case of a service subscriber originated voice group call, or the downlink channel in any one cell within the group call area, in the case of a dispatcher originated voice group call, is established. The MSC shall signal to the calling service subscriber that this has occurred so that he knows when to start speaking. If a voice group call does not meet the above conditions in a pre-set time (Txx) then the call shall be released.

The mobile station shall indicate connection to the subscriber. If channels could not be established in particular cells because of congestion, channels are allocated to these cells as soon as possible.

The MSC may retry the VGCS Assignment procedure to establish channels in cells where they are missing. If supported, the procedure may be initiated when:

i) congestion (i.e. a lack of A-interface circuits) prevented the VGCS ASSIGNMENT REQUEST message from being sent to the BSS;

ii) a VGCS ASSIGNMENT FAILURE message is received and the cause value indicates an acceptable reason for retry (e.g. no radio resource available);

iii) the radio and terrestrial resources for the group call channel are cleared (group call on-going) and the cause value indicates an acceptable reason for retry (e.g. pre-emption);

iv) no response to the VGCS ASSIGNMENT REQUEST message is received (this is determined by the MSC in an implementation-dependent manner such as expiry of a timer); or

v) no response is received following receipt of a VGCS QUEUING INDICATION (this is determined by the MSC in an implementation-dependent manner such as expiry of a timer).

NOTE: If A-interface link sharing or group call re-establishment by the BSS apply, the BSC is responsible for establishing/re-establishing channels in cells and scenario (ii) and (iii) do not apply, unless A-interface resources also need to be re-established (e.g. when the PCM system serving the A-interface circuit fails (equipment failure)). Scenario (v) does not apply to A-interface link sharing and group call re-establishment by the BSS.

The MSC may repeat the VGCS assignment procedure until a VGCS ASSIGNMENT RESULT message is received, the call is released or an unacceptable reason for retry (e.g. protocol error between BSS and MSC) is received by the MSC. The time between subsequent retries is implementation-dependent. The MSC shall send each retry of the VGCS ASSIGNMENT REQUEST over a new resource-controlling SCCP connection.

If a message is received with an unacceptable reason for retry then existing procedures apply. I.e. the MSC shall initiate the clearing of the radio and terrestrial resources, if necessary, and no further attempts to establish a group call channel in the cell are made.

While using the retry procedure the MSC shall maintain the call controlling SCCP connection to a BSS until the call is released or until the MSC decides to stop any further attempts to establish group call channels for the call on a BSS where no cells are established.

11.3.1.1.3 Release of the dedicated transmission means of the calling service subscriber

The calling service subscriber shall be given a dedicated connection up to the time where the network requests him to join the voice group call channel. If the calling service subscriber is not talking, the network requests him to join the voice group call channel as a listener by use of a channel release procedure. Otherwise, the network shall request him to join the voice group call channel as a talker by either a channel assignment procedure or a handover procedure or a channel mode modify procedure.

If the "talker channel parameter" is used and indicates that the network shall always establish and maintain a dedicated channel for the talking service subscriber, then the network shall not request the calling service subscriber to join the group call channel as long as he is talking.

For the time when the voice group call is established until the calling service subscriber becomes a listening service subscriber for the first time, the "uplink busy" flag is set (see subclause 11.3.8). Mobile stations shall be programmed such that if they originate the call, they indicate to the user that it is required that an indication of the desire to speak should be made if he wants to speak. If this is not done within a certain time, the mobile station sends an UPLINK_REL message to the network and the uplink shall become free.

11.3.1.1.4 Release of the dedicated transmission means of mobile stations responding to a notification

Mobile stations which respond to a notification for which no description of the voice group call channel was given in the notification message may be given a dedicated connection up to the time where the network requests the mobile station to join the voice group call channel. If the service subscriber is not talking, the network requests him to join the voice group call channel as a listener by use of a channel release procedure. Otherwise, the network shall request him to join the voice group call channel as a talker by either a channel assignment procedure or a handover procedure or a channel mode modify procedure.

11.3.1.1.5 void

11.3.1.1.6 void

11.3.1.2 Dispatcher call establishment

In the case of dispatchers originated calls the call request, in the form of an MSISDN number, shall be received at a GMSC. Such a call can be treated by the GMSC as a normal mobile terminated call. In this case, the GMSC shall interrogate an HLR, determined on the basis of the MSISDN number. The HLR in turn may either interrogate the appropriate MSC/VLR to obtain an MSRN, or may supply an MSRN predefined in the HLR and related to the respective group call reference in the MSC/VLR. If the HLR interrogates the MSC/VLR for the MSRN, the HLR shall provide this MSC/VLR with the related IMSI including the group call reference as defined in clause 9.

Alternatively, the call request can be forwarded directly to the related group call anchor MSC on basis of the GMSC’s internal routing table. In this case, the group call reference shall already be included in the requested MSISDN number as defined in clause 9.

When interrogated by the group call anchor MSC, the GCR shall check if the calling line identity is within the list of dispatcher identities allowed to establish the voice group call. If not the case, the call shall be rejected.

After reception of the call request, the group call anchor MSC checks whether an on-going call of the same group call reference exists, in which case for the following configurations

a) for configurations with or without RANflex;

b) for RANflex configurations; and

c) for RANflex configurations with group call redundancy where the group call is ongoing at the call request receiving MSC:

the group call anchor MSC shall add the dispatcher to the call, and for the following configuration

d) for RANflex configurations with group call redundancy where the group call is ongoing at another MSC within the redundancy pool:

the call request receiving MSC shall forward the call request to the MSC where the group call is ongoing.

At the point at which notification messages are sent to mobile stations, a tone is relayed to the calling dispatcher to inform the dispatcher that the message can commence.

11.3.1.3 Notification procedures

Different notification procedures shall be applied in relation to the mode of the mobile station as presented in Table 1 and Table 2 and defined in the following sections.

Table 1: Overview on different information messages for new or on-going calls

Incoming call type:	VBS or VGCS call	point-to-point call
MS states:
Idle mode	(section a)	(standard paging)
Group mode, dedicated channel	(section b)	(section c)
group receive mode and
group transmit mode	(section b)	(section c)
dedicated mode	(section b)	(standard Call Waiting) (note)
NOTE: only for point to point calls with certain restrictions as defined in 3GPP TS 22.083.

Table 2: Overview on different information messages for incoming point-to-point short messages

MS states:	Incoming point-to-point short messages
Idle mode	(standard paging)
group mode, dedicated channel	(standard SMS delivery) (note)
group receive mode	(section c)
group transmit mode	(standard SMS delivery or section c) (note)
dedicated mode	(standard SMS delivery) (note)
NOTE: see subclause 11.3.9.2.

a) Notification for mobile stations in idle mode

Once the voice group call channel has been established in a cell or the network is waiting to receive notification responses to establish a voice group call channel, notifications shall be broadcast on the NCH in that cell.

The position of the NCH is derived from the system information of the BCCH.

The notification messages shall include the group call reference and possibly the description of the voice group call channel, the call priority if eMLPP is applied, the group cipher key number, and the emergency mode indication, if applicable.

A notification message can contain no, one or more notifications.

The notification process needs to continue throughout the duration of the group call, in order to permit the "late entry" of other mobile stations. Mobile stations moving into the group call area which are in idle mode shall be directed to the voice group call channel by the notification messages, possibly by means of the notification response procedure.

The scheduling of the notification messages in a cell shall be managed by the BSS. Information can be added in the messages to limit the required reception of NCH messages. The following constraints shall be met:

– the three first initial notifications (i.e. the first for a given group call) shall have priority over subsequent notifications (i.e. the messages for an on-going group call) and must be sent as soon as possible;

NOTE 1: In addition initial notification messages for calls with or above an operator defined priority level can be sent on all possible paging or access grant channels to reduce the delay for those mobile stations which are not using Discontinuous reception (DRX).

– afterwards, an on-going group call in the cell shall be periodically notified on the NCH.

Since the information for the establishment of a voice group call is sent onto the NCH rather than on the PCH as for normal point-to-point calls, the mobile station must listen to the PCH as well as to the NCH. A "reduced NCH monitoring" mechanism can be used to save power in the mobile station when listening to the NCH.

A mobile station able to receive voice group calls either, depending on the implementation:

– can use the "reduced NCH monitoring" mechanism. When entering a cell, the mobile station shall listen to the NCH to get the notifications of the voice group calls on-going in the cell. Afterwards, the mobile station needs to listen to the NCH only if it is informed on the availability of a notification for a new voice group call. This shall be based on the NCH status information provided, as indicated in 3GPP TS 44.018;

– do not apply the "reduced NCH monitoring" mechanism and read all possible paging or access grant channels.

b) Notifications for mobile stations in group mode dedicated channel, group receive, group transmit or dedicated mode

In addition to sending initial notification messages on the NCH for the voice group call, the BSS can provide initial notification into on-going voice broadcast, group calls and point to point calls informing mobile stations partaking in these calls of new voice group calls that are being set-up in the cell.

NOTE 2: The additional notification into on-going voice broadcast and group calls and point to point calls should be provided by the BSS if the priority level of the new call is equal or higher than the O&M defined priority level.

In order to do this the BSS sends initial notification messages on FACCH to all other ongoing voice broadcast, group calls, and point to point calls in the cell. The initial notification message on FACCH shall contain the group call reference, the priority level if eMLPP applies, possibly the TCH description which allows the mobile station to connect directly to the new call without reading the NCH, and the emergency mode indication, if applicable.

An indication of change of notifications in the current cell may be provided on SACCH by the BSS.

When the emergency mode is set or reset for a voice group call ongoing in a cell, the BSS shall send additional notifications on FACCH to all on-going voice broadcast, group calls, and point-to-point calls in the respective cell.

As a mobile station option, the mobile station may read the NCH of the current cell while in group mode dedicated channel, group receive, group transmit or dedicated mode in order to be notified on other voice group calls.

NOTE 3: Mobile stations may require an additional receiver to read the NCH in order to ensure a higher probability of receiving notifications for all present voice group calls without degradation of the received speech quality.

Late entry of mobile stations into ongoing high priority group calls and voice group calls in emergency mode is covered by the following mechanisms:

– Late entrance in dedicated mode

If a mobile station in dedicated mode is moving into an area where a group call (VGCS or VBS) with priority level equal or higher to an operator specific setting or a voice group call in emergency mode is ongoing, the BSS shall resend the notification message to the mobile station on FACCH, if the mobile station has ASCI capabilities. This notification shall be triggered by completion of the dedicated channel assignment.

Sending periodical notification on FACCH to the mobile station in dedicated mode is optional, and is done as long as the group call (VGCS or VBS) with priority level equal or higher to an operator specific setting, is ongoing or as long as the emergency mode is set for the voice group call, with a repetition period given by an operator specific setting.

– Late entrance in group receive or group transmit mode

When a group call (VGCS or VBS) with priority level equal or higher to an operator specific setting, is established, or when the emergency mode is set for a voice group call, the BSS shall send periodical notification on FACCH to all ongoing voice broadcast and group calls in the cell, except on the FACCH of the group call (VGCS or VBS) which has initiated this periodical notification. By this method the mobile station in group receive or group transmit mode moving into this cell is notified. Periodical notification on FACCH is done as long as the group call (VGCS or VBS) with priority level equal or higher to an operator specific setting, is ongoing, or as long as the emergency mode is set for the voice group call, with a repetition period given by an operator specific setting.

NOTE 3a: The operator determined Periodical FACCH notification period shall be a BSS specific operator setting and be a minimum of 1s and maximum of 5s.

c) Paging into on-going voice group calls

Paging into on-going voice group calls shall be provided as an implementation option.

In addition to establishing the links for the voice group call, the network can provide paging information into on-going voice group calls informing mobile stations partaking in a voice group call of new incoming point-to-point calls. The network can also provide paging information into ongoing voice group calls informing mobile stations in group receive mode or group transmit mode of incoming point-to-point short messages.

The mobile station shall be ready to receive a paging message on the FACCH containing the mobile subscriber identity and the priority level if eMLPP applies.

In the event of a reorganisation of the PCH the BSS shall inform the mobile stations via the SACCH that paging reorganisation has occurred. A mobile station receiving this indication shall decode the BCCH in order to obtain the new paging configuration.

As a mobile station option, the mobile station may read its paging subchannel in the current cell in group receive mode or group transmit mode in order to receive paging messages.

NOTE 4: Mobile stations may require an additional receiver to read its PCH subchannel in order to ensure a higher probability of receiving all relevant paging messages without degradation of the received speech quality. The additional receiver may be the same as used for reception of the NCH described under b) above.

11.3.1.4 Destination service subscribers

Mobile stations of destination service subscribers which are in idle mode shall listen to notification messages on the NCH and move to the voice group call channel or respond to the notification.

Mobile stations which are busy shall either pre-empt the current call (if eMLPP is applied and the new call is of a sufficient priority), or shall provide the service subscriber with an indication similar to call waiting, when applicable.

If the mobile station supports group IDs with prefixes and has stored prefixes for the group ID in the notification, the mobile station shall only react as described previously in this subclause if the last digit of the group call area ID included in the group call reference matches the default prefix or one of the prefixes stored for the group ID. If the last digit of the group call area ID does not match then the mobile station shall not react as described previously in this subclause, but may still react by making an acknowledgement as defined in subclause 4.2.5.

NOTE: In a network supporting the use of prefixes together with the group ID, a mobile station not supporting the use of prefixes will react to notification messages and can participate in any voice group call for a group ID for which it has a subscription, regardless of any prefix used for setting up the voice group call.

11.3.1.5 Destination dispatchers

Destination dispatchers are connected into the voice group call as a standard point-to-point call. The notification of the identity of the received group call shall be supplied in the Calling Line Identity, formatted according to sub-clause 9.2.