7 Transmission

3GPP43.069Release 17Stage 2TSVoice Broadcast Service (VBS)

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

7.1 Transmission architecture

A distribution function, is required to distribute the voice broadcast call to the nominated cells and dispatchers, respectively. The distribution function is to be located within the group call anchor MSC.

Without a RANflex configuration: The group call anchor MSC is responsible for setting up all connections, both to the nominated cells (voice broadcast channels) in the group call anchor MSC and to any related group call relay MSC, and to the dispatchers. There shall be one common link for all cells within the group call relay MSC which is involved in the voice broadcast call, i.e. there shall be a secondary distribution function in the group call relay MSCs.

In a RANflex configuration: The group call anchor MSC is responsible for setting up all connections, both to those cells of the group call area that belong to a location area for which the group call anchor MSC is the group call serving MSC and to any related group call relay MSC, and to the dispatchers. Except when a calling service subscriber, served by a relay MSC, is on the initial dedicated link, there shall be one link towards every relay MSC and a distribution function in the relay MSCs and from there one link per cell of the group call area that belongs to a location area for which the group call relay MSC is the group call serving MSC which is involved in the voice broadcast call.

In a RANflex configuration with group call redundancy: The group call anchor MSC selected from the anchor MSC redundancy pool is responsible for setting up all connections, both to those cells of the group call area that belong to a location area for which the group call anchor MSC is the group call serving MSC and to any related group call relay MSC pool, and to the dispatchers. Except when a calling service subscriber, served by a relay MSC, is on the initial dedicated link, there shall be one link towards every relay MSC redundancy pool and a distribution function in the selected relay MSCs and from there one link per cell of the group call area that belongs to a location area for which the group call relay MSC is the group call serving MSC which is involved in the voice broadcast call.

7.1a Transmission architecture – A interface circuit sharing

7.1a.1 Transmission architecture – general

The MSC and BSC shall negotiate during the setup of a voice broadcast call whether A-interface circuit sharing is supported by both entities. When this optional feature is supported by both entities, the same A-interface circuit can be shared for all cells belonging to a BSC for a given voice broadcast call. When AoIP is used, the same AoIP transport layer information can be shared for all cells belonging to a BSC for a given voice broadcast call.

7.1a.2 Transmission architecture – control plane

The control plane signalling shall be the same as in sub-clause 7.1.

7.1a.3 Transmission architecture – user plane

The following distribution functions are required to distribute the voice broadcast call to the nominated cells and dispatchers, respectively:

– a distribution function within the group call anchor MSC. There will be one link from the group call anchor MSC to each of the relay MSCs in the voice broadcast call. Also, there will be one link from each of these relay MSCs and the group call anchor MSC to each BSC controlled by the respective MSC and involved in the voice broadcast call;

– a distribution function within the BSC to distribute the speech to each of the nominated cells.

7.1b Transmission architecture – A interface link sharing

7.1b.1 Transmission architecture – general

The MSC and BSC shall negotiate during the setup of a voice broadcast call whether A-interface link sharing is supported by both entities. When this optional feature is supported by both entities, the same A-interface link (user and control plane) can be shared for all cells belonging to a BSC for a given voice broadcast call.

A conference bridge is required to connect the transmission paths of the nominated cells. The bridge is to be located within the group call anchor MSC. The group call anchor MSC is responsible for setting up a single A-interface link (i.e. either A-interface circuit or A interface IP address and UDP port value in case of AoIP and resource controlling SCCP connection) to each BSC containing nominated cells in the group call anchor MSC and links to any group call relay MSCs and to the dispatchers.

Each BSC is responsible for setting up a speech and signalling connection (voice broadcast call channel) to each nominated cell in the group call area served by this BSC. The BSC contains a distribution function that distributes speech and resource control related signalling from the MSC to each of the nominated cells.

7.1b.2 Transmission architecture – control plane

The MSC shall inform the BSC of the cells required to be setup for the broadcast call in the VBS ASSIGMENT REQUEST. This message shall contain the list of group call area cells served by this BSC. If the entire list of cell identifiers does not fit into the message or if different methods of radio resource allocation (e.g. “immediate allocation” or “delay allowed”) are to be used for different cells controlled by the BSC, the MSC shall send one or more VBS AREA CELL INFO messages containing the remaining cell identifiers or the cell identifiers of cells using a different method of radio resource allocation. Once a channel could be established to the cell of origin or, if the cell of origin is not served by this BSC, to any other cell, the VBS ASSIGMENT RESULT shall be sent to the MSC and timer Tast shall be started. Timer Tast is used to measure the duration between periodic reports from the BSC to the MSC of group call area cells for which channels have been assigned or released since the last periodic report. When timer Tast expires, if new cells in the group call area have been established or existing ones have been released, pre-empted or failed, the MSC shall be informed of the changes. If no changes have taken place nothing shall be sent. Timer Tast shall be started again to measure the period of time until the next report. The timer shall be stopped when the call is released.

Once all cells for a given group call area served by a BSC are established this BSC shall immediately send a VBS ASSIGNMENT STATUS message to the MSC indicating this and restart timer Tast. This information shall be used by the MSC to determine the conditions for call set up as described in subclause 11.3.1.1.2.

The BSS is responsible to establish the channel to the different cells and manage the signalling accordingly (e.g. HO decisions, pre-emption, re-establishment of cells). The A-interface link (user and control plane) between MSC and BSC shall only be released when the call is released.

7.1b.3 Transmission architecture – user plane

The transmission architecture of the user plane as specified in subclause 7.1a.3 applies.

7.2 Radio channels

In each cell of the group call area one voice broadcast channel may be established consisting of a downlink received by all service subscribers’ mobile stations and also an uplink which shall be used by the listening subscriber’s mobile stations for the uplink reply procedure only.

The calling service subscriber’s mobile station shall use a dedicated standard uplink/downlink which is connected as input to the distribution function.

A listening subscriber’s mobile station which responds to a notification because no description of the voice broadcast channel was provided in the notification may be assigned a dedicated standard link up to the instant where the radio access network decides that the mobile station shall join the voice broadcast channel and the dedicated connection is released.

Voice broadcast channels shall be standard full rate or half rate speech channels, EFR speech channels, full rate AMR speech or half rate AMR speech channels. The support of voice broadcast channels other than full rate speech is a network option. A specific voice broadcast call can use either the same speech codec type in all cells of the group call area or different speech codec types in different cells of the group call area. Those implementations are optional for the network operator.

When establishing an AMR half rate or AMR full rate speech channel, the BSC shall select a suitable AMR codec configuration:

– for the dedicated channel used by the calling service subscriber, the BSC may select any configuration permitted for a point-to-point call;

– for a voice broadcast channel the BSC shall select one of the preferred configurations as defined in 3GPP TS 28.062 [16], Table 7.11.3.1.3-2. The BSC shall disable the rate adaptation mechanism and apply a single codec mode for the downlink until the channel is released.

Within a cell the BSC shall select the same codec configuration for all voice group and voice broadcast calls using the same AMR codec type, AMR FR or AMR HR, respectively. The selected configuration shall be broadcast on the NCH, as long as at least one voice group or voice broadcast call using the respective AMR codec type is active.

When A-interface circuit sharing or A-interface link sharing applies there is one A-interface circuit allocated for the broadcast call per BSC. In case of AoIP, there is one A-interface IP and port allocated for the broadcast call per BSC. Therefore the same speech codec is applied for all voice broadcast channels in the part of the group call area served by one BSC, if the TRAU is locatedbetween the MSC and the BSC-internal distribution function for speech (see subclause 7.1a.1 and 7.1b.1).

Full standard duplex channels shall be provided to all dispatchers listed in the GCR as for normal calls and connected to the distribution function although their speech shall not be added to the speech of the calling subscriber in the distribution function if they are destination subscribers. The links may be provided either via GSM, or via an external network.

Downlink and uplink radio channels are to be provided to all destination service subscribers, with one common downlink and one common uplink per cell. The uplink channel is used for the uplink reply procedure for VGCS and VBS (see subclause 4.2.2.1).

A separate standard duplex channel is to be provided to the calling service subscriber.

7.3 Data confidentiality

Data confidentiality on the radio link can be provided as a network option.

If data confidentiality is provided, the downlink of the voice broadcast channel within cell of the group call area shall be ciphered using broadcast group ciphering keys derived from the same group key, see 3GPP TS 43.020 [4].

The group key is related to the group ID. For each group ID, there is a number of group keys stored on the SIM which are identified by a group key number. The group key number identifying the group key to be used for a particular voice broadcast call is provided with the notification to the mobile stations. Mobile stations which have a responded to a notification shall be informed of the group key number before they join the voice broadcast channel.

USIM based VBS ciphering uses a concept of short term keys where the short term key is derived by the GCR and the USIM from the group key and a RAND (random number) parameter. The actual broadcast group ciphering key is then derived by the BSS and the ME from the short term key, the cell global identifier, and a Cell Global Count parameter.

To include a subscriber into a voice group the required group data (including the 2 master group keys) shall be stored on the USIM, e.g. during the personalisation process or via OTA (over-the-air). To exclude a subscriber from a voice group the group data shall be deleted from the USIM. If a USIM is lost or stolen, all USIMs of the remaining members of the voice groups that this USIM is a member of need to be changed (e.g. via OTA or manual provisioning).

Details on data confidentiality for voice broadcast calls are provided in 3GPP TS 42.009 [2] and 3GPP TS 43.020 [4].

NOTE 1: USIM based VBS ciphering is not compatible with SIM based VBS ciphering which has not been completely specified. The SIM specifications contain no support for the storage of the group keys. A pre-Rel-6 VBS capable mobile station will be able to participate in an un-ciphered group call, if it is part of that group.

If data confidentiality is provided, then for a mobile station in originator in dedicated channel mode the uplink and the downlink of the dedicated channel shall be ciphered using the individual ciphering key of the service subscriber.

NOTE 2: The individual ciphering key is the key generated during a previous authentication procedure.

If data confidentiality is provided, then for a mobile dispatcher the uplink and the downlink of the dedicated channel shall be ciphered using the individual ciphering key of the dispatcher.