7 Transmission

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

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

7.1 Transmission architecture

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.

Without a RANflex configuration: The group call anchor MSC is responsible for setting up all connections, both to the nominated cells (voice group call channels) in the group call anchor 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 within the group call relay MSC which is involved in the voice group call.

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 group 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 group call.

For all configurations the following applies:

While the calling service subscriber is on a dedicated link served by his VMSC, there is an additional link from the anchor MSC to the VMSC serving the calling service subscriber and an additional link from the VMSC serving the calling service subscriber to the cell serving the calling service subscriber. There shall be no secondary bridges in BSCs.

While a talker served by a relay MSC is on any other dedicated or group channel than the initial dedicated channel , the following applies: The distribution function shall be implemented using a secondary conference bridge at the relay MSC so that VGCS talker speech sent on the current channel uplink is transmitted to local relay cells as well as being transmitted over the link back to the anchor MSC, for distribution to the rest of the network, dispatchers and nominated cells at other relay MSCs.

The conference bridge shall not mute the uplink speech.

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 group 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 group 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 group 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 all connections, both to the nominated cells (voice group call channels) in the group call anchor MSC and in any related group call relay MSC, and to the dispatchers

The BSC contains a distribution function that distributes speech sent from the MSC to each of the nominated cells.

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

In the case of an originator that is not on the initial dedicated link, there shall be one link from the anchor MSC towards every relay MSC. 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 group call. Each of these BSCs contains a distribution function that distributes speech received from the MSC to each cell involved in the group call.

When an originator, served by a relay MSC, is on the initial dedicated link, there is an additional link from the anchor MSC to the relay MSC serving the originator and an additional link from the relay MSC serving the originator to the cell serving the originator.

When an originator, served by an anchor MSC, is on the initial dedicated link, there shall be one link from the anchor MSC towards every relay MSC. There will be one link from each of these MSCs and the group call anchor MSC to each BSC controlled by the respective MSC and involved in the voice group call. Each of these BSCs contains a distribution function, with one link to each cell and involved in the group call. There is an additional link from the anchor MSC to the cell serving the originator.

While a talker served by an anchor MSC is on any other dedicated or group channel than the initial dedicated channel, the following distribution functions shall be implemented:

– conference bridge at the anchor MSC so that VGCS talker speech sent on the current channel uplink is transmitted to local cells as well as being transmitted over the links to the relay MSCs, for distribution to the rest of the network, dispatchers and nominated cells at other relay MSCs;

– distribution point at the BSC so that speech sent from the MSC is distributed to each of the nominated cells.

While a talker served by a relay MSC is on any other dedicated or group channel than the initial dedicated channel, the following distribution functions shall be implemented:

– secondary conference bridge at the relay MSC so that VGCS talker speech sent on the current channel uplink is transmitted to local cells as well as being transmitted over the link back to the anchor MSC, for distribution to the rest of the network, dispatchers and nominated cells at other relay MSCs;

– distribution point at the BSC so that speech sent from the MSC is distributed to each of the nominated cells.

The conference bridge shall not mute the uplink speech.

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 group 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 group 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 group 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 group call in the VGCS 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 VGCS 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 VGCS 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 VGCS 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, priority uplink decision). The A-interface link (user and control plane) between MSC and BSC shall only be released when the call is released.

When the network supports uplink access option (i) as defined in subclause 7.2 (i.e. indication in uplink busy instructs the MS to use the group call channel to send the uplink access message if the talker on the group call channel is not in the same cell as the MS) the network shall include talker priority information in all relevant A-interface messages on the resource controlling SCCP connection, to distinguish between messages related to the current talker and messages related to the subscriber requesting the uplink.

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 group call channel may be established consisting of a downlink received by all service subscribers’ mobile stations and an uplink which shall be used by the talking subscriber’s mobile station only.

If the network allocates a dedicated traffic channel for the calling service subscriber, his mobile station shall use the dedicated standard uplink/downlink which is connected to the conference bridge up to the instant where the network decides that the mobile station shall join the voice group call channel and the dedicated connection is released.

The network may decide to switch a talking subscriber’s mobile station from the voice group call channel to a dedicated standard uplink/downlink at any time. The talking subscriber’s voice group call channel and the dedicated channel may belong to different cells within the group call area, i.e. the network may request the talker to perform a handover. The dedicated connection shall then be maintained up to the instance where the network decides that the mobile station shall join the voice group call channel again and the dedicated connection is released.

When the network indicates the uplink as free, the mobile station uses the group call channel uplink to signal an uplink request, "emergency mode reset request" or uplink request for sending application-specific data.

When the network does not indicate the uplink as free, there are two options for the mobile station how to signal an uplink request with talker priority higher than "normal subscriber", an "emergency mode reset request" or an uplink request for sending application-specific data:

i) if the priority uplink access parameter broadcast on the NCH indicates "RACH access" but the latest UPLINK BUSY message received in the cell indicates that the group call channel uplink shall be used, the mobile station sends an uplink access message on the group call channel uplink; otherwise, the mobile station temporarily leaves the group receive mode and sends a channel request message on the RACH. Once a dedicated connection has been established by the network, the mobile station sends a layer 3 message PRIORITY UPLINK REQUEST or DATA INDICATION 2. The PRIORITY UPLINK REQUEST message contains the MS identity (IMSI or TMSI), the group call reference, random reference, the type of request and the token broadcast by the BSS, if the BSS broadcasts a token. On receipt of this information, the network shall release the dedicated connection, and the mobile station shall return to the group receive mode and continue to listen on the downlink of the group call channel for further instructions from the network.

NOTE 1: The indication in uplink busy instructs the MS to use the group call channel uplink to send the uplink access message if the talker on the group call channel is not in the same cell as the MS.

If the mobile station releases the uplink on request of the subscriber, it shall use the group call channel uplink for signalling subsequent uplink requests, until it performs a cell change or receives either an UPLINK BUSY message indicating that RACH shall be used or a VGCS UPLINK GRANT message destined for a different mobile station.

ii) If the priority uplink access parameter broadcast on the NCH indicates "group call channel uplink access" then the mobile station always sends an uplink access message on the group call channel uplink. If this option is used, the network shall always establish and maintain a dedicated channel for the talking service subscriber.

NOTE 2: Otherwise, with option (ii) the BSC would not be able to detect the requests of higher privileged talkers in the cell where the current talking service subscriber is located.

Support of both options is mandatory for a mobile station supporting the use of talker priorities or transfer of application-specific data and optional for the network. A network supporting the use of talker priorities or transfer of time-critical application-specific data shall indicate the default procedure to be used on the NCH (i.e. either "RACH access" or "group call channel uplink access"). The indication shall have the same value throughout the network. If the "talker channel parameter" is used in a network, then the value of the indication shall be set to "RACH access".

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

Voice group call 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 group call channels other than full rate speech is a network option. A specific voice group 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 a dedicated channel used by a talking service subscriber, the BSC may select any configuration permitted for a point-to-point call;

– for a voice group call 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. For the downlink the BSC shall disable the rate adaptation mechanism and apply a single codec mode until the channel is released. If the talking service subscriber uses the voice group call channel uplink and the BSC selected a multi-mode configuration, the BSC shall apply the rate adaptation mechanism for the uplink.

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

Mobile station using the uplink are in group transmit mode. Signalling for this RR mode is specified in 3GPP TS 44.018. Mobile stations not using the uplink and not in dedicated mode shall ignore any signalling concerned only with uplink usage.

Full standard duplex channels shall be provided to all dispatchers listed in the GCR. These may be provided either via GSM, or via an external network. The links to the dispatchers are connected to the conference bridge.

If the mobile station of the talking service subscriber joins the voice group call channel, it will transmit on the uplink of the voice group call channel.

7.3 Data confidentiality

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

If data confidentiality is provided, both the uplink and the downlink of the voice group call channel within a cell of the group call area shall be ciphered using voice group ciphering keys derived from the same group key, see 3GPP TS 43.020 [10].

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

USIM based VGCS 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 voice 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 group calls are provided in 3GPP TS 42.009 [9]and 3GPP TS 43.020 [10].

NOTE 1: USIM based VGCS ciphering is not compatible with SIM based VGCS ciphering which has not been completely specified. The SIM specifications contain no support for the storage of the group keys. A pre-Rel-6 VGCS 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 group mode dedicated channel 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.

In order to start the ciphering for the calling service subscriber, the MSC serving the mobile station shall initiate a cipher mode control procedure during call setup, while the mobile station is in group mode dedicated channel. When ciphering was started successfully, the mobile station shall apply the individual ciphering key until it leaves group mode dedicated channel or a new cipher mode control procedure is performed successfully.

In order to start the ciphering on the dedicated channel, if the network decides to move a talking subscriber’s mobile station from group transmit mode to group mode dedicated channel, the network shall include cipher mode setting information in the assignment command or handover command message (see 3GPP TS 43.020 [10], Annex F 3.2). On the dedicated channel the mobile station shall apply the individual ciphering key until it leaves group mode dedicated channel or a new cipher mode control procedure is performed successfully.

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.

If data confidentiality is provided, the priority uplink request procedure can be validated by a handshake between the requesting MS and the BSS. A32 bit token is generated randomly and broadcast by the BSS on the ciphered downlink of the voice group call channel and stored in the MS. When the MS sends a priority uplink request it includes the token as a parameter and the BSS checks this against its stored value. If it matches the request is processed further. If it does not match, the request is ignored. A new token is broadcast on the ciphered downlink of the voice group call channel each time a match occurs and each time a periodic UPLINK_BUSY message is sent (i.e. every T1 seconds) (refer to Figure 7e in section 11.3.8).

If data confidentiality is provided for point-to-point SMS over the CS domain, the network shall broadcast an SMS data confidentiality indication in NOTIFICATION/ NCH messages, NOTIFICATION/ FACCH messages and in the Connect message sent to the calling service subscriber of a voice group call.

If guaranteed privacy is provided for SMS over the CS domain (see 3GPP TS 42.068 [2]), the network shall broadcast an SMS guaranteed privacy indication in NOTIFICATION/ NCH messages, NOTIFICATION/ FACCH messages and in the Connect message sent to the calling service subscriber of a voice group call.

NOTE 3: For backward compatibility with legacy networks the coding of these indications in NOTIFICATION messages is chosen so that the default settings are SMS data confidentiality = "on" and SMS guaranteed privacy = "on".