03.683GPPStage 2TSVoice Group Call Service (VGCS)
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. 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. 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, while the calling service subscriber is on a dedicated link served by a relay MSC, there is an additional link from the anchor MSC to the relay MSC serving the calling service subscriber and an additional link from the relay MSC 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.
NOTE 1: The conference bridge shall not mute the uplink speech.
A mechanism is required to indicate the downlink muting and uplink busy when the dispatcher is talking. DTMF should be used for this mechanism. [FFS]
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 subscriber’s mobile stations and an uplink which shall be used by the talking subscriber’s mobile station only.
The calling service subscriber’s mobile station shall use a 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. This 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.
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 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.
Voice group call channels shall be standard full rate or half rate speech channels. A specific voice group call can have cells in the group call area where the voice group call channels are either only half rate speech or only full rate speech or there are cells with half rate speech and cells with full rate speech. Those implementations are optional for the network operator.
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.
The mobile station of the talking service subscriber will transmit on the uplink related to the downlink of the voice group call channel. The downlink of this channel which is also received by the mobile station using the uplink will typically echo the uplink unless one or more dispatchers are talking simultaneously. The mobile station of the talking service subscriber shall mute the downlink speech unless more than one speaker is talking. In this case, an indication shall be provided to the mobile station, and the mobile station shall no longer mute the downlink. When the downlink is not muted it is acceptable for the talking subscriber to hear an echo, and possibly other distortions which may occur, as the intention is to alert the talking subscriber to the fact that someone else is talking, rather than allow them to hear the message from the dispatcher. If no dispatcher is talking anymore and the talking service subscriber still has access to the uplink, an indication shall be provided to the mobile station, and the mobile station shall mute the downlink again.
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 channels in each cell of the group call area shall be ciphered using the same group key.
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 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.
The distribution of the group keys to the related SIMs shall be done off-line.
NOTE: The distribution of group keys shall be done by the service provider when editing the SIM. Distribution of group keys via the radio interface may be possible with future features as the SIM-toolkit. Those distribution schemes are for further study. [FFS]
Details on data confidentiality for voice group calls are provided in 3GPP TS 02.09 and 3GPP TS 03.20.