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IMPORTANT NOTICE: This page contains information about a feature as it is specified in the relevant 3GPP/GSM/ITU/ETSI or other specifications. This does not mean that feature is already implemented in Osomocom, or that implementation is work in progress.

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Voice Group Call (VGCS)¶

Voice Group Calls are part of the ASCI features introduced primarily for GSM-R. They're basically the bigger brother of Voice_Broadcast_Call (VBS). So if you're unfamiliar with the former, it is suggested to first read up on VBS.

While VBS calls are unidirectional, VGCS calls are bi-directional between multiple parties. There's always one talker and many listeners. The talker can change dynamically over time, basically creating a user experience of a classic two-way radio. There is always only one talker active at any moment.

Like VBS, VGCS works with the concept of groups. Eachs subscriber can be part of up to 50 groups, and calls are made to a specific group. There can be any number of parallel groups active in parallel next to each other. Each group has an area describing a list of cells in which subscribers can participate in the group call.

• notification works via NCH, like VBS
• there still is only one TCH per BTS, like VBS
  • however, that TCH has uplink: For voice, in case the talker is in the current cell
  • for all listeners, the TCH is downlink-only, like in VBS

Seizure of the uplink¶

• the NCH contains information if the uplink of a given group is currently free
  • if it is, a subscriber wanting to talk can send an uplink request on the group TCH uplink
  • if it is not free, the subscriber must request via RACH

Arbitration¶

A subscriber who wishes to talk signals this to the network. The right to talk is allocated on a first-come-first-serve basis without queuing.

Specifications¶

For more information, see

• 3GPP TS 42.068 (Stage 1)
• 3GPP TS 43.068 (Stage 2)
• 3GPP TS 44.068 (Stage 3; GCC Layer 3 Protocol)