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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.


Voice Broadcast Call (VBCS)

Voice Broadcast Calls are part of the ASCI features introduced primarily for GSM-R.

They allow the distribution of speech (or other signals which can be transmitted via the speech codec), generated by a service subscriber, into a predefined geographical area to all or a group of service subscribers located in this area.

VBS is an uni-directional service.

Voice can originate from any subscriber (with related privileges).

VBS contains the notion of a VBS group. Subscribers are either part of a group or not. Each subscriber can be part of up to 50 groups. Every group is identified by a Group ID. Every group is realted to a Group Call Area (a group of cells in).

Only subscribers that are members of the Group ID which are currently within the Group Call Area will receive the voice information. If they leave the area, they loose the call.

Subscribers can activate/deactivate their group membership; that information is stored on the SIM card.

Security

Normal GSM authentication is only performed for the calling subscriber, but not for the called (receiving) subscribers; those have uni-directional downlink signals only with no return channel.

Before Release 6, VBS calls were always in cleartext.

Release 6 introduced the optional ciphering of VBS. The group specific key material are stored on the USIM (e.g. via OTA).

Details

NCH channel

One of the SDCCH of each BTS C0 is re-appropriated as NCH (notification channel); this is similar to how CBCH works.

NCH contain NOTIFICATION messages, something similar to PCH with its PAGING messages. A NOTIFICATION contains the Group ID of the VBS call, and all MS autonomously determine whether or not they are part of the group and hence need to activate the receive side of the VBS call.

There are two types of notifications:

  • notification describing a TCH
  • notification without TCH / channel description (optional)
    • this avoids early allocation of a downlink TCH without knowing if any subscribers are actually around. The network can use this to wait and see if there is somebody interested
    • if a MS is part of the group and camping in the cell, it will start the notification response procedure to infrom the network of its presence. The network will now perform late allocation of the downlink TCH

The NCH needs to repeat the NOTIFICATION throughout the call, as there might be subscribers moving into the cell which didn't see the NOTIFICATION at the start of the call.

Specifications

For more information, see
  • 3GPP TS 42.069 (Stage 1)
  • 3GPP TS 43.069 (Stage 2)
  • 3GPP TS 44.069 (Stage 3; BCC Layer 3 Protocol)

Updated by laforge 2 months ago · 5 revisions

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