Cellular Network Infrastructure

h1. 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.

h2. 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).

h2. Details

h3. 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.

h2. 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)