Advanced Speech Call Items ASCI

GSM systems of Phase 2 offer inadequate features for group communications. For example, group call or "push-to-talk" services with fast connection setup as known from private radio or digital trunked radio systems (e.g. TETRA), are not offered. However, such services are indispensable for most closed user groups (e.g. police, airport staff, railroad or taxi companies). In particular railroad operators had a strong request for such features. In 1992, their international organization, the Union Internationale des Chemins de Fer (UIC), selected the GSM system as their standard [45]. This GSM-based uniform international railway communication system should replace a multitude of incompatible radio systems.

In this section we describe the standardized speech teleservices that offer functionality for group communication: the Voice Broadcast Service (VBS) and the Voice Group Call Service (VGCS). In addition, the Enhanced Multi-Level Precedence and Pre-emption Service (eMLPP) is used to assign and control priorities to users and their calls (e.g. for emergency calls). All those services together are referred to as Advanced Speech Call Items (ASCI).

12.3.2.1 Voice Broadcast Service (VBS)

The Voice Broadcast Service (VBS) allows a user to broadcast a speech message to several other users within a certain geographical area. The user who initiates the call can only send ("speaker"), and all others can only listen ("listeners").

Closed User Group Scenario
Figure 12.4: VBS scenario (schematic illustration)

Figure 12.4 gives a schematic illustration of a VBS scenario. Mobile users who are interested in a certain VBS group subscribe it and will then receive broadcast calls of this group. A special permission is needed, however, for the right to send broadcast calls, i.e. for the right to act as a speaker. The subscribed VBS groups are stored on the user's SIM card, and if a subscriber does not want to receive VBS calls for a certain time, he or she can deactivate them. Besides mobile GSM users, also a predefined group of fixed telephone connections can participate in the VBS service (e.g. dispatchers, supervisors, operators, or recording machines).

Figure 12.5: Some examples of group call areas

System Concept and Group Call Register - The area in which a speech broadcast call is offered is referred to as group call area. As illustrated in Figure 12.5, in general, this area consists of several cells. A group call area may comprise cells of several MSC areas and even of several PLMNs. One MSC is responsible for the handling of the VBS. It is called Anchor MSC. In case a voice broadcast should also be transmitted in cells that are not within the service area of this MSC (i.e. if the group call area contains also cells belonging to other MSCs), the MSCs of those cells are also involved. They are then denoted as Relay MSCs.

The VBS-specific data is stored in a Group Call Register (GCR). Figure 12.6 shows the extended GSM system architecture. The GCR contains the broadcast call attributes for each VBS group, which are needed for call forwarding and authentication. For example:

• Which cells belong to the group call area?

• Which MSC is the responsible anchor MSC?

• In which cells are group members currently located, i.e. in which cells is a voice message to be broadcast?

• To which other MSCs is the voice message to be forwarded to reach all group members who are currently located in the group call area?

• To which external fixed telephone connections is the broadcast message addressed?

• Which fixed telephone connections are allowed to act as speakers?

Gsm System Using Anchor
Figure 12.6: Extension of the GSM system architecture with the GCR

Call Establishment and Logical Channels - A mobile station that intends to initiate a voice broadcast call sends a service request to the BSS. The request contains the Group ID of the VBS group to be called. Thereupon, the responsible MSC queries the user's profile from the VLR and checks whether the user is allowed to act as speaker for the stated group. Afterward, some VBS-specific attributes are requested from the GCR. If the broadcast call should also be transmitted in cells that do not belong to the current MSC, an anchor MSC is determined. The anchor MSC then forwards the VBS attributes to all relay MSCs, which then request all affected BSCs to allocate a traffic channel in the respective cells, and to send out notification messages on the NCH (see Section 5.1). When a mobile station receives such a message and it is also subscribing to the respective VBS group, it changes to the given traffic channel and listens to the voice broadcast in the downlink. The speaker is then informed about the successful connection setup and can start talking. The notification message is periodically repeated on the NCH until the speaker terminates the call.

In contrast to the paging procedure in conventional GSM calls, the individual mobile users and their mobile stations are not explicitly addressed by an IMSI or TMSI but with the Group ID of the VBS group. Furthermore, the mobile stations do not acknowledge the reception of VBS calls to the network. To realize the service, traffic channels are not allocated to individual subscribers, but the voice signal of the speaker is broadcast to all listening participants in a cell on one group channel. Thus, in each participating cell, only one full-rate channel is occupied (as in regular voice calls).

12.3.2.2 Voice Group Call Service (VGCS)

Another group communication service is the Voice Group Call Service (VGCS). The VGCS defines a closed user group communication service, where the right to talk can now be passed along within the group during a call by using a push-to-talk mechanism as in mobile radio. This principle is illustrated in Figure 12.7: User 1 initializes a group call and speaks, while the other users listen. Afterward, User 1 releases the channel and changes into listener mode. Now, each of the subscribers may apply for the right to become speaker. For example, User 4 requests the channel, and the network assigns it to him/her. He or she talks, releases the channel, and changes back to listener mode. Finally, the group call is terminated by the initiator (in general). Whereas the information flow in the VBS is simplex, the VGCS can be regarded as a half-duplex system (compare Figures 12.4 and 12.7).

Advanced Speech Call Items
Figure 12.7: Group call scenario (schematic illustration)

The fundamental concepts and entities of the VBS, e.g. the definition of group call areas, group IDs, the GCR, and anchor and relay MSCs are also used in the VGCS.

Logical Channels - A traffic channel is allocated in each cell of the group call area that is involved in the VGCS. All group members listen to this channel in the downlink, and only the speaker uses it in the uplink. Therefore, in addition to the tasks for VBS calls, the network must also control uplink radio resources. The network indicates in the downlink to all mobile stations whether the uplink channel is in use or not. If the channel is free, the group members may send access bursts. Collisions that occur with simultaneous requests are resolved, and the network chooses one user who obtains the channel and thus has the right to talk.

12.3.2.3 Enhanced Multi-Level Precedence and Pre-emption (eMLPP)

Priority services enable a network to process calls with a priority class (precedence level). If the network load is high, calls with high priority can then be treated in a preferred manner, and resources for low priority calls can be deallocated. In the extreme case, a call with low priority can be dropped because a call with high priority arrives (pre-emption).

Table 12.2: Priority classes in eMLPP

Class

Used by

Connection setup

Call interruption (pre-emption)

Example

A

Operator

Fast (1-2 s)

Yes

Highest priority; VBS/ VGCS emergency calls

B

Operator

Normal (<5 s)

Yes

Calls of operator

0

Subscriber

Normal (<5 s)

Yes

Emergency calls of users

1

Subscriber

Slow (<10 s)

Yes

2

Subscriber

Slow (<10 s)

No

3

Subscriber

Slow (<10 s)

No

Standard priority

4

Subscriber

Slow (<10 s)

No

Lowest priority

The control of priorities in GSM is called Enhanced Multi-Level Precedence and Preemption (eMLPP). It is a supplementary service for point-to-point speech services as well as for VBS and VGCS. The principle of eMLPP is based on the Multi-Level Precedence and Pre-emption (MLPP) [33] method used in SS#7. In doing so, MLPP has been enhanced with functions for priority control at the air interface. Table 12.2 lists all priority classes of eMLPP. Besides the five precedence levels that are used in MLPP (Classes 0-4), two additional levels with higher priority are defined (Classes A and B). The table also shows whether a call with higher priority may terminate a call with lower priority. It is important to note that only the operator may use calls of Class A and B, such that for example an emergency call over VBS or VGCS can be initiated in disaster situations. Calls of this class can only be employed within the service area of one MSC. The other five classes can be utilized within the entire PLMN and also in combination with the MLPP of ISDN. The highest priority call that a subscriber is allowed to use is stored on his or her SIM card and in the HLR.

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