Advanced Speech Call Items

Description

Advanced Speech Call Items (ASCI) is a comprehensive framework defined in 3GPP specifications that introduces three specialized voice communication services to GSM and UMTS networks: Voice Group Call Service (VGCS), Voice Broadcast Service (VBS), and enhanced Multi-Level Precedence and Preemption (eMLPP). These services were designed to extend traditional circuit-switched voice capabilities to support professional and mission-critical communication requirements that were previously only available in specialized trunked radio systems like TETRA or P25.

VGCS enables half-duplex group calling where one user speaks while multiple users listen, similar to a walkie-talkie system but utilizing the cellular network infrastructure. The service establishes a group call area based on cell configuration, and users within that area can join the call automatically or manually. The network manages the uplink contention where multiple users might attempt to speak, granting the floor to one speaker at a time. VBS provides a one-way broadcast service where an authorized user transmits voice to all listeners in a defined broadcast area, with no uplink capability for listeners. This is particularly useful for announcements, emergency alerts, or dispatch instructions.

Enhanced Multi-Level Precedence and Preemption (eMLPP) introduces priority mechanisms for voice calls, allowing high-priority calls to preempt lower-priority calls when network resources are congested. The system defines multiple priority levels (typically 1-7, with 1 being highest) and includes both precedence (queuing priority) and preemption (resource takeover) capabilities. The architecture involves modifications across multiple network elements: the Mobile Switching Center (MSC) handles call prioritization and preemption decisions, the Base Station Controller (BSC) or Radio Network Controller (RNC) manages radio resource allocation based on priority, and the Mobile Station (MS) must support the priority indicators and preemption notifications.

ASCI services require specific network configurations including group/broadcast call area definitions, subscriber profiles with service authorization, and priority level assignments. The implementation involves signaling enhancements on the Um/Uu air interface, A interface between BSC and MSC, and core network interfaces. For VGCS and VBS, the network establishes a common traffic channel in cells belonging to the service area, allowing efficient use of radio resources compared to establishing individual calls to each participant. eMLPP integrates with existing call setup and handover procedures, adding priority information elements to signaling messages and implementing resource management algorithms that consider priority levels during congestion scenarios.

Purpose & Motivation

ASCI was created to address the growing demand for professional mobile radio (PMR) capabilities within public cellular networks. Before ASCI, organizations requiring group communications, broadcast announcements, or priority calling had to deploy and maintain separate dedicated networks like TETRA, which involved significant infrastructure costs and limited coverage. By integrating these features into GSM/UMTS networks, ASCI enabled a cost-effective alternative with wider coverage, leveraging existing cellular infrastructure while providing similar functionality to traditional PMR systems.

The primary motivation came from public safety organizations, transportation companies, and industrial enterprises that needed reliable group communication and priority mechanisms for mission-critical operations. Traditional cellular voice services only supported point-to-point calls without group management or priority features, making them unsuitable for dispatch operations, emergency response coordination, or industrial team communications. ASCI solved these limitations by introducing standardized mechanisms for group call establishment, broadcast distribution, and call prioritization within the 3GPP framework.

Another key problem ASCI addressed was spectrum efficiency for group communications. Instead of establishing individual circuit-switched connections to each group member (which would consume excessive network resources), VGCS and VBS utilize shared radio resources within a defined geographical area. This efficient resource usage, combined with the wide-area coverage of cellular networks, made ASCI particularly attractive for applications like railway communications, where trains moving across large areas need continuous group communication capabilities without frequent reconfiguration.

Key Features

• Half-duplex group calling with dynamic floor control (VGCS)
• One-way broadcast service for announcements and alerts (VBS)
• Multi-level priority with preemption capabilities (eMLPP)
• Geographical service area configuration for group/broadcast calls
• Efficient radio resource usage through shared channels
• Integration with existing GSM/UMTS authentication and billing systems

Evolution Across Releases

Defining Specifications