Voice Call (Circuit-Switched)

Description

A voice call in 3GPP terminology refers to a circuit-switched (CS) call that can carry various teleservices, including speech, 3.1 kHz audio, fax, and CS data. It explicitly excludes circuit-switched video telephony calls (BS 37, 64 kbit/s unrestricted digital info mode) and Voice over LTE (VoLTE). This definition is used in management specifications such as 32.276 and 32.849 for performance and fault management of CS voice services.

The architecture for a CS voice call involves the mobile station (MS), the base station subsystem (BSS) comprising base transceiver stations (BTS) and base station controllers (BSC), the mobile switching center (MSC), and the gateway MSC (GMSC) for interconnection with external networks. The call setup uses signaling protocols like ISUP or BICC over the core network, while the radio interface employs channel coding and modulation suitable for speech or data. The MSC manages call routing, supplementary services, and handovers between cells.

In operation, a voice call is established through a series of signaling messages: the MS requests a channel, the BSS allocates radio resources, and the MSC sets up the circuit to the destination. Once connected, the call maintains a dedicated circuit for the duration, ensuring a constant bit rate and low latency. Handover procedures ensure continuity when the MS moves between cells or MSCs. The circuit-switched nature guarantees quality of service but is less efficient for bursty data.

With the advent of LTE and 5G, voice services have migrated to packet-switched domains using VoLTE and VoNR. However, CS voice calls remain relevant in legacy networks and for fallback scenarios (e.g., CS fallback). The definition in 3GPP management specs helps operators monitor and manage CS voice performance separately from VoLTE, ensuring accurate reporting and troubleshooting.

Key performance indicators (KPIs) for voice calls include call setup success rate, drop call rate, and voice quality metrics. These are defined in specifications like 32.276, which outlines the performance measurements for CS voice. The exclusion of VoLTE from this definition is crucial because VoLTE is managed under different KPIs and architectures, reflecting the technological shift in voice delivery.

Purpose & Motivation

The concept of a voice call as a circuit-switched service exists to provide reliable, real-time voice communication with guaranteed quality of service. In early mobile networks (2G/3G), circuit-switching was the only way to deliver voice, as packet-switching lacked the mechanisms for low-latency, constant-bit-rate transmission. This definition was created to clearly delineate CS voice services from emerging packet-based voice solutions, enabling separate management and performance measurement.

The exclusion of CS video telephony and VoLTE addresses the need for precise categorization in management specifications. CS video telephony, though circuit-switched, has different resource requirements and KPIs, while VoLTE operates over IP with distinct architecture and management. By defining voice call narrowly, 3GPP ensures that performance counters and fault management for traditional voice services are not conflated with newer technologies.

Historically, as networks evolved from 2G to 3G and then to LTE, the voice call definition remained stable to support legacy systems. It allows operators to maintain and optimize CS voice while transitioning to all-IP networks. The definition also aids in interoperability and roaming, as CS voice is still used in many regions. Thus, it serves as a foundational term for managing the coexistence of CS and PS voice services.