Call

Description

In 3GPP terminology, a call is a fundamental concept that represents the execution of actions to establish, release, and maintain a communication between two or more parties on behalf of an initiating party. It encompasses both the signaling procedures and the actual communication itself, based on the required service. A call can also be referred to as a service invocation, highlighting its role as the instantiation of a service. The call is characterized by lower-layer attributes, such as the bearer(s) it uses, and higher-layer attributes at the service level, such as the type of media or application.

The concept of a call has evolved significantly across 3GPP releases, reflecting the transition from circuit-switched (CS) to packet-switched (PS) and IP Multimedia Subsystem (IMS) architectures. In early releases (R99), the definition focused on the execution of actions to establish and release communication, emphasizing the service invocation aspect. In Rel-4, the definition was changed to describe a call as a logical association between several users, which could be connection-oriented or connectionless. This definition was more abstract and applicable to both CS and PS domains. Rel-5 further clarified that this logical association pertains to the CS CN domain, the PS CN domain, and the IMS.

In Rel-8, the definition was again revised to describe a call as an informal term that refers to a dialog between peers, generally set up for the purposes of a multimedia conversation. This definition reflected the growing importance of multimedia services and the peer-to-peer nature of IMS-based communications. However, in Rel-10, the definition reverted to the logical association between several users, which remained consistent through Rel-19. This stability suggests that the concept of a call as a logical association between users is sufficiently broad to encompass various service types, including voice, video, and data.

Architecturally, a call involves multiple network elements depending on the domain. In the CS domain, a call is managed by the Mobile Switching Center (MSC) and involves the setup of a dedicated circuit between the parties. In the PS domain, a call is typically a session established over a Packet Data Network (PDN) connection, managed by the Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN) in earlier releases, or by the Session Management Function (SMF) and User Plane Function (UPF) in 5G. In the IMS, a call is a session established using SIP signaling, managed by Call Session Control Functions (CSCFs).

The call concept is central to many 3GPP specifications, including TS 21.905 (Vocabulary for 3GPP Specifications), TS 22.031 (Fraud Information Gathering System), TS 22.105 (Services and Service Capabilities), TS 22.127 (Service Requirement for the Open Service Access), TS 22.273 (Location Services), TS 22.960 (Mobile Multimedia Services), TS 23.050 (Location Services), TS 27.001 (General on Terminal Adaptation Functions), and TS 29.998 (Open Service Access). These specifications define the requirements, architecture, and protocols for various call-related services, including voice calls, video calls, emergency calls, and location-based services.

In the context of 5G, the concept of a call has expanded to include new service types such as ultra-reliable low-latency communication (URLLC) and massive machine-type communication (mMTC). A call in 5G can be a voice call over IMS, a data session for IoT devices, or a mission-critical push-to-talk (MCPTT) call. The underlying principles remain the same: a call is a logical association between parties, characterized by its bearer and service attributes, and managed by the network to ensure the required quality of service.

Purpose & Motivation

The concept of a call exists to provide a standardized way to describe and manage communication sessions between users in a telecommunications network. It serves as a fundamental building block for defining services, protocols, and procedures across different domains (CS, PS, IMS). The call abstraction allows network operators to design, implement, and manage services consistently, regardless of the underlying technology.

Historically, in circuit-switched networks, a call was a dedicated physical connection between two parties, and the term was well-understood. With the advent of packet-switched networks and the IMS, the concept of a call evolved to encompass logical associations and multimedia dialogs. The 3GPP definitions have adapted to these changes, ensuring that the term remains relevant and precise. The call concept solves the problem of describing complex communication sessions in a technology-agnostic way, enabling interoperability and service portability across different network generations.

Furthermore, the call concept is essential for charging, policy control, and quality of service management. Network elements use call attributes to apply appropriate charging models, enforce QoS policies, and manage resources. For example, an emergency call may be prioritized over a regular call, and a video call may require higher bandwidth than a voice call. The call abstraction provides a common language for these functions, facilitating seamless service delivery and network operation.