Mobile Originated Call (attempt)

Description

A Mobile Originated Call (MOC) attempt is the procedural initiation of a voice call by a mobile User Equipment (UE). It represents the point where a subscriber dials a number and presses the call button, triggering a series of network signaling procedures. From a technical standpoint, an MOC attempt begins with the UE sending a service request (e.g., an Extended Service Request for CS fallback or a SIP INVITE for VoLTE/VoNR) to the network, requesting resources to establish a voice bearer. The process encompasses all steps from initial access through to the point where alerting (ringing) is indicated at the called party's side or until the attempt fails.

The architecture involved in processing an MOC spans multiple network domains. In a 5G Standalone (SA) network using Voice over New Radio (VoNR), the UE initiates a PDU Session Modification request for an IMS Voice bearer. The Session Management Function (SMF), in coordination with the Policy Control Function (PCF), establishes the appropriate QoS flows. The IMS core then handles the SIP signaling. In 4G LTE or 5G Non-Standalone (NSA) scenarios using VoLTE or Circuit-Switched Fallback (CSFB), the MOC triggers different procedures involving the MME, MSC Server, or IMS. Key network components include the UE (originator), the RAN (gNB/eNB), the core control plane functions (AMF/MME, SMF), the IMS core (P-CSCF, S-CSCF), and potentially a Media Gateway.

MOC is not just a user action but a critical measurable event. In 3GPP specifications, particularly those from the Technical Specification Group Services and System Aspects (TSG SA) and TSG Core Network and Terminals (TSG CT), MOC attempts are used to define test cases for network equipment and UE conformance. They are also fundamental to Performance Management (PM) and Fault Management (FM) standards, where counters for MOC attempt success and failure rates are crucial Key Performance Indicators (KPIs) for network operators to monitor service health and accessibility. Furthermore, Charging Data Records (CDRs) are generated based on successful MOC events, forming the basis for billing.

Purpose & Motivation

The concept of a Mobile Originated Call attempt exists as a fundamental, atomic unit of measurement for voice telephony service in mobile networks. It serves several critical purposes: defining test procedures, enabling performance monitoring, and facilitating accurate charging. Without a standardized definition of what constitutes a call attempt, from which point to which point, it would be impossible to consistently measure call setup success rates (a primary metric of network quality) or generate comparable charging records across different network vendors and operators.

Historically, in pure circuit-switched (2G/3G) networks, the MOC attempt was clearly defined by the initiation of a call setup message on the radio channel. With the evolution to packet-switched voice (VoIP via IMS), the definition needed to be adapted to the new signaling paradigm (SIP), but the core need remained. 3GPP specifications meticulously define the MOC procedure across all generations and voice solutions (CS, VoLTE, VoNR, CSFB) to ensure consistency. This allows operators to benchmark performance, engineers to diagnose problems using common terminology, and billing systems to apply tariffs correctly based on the service invocation. It addresses the limitation of having vendor or implementation-specific interpretations of a 'call attempt,' which would hinder interoperability, roaming, and fair charging.