Mobile-To-Mobile (call)

Description

Mobile-To-Mobile (MTM) is a foundational service type in 3GPP systems, denoting a communication session where both the calling party (originating UE) and the called party (terminating UE) are mobile stations operating within a cellular network. The term broadly encompasses voice calls, and later video calls, that are routed entirely through the mobile network infrastructure without traversing the traditional Public Switched Telephone Network (PSTN) for termination. An MTM call involves a series of standardized signaling and bearer path procedures across the radio access network (RAN) and core network (CN) to establish, maintain, and release a bidirectional communication channel between the two UEs.

The technical process for an MTM call begins with the originating UE sending a call setup request (e.g., a SETUP message in Circuit-Switched (CS) domain or a SIP INVITE in Packet-Switched (PS)/IMS domain) via the serving base station to the core network. In traditional 2G/3G CS calls, the Mobile Switching Centre (MSC) receives the request, performs number analysis, and determines that the called party is a mobile subscriber. It then queries the Home Location Register (HLR) to obtain routing information (the address of the serving MSC/VLR for the called party). The originating MSC routes the call signaling to the terminating MSC, which pages the called UE in its location area. Upon answer, a voice bearer path is established: in CS, this is a dedicated timeslot or circuit; in 4G/5G using Voice over LTE (VoLTE) or Voice over NR (VoNR), it is an IP-based bearer over the PS domain with media handled by the IP Multimedia Subsystem (IMS).

The end-to-end path for an MTM call typically involves the radio bearers between each UE and its respective base station (BTS, NodeB, eNB, gNB), backhaul connections to the core network, and the inter-core network links (e.g., using ISUP signaling in CS or SIP routing in IMS). In roaming scenarios, the call may traverse the visited networks of both parties and potentially involve inter-operator interfaces. Key network functions involved include the MSCs (for CS), the Mobility Management Entity (MME) and Serving Gateway (SGW) for LTE control and user plane, the IMS Core (CSCF, MGCF, etc.), and the Policy and Charging Rules Function (PCRF/PCF) for policy control on PS bearers. Charging systems generate Mobile Originated (MO) and Mobile Terminated (MT) call detail records (CDRs) for billing.

With the evolution to all-IP networks, MTM calls have transitioned from being predominantly circuit-switched to packet-switched via IMS. This allows for enhanced services like High Definition (HD) voice (using AMR-WB codec), video calling, and rich communication services (RCS). The call control signaling uses SIP over the PS control plane, while the voice/video media streams are carried as RTP/UDP/IP packets over the PS user plane bearers (dedicated bearers with appropriate QoS Class Identifiers for conversational voice/video). The 5G system further optimizes this with a service-based architecture where the Session Management Function (SMF) and User Plane Function (UPF) establish the PDU session for IMS traffic.

Purpose & Motivation

The Mobile-To-Mobile call service is the cornerstone of cellular telephony, enabling direct communication between mobile subscribers. Its purpose is to provide ubiquitous, reliable, and manageable voice connectivity without requiring fixed-line intermediaries. Initially, mobile networks were seen as an extension of the PSTN, with calls often routed to/from fixed lines. As mobile penetration increased, a significant portion of traffic became mobile-to-mobile, driving optimizations in network routing, signaling efficiency, and tariff structures specifically for MTM calls.

The concept addressed the fundamental need for person-to-person wireless communication. Early cellular systems (1G) provided basic MTM capability but with limited scalability and no digital security. The standardization of MTM procedures in 2G (GSM) through 3GPP provided interoperability, allowing subscribers on different networks (national and international) to connect seamlessly. This fostered competition and growth. The service also solved the problem of mobility management during a call, with handover procedures ensuring call continuity as users move between cells.

With the rise of IP networks, the motivation evolved to integrate MTM calls into the packet-switched domain to reduce costs, simplify network architecture, and enable convergence with data services. The transition to VoLTE/VoNR under IMS was driven by the need to support high-quality voice on LTE/5G networks that lack a native CS domain. MTM in this context leverages the efficiency of IP transport and enables service innovation (e.g., video add-ons, file sharing during calls). It remains critical for operator revenue and as a baseline service, even as Over-the-Top (OTT) alternatives exist, due to its guaranteed quality, reliability, and seamless integration with network features like emergency calling and lawful interception.

Key Features

• Direct call establishment between two mobile user equipment (UEs) within cellular networks.
• Supports both circuit-switched (CS) domain (2G/3G) and packet-switched/IMS domain (4G/5G) implementations.
• Involves core network routing and mobility management (HLR/HSS query, MSC/MME signaling).
• Enables end-to-end voice and video media bearers with appropriate QoS (e.g., conversational class).
• Generates specific charging records (MO-MT CDRs) for billing purposes.
• Foundation for advanced communication services like VoLTE, VoNR, and Rich Communication Services (RCS).

Evolution Across Releases

Defining Specifications