Mobile Terminated

Description

Mobile Terminated (MT) describes the direction of a communication flow where the mobile user equipment (UE) is the recipient or destination. This applies across all telecommunication services: a voice call received by the mobile phone (MT call), an SMS message delivered to the UE (MT SMS), or a data packet session initiated by a network server towards the UE (MT data). The term is used in technical specifications to define procedures, signaling flows, charging scenarios, and performance requirements specific to the terminating leg of a service.

From a network architecture perspective, an MT service involves a multi-step process. For a voice call, the calling party's signal routes through various networks (PSTN, other mobile networks) to the recipient's home mobile network. The Home Public Land Mobile Network (HPLMN) must locate the UE, which may be roaming, through procedures like paging and location update. The Mobile Switching Center (MSC) for circuit-switched calls, or the IP Multimedia Subsystem (IMS) for VoLTE/VoNR calls, then establishes the connection to the UE via the serving radio access network. For packet data, an MT session might be triggered by a downlink data notification from a Packet Data Network Gateway (PGW) or SMF to the Mobility Management Entity (MME) or Access and Mobility Management Function (AMF), which then pages the UE to re-establish the user plane.

Key technical aspects governed by MT specifications include paging strategies, mobile-terminated call setup times (a key performance indicator), authentication and security procedures for incoming requests, and charging data generation (e.g., generating Call Detail Records (CDRs) for MT calls). The distinction between MT and Mobile Originated (MO) is crucial for asymmetric service features, such as different charging rates, barring options (e.g., barring of all incoming calls), and lawful interception. In IMS, MT sessions involve specific SIP message flows (e.g., INVITE request sent to the UE) and service triggering rules in the Telephony Application Server (TAS).

Purpose & Motivation

The Mobile Terminated concept exists to delineate and standardize the procedures, network behaviors, and business logic associated with delivering services to a mobile subscriber. In early cellular systems (GSM), the ability to reliably receive calls was as important as making them, and the network needed distinct processes for handling incoming calls versus outgoing ones. Defining MT separately from MO allowed for specialized functions like call forwarding, missed call notification, and differential charging (often, receiving calls was free for the recipient while the caller paid).

It solves the problem of asymmetric network state management. A mobile device, to conserve battery, may be in an idle state with no active radio connection. An MT service requires the network to locate and alert the device, a process not needed for MO where the device initiates contact. Standardized MT paging and location update procedures ensure efficient use of radio resources and reliable delivery. Furthermore, from a service and regulatory perspective, features like incoming call barring, number presentation, and lawful interception for received communications require clear technical definitions of the MT phase.

Historically, as services evolved from circuit-switched voice to packet-switched data and IMS-based multimedia, the MT concept persisted and expanded. For always-on packet data, the network's ability to initiate downlink data transfer (MT data) is essential for push notifications, instant messaging, and email. The creation of MT specifications ensures interoperability between network elements from different vendors and enables consistent user experience across diverse networks and service types, from 2G to 5G.