Multi-Universal Subscriber Identity Module

Description

MUSIM (Multi-Universal Subscriber Identity Module) is a 3GPP service feature that allows a single User Equipment (UE) to manage multiple active Universal Subscriber Identity Module (USIM) profiles concurrently. Architecturally, it involves enhancements across the UE, the Radio Access Network (RAN), and the Core Network (CN) to support the independent operation and resource management for each subscription. The UE must possess the hardware and software capability to host multiple USIM applications and maintain separate protocol stacks, including distinct Radio Resource Control (RRC) states and Non-Access Stratum (NAS) contexts, for each active profile. The network, including the gNB in 5G NR or eNB in LTE, must be aware of the MUSIM capability to efficiently schedule paging and manage UE activity states, as the device may be temporarily unreachable on one subscription while actively communicating on another.

From an operational perspective, MUSIM enables a UE to be simultaneously registered on one or more Public Land Mobile Networks (PLMNs) using different subscriber identities. Each USIM profile has its own International Mobile Subscriber Identity (IMSI), security credentials, and subscription parameters. The UE and network coordinate through specific RRC and NAS procedures to indicate MUSIM capability and negotiate parameters like paging cycles and discontinuous reception (DRX) configurations tailored for multi-SIM operation. This coordination is essential to prevent service disruption, as the UE's radio transceiver can typically only serve one subscription's active data session or call at a time, requiring time-sharing or suspension mechanisms.

The primary role of MUSIM in the network is to facilitate advanced user services without requiring multiple physical devices. It impacts mobility management, as location updates and tracking area management must be handled per subscription. It also influences connection management, with the UE potentially establishing and maintaining multiple Protocol Data Unit (PDU) Sessions or Packet Data Network (PDN) Connections across its profiles. For the core network, each subscription is treated as a separate user with its own authentication, session management, and policy control, even though they originate from the same physical device. This separation ensures billing, security, and service differentiation remain intact for each profile, making MUSIM a foundational enabler for converged personal and enterprise communication services.

Purpose & Motivation

MUSIM was created to address the growing user and market demand for a single mobile device to seamlessly support multiple active network subscriptions. Historically, dual-SIM devices often used a 'Dual-SIM Dual-Standby' (DSDS) approach, where only one SIM could be active in a call or data session at a time, with the other being unreachable. This limitation was problematic for users needing constant availability on both numbers, such as for separate personal and work lives. The evolution towards 'Dual-SIM Dual-Active' (DSDA) hardware posed new challenges for network efficiency and UE power consumption, necessitating standardized network support to manage the radio resource conflicts and signaling overhead intelligently.

The technical motivation for standardizing MUSIM in 3GPP Release 17 was to define a unified, network-aware framework that goes beyond proprietary handset implementations. Previous non-standardized implementations could lead to inefficient network behavior, such as excessive paging attempts when a UE is busy on another subscription, wasting radio resources and battery life. By introducing MUSIM, 3GPP enables the UE to explicitly inform the network of its multi-SIM capability and patterns, allowing the network to optimize paging strategies, mobility procedures, and connection release timers. This cooperation improves overall system capacity, reduces signaling congestion, and enhances the user experience by providing more reliable simultaneous service across subscriptions.

Furthermore, MUSIM supports emerging service paradigms like network slicing, where different subscriptions might be associated with different network slices (e.g., one for enhanced Mobile Broadband and another for a dedicated IoT service). It also caters to the subscription flexibility required for IoT devices that may need to switch between operator profiles for coverage or cost reasons. By standardizing MUSIM, 3GPP provides a future-proof foundation for devices that must juggle multiple identities, whether for consumer convenience, enterprise device management, or innovative IoT deployments, ensuring interoperability and efficient network operation across vendors and operators.