Alternate Connectivity

Description

Alternate Connectivity (ALTC) is a sophisticated mobility management mechanism defined in 3GPP specifications, primarily within TS 23.334 (Proximity-services (ProSe) architecture) and TS 29.162 (Interworking between the IMS and non-IMS networks). It operates at the network layer, enabling User Equipment (UE) to establish and maintain multiple active connections to different access networks simultaneously. These networks can include 3GPP access (like LTE or 5G NR) and non-3GPP access (such as Wi-Fi or fixed broadband), managed through core network functions like the Access Network Discovery and Selection Function (ANDSF) or the Policy Control Function (PCF). The architecture involves coordination between the UE, the serving network, and potentially a ProSe Function or Application Function to discover, authenticate, and manage the alternate connectivity paths.

At its core, ALTC works by allowing the UE to register multiple Packet Data Network (PDN) connections or PDU Sessions via different access types. The network provides policies to the UE, instructing it on when and how to use these alternate paths. For instance, the network can specify that certain IP flows or services should be routed via a primary 3GPP access, but if that access's quality falls below a threshold (e.g., signal strength or latency), the UE should seamlessly switch those flows to an established alternate non-3GPP connection without dropping the session. This is managed through IP flow mobility (IFOM) or multi-access PDU session (MA PDU) mechanisms, where the same IP address can be maintained across the handover to preserve application continuity.

Key components in the ALTC framework include the UE, which must support multiple radio interfaces and the ability to execute network-provided policies; the core network policy functions (ANDSF/PCF) that generate and provision these connectivity policies; and the gateway nodes (e.g., PGW in EPC or UPF in 5GC) that anchor the user plane and facilitate traffic routing across the different accesses. In scenarios involving proximity services, a ProSe Function may assist in discovering nearby UEs or networks that can serve as alternate connectivity points, enabling device-to-device or UE-to-network relay paths. The role of ALTC is to create a robust, multi-homed connectivity environment that maximizes resource utilization, enhances user experience through seamless mobility, and provides redundancy for critical communications.

From a protocol perspective, ALTC relies on extensions to existing mobility and session management protocols. For example, in 5G, the Non-3GPP Interworking Function (N3IWF) enables secure integration of untrusted non-3GPP access into the 5G Core. Policies are communicated using protocols like S14 for ANDSF or RESTful APIs for PCF. The UE continuously monitors the performance of its active accesses and applies policy rules to make routing decisions, potentially triggering handovers at the flow level. This granular control allows for optimized load balancing and quality-of-service (QoS) management, ensuring that high-priority traffic always uses the best available path.

Purpose & Motivation

ALTC was created to address the growing demand for always-best-connected services and network resilience in mobile communications. Prior to its introduction, UEs typically relied on a single active connection at a time, with handovers between accesses causing service interruptions or packet loss. This was insufficient for emerging applications like real-time video, voice over IP (VoIP), and public safety communications, which require uninterrupted connectivity even in challenging radio conditions. The limitations of single-path connectivity became particularly evident with the proliferation of heterogeneous networks, where 3GPP cellular coverage overlaps with dense Wi-Fi hotspots, yet users could not leverage both simultaneously for redundancy or aggregation.

Historically, solutions like Mobile IP provided some level of multi-homing but were often inefficient, involving triangular routing and significant latency. The motivation for ALTC within 3GPP was to develop a standardized, network-controlled approach that enables seamless flow mobility across different access technologies. By introducing ALTC in Release 12, 3GPP aimed to enhance service continuity, improve resource utilization across multiple radio accesses, and support mission-critical applications that demand high availability. This was part of a broader trend toward network densification and integration of non-3GPP accesses, paving the way for converged 5G systems.

Furthermore, ALTC solves problems related to coverage gaps and congestion. In areas with weak cellular signal, an alternate Wi-Fi connection can maintain service. For network operators, it allows traffic offloading from congested cellular networks to other available accesses, optimizing overall network performance. The technology also supports scenarios where direct network connectivity is unavailable, such as in public safety situations, by allowing UEs to use other nearby UEs as relays to reach the network, thereby extending coverage. Thus, ALTC is a foundational enabler for reliable, efficient multi-access connectivity in modern mobile networks.

Key Features

• Enables simultaneous multi-access connectivity (3GPP and non-3GPP)
• Supports network-controlled IP flow mobility (IFOM) for seamless handover
• Allows service continuity across different access types without session break
• Provides policy-based traffic steering and routing decisions
• Facilitates discovery and utilization of alternate connectivity paths, including UE-to-network relays
• Enhances reliability and availability for mission-critical applications

Evolution Across Releases

Defining Specifications