Single-Connection Mode

Description

Single-Connection Mode (SCM) is a core concept in 3GPP network-based mobility management, formally defined in TS 23.402 for the System Architecture Evolution (SAE). It describes the state of a User Equipment (UE) when it is simultaneously connected to the core network via exactly one access network. This single connection is the active point of attachment for the UE's IP traffic. In the context of Evolved Packet Core (EPC), this typically means the UE has one active PDN connection established through a single Packet Data Network Gateway (PGW). The access network for this connection can be a 3GPP access like E-UTRAN, UTRAN, or GERAN, or a trusted/untrusted non-3GPP access like Wi-Fi.

The technical operation of SCM is governed by the mobility management protocols. For a UE in SCM, all of its IP traffic flows over this single bearer path. The mobility management entity (e.g., MME for 3GPP, ePDG for untrusted non-3GPP) tracks the UE's location and manages the handover procedures for this sole connection. The UE's IP address(es) are anchored at the single PGW serving this PDN connection. Key procedures like Attach, PDN Connectivity, and Bearer Management are executed with the understanding that the UE is in this singular connectivity state. The network's policy and charging rules (PCRF) are applied to this single data path.

SCM is often discussed in contrast to Multi-Access PDN Connectivity (MAPCON) or IP Flow Mobility (IFOM), where a UE can maintain multiple simultaneous PDN connections over different accesses. In SCM, the network handles mobility events (e.g., handover from LTE to Wi-Fi) by transferring the entire PDN connection and its associated bearers from one access to another, potentially involving an inter-access handover procedure. The concept is foundational for understanding how a UE registers, connects, and is managed in its most basic and common operational state within EPS and 5GS architectures, where simplicity and single-path routing are required.

Purpose & Motivation

SCM was defined to establish a clear and fundamental baseline for UE connectivity behavior within the new, access-agnostic Evolved Packet System (EPS) introduced in 3GPP Rel-8. Prior to EPS, connectivity was more siloed (e.g., GPRS attach for 2G/3G). EPS aimed to provide seamless mobility and service continuity across heterogeneous access technologies (LTE, 3G, 2G, Wi-Fi) under a unified core. A formal definition of the state where a UE uses only one of these accesses at a time was necessary for specifying standard procedures.

It solves the problem of ambiguity in mobility management specifications. By defining SCM as the default and simplest mode, 3GPP could precisely specify attach, detach, handover, and bearer management procedures for this case. This created a stable reference model. More advanced features like simultaneous multi-access connectivity (e.g., MAPCON) were then defined as extensions to or deviations from this baseline SCM behavior. The concept is crucial for network implementations to correctly handle resource allocation, routing, charging, and policy enforcement for the vast majority of UEs that are not using advanced multi-connectivity features, ensuring predictable and efficient network operation.