S101 Interface

Description

The S101 interface is a critical component defined within 3GPP specifications to facilitate interoperability and mobility between 3GPP LTE networks and 3GPP2 CDMA2000 HRPD networks. It is a signaling-only interface that connects the Mobility Management Entity (MME) in the LTE Evolved Packet Core (EPC) to the HRPD Access Network (AN), which includes entities like the HRPD Base Station Controller (BSC) or Access Network Gateway (ANGW). The interface does not carry user plane traffic; its sole purpose is to exchange control plane messages necessary for preparing and executing handovers. This preparation is crucial for minimizing service interruption when a User Equipment (UE) moves from LTE coverage to a CDMA HRPD coverage area, or vice-versa.

Architecturally, the S101 interface operates over an IP-based transport network. The communication is governed by the S101 Application Protocol (S101-AP), which defines the specific messages and procedures. Key procedures enabled over S101 include the transfer of HRPD session information from the UE to the target network, the pre-registration of the UE with the HRPD network while it is still camped on LTE, and the actual handover command signaling. The MME acts as the 3GPP endpoint, while the HRPD AN (or a dedicated HRPD Serving Gateway, HSGW, in some signaling flows) acts as the 3GPP2 endpoint.

From a network perspective, the S101 interface is a cornerstone of the Non-3GPP Interworking architecture specified for CDMA2000 networks. It allows the EPC to treat a pre-configured, trusted CDMA HRPD network as an extension of its own radio access, enabling tighter integration than generic untrusted non-3GPP access. The interface works in concert with the S103 interface, which handles the forwarding of user plane data during the handover process. By separating control (S101) and data (S103) paths, the architecture allows for optimized, low-latency signaling for handover decisions while efficiently managing data path switching.

Purpose & Motivation

The S101 interface was created to solve a critical market need in the late 2000s during the initial deployment of 4G LTE networks. Many operators, particularly in North America, had existing large-scale investments in 3GPP2 CDMA2000 (e.g., EV-DO) networks. A smooth migration path to LTE was essential, requiring the ability for dual-mode devices to seamlessly transition between the new LTE technology and the legacy CDMA HRPD technology without dropping data sessions. This was a key requirement for providing a consistent user experience and encouraging early adoption of LTE services.

Prior to standardized interfaces like S101, inter-technology mobility was either non-existent or relied on proprietary, vendor-specific solutions that limited interoperability and increased complexity. The alternative for a UE would be a "break-before-make" handover: completely disconnecting from LTE, performing a full CDMA network acquisition and session setup, and then re-establishing the data session. This could result in interruptions of several seconds, which was unacceptable for real-time services like Voice over IP or video streaming.

Therefore, the S101 interface, introduced in 3GPP Release 8 as part of the initial EPS architecture for non-3GPP interworking, provided a standardized, optimized handover mechanism. It addressed the limitation of previous approaches by enabling the network to prepare the resources in the target (HRPD) system *before* the UE disconnects from the source (LTE) system. This "make-before-break" principle, enabled by S101 signaling, drastically reduced handover latency and packet loss, making LTE-to-CDMA handovers virtually seamless for the end user.

Key Features

• Enables optimized handover (HO) procedures between LTE and CDMA HRPD access technologies.
• Carries only control plane signaling for session transfer and pre-registration.
• Uses the S101 Application Protocol (S101-AP) for message exchange.
• Supports UE context transfer from MME to HRPD Access Network.
• Facilitates pre-registration of the UE with the HRPD network while connected to LTE.
• Works in conjunction with the S103 interface for user plane handling during HO.

Evolution Across Releases

Defining Specifications