S103 Interface

Description

The S103 interface is the user plane counterpart to the control plane S101 interface within the 3GPP-defined architecture for interworking with CDMA2000 HRPD networks. It provides a temporary data forwarding tunnel between the LTE Evolved Packet Core (EPC) and the CDMA HRPD network core. Specifically, it connects the Serving Gateway (SGW) – the EPC anchor for the user plane during mobility – to the HRPD Serving Gateway (HSGW) – the gateway that provides IP connectivity for the UE in the HRPD network. The primary, and almost exclusive, function of S103 is to support downlink data forwarding during an optimized handover from LTE to HRPD.

Architecturally, S103 is implemented as a GTP-based (GPRS Tunneling Protocol) tunnel. When a handover from LTE to HRPD is decided and prepared via S101 signaling, the MME instructs the SGW to establish a temporary indirect forwarding path for the UE's downlink data. Instead of sending packets to the eNodeB (which the UE is about to leave), the SGW starts duplicating and forwarding them through the S103 GTP tunnel to the HSGW. The HSGW buffers these packets until the UE successfully completes its attachment to the HRPD radio network. Once the UE's radio link is up in HRPD and its data path is switched to the HSGW, the buffered packets are delivered, and the S103 tunnel is subsequently released.

This mechanism is crucial for a seamless handover experience. While the UE is switching its radio transceiver and performing access procedures on the HRPD side (a process that can take hundreds of milliseconds), downlink data from the internet continues to flow to the SGW. Without S103, these packets would be dropped at the SGW because the path to the LTE eNodeB is no longer valid. The S103 tunnel ensures these in-flight and newly arriving packets are preserved and delivered immediately after handover completion, effectively eliminating downlink packet loss. It's important to note that S103 typically handles only downlink forwarding; uplink data from the UE during the transition is usually handled via buffering in the UE itself or treated as new data after connection.

Purpose & Motivation

The S103 interface was created to address a specific weakness in inter-RAT handovers: data loss during the switching period. While the S101 interface solved the control plane coordination problem, ensuring the target network was ready to accept the UE, a gap remained for user data already in transit. In a high-speed data session, even a few hundred milliseconds of interruption can result in the loss of numerous packets, causing TCP timeouts, video buffering, or call drops in VoLTE. The S103 interface was the engineered solution to this problem.

Its design was motivated by the principle of "seamless" mobility, a key goal for LTE/EPC. A handover should be imperceptible to the application layer. Previous systems without such a data forwarding mechanism could only achieve "break-before-make" handovers with noticeable disruption. S103 enables a "make-before-break" characteristic for the user plane, at least in the downlink direction. By leveraging the existing GTP protocol already used extensively within the EPC (e.g., on the S5/S8 interfaces), the solution was elegant and reused known network elements and protocols.

The interface specifically solves the problem of downlink data path continuity. During the handover execution phase, the UE's point of attachment changes from the LTE eNodeB to the HRPD Base Station. The core network anchor (PGW) remains the same, but the intermediate routing point (from SGW to access) must be re-established. S103 provides a temporary bridge for this re-routing, ensuring that data is never without a path to its destination. This was a critical enhancement that, combined with S101, allowed operators to market LTE with true seamless mobility into their legacy CDMA coverage areas, protecting the user experience for real-time services.

Key Features

• Provides a temporary user plane tunnel for downlink data forwarding during LTE-to-HRPD handover.
• Uses GTP protocol (GTP-U) over IP transport, consistent with EPC architecture.
• Connects the LTE Serving Gateway (SGW) to the HRPD Serving Gateway (HSGW).
• Minimizes packet loss and service interruption for downlink traffic.
• Operates in conjunction with S101 control plane procedures; established and released based on MME commands.
• Typically used for indirect forwarding where data is tunneled via the core network gateways.

Evolution Across Releases

Defining Specifications