Interrupted Transmission Indication

Description

The Interrupted Transmission Indication (ITI) is a specific control mechanism embedded within the Iu user plane protocol, which is part of the Radio Access Network Application Part (RANAP) signaling. It operates at the interface between the Radio Network Controller (RNC) in UMTS and the Serving GPRS Support Node (SGSN) in the core network. The primary function of the ITI is to signal an interruption in the flow of user data packets over the Iu-PS (Packet Switched) interface. This interruption is not a random failure but a controlled event, most commonly triggered by mobility procedures like Serving Radio Network Subsystem (SRNS) relocation or inter-RNC handover. During such procedures, the data path for a user equipment (UE) is reconfigured, which can temporarily halt the forwarding of packets from the source RNC to the SGSN.

Technically, the ITI is conveyed using a specific control frame within the Iu user plane protocol data units. When an SRNS relocation is initiated, the source RNC sends an ITI control frame to the SGSN. This frame acts as a marker inserted into the stream of user data packets. It informs the SGSN that the transmission of user data from this particular RNC for this specific UE context has been intentionally interrupted. The SGSN, upon receiving this indication, understands that any packets received after the ITI might be out of sequence, duplicated (if forwarded from both old and new paths), or that there may be a gap in the data sequence numbers. This allows the SGSN to perform necessary internal adjustments, such as buffering management or sequence number realignment, to handle the subsequent data flow from the new RNC correctly.

The role of ITI is crucial for maintaining the integrity of packet data services during mobility events. Without it, the core network might misinterpret the interruption as a link failure or incorrectly process the arriving packets, leading to potential data loss, application errors, or degraded user experience. By providing this explicit in-band signaling, ITI enables a smoother and more predictable handover process. It is a foundational element for ensuring that packet-switched services like internet browsing or video streaming remain robust even as the user moves and the network redirects their data path through different network nodes.

Purpose & Motivation

The ITI was created to address a specific challenge in UMTS networks related to packet data continuity during active mobility procedures. In early 3GPP releases, as networks evolved to support more sophisticated packet-switched services, it became clear that handovers and relocations could cause disruptions in the data flow that were not adequately signaled to the core network. The SGSN, responsible for managing the UE's session and forwarding data to the GGSN, needed a way to distinguish between a normal, controlled interruption of data (due to a planned handover) and an abnormal, error-based interruption (like a link failure).

Prior to the standardization of ITI, the SGSN could only infer such events from other signaling messages, which might not provide precise timing or context for the user plane interruption. This ambiguity could lead to inefficient processing, such as unnecessary retransmissions from higher layers or incorrect assembly of data packets. The introduction of ITI in Release 8 provided a direct, in-band signaling mechanism on the user plane itself. It solved the problem by giving the RNC a standardized way to explicitly notify the SGSN the moment data transmission is halted for a controlled reason. This allowed the core network to optimize its handling of the subsequent data flow, improving the reliability and performance of real-time and non-real-time packet services during critical mobility events, forming a key part of the overall QoS and mobility management framework for UMTS.