Originating User Equipment

Description

The Originating User Equipment (O-UE) is a logical role assigned to a mobile device (such as a phone or data terminal) at the moment it initiates a request to establish a communication session. This is a foundational concept in 3GPP call models and service architecture, applicable across circuit-switched (CS), packet-switched (PS), and IMS-based services. The O-UE is the endpoint that originates a SIP INVITE for an IMS voice call, sends a SETUP message for a CS call, or activates a Packet Data Protocol (PDP) context for a data session. The network identifies the device as the O-UE based on the direction of the session establishment request.

When the O-UE sends an initial service request, it triggers a cascade of network procedures. In the control plane, the request is routed to the appropriate core network control node—the MSC Server for CS calls, the MME for LTE attach, or the S-CSCF for IMS sessions. This node then authenticates the subscriber using credentials stored on the UE's Universal Subscriber Identity Module (USIM). The network checks the subscriber's service profile from the HSS to determine what services are allowed (e.g., can this user make international calls?). For charging, the network begins generating charging records associated with the O-UE's identity (e.g., MSISDN).

The role of the O-UE is critical for the execution of originating service logic. In Intelligent Network (IN) architectures like CAMEL, a Detection Point (DP) for 'Collected_Info' or 'Analysed_Information' is triggered at the O-UE's serving network node. This allows the home network's Service Control Point (SCP) to apply custom logic, such as prepaid balance checks or number translation, before the call is routed further. In IMS, the originating side's S-CSCF applies initial Filter Criteria (iFC) to trigger application servers for services like call screening or voicemail. The physical and protocol layers of the O-UE are identical to any other UE; its designation is purely situational within a specific dialog or transaction.

Purpose & Motivation

The term O-UE exists to provide a clear and consistent reference point in 3GPP specifications for all procedures and service logic that depend on which party initiated a session. In telecommunications, the calling party and called party are subject to different regulations, charging models, and service treatments. By formally defining the O-UE role, 3GPP standards ensure that network behavior is unambiguous. For example, lawful interception mandates capturing different data for the originator versus the recipient. Similarly, charging systems must apply tariffs to the O-UE's account, not the terminating UE's.

Historically, this distinction was implicit in telephony but became explicitly modeled in digital mobile networks to support advanced features. As networks evolved from basic voice to support prepaid services, number portability, and complex call forwarding rules, the need to anchor logic to the originator became paramount. The O-UE concept allows the network to isolate and process the 'originating leg' of a call independently. This is especially important in roaming scenarios, where the O-UE is served by a visited network but its service profile resides in the home network. The visited network must recognize the device as an O-UE to properly interact with the home network for credit control and service authorization, ensuring a consistent user experience regardless of location.

Key Features

• Defines the UE that initiates a session establishment request (call, data session, IMS dialog)
• Triggers originating-side authentication, authorization, and accounting (AAA) procedures in the network
• Determines the application of originating call/service controls (e.g., barring, prepaid checks)
• Its identity (e.g., IMSI, MSISDN) is used as the charging party for the session
• Anchors the execution of originating service logic in CAMEL or IMS application servers
• A logical role; any UE can become the O-UE when it initiates communication

Evolution Across Releases

Defining Specifications