EPC ProSe User ID

Description

The EPC ProSe User ID (EPUID) is a critical identifier within the 3GPP architecture for Proximity Services (ProSe), defined across specifications such as TS 23.303, TS 29.343, and TS 29.345. It uniquely identifies a user (more precisely, a ProSe-enabled subscriber) within the context of ProSe functions in the EPC. The EPUID is used by network-based ProSe entities—primarily the ProSe Function—to manage ProSe-related subscriber data, authorize ProSe capabilities, and facilitate direct discovery and communication between nearby UEs. It is distinct from other subscriber identifiers like IMSI or MSISDN, as it is specific to the ProSe service layer.

Architecturally, the EPUID is stored and managed within the ProSe Function in the subscriber's home network. When a UE registers for ProSe services, the ProSe Function allocates or maps an EPUID for that user. This identifier is then used in signaling between network functions, such as between the ProSe Function and the ProSe Application Server, or between ProSe Functions in different networks (for roaming scenarios). For direct discovery, a UE may broadcast a ProSe Application Code derived from its ProSe Application ID, which can be resolved by the network (using the ProSe Function) to the corresponding EPUIDs to enable discovery without revealing permanent identifiers over the air.

The EPUID plays a central role in the security and privacy of ProSe. It allows the network to authorize and track ProSe activities without necessarily exposing the user's permanent identity (IMSI) to other UEs or even to the application layer. The ProSe Function uses the EPUID to retrieve the subscriber's ProSe subscription profile from the HSS and to enforce policies. In direct communication (ProSe Direct Communication), the EPUID can be used within the network to establish security contexts and manage one-to-one or one-to-many communication groups. Its format and structure are defined to ensure global uniqueness within the ProSe domain, often constructed based on the user's subscription data.

Purpose & Motivation

The EPUID was introduced in 3GPP Release 12 as part of the standardization of Proximity Services (ProSe), which enables device-to-device discovery and communication. Its creation was motivated by the need for a dedicated, service-layer identifier that could support new use cases like public safety communications, social networking discovery, and commercial D2D services, without compromising user privacy or overloading existing core network identifiers.

It solved the problem of how to uniquely and securely identify users within the ProSe service framework. Using permanent identifiers like IMSI for over-the-air discovery would pose significant privacy risks. The EPUID provides an abstraction layer, allowing the network to map temporary ProSe codes or identifiers used over the radio interface to a stable network-internal user identity for authorization and management. This enables controlled discovery where the network can verify permissions before revealing a user's presence or contact information. The EPUID is fundamental to the network-controlled architecture of ProSe, distinguishing it from ad-hoc Wi-Fi Direct by providing carrier-managed security, billing, and interoperability.