User Plane Location Protocol

Description

The User Plane Location Protocol (ULP) is a client-server protocol standardized jointly by 3GPP and the Open Mobile Alliance (OMA). It operates over IP-based user plane connections, typically using HTTP/HTTPS or SUPL as a transport layer, to facilitate the exchange of positioning-related information between a Secure User Plane Location (SUPL) Enabled Terminal (SET) – which is typically the User Equipment (UE) – and a SUPL Location Platform (SLP). The protocol carries messages for positioning procedures, including capabilities exchange, positioning method negotiation, and the delivery of assistance data or location measurements and results.

Architecturally, ULP messages are encapsulated within SUPL sessions. The SLP can act in two primary roles: the SUPL Location Center (SLC), which handles session management, privacy, and assistance data delivery, and the SUPL Positioning Center (SPC), which performs position calculation. The SET (UE) contains the SUPL Agent, which is the application requesting location, and the positioning engine. ULP defines a comprehensive set of message types, including SUPL START, SUPL RESPONSE, SUPL POS INIT, and SUPL END, which orchestrate the entire location session.

How it works involves a sequence of message exchanges. A location request triggers the SUPL Agent on the SET to initiate a ULP session by sending a SUPL START message to the SLP. This message includes the SET's capabilities and the desired Quality of Service (QoS). The SLP responds with a SUPL RESPONSE, indicating the chosen positioning method (e.g., A-GNSS, OTDOA, E-CID). Subsequently, a SUPL POS session is established where the SET and SLP exchange positioning-specific messages (using RRLP, RRC, or LPP/LLP protocols) to transfer assistance data, measurement data, and finally the computed location estimate. The session concludes with a SUPL END message.

Its role in the network is critical for commercial and regulatory location services. By using the user plane, it bypasses the need for core network signaling plane integration for every positioning technology, offering greater flexibility and faster deployment of new location features. It supports multiple positioning technologies, network-initiated and mobile-terminated location requests, and is foundational for emergency services like E911 and E112, as well as commercial applications like navigation and asset tracking.

Purpose & Motivation

ULP was created to provide a standardized, IP-based protocol for delivering location services over the user plane, addressing the limitations of control plane positioning methods. Prior to ULP, location services primarily relied on control plane signaling within the circuit-switched or packet-switched core network (e.g., using RRLP over the GSM/UMTS radio interface). This approach was tightly coupled to specific network architectures, making it complex to deploy new positioning methods and services across different operator networks and device types. The control plane methods could also incur significant signaling load on the core network.

The motivation for ULP, developed under the OMA SUPL standard and adopted by 3GPP, was to leverage the ubiquitous IP connectivity of data networks. By moving location signaling to the user plane, it decouples location services from the underlying radio access and core network technology. This allows for more rapid innovation and deployment of advanced positioning techniques (like A-GNSS and OTDOA) and services. It simplifies interoperability between handsets and location servers from different vendors.

Furthermore, ULP solves the problem of enabling high-accuracy, low-latency location services for emergency calls and commercial applications without overburdening the legacy circuit-switched infrastructure. It provides a secure, scalable framework (using HTTPS) that supports privacy controls, roaming scenarios, and a wide range of positioning protocols, making it the de facto standard for network-assisted GPS and hybrid positioning in modern mobile networks.