Description
The VoIP Positioning Centre (VPC) is a critical functional entity within the 3GPP architecture for supporting location-based services, specifically emergency calls (e.g., E911, eCall) originating from IP-based voice services like VoLTE, VoWiFi, and VoNR. Defined in TS 23.167, it acts as a specialized Location Server (LS) tailored for the IP Multimedia Subsystem (IMS) environment. Its primary role is to interface between the IMS core, which handles the call signaling, and the various location determination systems to acquire a caller's geographical position.
Architecturally, the VPC sits in the home network or a trusted third-party network. It interfaces with several key nodes. It receives location requests via the MLP (Mobile Location Protocol) over the Le interface, typically from a Gateway Mobile Location Centre (GMLC) or directly from an Emergency Call Session Control Function (E-CSCF). The VPC then orchestrates the process of obtaining location information. It may query the subscriber's Home Subscriber Server (HSS) to determine the serving network and available location capabilities. For network-based positioning, it can interact with location servers in the visited access network (e.g., an E-SMLC in LTE for UEs) via standardized interfaces (e.g., SLg, SLs in 4G/5G). It can also handle device-based location if the UE provides it in the SIP signaling (e.g., in a Geolocation header).
The VPC's operation involves several steps. Upon an emergency call setup, the E-CSCF identifies the need for location and sends a request to the VPC. The VPC initiates location retrieval procedures. It validates the request and determines the best method to obtain a fix—this could be leveraging the underlying 3GPP access network's positioning methods (e.g., OTDOA, ECID in LTE; NR positioning in 5G), using assisted-GNSS data, or accepting a UE-provided location estimate. The VPC then formats the location information (e.g., into civic address or geodetic coordinates) and returns it to the requesting entity. This location is used for two key purposes: routing the emergency call to the correct Public Safety Answering Point (PSAP) based on the caller's location (location-based routing), and providing the dispatcher with the caller's position for responder dispatch.
Furthermore, the VPC is responsible for location validation and privacy handling. It must ensure the location data meets accuracy requirements set by regulatory authorities. It also enforces privacy rules, ensuring location is only disclosed for authorized emergency purposes. In scenarios where the caller is roaming or using non-3GPP access (like Wi-Fi), the VPC's role becomes even more complex, requiring coordination with visited network elements or alternative location sources (like Wi-Fi positioning systems) to fulfill its function.
Purpose & Motivation
The VPC was created to solve a fundamental problem introduced by the transition to VoIP and IMS-based telephony: how to reliably and accurately determine the location of a caller for emergency services when there is no fixed, line-associated address as in traditional landline telephony, and when the caller may be mobile or using IP access from anywhere. Legacy circuit-switched mobile networks had a well-established architecture for location services (LCS) centered on the GMLC. However, IMS decouples the service layer from the access network, complicating location retrieval for emergency calls originating over IP.
The primary motivation was regulatory compliance and public safety. Governments worldwide mandate that emergency calls must deliver the caller's location to enable rapid response. The existing mobile LCS architecture was designed for non-emergency services and was not fully integrated with the IMS call control procedures. The VPC was standardized to bridge this gap, providing a dedicated, access-agnostic function that could interface the IMS emergency call framework with the variety of location determination technologies available across different access types (LTE, 5G, WLAN).
It addressed the limitations of previous ad-hoc or non-standardized approaches. Before VPC standardization, early VoIP implementations struggled with emergency calling, often providing no location or only coarse network-derived location. The VPC provided a standardized, scalable, and secure method to obtain validated location. It also future-proofed the architecture for new access technologies and hybrid scenarios (e.g., a VoWiFi call from a device that also has cellular radio). By centralizing the location coordination logic for IMS emergency calls, the VPC simplifies network design, ensures consistent behavior across operators, and is crucial for meeting stringent emergency service requirements like those defined by NENA i3 and ETSI EMTEL standards in the IP era.
Key Features
- Specialized Location Server for IMS-based emergency and location-based services
- Orchestrates retrieval of caller location from network-based (e.g., E-SMLC), device-based, or hybrid methods
- Interfaces with IMS core (E-CSCF) via MLP over the Le interface and with access network location servers
- Provides location validation, formatting (civic/geodetic), and assurance to meet regulatory accuracy requirements
- Supports location-based routing of emergency calls to the correct PSAP
- Enforces privacy policies for location disclosure and handles roaming/non-3GPP access scenarios
Evolution Across Releases
Initial introduction of the VPC concept in TS 23.167 for IMS emergency sessions. Defined its basic role as a location server for VoIP emergency calls, specifying interfaces (Le) with the IMS core (E-CSCF) and its relationship with the GMLC and HSS. Established the need for it to support both network-based and terminal-based location methods.
Defining Specifications
| Specification | Title |
|---|---|
| TS 23.167 | 3GPP TS 23.167 |