Handover Interface Port 2

Description

Handover Interface Port 2 (HI2) is a key interface defined within the 3GPP lawful interception (LI) architecture, standardized in technical specification 33.108. It serves as the delivery mechanism for Intercept Related Information (IRI), which is the call-associated data or signaling information related to a target's communication. The HI2 interface connects the Mediation Function (MF) or Delivery Function (DF) within the network operator's domain to the Law Enforcement Monitoring Facility (LEMF) operated by a government agency. The primary role of HI2 is to transport structured IRI data, which includes details such as the calling and called party identities (e.g., MSISDN, IMSI), call start and end times, duration, location information (when available), and supplementary service events. This data provides the context and metadata of a communication session without the actual voice or data content.

The architecture for HI2 is designed to ensure secure, reliable, and standardized delivery of interception data. The interface uses specific protocols and encoding formats, typically based on ASN.1 and encoded in BER (Basic Encoding Rules) or PER (Packed Encoding Rules), to ensure interoperability between different vendors' equipment and the LEMF systems. The communication over HI2 is generally initiated by the network operator's interception system upon detection of an event related to a monitored target. The data is packaged into defined messages (e.g., call-related IRI, SMS-related IRI, location update IRI) and transmitted to the LEMF. Security is paramount, so the interface specification includes requirements for secure transmission, often implemented over dedicated, protected networks or using VPNs and strong encryption to prevent unauthorized access to sensitive interception data.

HI2 operates in conjunction with HI3 (for Content of Communication) and HI1 (for interception activation and deactivation). While HI3 carries the actual voice call audio or packet data content, HI2 provides the essential metadata that makes the intercepted content intelligible and actionable for law enforcement. The separation of control/data (IRI on HI2) and user plane (content on HI3) follows a clear architectural principle within 3GPP LI. The interface must support high reliability and accurate sequencing of events to maintain a legally admissible audit trail. Its implementation is mandatory for network operators in jurisdictions with lawful interception mandates, making it a fundamental compliance component within the core network's security and mediation layers.

Purpose & Motivation

HI2 was created to standardize the delivery of Intercept Related Information (IRI) from telecommunications networks to law enforcement agencies. Prior to standardization, various countries and network equipment vendors used proprietary interfaces and formats for delivering interception data, leading to interoperability problems, high costs for law enforcement to integrate with multiple networks, and potential legal challenges regarding the admissibility of evidence. The 3GPP standardization, starting in Release 8, aimed to create a uniform, vendor-neutral interface that would simplify lawful interception for operators and ensure reliable, consistent data delivery for authorities.

The driving force behind HI2 and the broader 3GPP LI framework includes legal mandates in many countries requiring network operators to provide technical capabilities for lawful interception. These laws are designed to support national security and criminal investigations. By defining HI2, 3GPP addresses the operational need for a secure and efficient channel to transmit call-associated metadata. This metadata is often as critical as the communication content itself for investigations, as it establishes the who, when, and where of a communication. The interface solves the problem of fragmented, non-interoperable solutions, enabling law enforcement to receive standardized data feeds from any compliant network, whether 2G, 3G, 4G, or 5G, thereby future-proofing interception capabilities as network technology evolves.

Key Features

• Standardized delivery of Intercept Related Information (IRI) to Law Enforcement
• Transports call-associated metadata (e.g., parties, timestamps, location)
• Uses ASN.1-based protocols (e.g., ETSI LI standards) for encoding
• Designed for secure and reliable transmission over dedicated links
• Works in tandem with HI1 (activation) and HI3 (content) interfaces
• Ensures legal compliance for network operators in regulated markets

Evolution Across Releases

Defining Specifications