The X1 X2 X3 5G interface architecture represents a fundamental evolution in lawful interception design. The arrival of 5G has brought fundamental changes to how telecommunications networks are designed, deployed, and operated. For lawful interception, these changes are equally profound. The 3GPP LI architecture for 5G builds on the X1, X2 and X3 interfaces already known from 3G and 4G, but now with standardised protocols (ETSI TS 103 221-1 and TS 103 221-2) instead of the vendor-specific implementations used in previous network generations. For operators deploying 5G networks, understanding the X1/X2/X3 architecture is essential to building LI capabilities that meet regulatory requirements and function effectively in the new network environment.
本文详细阐述了5G的3GPP LI架构，重点介绍了X1、X2和X3接口，它们与ETSI HI接口之间的关系，以及这对运营商和 LI 解决方案 服务提供商。.
X1、X2、X3 5G 架构
根据 3GPP 的定义，5G 核心网采用基于服务的架构（SBA），这与 3G 和 4G 的基于节点的架构有着根本性的区别。 5G核心网不再依赖执行特定功能的专用网络元素，而是由一组网络功能（NFs）组成，这些功能通过基于服务的接口相互通信。 关键网络功能包括接入与移动性管理功能（AMF）、会话管理功能（SMF）、用户平面功能（UPF）以及许多其他功能。.
这一架构转变对LI具有重大影响。在以往的网络世代中，截获通常在特定的网络节点上实现——例如3G网络中用于语音的MSC，4G网络中用于数据的P-GW——这些节点是目标信号和用户平面流量汇聚的明确点。 在5G中，网络功能的分布式和虚拟化特性意味着必须采取不同的截获方法。3GPP LI架构通过定义LI系统与网络功能之间的标准化接口来解决这一问题，而不是依赖于特定节点的截获机制。.
3GPP TS 33.127 和 TS 33.128 中的 LI 架构
The 3GPP LI architecture for 5G is defined primarily in three specifications: TS 33.126 (Lawful Interception requirements), TS 33.127 (Lawful Interception architecture and functions) and TS 33.128 (Lawful Interception protocols and procedures, stage 3). These specifications define the functional components of the LI system within the 5G network, the interfaces between them, and the interfaces to the law enforcement domain.
The key functional components defined in TS 33.127 are the Administration Function (ADMF), which receives warrants (in its LI Control Function, LICF) and provisions the LI system (through its LI Provisioning Function, LIPF); the Points of Interception (POIs) inside or alongside the network functions, split into IRI-POIs and CC-POIs; Triggering Functions (TFs), which activate interception at POIs such as the UPF on demand; and the Mediation and Delivery Function (MDF), split into MDF2 for IRI and MDF3 for content, which delivers the result to law enforcement.
The interfaces between these components are designated X1, X2, and X3. These interfaces are internal to the operator’s domain — they connect the LI management and delivery functions to the network functions — and are distinct from the ETSI HI interfaces, which connect the operator to the LEMF. In practice, X2 and X3 feed the handover interfaces: the MDF2 turns xIRI received over X2 into IRI for HI2, and the MDF3 turns xCC received over X3 into CC for HI3. X1 has no HI counterpart: it is the operator’s internal provisioning channel, while HI1 carries the warrant from the authority to the operator.
X1：管理界面
X1 is the interface over which the ADMF provisions and manages the POIs, the triggering functions and the MDF. Through X1, the ADMF sends instructions to the POIs to activate, modify, or deactivate interceptions. The X1 interface carries the target identification parameters, the scope of the interception, and any technology-specific configuration information needed by the POI.
X1 uses ETSI TS 103 221-1: XML messages sent as HTTP POST requests over HTTPS (HTTP/1.1 or HTTP/2), with mutual TLS authentication using X.509 certificates. X1, X2 and X3 are not new as concepts (3GPP TS 33.107 already used them for 3G and 4G), but in previous generations their protocols were left to vendors. For 5G, TS 33.128 mandates the standardised ETSI protocols TS 103 221-1 (X1) and TS 103 221-2 (X2/X3), which can also be used for 4G and IMS.
The X1 interface supports various target identification methods, including SUPI (Subscription Permanent Identifier), GPSI (Generic Public Subscription Identifier), PEI (Permanent Equipment Identifier), and IP addresses. Because 5G conceals the permanent identifier on the radio interface (SUCI, 5G-GUTI), TS 33.127 adds identifier-association functions (IEF, ICF and IQF) that let the operator and the LEA map temporary identifiers to the target’s SUPI.
X2：IRI 传输接口
X2 is the interface for delivering intercept-related information from the Points of Interception to the LI Mediation and Delivery Function. When a target’s communications trigger events at the POI — such as registration, session establishment, mobility events, or session termination — the IRI-POI generates xIRI records and sends them to the MDF2 over X2.
The X2 IRI records in the 5G architecture are significantly richer than those in previous generations, reflecting the more complex signalling and session management in 5G networks. X2 records may include information about PDU (Protocol Data Unit) session establishment, QoS (Quality of Service) flow creation and modification, handover events, network slice selection, and service-specific parameters. The X2 records (xIRI) are defined in TS 33.128 as BER-encoded ASN.1 and carried in the binary X2 PDU format of ETSI TS 103 221-2.
The X2 interface operates in a push model — the POI generates and sends xIRI records to the MDF2 as events occur, without waiting for a request. The delivery must be timely and reliable, as the IRI forms a critical part of the intercepted material delivered to law enforcement. The MDF2 receives X2 records from potentially multiple POIs across the network, correlates them, and formats them for delivery to the LEMF over the HI2 interface.
X3：CC 交付接口
X3 carries content (xCC) from the CC-POIs to the MDF3. For 5G data sessions the CC-POI is in the UPF: when the target’s PDU session is set up, the triggering function in the SMF instructs the UPF over LI_T3 to duplicate the session’s user-plane packets — IP packets carrying voice, data, messaging, and other services — and send them over X3.
The X3 interface handles potentially large volumes of data, particularly for data-heavy targets. The interface must be dimensioned to handle the throughput requirements without packet loss, and the delivery mechanism must maintain the integrity and ordering of the intercepted packets. The MDF3 receives the X3 traffic, processes it as needed, and delivers it to the LEMF over the HI3 interface.
In the 5G architecture, the UPF is the primary POI for user-plane interception. However, the UPF may be deployed in different locations within the network — at the edge, in regional data centres, or in central facilities — depending on the network architecture and the services being provided. The LI system must be capable of interacting with UPFs wherever they are deployed, which may require distributed X3 collection points and centralised processing at the MDF3.
网络切片为 X3 拦截增添了新的维度。单个目标的流量可能流经多个网络切片，每个切片都有自己的 UPF 实例。LI 系统必须识别并拦截该目标在所有相关切片中的流量，以确保对目标通信的全面覆盖。.
X1/X2/X3 与 HI1/HI2/HI3 之间的关系
The X interfaces and the HI interfaces serve different but complementary roles in the overall LI architecture. The X interfaces operate within the operator’s domain, connecting the LI management and delivery functions to the network infrastructure. The HI interfaces operate at the boundary between the operator and law enforcement: HI1 carries the warrant to the operator, and HI2 and HI3 deliver the intercepted IRI and content to the LEMF.
The LI Mediation and Delivery Function serves as the bridge between these two interface sets. It receives X2 IRI records from the POIs, processes and formats them, and delivers the resulting ETSI-compliant IRI over the HI2 interface to the LEMF. Similarly, it receives X3 content (xCC) from the CC-POIs, such as the UPF, processes it, and delivers it over the HI3 interface. On the administration side, the ADMF receives the warrant over HI1 (typically ETSI TS 103 120), and its provisioning function sends X1 messages (ETSI TS 103 221-1) to the POIs, the triggering functions and the MDF itself.
This layered architecture provides a clean separation between the internal network interception mechanisms and the external handover to law enforcement. It allows the network functions to implement interception using standardised X interfaces without needing to understand the specific requirements of the national HI interface implementation, and it allows the MDF to adapt between different internal and external interface versions as standards evolve.
对运营商的实际影响
For operators deploying 5G networks, the X1/X2/X3 architecture has several practical implications. First, LI must be considered from the earliest stages of 5G network design. The POI interfaces within the network functions must be planned and provisioned, the MDF must be dimensioned for the expected interception volumes, and the connectivity between the POIs and the MDF must be established.
Second, operators must ensure that their 5G network function vendors support the X1, X2, and X3 interfaces as defined in TS 33.127 and TS 33.128 (ETSI TS 103 221-1/-2 as profiled by TS 33.128). TS 33.128 also makes ETSI TS 103 120 the HI1 protocol and ETSI TS 102 232-1/-7 the HI2/HI3 transport for 5G. Vendor support for the LI interfaces has historically been uneven, and operators should include LI interface compliance as a requirement in their procurement specifications.
Third, the virtualised and cloud-native nature of 5G networks introduces new challenges for LI deployment. The MDF and POIs may need to operate in containerised environments, scale dynamically, and maintain performance and reliability across distributed deployments. Operators must ensure that their LI solutions are designed for cloud-native operation and can keep pace with the dynamic nature of the 5G network.
结论
X1/X2/X3 接口代表了 3GPP 法律拦截（LI）架构的一次重大演进，反映了 5G 为网络设计和运营带来的根本性变革。X1 提供用于管理拦截操作的管理信道，X2 传输执法部门开展调查所需的丰富元数据，而 X3 则传输被拦截通信的内容。 这些接口与 ETSI HI 接口共同构成了一个全面的分层架构，可在最先进的电信网络中实现合法截取。对于运营商而言，理解并实施这些接口对于履行其法律义务，以及确保其 5G 网络支持有效且合规的合法截取能力至关重要。.
随着5G独立组网部署的加速，X1、X2、X3 5G接口变得越来越重要。运营商必须确保其X1、X2、X3 5G实施方案符合最新的3GPP规范。.
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