White Paper - Lawful Intercept Overview
Introduction
The FCC's ruling that US VoIP service providers must comply with CALEA's Lawful Intercept requirements by May 2007 has accelerated the need to understand what is involved in achieving compliance. This White Paper looks at the general architecture of Lawful Intercept in VoIP networks.
Lawful Interception (LI) is a requirement placed upon service providers to provide legally sanctioned official access to private communications. With the existing Public Switched Telephone Network (PSTN), Lawful Intercept is performed by applying a physical 'tap' on the telephone line of the target in response to a warrant from a Law Enforcement Agency (LEA). However, Voice over IP (VoIP) technology has enabled the mobility of the end-user, so it is no longer possible to guarantee the interception of calls based on tapping a physical line.
Whilst the detailed requirements for VoIP Lawful Interception may differ from one jurisdiction to another, the general requirements are the same. The LI system must provide transparent interception of specified traffic only and the subject must not be aware of the intercept. The service provided to other users must not be affected during interception.
Architecture Overview
Although the detail of LI may vary from country to country we can look at the general logical and physical requirements and also explain much of the common terminology used. The primary purpose of the service provider network is to enable private communications between individuals; any interception functionality built into the network must not affect the normal service to those individuals. The architecture requires a distinct separation of the Public Telecom Network (PTN) and the networks used for distribution and processing of LI information. The interfaces between the PTN and the Law Enforcement Monitoring Facility (LEMF) are standardised within a particular territory.
Figure 1 - General Network Arrangements for Interception (ETSI)
Lawful Interception deals with two 'products', these are; Contents of Communications (CC) and Intercept Related Information (IRI). Contents of Communications is exactly what it sounds like: the voice, video or message contents. Intercept Related Information refers to the signalling information, the source and destination of the call etc.
Figure 1 shows the logical flow of Intercept Related Information (IRI) and Contents of Communications (CC) from its collection in the Public Network to the handover interface to the Law Enforcement Monitoring Facility (LEMF) as defined by ETSI. In North America CALEA (Communications Assistance for Law Enforcement Act) requires operators to provide LI capabilities. The network architecture and handover specifications are based on the PacketCable™ surveillance model shown in Figure 2 below, the general architectural similarities can be seen.
Figure 2 - PacketCable™ Surveillance Model
Figure 3 below shows the high-level functions and interfaces as defined by ETSI, the Mediation Function (MF) provides standardized interfaces, HI2 - Intercept Related Information, and HI3 - Call Contents, from the Public Telecom Network to the LEA Network.
Figure 3 - Distinction between PTN and Law Enforcement Network
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