VoLTE – Introduction
VoLTE Lawful Interception has been the main telecom service since the inception of mobile telephony. It is still relevant and is increasing as well in this day and age of mobile data and IP based telecom technology. In Evolved Packet System (EPS), which is completely IP based technology, delivering voice service can only be realized using either Circuit-Switched Fallback (CSFB) or Voice over LTE (VoLTE).
While CSFB offers voice services to LTE users without utilizing IMS by temporarily moving the user from LTE network to a legacy network i.e. 2G/3G system which can then serve voice call over circuit switched network. Whereas VoLTE is a digital packet voice service that is delivered over IP via an LTE access network utilizing IMS (IP Multimedia Subsystem) technology.
VoLTE: Lawful Interception
Lawful Interception of VoLTE is a complex process as a VoLTE call invokes two systems, i.e. EPC and IMS. Moreover, there are multiple network elements in these two systems which can provide IRI and CC of the target. PertSol offers two different approaches for the interception of VoLTE calls depending upon the MNO’s network and requirement:
i. By Interception of EPC nodes
ii. By Interception of both EPC and IMS nodes
MNO’s which already has an existing LI solution for its IMS system can use this approach. In this approach, PertSol will provide its LI solution (iNterceptor) which will intercept the EPC traffic (including VoLTE) by integrating with the EPC nodes (Intercept Access Points) like MME, SGW and PGW. It also facilitates the VoLTE interception for inbound roamers using S8HR interception as explained in below section.
PertSol iNterceptor can also be implemented as a combined LI solution for both IMS and LTE network of an MNO. This approach will involve integration with IMS IAPs in addition with the EPC IAPs explained in the first approach. The IMS access points that will be integrated for VoLTE interception are CSCF, BGF, TAS and MGW.
Lawful Interception: VoLTE Roaming Scenario
VoLTE roaming can be carried out using two ways, as defined in 3GPP specifications:
i. Local Breakout (LBO): Local Breakout mechanism requires the visited network to have an IMS system in its network. In LBO the roaming user is connected to the PDN-GW of the visited network and all the traffic is transported via visited network’s PDN-GW, without using the PDN-GW of home network.
ii. S8 Home Routing (S8HR): S8HR makes use of the home network PDN-GW and does not require the visited network to have an IMS system. The roaming user traffic is to the PDN-GW of home network, via S-GW of visited network, which then forwards it to the IMS network for completing the call.
S8 Home Routing (S8HR) is the preferred VoLTE roaming architecture of most of the mobile network operators as it does not dependent on the IMS system of the visited network. PertSol offers two ways of intercepting VoLTE calls for inbound roamer in S8HR roaming scenario:
i. Active Interception – By using BBIFF functionality of SGW
ii. Passive Interception – By intercepting S8 interface
S8HR Active Interception
PertSol iNterceptor is capable of intercepting the VoLTE calls of inbound roamer by providing LI Mirror IMS State Function (LMISF). For active interception of VoLTE calls, the SGW shall support BBIFF functionality (Bearer Binding Intercept and Forward Function). This approach is as per 3GPP TS 33.107 specifications and the recommendations of GSMA PRD IR.65. It requires implementation of proprietary interfaces Xia / Xib. Moreover this approach can also be extended to 5G with minor changes.
S8HR Passive Interception
In S8HR Passive Interception, PertSol uses its iNteliProbe which functions as an LI Access Point and investigates the traffic data tapped from S8 interface between S-GW of visited network and PDN-GW of home network. It requires passive taps at appropriate point in the VPLMN to duplicate the complete S8 traffic. This approach can also be used for 5G (N9 interface).