The VoLGA service resembles the 3GPP Generic Access Network (GAN). GAN provides a controller node - the GAN controller (GANC) - inserted between the IP access network (i.e., the EPS) and the 3GPP core network. The GAN provides an overlay access between the terminal and the CS core without requiring specific enhancements or support in the network it traverses. This provides a terminal with a 'virtual' connection to the core network already deployed by an operator. The terminal and network thus reuse most of the existing mechanisms, deployment and operational aspects.

According VoLGA specifications, The aim of VoLGA is to make traditional GSM/UMTS circuit switched (CS) services available to UEs accessing the EPS network via E-UTRAN.

GAN services and goals are reused in VoLGA wherever beneficial, and no re-invention of existing functionality shall occur.

VoLGA supports two modes of operation:

• VoLGA A-mode
• VoLGA Iu-mode

VoLGA A-mode supports an extension of GSM CS services that is achieved by tunnelling Non Access Stratum (NAS) protocols between the UE and the Core Network over EPS bearers and the A interface to the MSC.

VoLGA Iu-mode supports an extension of UMTS CS services that is achieved by tunnelling Non Access Stratum (NAS) protocols between the UE and the Core Network over EPS bearers and the Iu-CS interface to the MSC.

Non-roaming architecture

The operator reuses the existing CS domain entities (e.g., MSC/VLR) that control establishment of CS services under E-UTRAN coverage. The VANC enables the UE to access the MSC/VLR using the generic IP connectivity provided by the EPS. The VANC can be connected to the MSC/VLR using either the A-interface ("VoLGA A-mode") or the Iu-CS interface ("VoLGA Iu-mode"). From the EPS point of view, the VANC is viewed as an Application Function and the "Z1" interface between the UE and the VANC is based on the Up interface.

Roaming architecture

If the Visited PLMN supports "VoLGA service", the above architectures would apply, where the PDN Gateway (PGW), the Serving VANC and the MSC/VLR are located in the VPLMN.

Reference points

• SGi: SGi is defined in TS 23.401. It is the reference point between the P-GW and the packet data network. The "packet data network" is the CS core network connected by VANC in this specification.
• Sv: Sv is defined in TS 23.216, where it is defined as the reference point between the MME/SGSN and  MSC Server. In this specification, Sv applies to two interfaces: (1) between the MME and HOSF, and (2) between the HOSF and MSC Server.
• Z1: Z1 is the reference point between the UE and VANC, which is based on the Up interface defined in TS 43.318.
• Z3: Z3 is the reference point between the VANC and HOSF. It is based on GTPv2-C as specified in TS 29.274. The Z3 reference point is used for the creation and deletion of VANC-UE bindings in the HOSF, and to route the SRVCC PS to CS Handover Request message to the VANC.

VANC
The VANC behaves like a BSC (VoLGA A-mode) or RNC (VoLGA Iu-mode) towards the CS domain. The VANC also behaves like an Application Function (AF) towards the PCRF. The VANC includes a Security Gateway (SeGW) function that terminates a secure remote access tunnel from each UE, providing mutual authentication, encryption and integrity protection for signalling traffic.