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The Voice over Wi-Fi (VoWiFi) service enables subscribers to access EPC networks using Wi-Fi (untrusted, non-3GPP access) and register with IMS networks so that they can use IMS services such as voice call, SMS, and supplementary services. Wi-Fi connections are supplementary to LTE network coverage. When LTE network signals are unavailable, subscribers can use Wi-Fi to access EPC networks. The LTE and Wi-Fi networks provide the handover function to ensure voice continuity for subscribers moving between the two networks.  Non-3GPP networks include WLAN (Wi-Fi), CDMA, HRPD, and WiMax, and support the following access types: Untrusted access includes any type of Wi-Fi access that the carrier has no control over, for example, subscribers' home Wi-Fi access. Trusted access is a carrier-built Wi-Fi access.
BenefitsTable 1 Benefits of the VoWiFi serviceFor... | Benefits |
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Carriers | Provide IMS services for subscribers accessing EPC networks using Wi-Fi connections, improving subscriber satisfaction in areas with poor LTE network signals. Provide subscribers with rich voice service experience, improving subscriber loyalty and registration rate.
| Subscribers | | Application ScenarioA VoWiFi UE accesses a Wi-Fi network, uses the evolved packet data gateway (ePDG) to attach to an EPC network, and then registers with an IMS network to use services. Figure 1 VoWiFi service networking
The access point (AP) converts wired signals into wireless signals to provide Wi-Fi access for UEs. The access control (AC) manages and controls all APs on a Wi-Fi network. RequirementsTable 2 Requirements for implementing the VoWiFi serviceNE | Version | License Requirements | Function |
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ePDG | UGW9811 V900R011C80 or later | This is an optional service provided by the UGW9811 and requires the license control item Supporting VoWiFi. | Interworks with the authentication, authorization, and accounting (AAA) server and PDN gateway (P-GW) and provides signaling and data channels for UEs. | P-GW | UGW9811 V900R010C07 or later | This is a basic service provided by the UGW9811 and does not require a license. | Provides basic bearers for UEs to access external networks to use data services. Receives data from subscribers on untrusted non-3GPP networks and sends received data to corresponding external networks; or receives data from external networks, selects transmission paths on EPC networks based on the destination addresses, and sends data to the corresponding ePDG. | MME | USN9810 V900R012C02 or later | This is an optional service provided by the USN9810 and requires the license control item PS Handover Between LTE and WiFi. | Sets up or deletes bearers on LTE networks during subscriber handovers between Wi-Fi networks and LTE networks. | AAA server | UIM V900R003C00 or later | This is an optional service provided by the UIM and requires the following license control items: VoWiFi Untrusted Access of USIM Card Session VoWiFi Untrusted Access of Non-USIM Card Session VoWiFi AAA Without USIM Cards Authentication Algorithm
| Functions as the unified authentication center. It interworks with the ePDG over the SWm interface to authenticate UEs. Interworks with the P-GW over the S6b interface and interworks with the convergent home location register (HLR)/home subscriber server (HSS) over the SWx interface. The AAA server updates the P-GW addresses stored on the convergent HLR/HSS to ensure uninterrupted PDN connections for UEs roaming between LTE and Wi-Fi networks.
| Convergent HLR/HSS | HSS9860 V900R009C00 or later | This is an optional service provided by the HSS9860 and requires the license control item Non-3GPP Access. | Stores subscription and authentication data of UEs. | Policy and charging rules function (PCRF) | UPCC V300R006C30 or later | This is a basic service provided by the UPCC and does not require a license. | Sends Policy and Charging Control (PCC) rules for bearer establishment and termination. | Proxy-call session control function (P-CSCF) | SE2900 V300R002C00 or later | This is an optional service provided by the SE2900 and requires the license control item VoWiFi Access (per concurrent session). | Provides an interface for UEs to access IMS networks. | Serving-call session control function (S-CSCF) | CSC3300 V100R010C30 or later | This is a basic service provided by the CSC3300 and does not require a license. | Functions as the central node of IMS networks and provides the following functions: Performs session routing control. Contacts the advanced telephony server (ATS), after checking all messages transmitted by itself based on iFC data and determining that conditions for contacting the ATS are met.
| Interrogating-call session control function (I-CSCF) | CSC3300 V100R010C30 or later | This is a basic service provided by the CSC3300 and does not require a license. | Queries the convergent HLR/HSS to determine the S-CSCF with which a called party has registered. | ATS | ATS9900 V100R006C30 or later | This is an optional service provided by the ATS9900 and requires the license control item VoWiFi Service. | Functions as the IP short message gateway (IP-SM-GW) to provide SMS interworking between VoWiFi UEs and CS UEs and to provide enhanced IP SMMT. Functions as the multimedia telephony application server (MMTel AS) to provide basic and supplementary services for multimedia calls. Performs terminating access domain selection (T-ADS) to implement domain selection on the network side. Functions as the IP multimedia service switching function (IM-SSF) to process intelligent network (IN) services for UEs. Performs handovers and updates media information about remote UEs.
| Entitlement server | CDM V100R001C10 or later | This is a basic service provided by the CDM and requires the following license control items: VoWiFi Entitlement Server Basic Package VoWiFi Entitlement Server Network Reliability Value Package
| Provides the following functions: The two entitlement servers use 1+1 active-standby mode. If the active server fails, the standby server immediately takes over services. | Impact on the SystemNone. Application LimitationsWhen CAP or INAP online charging is used, the CAP or INAP interface cannot distinguish between VoWiFi and VoLTE subscribers. Consequently, the ATS cannot report handover events between Wi-Fi and LTE networks. When the ePDG is deployed on the home network, the following services are unavailable to VoWiFi subscribers: Barring of Outgoing International Calls Except Those Directed to the Home PLMN Country (BOIC-exHC) Barring of Incoming Calls When Roaming Outside Home PLMN Country (BIC-ROAM) ODB Barring Outgoing International Communications Except Those Directed to the Home PLMN Country (ODB_BOIC_EXHC) ODB Barring of All Outgoing Calls When Roaming Outside the HPLMN Country (ODB_BOC_ROAM) ODB Barring Incoming Communications When Roaming Outside the Home PLMN Country (ODB_BAIC_ROAM)
When only Wi-Fi signals are available, the VoLTE or VoWiFi network cannot obtain Wi-Fi location information about VoWiFi UEs, because Wi-Fi locations are not defined in 3GPP, IETF, or GSMA standards. Typically, the short message service center (SMSC) generates call detail records (CDRs) when they use IP short messages. However, VoWiFi and VoLTE CDRs cannot be distinguished due to restrictions of MAP. To support international roaming for VoWiFi subscribers, the ePDG must be deployed on the home EPC network. No additional fees are charged for international roaming. If VoWiFi subscribers report information about the roamed-to country, the ATS records the information in ACR messages but does not charge the subscribers for international roaming. If an external location retrieval function (LRF) is deployed, the emergency call service based on longitude and latitude cannot be implemented. The SBC validates and modifies the access-type contained only in the P-Access-Network-Info header field of REGISTER, INVITE/200, 180/183, and MESSAGE/200 messages. The ePDG does not support E2E signaling trace, and the P-GW does not support E2E signaling trace over S6b and S2b interfaces. The rich communication suite application server (RCS AS) cannot be used as an IP-SM-GW. CS Retry is not supported when the SCC AS, IM-SSF, and MMTel AS are separately deployed, or the SCC AS and the IM-SSF/MMTel AS are separately deployed. IMEI information of access UEs cannot be obtained in calls with anchoring triggered for VoLTE subscribers, and IMEI information is not included in CDRs generated on IMS networks. Therefore, CDRs generated on the CS network must be associated to.
Feature InteractionsTable 3 Interactions between the VoWiFi service and other servicesService | Interaction |
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VoLTE International Roaming Service Flexible Anchoring | The ATS does not trigger the VoLTE International Roaming Service Flexible Anchoring service for an international roaming subscriber who has subscribed to the VoWiFi service. | VoLTE Supplementary Service Self-Provision | When a UE accesses the ePDG using Wi-Fi, a bearer established using a Ut-dedicated APN must be used to carry VoLTE supplementary service setting requests. This bearer is also used for supplementary service setting on VoLTE+VoWiFi networks. | IN service | If an IN call fails to be connected on the Wi-Fi network and the called party has ICS registration information, ICS Retry is performed before the IN service is triggered. If the called party does not have ICS registration information, CS Retry is performed before the IN service is triggered. If the IN service is triggered for subscribers who are roaming internationally, CDRs are generated by the CS domain. | Technical DescriptionOverview To support VoWiFi, the ePDG, AAA server, and entitlement server are added to the existing VoLTE network architecture. The ePDG provides EPC network access for UEs using Wi-Fi connections. The AAA server authenticates the UEs and obtains subscription data from the convergent HLR/HSS. The entitlement server implements auto-authentication and authorization on VoWiFi service for UEs. A UE accesses the ePDG through a Wi-Fi network. The ePDG interworks with the AAA server to authenticate the UE. After the UE is authenticated, the ePDG forwards a service request to the P-GW over the S2b interface. After the UE is attached to the EPC network, it registers with an IMS network. After the registration, the UE can use IMS services. Table 4 Typical scenarios of the VoWiFi serviceScenario | Description |
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Service provisioning | Authentication | Authentication is implemented in the following ways: | VoWiFi Activation | The business support system (BSS) queries or activates the VoWiFi service. | VoWiFi deactivation | When the BSS receives a deactivation request from the entitlement server, it deletes VoWiFi subscriber data. | Registration | EPC attach | If only Extensible Authentication Protocol Authentication and Key Agreement (EAP AKA) is used, algorithm negotiation is optional. | Registration | After receiving a REGISTER message, the P-CSCF obtains the RAT-Type from the PCRF over the Rx interface and compares it with the access-type in the P-Access-Network-Info header field of the REGISTER message. | Deregistration | The deregistration procedure is the same as that on VoLTE networks. Deregistration is classified into UE- and network-initiated deregistration. | EPC Detach | PDN connections need to be deleted in any of the following scenarios: | Call | MO Calls | After receiving an INVITE message, the P-CSCF obtains the RAT-Type from the PCRF over the Rx interface and compares it with the access-type in the P-Access-Network-Info header field of the INVITE message. If a UE accessing an EPC network using Wi-Fi originates a call and data configuration in MOD VOWIFICFG enables the ATS to obtain a UE's location information, the ATS queries the HLR/HSS for the UE's location information.
| MT Calls | During T-ADS, the convergent HLR/HSS determines whether the called party has VoWiFi registration information. If the called party has, the S-CSCF routes the call to a Wi-Fi network. If the call fails to be connected on the Wi-Fi network and the called party has ICS registration information, the S-CSCF reselects an ICS network to route the call. If the called party does not have ICS registration information, the S-CSCF routes the call to a CS network. If a UE accessing an EPC network using Wi-Fi receives a call and data configuration in MOD VOWIFICFG enables the ATS to obtain a UE's location information, the ATS queries the HLR/HSS for the UE's location information.
| IN Calls | If a UE accesses an EPC network using Wi-Fi, the IM-SSF obtains the UE's location information using the following methods: Querying the HLR/HSS for the UE's IN location information, if data configuration in MOD INCRTCTRL and MOD VOWIFICFG enables the IM-SSF to obtain a UE's IN location information. Using ADD INLOCINF to configure the UE's default location information, if the IM-SSF does not obtain or fails in obtaining the UE's location information on the CS or LTE network.
The IM-SSF constructs an IDP message carrying the UE's location information and sends the message to the service control point (SCP) to trigger IN services. | SMS | SMMO Procedure | After a UE initiates an SMS message, the IP-SM-GW forwards the SMS message to the short message service center (SMSC), which then forwards the SMS message to the recipient. | SMMT Procedure | The convergent HLR/HSS determines whether the recipient has VoWiFi registration information. If so, the IP-SM-GW routes the SMS message to a Wi-Fi network. If sending the SMS message on the Wi-Fi network fails, the IP-SM-GW reselects a CS network for sending the SMS message. | Handover | Handover from Wi-Fi networks to LTE networks | A UE initiates a handover based on the signal strength and the P-GW remains unchanged before and after the handover. | Handover from LTE networks to Wi-Fi networks | Emergency call | Emergency Call Procedure | A 2G, 3G, or 4G network is preferentially selected to carry emergency calls. A Wi-Fi network is selected when only Wi-Fi signals are available. | Supplementary service setting | Supplementary service setting | When a UE accesses the EPC network using Wi-Fi, a default bearer using a Ut-dedicated APN must be established to carry supplementary service setting requests. The default bearer is established when the UE sends a supplementary service setting request. | International roaming restriction | VoWiFi international roaming restriction | International roaming calls from UEs are restricted based on the country information carried in messages sent by the UEs. | Charging and CDRsWhen a VoWiFi subscriber is in the international roaming state, no roaming fees are charged since the ePDG and session border controller (SBC) are deployed on the home network and the subscriber does not use network resources of carriers in the roaming area. It is recommended that the same tariff should be used for VoWiFi and VoLTE subscribers and the ATS should not generate intermediate CDRs. After receiving a re-registration request from a VoWiFi subscriber, the ATS determines whether the following conditions are met: If all the conditions are met, the ATS sends a CCR [Update] message to the online charging server (OCS). The message contains the Access-Network-Information, Trigger, and Trigger-Type attribute-value pairs (AVPs). The VoWiFi subscriber is re-registering with the network, and the P-Access-Network-Info header field in the re-registration request contains a different value from that in the initial registration flow. The ATS has sent a CCR [Initial] message. CCR Update upon VoWiFi Switch is set to Yes by running MOD VOWIFICFG.
If Include location info in ACR/CCR in MOD VOWIFICFG is set to YES, the ATS populates the ACRs or CCRs with the subscriber location information and access type obtained from the HSS. The ATS does not generate interim CDRs specifically for a call transfer. It only records the call transfer related information in the first CDR generated after the call is transferred.
When a VoWiFi subscriber initiates a call, the ATS generates CDRs. In the CDRs, the Access-Network-Information AVP records access network information, and the TADS-Indication AVP records that the T-ADS result is Wi-Fi. When a VoWiFi subscriber initiates an SMS message, the SMSC cannot determine whether the subscriber accesses through a Wi-Fi or LTE network. Therefore, it is recommended that the same charging mode be used for the subscriber, regardless of whether the subscriber accesses through a Wi-Fi or LTE network. If distinctive charging is required, the IP-SM-GW, rather than the SMSC, generates CDRs. When a VoWiFi subscriber performs a handover between Wi-Fi and LTE networks, the ATS can generate intermediate CDRs. You are advised to configure the ATS not to generate intermediate CDRs. By default, the CCF does not record segment CDRs for a call in which a handover between Wi-Fi and LTE networks occurs. To enable the function, set GenPCDRbyNetworkTransfer in the CDR format engine package on the CCF to 1. To enable the ATS to generate an ACR [Interim] for a handover between Wi-Fi and LTE networks, run MOD VOWIFICFG to set ACR INTERIM for RF When VOWIFI Switch to YES. To enable the ATS to fill in the PANI of an ACR [Interim] with the access network type after a handover, run MOD RFPARAM in the MML Command - ATS9900 window of the OMU client to set VCU AVP Control List to Access-Network-Information and run MOD RFPARAMEX to set VCU Interim AVP Control List and VCU Stop AVP Control List to Access-Network-Information.
If CS Retry has been triggered when the IN service fails to be connected over Wi-Fi, the system suppresses the ATS from triggering the IN service and triggers the IN service on the CS domain again. If the IN service is triggered for subscribers who are roaming internationally, CDRs are generated by the CS domain. To trigger the IN service after CS Retry, run MOD CSRETRYCFG in the MML Command - ATS9900 window of the OMU client and set Trigger IN service after CS Retry to YES.
CDRs The CDRs generated by the ATS for the VoWiFi service carry the following key AVPs: [Access-Network-Information]: carries the P-Access-Network-Info header field to record the access network information. [Trigger-Type]: specifies the type of the event for which the ATS generates the CCR. When the access network type changes, the CCR [Update] message contains this AVP. [Service-Specific-Type]: specifies the country code information. [Service-Specific-Data]: contains the country information if the [Service-Specific-Type] AVP is set to 502.
Standards ComplianceTable 5 Standards with which the VoWiFi service compliesCategory | Name | Purpose | Remarks |
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IETF | RFC6442 Location Conveyance for the Session Initiation Protocol | Defines the location conveyance for SIP. | None. | 3GPP | 3GPP23.402 Architecture enhancements for non-3GPP accesses (Release 11) | Defines an enhanced architecture for non-3GPP accesses. | None. | 3GPP | 3GPP33.402 Security aspects of non-3GPP accesses (Release 11) | Defined security aspects of non-3GPP accesses. | None. | 3GPP | 3GPP29.273 3GPP EPS AAA interfaces (Release 11) | Defines interfaces of the AAA server. | None. | 3GPP | 3GPP29.274 GTP (Release 11) | Defines the GPRS tunneling protocol (GTP). | None. | IETF | RFC 3891 The Session Initiation Protocol (SIP) Replaces Header | The Replaces header field is used to replace an existing session with a new session. The header field is used to implement attended transfers or call pickups. | None. | IETF | RFC 4306 Internet Key Exchange (IKEv2) Protocol | Define the IKEv2 protocol. | None. | IETF | RFC 5996 Internet Key Exchange Protocol Version 2 (IKEv2) | None. | IETF | RFC 7926 Internet Key Exchange Protocol (IKEv2) | None. | GSMA | GSMA IR.51 IMS over Wi-Fi | None. | None. | GSMA | GSMA IR.61 Wi-Fi Roaming Guidelines | None. | None. | EnhancementsTable 6 Enhancements of the VoWiFi serviceFeature Version | Release Version | Details |
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05 | VoLTE V500R018C00 | The ATS bars registration of VoWiFi subscribers based on the location information. | 04 | VoLTE V500R005C10 | The following functions were added: The ATS queries the HLR/HSS for the UE's location information on the CS or LTE network. The ATS includes the VoWiFi subscriber's location information and access type obtained from the HSS/HLR in UE's ACR or CCR messages.
| 03 | VoLTE V500R005C00 | The following functions were added: The IM-SSF queries the HLR/HSS for the UE's location information on the CS or LTE network. The IM-SSF records the Charge-Free indicator in CDRs for MT IN service suppression. The SCC AS instructs the IM-SSF to terminate all IN relationships in the IMS domain and re-trigger these terminated IN services in the CS domain if the MT call flow encounters CS retry. The ATS supports the VoWiFi BOC_ROAM Service.
| 02 | VoLTE V500R003C20 | The following functions were added: The ATS9900 is enhanced to record the T-ADS result (Wi-Fi) in the TADS-Indication AVP. The P-CSCF address remains unchanged when a VoWiFi subscriber is handed over from a Wi-Fi network to an LTE network. If the subscriber's UE is engaged in a call that is not in the ringing or active state, a handover is not initiated. The AAA server can provision services through the SPG.
| 01 | VoLTE V500R003C10 | First release. |
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