The UE detects the LTE network and determines to perform a handover from Wi-Fi to LTE.
When a VoLTE UE attaches to LTE, it executes the normal attach procedure as defined in 3GPP TS 23.401 [6] section 5.3.2.
LTE Radio capabilities are described in GSMA PRD IR.92 [54]; radio bearer capabilities – section 4.2.1, DRX mode of operation – section 4.2.2, and RLC configuration section 4.3.2.
The UE initiates the Attach Request to the eNodeB including (inter alia) the EPS Attach Type (set to “handover”), the IMSI and any one of the APNs, corresponding to the PDN connections in the source WLAN access.
The eNodeB selects the MME and forwards the Attach Request to the MME with the Selected Network and the TAI+ECGI location information of the cell where it received the message.
Authentication and security mechanisms are performed to activate integrity protection and NAS ciphering. The MME shall initiate the Security Mode Command to the UE containing the Selected NAS algorithms, eKSI, ME Identity request, and UE Security Capability. The UE responds with the Security Mode Complete with the NAS-MAC and ME Identity. After the completion, all NAS messages are protected by the NAS security functions (integrity and ciphering).
The MME performs an Update Location to the HSS to retrieve the subscriber profile.
Additional information includes the IMSI, MME Identity, ME Identity and MME capabilities.
The HSS confirms the Update Location to the MME with the related IMSI and subscriber data containing a PDN subscription context with a subscribed QoS profile and subscribed APN-AMBR (Aggregate Maximum Bit Rate). Since the Request Type is "Handover", the PDN GW identity conveyed to the MME will be stored in PDN subscription context. The MME also receives information on any other PDNs the UE is connected to over the WLAN access as well as the corresponding PGW identities.
The MME selects a serving GW and sends a Create Session Request (including IMSI, MME Context ID, PDN-GW address, Handover Indication, APN, APN-AMBR, user location
information (e.g. TAI+ECGI), RAT Type etc.) message to the selected Serving GW.
The SGW creates a new entry in the EPS Bearer table, allocating a relevant TEID for the control plane and the user plane, which enables it to route GTP control plane traffic between the MME and the PGW, and forwards the request to the PGW.
Due to the presence of the Handover Indication, the PGW re-uses the current IP address that was allocated as part of the original Wi-Fi access attachment. The PGW utilises dynamic PCC to initiate a Credit Control Request to the PCRF to obtain the default PCC rules for the default bearer to be used for IMS signalling. Included in the message are the IMSI, UE IP Address, default bearer QoS parameters (i.e. QCI=5, ARP, APN-AMBR), user location information, time zone information, RAT type (EUTRAN), etc. The PCRF binds the related policy rules to the IP Address of the default bearer, and responds to the PGW with the default TFT (traffic flow template) and potentially modified QoS parameters. In the message to the PGW, the PCRF shall also subscribe to modifications related to the default
bearer in the PGW (e.g. RELEASE_OF_BEARER. The PGW stores the new PCC Rules for E-UTRAN access as well as the old PCC Rules for the Untrusted Non-3GPP IP access and still applies the old PCC Rules for charging.
The PGW creates a new entry in the EPS Bearer table, allocating relevant TEID for the control plane and the user plane, which enables it to route user plane data between the SGW and the IMS network with the related policy rules obtained from the PCRF applied.
The PGW sends a Create Session Response to the SGW with the IP Address for the UE, QoS parameters, PCO, relevant TEID's for the GTP control plane and GTP user plane, etc.
The PGW maps the IMS-APN received in the request to a pre-configured IMS P-CSCF IP address and inserts this into the PCO as described in GSMA PRD IR.92 [54] section 4.4.
The response also contains the Charging Id previously assigned to the PDN connection in the non-3GPP access although the Charging Id still applies to the non-3GPP access.
The SGW returns the Create Session Response to the MME.
The MME sends an Attach Accept to the eNodeB with the IMS-APN, IP Address for the UE, QoS parameters, PCO, IMS Voice over PS supported indication, TAI list, ESM message container, etc. The eNodeB communicates with the UE to update the RRC configuration and includes the information received from the core network as part of the create session request.
The UE sends the Attach Complete message to the eNodeB, which forwards to the MME.
At this time, the UE is capable of sending uplink packets. The MME initiates a Modify Bearer Request to the SGW including the EPS Bearer Identity, eNodeB address, and eNodeB TEID.
The MME sends a Modify Bearer Request (eNodeB address, eNodeB TEID, Handover Indication) message to the Serving GW.
Since the Handover Indication was previously included, the Serving GW sends a Modify Bearer Request message to the PDN GW to prompt the PDN GW to tunnel packets from non 3GPP IP access to 3GPP access system and immediately start routing packets to the Serving GW for the default and any dedicated EPS bearers established. The PDN GW also removes the old PCC Rules for the Trusted or Untrusted Non-3GPP IP access and applies the new Rules for E-UTRAN access for charging. The Charging Id previously in use for the PDN connection in the non-3GPP access now only applies to the default bearer in use in E-UTRAN access.
The PDN GW acknowledges by sending Modify Bearer Response to the Serving GW.
The SGW acknowledges the request to the MME and is capable of sending downlink packets.
Finally, the PDN GW shall initiate resource allocation deactivation procedure for the
trusted/untrusted Wi-Fi access as defined in clause 16.4.1 or clause 7.9 of 3GPP TS 23.402 [81] and sends a Delete Bearer Request to the TWAG/ePDG. For the trusted case, in multi-connection mode, if all bearers belonging to a PDN multi-connection are released, then the UE is informed of the PDN connection release by means of a WLCP PDN Disconnection Request (PDN Connection ID). For the untrusted case, if all bearers belonging to a PDN connection
are released, the ePDG initiates an IKEv2 tunnel release procedure as described in RFC 5996 [89].
The TWAG/ePDG initiates a Delete Bearer Response to the PDN GW.
The above steps are repeated for all PDN connections that are transferred from the Wi-Fi access to the LTE access.
At this stage, the PDN Connection(s) of the UE has/have been handed over to LTE.
6.3 LTE to Wi-Fi HO
Handovers from LTE to untrusted/trusted Wi-Fi are described in sections 8.6.2 and 16.10.2 respectively of 3GPP TS 23.402 [81].
Users are assumed to have previously attached to LTE as described in sections 3.2.1 and 5.2.1 of GSMA PRD FCM.01 [95]. Users may or not be IMS registered, and in the former case may or may not have IMS sessions established. As IP address preservation is maintained during the HO, there is no impact on IMS due to the HO procedures.
In all cases, it is assumed that there is IP connectivity between cited elements as appropriate to each of the scenarios.
General 6.3.1
This section describes the HO procedures from LTE to Wi-Fi. The procedures are described separately for trusted (S2a) and untrusted (S2b) WLANs.