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Single Voice Core Product Documentation
Product Version: V500R021C10
Library Version: 07
Date: 2022-09-30
For any question, please contact us.
Copyright © Huawei Technologies Co., Ltd. 2023. All rights reserved.
Handover Between Wi-Fi and LTE Networks
Contents
4.4.17.1.5 Handover Between Wi-Fi and LTE Networks
4.4.17.1.5 Handover Between Wi-Fi and LTE Networks
Handover from Wi-Fi Networks to LTE Networks
Handover from LTE Networks to Wi-Fi Networks
Re-registration
When a Voice over Wi-Fi (VoWiFi) subscriber moves from a Wi-Fi network to an LTE network or from an LTE network to a Wi-Fi network,
the subscriber's UE detects network signals and initiates a handover. To prevent voice services from being interrupted during the handover, the
PDN gateway (P-GW) must keep the UE's IP address and port number unchanged.
If the UE is not engaged in a call during a handover, a default bearer must be established on the target network, and the bearer established before the handover
must be deleted.
If the UE is engaged in a call during a handover and the session is in the ringing or active state, default and dedicated bearers must be established on the target
network, and the default and dedicated bearers established before the handover must be deleted. For details on how a dedicated bearer is established on the LTE
network, see Voice Call Procedure.
Upon detecting LTE signals during a handover, the UE requests to attach to the PS network and establish a data bearer.
Handover from Wi-Fi Networks to LTE Networks
When a UE accessing through a Wi-Fi network detects LTE signals and determines that the LTE signal strength reaches a specified handover
threshold, the UE initiates a handover from the Wi-Fi network to the LTE network. Figure 1 shows how the handover is implemented, and Figure
2 shows the corresponding message flow.
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Figure 1 Implementation of a handover from a Wi-Fi network to an LTE network
Figure 2 Message flow of a handover from a Wi-Fi network to an LTE network
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The message flow is as follows:
1: A UE accesses an EPC network through the evolved packet data gateway (ePDG). The ePDG establishes a GTP tunnel to the P-GW over the
S2b interface. During the establishment of the GTP tunnel, the Authentication, Authorization, and Accounting (AAA) server synchronizes the
P-GW information to the system architecture evolution home subscriber server (SAE-HSS).
The UE detects LTE signals and determines that the LTE signal strength is higher than a specified handover threshold and the Wi-Fi signal
strength is lower than a specified threshold. The UE then initiates a handover.
2: The UE sends an Attach Request or PDN Connectivity Request message to the mobility management entity (MME), requesting to establish
the default bearer. The message carries Request Type, which is set to handover, and the access point name (APN) used for access on the WiFi network. Figure 3 shows an example Attach Request message.
The APN used for the handover must be an IMS APN. Typically, the default bearer for data APN is established before the default bearer for IMS APN. (ADD
OPTGPRSTPL on the HSS9860 can be used to determine whether to preferentially establish the default bearer for data APN or the default bearer for IMS APN.) If the
default bearer for data APN needs to be preferentially established, the UE sends a PDN Connectivity Request message that carries handover; if the default bearer for IMS
APN needs to be preferentially established, the UE sends an Attach Request message that carries handover.
Figure 3 Example Attach Request message
3: The MME checks that the license item PS Handover Between LTE and WiFi has been activated (by running SET LICCTRL) and obtains
the UE's authentication data from the SAE-HSS to authenticate the UE.
4: The MME initiates a location update procedure and obtains subscription data. Based on the subscription data, the MME learns the P-GW
information mapping to the IMS APN before the handover occurs.
5: The MME sends a Create Session Request message carrying the IMSI, Handover Indication, P-GW address, and APN to the serving
gateway (S-GW) to establish a PDN connection. The S-GW forwards the message to the P-GW. After receiving the message, the P-GW neither
deletes the PDP context used for accessing through the Wi-Fi network nor changes the data packet tunnel from the Wi-Fi network to the LTE
network. Instead, the P-GW uses the IMSI and APN to query PDN connections. If multiple PDN connections are established for the same
APN, the P-GW determines the PDN connection to be handed over based on the UE's IP address. Figure 4 and Figure 5 show an example Create
Session Request message, in which hi indicates handover indication.
Figure 4 Example Create Session Request message
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Figure 5 Example Create Session Request message (continued)
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6: (Optional) The P-GW initiates a PCEF-Initiated IP CAN Session Modification procedure to obtain the PCC rule mapping to the new IPCAN type from the PCRF. The P-GW stores the new PCC rule for the LTE network and the old PCC rules for the Wi-Fi network. The P-GW
has not switched the data packet tunnel from the Wi-Fi network to the LTE network. Therefore, the old PCC rule is still used for charging.
7: The P-GW returns a Create Session Response to the S-GW. The message carries IP addresses of the UE and P-CSCF, which are the same as
the IP addresses assigned to the UE on the Wi-Fi network. The S-GW forwards the message to the MME. Figure 6 shows an example Create
Session Response message.
Figure 6 Example Create Session Response message
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8: The MME requests the E-UTRAN NodeB (eNodeB) to establish a radio bearer. The MME, eNodeB, and UE interact with each other to
establish the default bearer on the LTE network. For details on how a radio default is established, see Flow for Establishing a Default Bearer.
9: The MME sends a Modify Bearer Request message to the S-GW to update the bearer. The message carries the eNodeB address, eNodeB
TEID, and Handover Indication. The S-GW forwards the message to the P-GW. The P-GW changes the data packet tunnel from the Wi-Fi
network to the LTE network. The P-GW deletes the old PCC rule and uses the new PCC rule for charging.
Figure 7 Example Modify Bearer Request message
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10: The P-GW sends a Delete Bearer Request message, requesting the ePDG to release the bearer on the Wi-Fi network. In the message,
instance in ebi is set to 0, indicating that the default bearer must be deleted (the dedicated bearer is deleted after the default bearer is deleted).
In addition, the message carries cause-result that is set to access-changed-from-non-3gpp-to-3gpp. Figure 8 shows an example Delete Bearer
Request message.
Figure 8 Example Delete Bearer Request message
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11: The P-GW returns a Modify Bearer Response message to the S-GW. Then, the S-GW forwards the message to the MME. The message
carries EPS Bearer Identity, notifying the MME of a handover from the Wi-Fi network to the LTE network.
Figure 9 Example Modify Bearer Response message
12: If the UE is engaged in a call during the handover and the new PCC rule requires that a dedicated bearer be established for the UE, the SGW sends a Create Bearer Request message to the MME to establish a dedicated bearer.
Figure 10 Example Create Bearer Request message
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13: The ePDG returns a Delete Bearer Response message to the P-GW, indicating that the bearer on the Wi-Fi network has been deleted. The
P-GW instructs the AAA server to update the P-GW IP address and unbind the UE from the P-GW IP address. The AAA server sends an SAR
message to the SAE-HSS to deregister the UE. In the message, serverAssignmentType is set to userDeregistration.
14: The MME returns a Create Bearer Response message to the S-GW, which forwards the message to the P-GW to establish a dedicated
bearer on the LTE network.
Figure 11 Example Create Bearer Response message
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15: The dedicated bearer between the UE and S-GW is established, and the GTP tunnel between the S-GW and P-GW is established. The UE
receives and sends data over the LTE network.
16: (Optional) If multiple PDN connections need to be created, steps 2 to 10 are repeated to establish all of them.
A UE can simultaneously create multiple PDN connections to access different networks and use different APNs on the network side to access different networks.
For example, a UE uses the IMS APN to access a Wi-Fi network and the Internet APN to access an LTE network. The SAE-HSS, AAA server, ePDG, and MME
must notify the corresponding access network of the P-GW address mapping to an APN.
A UE can use multiple APNs to simultaneously access different PDNs. After the UE initiates requests to create PDN connections, the ePDG determines the PGW to be used for each PDN connection. During or after the establishment of the default bearer, the P-GW assigns an independent IP address to the UE for each
PDN connection. The UE uses the IP address to access the corresponding PDN. One or more dedicated bearers can be established for each PDN connection.
17: The P-GW initiates a Non-3GPP EPS Bearer Release procedure, requesting the non-3GPP network to release the WiFi network resources.
After the UE is handed over to the LTE network, the UE initiates a re-registration to update the registration information. The P-CSCF and ATS
generate intermediate CDRs based on the re-registration message.
Handover from LTE Networks to Wi-Fi Networks
When a UE accessing through an LTE network detects Wi-Fi signals and determines that the Wi-Fi signal strength reaches a specified handover
threshold, the UE initiates a handover from the LTE network to the Wi-Fi network. Figure 12 shows how the handover is implemented, and Figure
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shows the corresponding message flow.
Figure 12 Implementation of a handover from an LTE network to a Wi-Fi network
Figure 13 Message flow of a handover from an LTE network to a Wi-Fi network
The message flow is as follows:
1: A UE accesses an EPC network through an LTE network. A GTP tunnel is established between the S-GW and P-GW. After the UE is
attached to the EPC network, the SAE-HSS obtains the P-GW information either from the MME or the local subscription data.
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P1: The UE detects Wi-Fi signals and determines that the Wi-Fi signal strength is higher than a specified handover threshold and the LTE
signal strength is lower than a specified threshold. The UE then initiates a handover.
2: The UE authenticates the Wi-Fi network, and the Wi-Fi network authorizes the UE. For details on the authentication procedure, see EPC
Attach.
3: The EPC attach flow is triggered. The UE and AAA server authenticate each other as follows:
1. The UE sends an IKE_AUTH-Request message to initiate an authentication request to the ePDG.
2. The AAA server obtains the P-GW information and IP address of the UE on the LTE network from the SAE-HSS.
3. The ePDG requires the AAA server to provide the PDN information, P-GW information, and IP address before the handover.
4: The ePDG selects a P-GW based on the P-GW address before the handover and sends a Create Session Request message to the P-GW to
establish a PDN connection. The message carries the RAT Type, Handover Indication, and UE's IP address. After receiving the message, the PGW changes the data packet tunnel from the LTE network to the Wi-Fi network. Figure 14 shows an example Create Session Request message,
in which hi indicates handover indication.
Figure 14 Example Create Session Request message
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5: (Optional) The P-GW initiates a PCEF-Initiated IP CAN Session Modification procedure to obtain the PCC rule mapping to the new IPCAN type on the Wi-Fi network from the PCRF.
6: The P-GW sends an AAR message carrying its ID and APN to the AAA server. The AAA server synchronizes the information to the SAEHSS, and the SAE-HSS synchronizes the P-GW information mapping to the APN to the MME.
7: Upon receiving an AAA message from the AAA server, the P-GW sends a Delete Bearer Request, requesting the MME to delete the bearer
on the LTE network. In the message, instance in ebi is set to 0, indicating that the default bearer must be deleted (the dedicated bearer is
deleted after the default bearer is deleted). In addition, the message carries cause-result that is set to rat-changed-from-3gpp-to-non-3gpp.
Figure 15 shows an example Delete Bearer Request message.
Figure 15 Example Delete Bearer Request message
8: The P-GW returns a Create Session Response message to the ePDG. The message carries the UE's IP address, Charging Id, and P-CSCF
address. Figure 16 shows an example Create Session Response message.
Figure 16 Example Create Session Response message
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9: The ePDG returns an IKE_AUTH-Answer message to the UE. The ePDG sends the IP address assigned by the P-GW to the UE to complete
the establishment of an IPsec tunnel. Now the UE is attached to the EPC network.
10: If the UE is engaged in a call during the handover, the P-GW sends a Create Bearer Request message to the ePDG to establish a dedicated
bearer. An IPsec tunnel is established between the UE and ePDG, and a GTP tunnel is established between the ePDG and P-GW. The UE sends
and receives IP data packets over the Wi-Fi network.
Figure 17 Example Create Bearer Request message
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11: The MME sends a Delete Bearer Response to the P-GW, indicating that the bearer on the LTE network has been deleted.
12: The ePDG sends a Create Bearer Response to the P-GW, requesting to establish a dedicated bearer on the Wi-Fi network.
Figure 18 Example Create Bearer Response message
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13: The IPsec tunnel between the UE and ePDG is established, and the GTP tunnel between the ePDG and P-GW is established. The UE
receives and sends data over the Wi-Fi network.
14: (Optional) If multiple PDN connections need to be created, steps 3 to 8 are repeated to establish all of them.
15: The P-GW initiates a PDN GW Initiated Bearer Deactivation procedure to the 3GPP network, requesting the P-GW, S-GW, and MME to
release the EPS bearer information used before the handover.
After the UE is handed over to the Wi-Fi network, the UE initiates a re-registration to update the registration information.
Table 1
describes the key processing of related NEs for a handover from an LTE network to a Wi-Fi network.
Table 1 Key processing of related NEs for a handover from an LTE network to a Wi-Fi network
NE
Key Processing
UE
Supports handovers between Wi-Fi and LTE networks.
ePDG
Provides signaling and data channels for UEs.
MME
Supports handovers between Wi-Fi and LTE networks.
P-GW
Provides the basic bearer function for UEs to access external data networks.
Receives data from Wi-Fi subscribers and routes messages to the corresponding external networks.
PCRF
Provides UEs' default PCC rules for the P-GW.
SAE-HSS
Authenticates and manages UEs.
AAA Server
Provides static QoS information for UEs.
Obtains subscription data from the SAE-HSS and authenticates UEs.
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Re-registration
After a UE is handed over to an LTE or Wi-Fi network, the UE initiates a re-registration to the IMS network. Figure 19 shows the re-registration
message flow.
Figure 19 Re-registration message flow after a handover
After a UE is handed over to an LTE or Wi-Fi network, the UE initiates a re-registration to the P-CSCF on the IMS network. The reregistration message flow is similar to the registration message flow, since the UE's IP address remains unchanged after the handover.
Therefore, only the differences between the two message flows are described. For details on the registration flow, see Registration.
P1: The P-CSCF compares access-type in the P-Access-Network-Info header field of the REGISTER message with RAT-Type obtained over
the Rx interface. If they are different, the P-CSCF replaces access-type with RAT-Type. If the REGISTER message does not carry the PAccess-Network-Info header field, the P-CSCF adds the header field and access-type.
P2: After receiving the third-party REGISTER message, the ATS determines whether the UE accesses through a Wi-Fi or LTE network based
on the P-Access-Network-Info header field and updates the registration status.
When a UE initiates a re-registration after a handover, the S-CSCF must trigger a third-party registration to the ATS.
Parent Topic: Description of the VoWiFi Service
Copyright © Huawei Technologies Co., Ltd.
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