LTE to WLAN (WiFi) Handover: Process and Message Flow

This page covers LTE to WLAN (WiFi) handover, and WLAN (WiFi) to LTE handover, including a message flow diagram.

Due to the substantial increase in data traffic, telecom operators are adopting technologies like LTE and LTE-Advanced. However, these technologies are not always sufficient to address the rising demand. Furthermore, cellular spectrum is a scarce and licensed resource. To create a viable solution, operators and technologists have decided to offload cellular traffic onto WLAN-based networks, often referred to as WiFi hotspots.

This approach has gained traction because of the widespread availability of low-cost WiFi hotspots and affordable WLAN-compliant smartphones. Crucially, WiFi spectrum is unlicensed, unlike LTE spectrum.

Figure 1: Integrated Network Supporting WLAN and LTE

Figure 1 depicts an integrated network supporting both WLAN and LTE. As illustrated, the PDN gateway interfaces with both the PCRF (Policy and Charging Rules Function) and AAA (Authentication, Authorization, and Accounting) servers, as well as the Serving Gateway (of UMTS) and the WiFi Gateway (of WLAN).

Therefore, the PDN gateway facilitates handover between the two systems.

LTE to WLAN Handover Process

Here’s a breakdown of the events that occur during the LTE to WLAN handover process:

• Initial LTE Connection: Assume the User Equipment (UE) is initially connected to the LTE network.

• WLAN AP Detection: The UE detects available WLAN Access Points (APs), i.e., WiFi hotspots, and selects the one that can provide connectivity. IEEE 802.11u, a WLAN standard version, enhances the capabilities of various frames to support interworking between LTE and WLAN. Moreover, UEs are equipped with features to support this interworking.

• Authentication: The UE initiates an authentication sequence using its existing cellular network credentials (e.g., USIM or SIM).

• DHCP Request: After successful authentication, the UE initiates a DHCP (Dynamic Host Configuration Protocol) sequence by sending a discover message. The WiFi Gateway (WiGW) receives this request and, in turn, initiates GTP (GPRS Tunneling Protocol) tunnel creation with the PGW (PDN Gateway).

• IP Address Assignment: If the GTP tunnel creation is successful, the PGW provides the same IP address to the WiGW, which then offers it to the UE via the WiFi network.

• DHCP Acknowledge: Upon receiving the same IP address (as used in the LTE network) from the WiGW, the UE responds with a DHCP ACK (Acknowledge) message. This triggers the PDN Gateway to send a NAS (Non-Access Stratum) Detach message for the LTE connection.

• Traffic Redirection: The moment the UE receives the network detach message, it begins redirecting all existing IP traffic to the WiFi network. This completes the LTE to WLAN handover.

• Seamless Handover: This seamless handover is achieved because:

  • It doesn’t require any user intervention.
  • It uses the same cellular network credentials for automatic authentication (e.g., USIM or SIM).
  • It obtains the same IP address, received on the LTE side, from the Wi-Fi side. As a result, applications are unaware of the handover.

WLAN (WiFi) to LTE Handover

Here’s a description of the events in the WLAN to LTE handover process:

• Initial WiFi Connection: The UE is already connected to the WiFi network, and IP traffic is flowing.

• LTE Connection Request: The UE decides to move back to the LTE network and sends a connection request to the LTE network.

• Traffic Routing: Once the LTE PDN becomes active, it starts routing traffic back to the LTE network.

• WiFi Connection Termination: Once traffic starts flowing towards the LTE network, the UE terminates the WiFi connection and releases all the resources it was using.