LTE to WLAN(wifi) Handover
This page covers LTE to WLAN(WiFi) handover.It also mentions WLAN(WiFi) to LTE handover with message flow diagram.
Due to huge increase of data traffic, telecom operators are adopting technologies like LTE, LTE advanced. Though it is not sufficient to address rise in demand. Moreover cellular spectrum is scarce resource and is licensed. Hence to have viable solution, operators and technologists have decided to offload cellular traffic on to WLAN based networks. WLAN based networks are referred as wifi hotspots.
The idea of network operators have got support due to availability of low cost wifi hotspots everywhere and WLAN compliant cheap smart phones. Moreover wifi spectrum is not licensed like LTE.
Figure-1 depicts integrated network supporting both WLAN and LTE. As shown PDN gateway is the interface with both PCRF and AAA as well as serving gateway(of UMTS) and WiFi gateway(of WLAN). Hence PDN gateway helps in handover between the two systems.
Following events happen in LTE to WLAN handover process:
• Assume that UE is connected with LTE network.
• UE detects existing WLAN APs (wifi hotspots), it selects the one which can provide connectivity to its mobile subscriber. The WLAN standard version IEEE 802.11u enhances capabilities of various frames to support interworking between LTE and WLAN. Moreover UE has been provided with more features to support this.
• UE initiates authentication sequence using available cellular network credentials.
• After authentication process is completed , UE initiates DHCP sequence by sending discover message. WiGW receives this request and in-turn initiates GTP tunnel creation with PGW(PDN Gateway).
• Once it is successful, PGW provides same IP address to WiFi Gateway(WiGW) which in turn offers it to UE via WiFi network.
• On receiving the same IP address, as that of LTE network, from WiFi Gateway, replies back with DHCP ACK. This lets PDN Gateway trigger a NAS Detach message for LTE connection.
• The moment UE receives a network detach, it starts re-directing the already existing IP traffic to WiFi network. This completes successful LTE to WLAN(WiFi) handover.
• This seamless handover is the result of following:
-It does not require any user intervention.
-It uses same cellular network credentials for automatic authentication for example, USIM or SIM
-It obtains same IP address, as received in LTE side, from Wi-Fi side. Thus applications do not even come to know about this handover.
WLAN(wifi) to LTE handover
Following events happen in WLAN to LTE handover process.
• UE is already connected to WiFi network and IP traffic is running.
• UE decides to move back to LTE network and sends a connection request to LTE network.
• Once LTE PDN becomes active, it starts sending traffic back to LTE network.
• Once traffic starts flowing towards LTE network, UE terminates the WiFi connection and releases all the resources which where occupied before.
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