UMTS(3G) to WLAN Handover
This page covers UMTS(3G) to WLAN handover.It also mentions WLAN(WiFi) to UMTS(3G) handover with message flow diagram.
The handover to WiFi network has become popular due to following benefits of wifi based networks.
• All the smart mobile devices come with WLAN support
• WiFi networks are low cost.
• WiFi spectrum is unlicensed and reusable.
• Availability of wifi hotspots everywhere.
Hence WiFi has become choice for network operators to offload the cellular data traffic. This will increase the cellular coverage. Also as per research, most of the users of data are stationary who even do not require any mobility. Hence WLAN is a optimum choice for them.
Figure-1 depicts UMTS to WLAN handover. All the WLAN compliant APs(Access Points) transmit beacon frames periodically. These are received by all the mobile nodes(terminals) within the coverage area. From the beacon frame mobile nodes will come to know whether to move to UMTS or not. This triggers handover or handoff.
• As shown in the figure, association handshake is first performed using layer-2 messages (association request and association response).
This completes layer-2 connectivity.
• Now Access Point transmits router advertisement message to Mobile terminal. This triggers layer-3 handover.
• Binding of mobile node is done using IP address of Access point.
• Using binding process WLAN AP will come to know about mobile subscribers(mobile nodes) contexts with SGSN and other useful informations.
• Using these informations, WLAN AP authenticates mobile node with AAA and assigns CoA(Care of Address).
• Now WLAN AP sends out WLAN RAU(Route Area Update) message to SGSN.
• SGSN replies with RAU accept message to WLAN AP. Now GMM mobility context is changed to WMM mobility context.
• After RAU is completed, Mobile node is binded to WLAN AP. Mobile node is informed by AP using ACK message for the same. After receiving ACK mobile can reserve the resources.
During the UMTS to WLAN handover process, UMTS resources are freed using RAB release message as shown in the figure-1.
WLAN(WiFi) to UMTS(3G) handover
Figure-2 depicts WLAN to UMTS handover. This happens when mobile node moves out of the WLAN AP coverage.
There will be two situations.
1. Mobile node will not detect beacon frames 2. mobile node sends probe request
to detect AP but does not get probe response.
During these situations Mobile node will infer that it has moved out of WLAN coverage and handover to cellular network such as UMTS is essential.
Mobile node sends out RAU message to SGSN using UMTS interface. It also indicates disconnection with WLAN. SGSN transmits RAB assignment and obtains UMTS resources from UMTS serving RNC. At this stage WLAN to UMTS handover is complete. WLAN resources are freed up.
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