UMTS (3G) to WLAN Handover Explained

This page covers UMTS (3G) to WLAN handover and WLAN (WiFi) to UMTS (3G) handover. It includes a message flow diagram. Handover to a WiFi network has become popular due to the following benefits:

• All smart mobile devices come with WLAN support.
• WiFi networks are low cost.
• The WiFi spectrum is unlicensed and reusable.
• WiFi hotspots are widely available.

Hence, WiFi has become the choice for network operators to offload cellular data traffic, increasing cellular coverage. Research also suggests that most data users are stationary and don’t require mobility, making WLAN an optimal choice for them.

Figure 1: UMTS to WLAN handover

Figure 1 depicts UMTS to WLAN handover. WLAN compliant Access Points (APs) periodically transmit beacon frames, which are received by mobile nodes (terminals) within the coverage area. The beacon frame informs mobile nodes whether to move to UMTS, triggering the handover (or handoff).

• As shown in the figure, an association handshake is first performed using layer-2 messages (association request and association response). This completes layer-2 connectivity.
• The Access Point then transmits a router advertisement message to the Mobile Terminal, triggering layer-3 handover.
• The mobile node is bound using the IP address of the Access Point.
• Using this binding process, the WLAN AP learns about mobile subscribers’ (mobile nodes’) contexts with the SGSN and other useful information.
• Using this information, the WLAN AP authenticates the mobile node with AAA and assigns a CoA (Care of Address).
• The WLAN AP then sends out a WLAN RAU (Route Area Update) message to the SGSN.
• The SGSN replies with an RAU accept message to the WLAN AP. Now the GMM mobility context is changed to WMM mobility context.
• After the RAU is completed, the Mobile node is bound to the WLAN AP. The Mobile node is informed by the AP using an ACK message. After receiving the ACK, the mobile can reserve the resources.

During the UMTS to WLAN handover process, UMTS resources are freed using a RAB release message, as shown in Figure 1.

WLAN (WiFi) to UMTS (3G) handover

Figure 2: WLAN to UMTS handover

Figure 2 depicts WLAN to UMTS handover. This happens when a mobile node moves out of the WLAN AP coverage. There are two situations:

1. The mobile node will not detect beacon frames.
2. The mobile node sends a probe request to detect the AP but does not get a probe response.

In these situations, the mobile node infers that it has moved out of WLAN coverage and handover to a cellular network such as UMTS is necessary.

The Mobile node sends out an RAU message to the SGSN using the UMTS interface, indicating disconnection with WLAN. The SGSN transmits a RAB assignment and obtains UMTS resources from the UMTS serving RNC. At this stage, the WLAN to UMTS handover is complete, and WLAN resources are freed up.