Handover Between IEEE 802.11 and UMTS

The motivation intertechnology (vertical handover) for the hybrid mobile
data networks arises from the fact that no one technology or service can provide
ubiquitous coverage, and it will be necessary for a mobile terminal to
employ various points of attachment to maintain connectivity to the network
at all times. There is a clear difference between the two types of handover—
namely, horizontal and vertical handover. Horizontal handover refers
to handover between node Bs or access points that are using the same kind of
network interface. Vertical handover refers to handover between a node B
and an access point (or vice versa) that are employing different wireless technologies.
In the case of a vertical handover, two distinctions are made:
• Upward vertical handover, which occurs from IEEE 802.11 WLAN
access points with small coverage to an UMTS node B with wider
coverage;
• A downward vertical handover, which occurs in the reverse
direction.
A downward vertical handover has to take place when coverage of a
service with a smaller coverage, as in WLAN service, becomes available when
the user still has connection to the service with the UMTS coverage. An
upward vertical handover takes place when an MS moves out of the IEEE
802.11 WLAN coverage to UMTS service when it becomes available but the
user still has a connection to the IEEE 802.11 WLAN coverage. In the case of
the upward vertical handovers, the mobile station/host decides that the current
network is not reachable and hands over to the higher overlay UMTS
network, when several beacons from the serving WLAN service are not available.
It instructs the WLAN to stop forwarding packets and routes this
request via a mobile IP registration procedure through the UMTS CN.
When it is connected to the UMTS network, the MS listens to the lower
layer WLAN access point. If several beacons are received successfully, it will
switch to the IEEE 802.11 WLAN network via the mobile IP registration process. The vertical handover decisions are thus made on the basis of the
presence or absence of beacon packets.