WLAN and UMTS Based on Mobile IP

Handover is the mechanism by which ongoing connection between MS and
CH is transferred from one point of access to another point while maintaining
the connectivity. When an MS moves away from an access point or from
a node B, the signal level degrades and there is a need to switch communications
to another point of attachment that gives access to the existing IEEE
802.11 WLAN network or UMTS network. Handover mechanism in an
overlay UMTS and underlay WLAN network could be performed so that the
users attached to the UMTS just occasionally check for the availability of the
underlay WLAN network. A good handover algorithm is needed to make the
decision when to make handover in order to avoid an unnecessary handover
(i.e., the ping-pong effect). The handover algorithm is beyond the scope of
this chapter. This section discusses the handover procedure and the mechanism
from WLAN to UMTS, and vice versa, based on the received signal
strength (RSS) metrics. It means that the handover initiation or the handover
triggering is sensitive to these signals. Figure 2.23 shows the handover procedure
by transition from one network to another.
An MS moving from the WLAN network coverage may suddenly experience
severe degradation of service and will have to perform handover very
fast to maintain the higher layer connection. The following stages occur
when an MS moves away from the coverage of WLAN within the UMTS
coverage:
1. The signals received from the access point in WLAN network is
initially strong and the MS is connected to the WLAN network,
which is also the home network of the MS and the HA in this
network.
2. The signals from the access point become weaker when the mobile
moves away. The mobile scans the air for another access point. If
no access point is available, or if the signal strengths from the available
access point are not strong enough, the handover algorithm
uses this information along with other possible information to
make a decision on handing over to the higher overlay UMTS network. Connection procedures are initiated to active the UMTS
PCMCIA card.
3. The handover algorithm in the MS decides to dissociate from the
WLAN and associate with the UMTS network.
4. The FA is activated and used by the MS dual PCMCIA card and
the mobile IP protocol, and the MS gets a COA due to visiting the
UMTS network as a foreign network.
5. The HA in the WLAN is informed about the new IP address
through a mobile IP registration procedure, and it does proxy
Address Resolution Protocol (ARP) and intercepts the datagram. The
HA encapsulates the datagram and tunnels any packets arriving for
the MS to the FA of the UMTS networks. At the end of the delivery,
the MS will deencapsulated and get the datagram.
In this case, the handover algorithm determines that there is no local
coverage available via WLAN and handover must be performed to the
UMTS network, assuming that a UMTS service is always available to the
MS.Once the MS is attached to the UMTS, it constantly monitors the air at
repeated intervals to see whether or not a high data rate WLAN service is
available. As soon as such a service becomes available, the handover algorithm
should initiate an association procedure to the newly discovered access point.
Figure 2.24 illustrates the handover procedure from WLAN to UMTS.
The procedure for this reverse handover from UMTS to WLAN network
is as follows:
1. The signal from the WLAN access point is initially not detected.
2. The MS then detects a beacon, which indicates that the underlay
WLAN network has become available.
3. The handover algorithm decides on making a handover from
UMTS to the WLAN network.
4. The FA in the UMTS network is deactivated, the mobile IP is
updated, and the home IP address is used. 5. The HA in the WLAN network is instructed by the MS to no
longer do a proxy ARP on its behalf through the mobile IP
protocol.