Handover mechanisms in next generation heterogeneous wireless ...

TRUST ASSISTED HANDOVER ALGORITHM FOR RELIABLE HANDOVER
especially in a low trust density scenario (A large portion of the POAs has no trust
association with mobile user’s home). If the THOA takes a median ETS value of 8, an
implementation of the THOA would see a reduction of 35% of the handover delay. The
much improvement is due to the fact that the THOA can always avoid unnecessary
handover attempts by checking accessibility of networks before triggering handover.
Such a preliminary check makes sure that each handover attempt would be effective.
While, the MHOA would have to deal with handover reselection(s) if an unsuccessful
handover takes place. Apparently, more handover attempts lead to longer handover
delay, and more signalling overheads. On the other hand, the changes in the ETS of the
THOA can result in different handover delay. Smaller handover delay is observed when
the THOA is given a low value of ETS. A decreased ETS means the network selection
scope is narrowed down.
In another experiment, the load balance factor LBF is examined. It is found that an
increased LBF was observed when the ETS was reduced in the sake of faster handover
as illustrated in Figure 5.7. Intuitively, a larger LBF means that the handover requests
are not evenly distributed to the available POAs. This is true because more access
networks are regarded as being “far away” from the MH’s home network during
handover decision making. Some of the networks would thus be excluded from network
selection. However, in favour of a fair balance of load, the THOA with a high value of
ETS (e.g. ETS=15) can achieve the similar performance as the MHOA in regards to the
LBF. The comparison is shown in Figure 5.7.
The level of trust association has an interesting impact on handover performance as
shown in both Figure 5.6 and Figure 5.7. In this simulation, the level of trust association
is determined by two parameters: the probability of no trust association ( p A ) and the
probability of authorisation failure ( p B ). The latter determines if the mobile user’s home
network would grant access through the selected foreign network. When the THOA
with a smaller ETS is applied, the LBF appears to be affected by the trust density of
access networks. The data line of the THOA with ETS=4 in Figure 5.7 shows this effect.
However, the MHOA shows a fairly smooth LBF curve due to its unbiased network
selection process. The THOA tends to give access networks “close” to mobile user’s
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