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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
dynamic-single replication strategy is equal to user's major
replica allocation.
As for the dynamic multi-replication strategy, the adaptive data
replication (ADR) with some modifications is adopted. The
original ADR, metaphorically, forms a variable-size that stays
connected at all times, and constantly moves towards the 'centre
of read-write activity'. The replication scheme expands as the
‘read’ activity increases and contracts as the ‘write’ activity
increases. In our model, we have assumed that the replicas are
allocated on the replica servers associated with registration
areas. That means, the dynamic multi-replication strategy does
not assume any connected situation.
However, as in the original ADR, ‘read’ the closest replica
serves requests and all access requests (including the updates)
are recorded. In each registration area, the access statistics are
periodically tested. A replica of data is made available in a
registration area if, during the access statistic evaluation period,
the number of its ‘read’ is greater than the number of its ‘write’.
Otherwise, the registration area will cease keeping the replica.
In this way, the replication level changes dynamically
following the read-write patterns but it is guaranteed that at
least one replica for each data exists in the fixed network.
In general, the average access cost increases in all strategies
when the network scales up. However, the way the access cost
increases in each strategy is slightly different, depending on the
‘move’ and ‘update’ frequencies.
6. CONCLUSIONS
In a wireless GIS environment, it is possible to access data
anytime and anywhere without a fixed network. In this paper,
we discussed distributed spatial data transferring strategies and
the replication strategies the wireless GIS, it is possible to
improve the availability of wireless GIS which such a
technology. Due to the use of wireless network, WGIS may
have very low availability without the effectively transferring
scheme and data storage strategies. This may lead inefficiency
in data sharing and interoperation among mobile users.
In this paper, we introduced the characteristics of wireless GIS
transferring and storage. The basic framework and the
environment of WGIS are deployed in an integrated network.
By analysing the distributed wireless data transferring scheme,
it was found that it depends on both software and hardware
technologies in order to improve WGIS transfers and increasing
transferring velocity. The possible solutions may include for
example, to spread software protocol and employ new mobile
equipments.
The performance of replication strategies depends on many
factors, such as network scale, mobility, access ratio and access
concentration. It was found that in most circumstances,
dynamic replication strategies excel to static replication
Strategies, and the performance of the dynamic multi-tiers
replication strategies is the best.
Nowadays, the wireless equipments become more and more
excessive and wireless GIS has been applied in many fields,,
for example, business, retail, medicine, etc. Spatial data
transferring and storage in distributed wireless GIS is a
challenge area to be further developed. More and more
309
comprehensive GIS application with 'wired velocity, infinity
freedom' will be realized.
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