More e?±ciently, in this kind of method, updates only take
place on location servers, a subset of N nodes inside the network, which are
selected through mapping with a well-known hashing function. Later, location
is retrieved from the same group of location servers. The availability of this
method lies in the common knowledge for producing a quorum intersection.
Hashing-based protocols can be further divided into hierarchical hashing
and flat hashing. In the hierarchical hashing-based protocols (for example [16],
[17]), networks are divided into several hierarchical grids. One or more nodes
240
in each level are designated as location servers for each node, so that location
updates and queries only need to traverse up or down the hierarchy to find
them. Because the height of the hierarchy is O(log n), e?®ectively each node??™s
location is disseminated to O(log n) location servers.
Another kind of hashing is flat hashing (for example [18], [19], [20]). In such
protocols, each node calculates for itself a home region, which may consist of
one or more nodes within a fixed area in the network, through a uniform and
well-known hash function. This home region, acting as location server, will
store the location update and reply to corresponding queries for that node.
The number of location servers for each node is independent of the total
number of nodes n, but the energy consumption on these O(1) servers is comparatively
high, which may cause some other problems such as a bottleneck
in a network.
Pages:
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394