1. Data dissemination in wireless computing environments

2. Introduction Characteristics of wireless computing environments
Limited wireless channel bandwidth
Unreliable transmission
Asymmetric communication environments
Limited effective battery lifespan
Threat to security
High cost

3. Design issues in wireless data dissemination
Efficient wireless bandwidth utilization
Efficient and effective scheduling strategies at the server
Energy-efficient data access for battery-powered portable devices
Support for disconnection
Support for secured and reliable transmission

4. Models for information dissemination
Point to point
Push-based
Pull-based
Hybrid
Static
dynamic

5. Data broadcast scheduling
Organization of broadcast for push-based broadcast system
Access time
Tuning time
Broadcast program
Flat program
Skewed non-flat program
Regular non-flat program
Broadcast cycle

7. Generation of broadcast programs
Flat program
The union of all objects that needed by clients are broadcast
The average access time is the same for all objects
20/80 rule
Broadcast the frequently accessed objects more regularly than those that are less popular
Naïve approach
Probabilistically pick an object for transmission
Problems?

8. Optimal broadcast program
Copies of an object are equally spaced
For any two objects x and y, fx/fy=
fx: number of copies of item x in a broadcast cycle
qx: access probabilities of item x
It’s not always possible to generate such broadcast program

10. Broadcast disk Data is split into n partitions
Data with similar access frequency is put in the same partition
Partitions with larger access frequencies will be broadcast more often than those with smaller access frequencies

12. Scheduling strategies for pull-based broadcast system
Methods
FCFS
LWF
MRF
R*W
Performance metrics
Responsiveness
Scheduling overhead
Robustness
Fairness

13. Indexing on air
Basic protocol for retrieving broadcast data

14. Flat broadcast programs with indexes
(1,m) index
A complete index is broadcast m times during a bcast
All buckets have an offset to the beginning of the next index segment
discussion
High average access time
Good tuning time
Consideration
Is it need to replicate the complete index between successive data blocks?

15. Tree-based index
A data file is associated with a B+ tree index structure
Broadcast media is a sequential medium, the data file and index must be flattened
Preorder traversal
First k levels of the index will be partially replicated in the broadcast, and the remaining levels will not be replicated
All non-replicated buckets contain pointers that will direct the search to the next copy of its replicated ancestors

18. Hash-based index Data are hashed into a set of partitions
Partitions may have different sizes (nonuniform distribution)
Hpartition(k) : determines the partition that that object k belong to
Hhash: determines the hash bucket that contains the shift pointer
The gap between hash buckets is given by the size of the smallest partition
Hhash = 1+(hpartition-1)*gap

19. Signature-based index
An abstraction of the information stored in an object or a file
K-levels of signature
The higher the level is, the coarser the granularity of the grouping is
Each integrated signature is broadcast before the corresponding group of objects.
To reduce the number of false drops, the hashing functions used in generating the signatures at different levels should be different.

20. Flexible indexing scheme
Split a sorted list of objects into several equal-sized segments
At the beginning of each segment, there is a control index
Global index
Local index

21. Discussion

22. Broadcast program generation for skewed data access
Access frequencies can be exploited to design index methods that further minimize the average number of index probes
Two kinds of approach
Imbalanced tree approach
Non-flat broadcast programs with indexes

24. Non-flat broadcast programs with indexes
Segment-level index
Similar to the (1,m) index scheme
Broadcast a full index at the beginning of each segment
The broadcast program is generated under broadcast disks
Problems
The cost to find the index may be very large

25. Distributed indexing
Each segment index is split into sub-indexes that are distributed within its corresponding segment