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Mobile computing?– Distributed system– Wireless communications– Mobility of communications devices
Difference between mobile computing and mobile communications?
– Ex. “Italian restaurant” through search engine.– Ex. Video streaming over the Internet– Limitations of mobile computing devices: energy, screen, …– Security or privacy– Middleware layer
What is Mobile Computing
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The vision of mobile computing– Roam seamlessly with your computing devices while continuing
to perform computing and communication tasks uninterrupted.
Transparency– The ability of a system to hide some characteristics of its
underlying implementation from users– Access transparency– Location transparency: name transparency, user mobility– Failure transparency– Mobile computing: mobility transparency
Adaptability – The key to Mobile Computing
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Constraints of mobile computing environments– Mobile computers can be expected to be more resource-poor
than their static counterparts: e.g., battery– Mobile computers are less secure and reliable.– Mobile connectivity can be highly variable in terms of its
performance (bandwidth and latency) and reliability. Fig 1.1
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Application-aware adaptation– Application-transparent (the system is fully responsible for
adaptation)– Laissez-faire (the system provides no support at all)– E.g., bandwidth, battery– Fig 1.2
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What can be adapted?– The functionality and the data
How to adapt?– Client-server (CS) model
Adapting functionality– CS model– A server with soft or hard state about the clients– Coda File servers (Saty 1996a)
A few trusted servers act as the permanent safe haven of the data. A large number of un-trusted clients can efficiently and securely acc
ess the data. Good performance is achieved by using techniques such as caching
and prefetching. Security of data is ensured by employing end-to-end authentication
and encrypted transmissions.
Mechanisms for Adaptation
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– Impact of mobility on the CS model: a resource-poor mobile client = thin clients
Adapting data– Fidelity: the degree to which a copy of data presented for use at t
he client matches the reference copy at the server. Video data – frame rate and image quality Spatial data – minimum feature size Telemetry data – sampling rate and timeliness
– QoS requirements Information quality Performance
– Agility: the speed and accuracy with which an adaptive application detects and responds to changes in its computing environments, e.g., change in resource availability.
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Software sensors– e.g., TCP’s congestion control
Coda (continued data availability) distributed file system– Coda client (called Venus) maintains a local cache.– Hoarding: the client prefetches files from the server to store local
ly.– Emulating: read and write access to local files– Write-disconnected: whether to fetch files from the server or to al
low local access– Reintegration: the log of operation is used.
How to develop or incorporate adaptations in applications
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– Fig 1.3
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What can adaptations be performed?– Proxy: an intermediate software entity– Adapting to the hw/sw capabilities of the mobile device– e.g., security firewalls, NAT, transcoding proxies
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– Fig. 1.4
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WebExpress– Fig. 1.6
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– Caching– Differencing: for common graphic interface processing– Protocol reduction: reduction of TCP/IP connection overhead
using virtual sockets.– Reduction of HTTP headers: e.g., MIME
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Odyssey [Noble 1997]– Between application and the operating system in handing the pr
esence of wireless links– Network bandwidth drops -> Odyssey informs the video, audio, a
nd other applications of the change -> proper adaptations in network usage
– When network conditions change, the os notifies the application of what has happened.
Support for building adaptive mobile applications
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Rover [Joseph 1995]– Object-based sw toolkit for developing both mobility-aware and
mobility-transparent CS distributed applications– Relocatable dynamic objects(RDO): loadable dynamically from t
he server to the client– Queued Remote Procedure Call (QRPC): nonblocking rpc suppo
rting split-phase operations– Optimize use of expensive links– Make use of asymmetric links– Stage messages near the destination
