Introduction Issues in Mobile Computing
• There are several characteristics of wireless networks and mobile devices that give rise to issues that do not pose a problem in fixed, wired infrastructure. – Adaptive behaviour p – Battery power – Limitation on local resources – Small display size – Disparity in the availability of remote services – Unpredictable variation in QoS.
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• Adaptability is critical to support pervasive computing. • Pervasive computing extends the problem domain:
– – – – Effective use of smart space space. Invisibility. Localised scalability. Masking uneven conditioning.
• Adaptive behaviour arises due to a ...view middle of the document...
• Adaptation decisions require timely knowledge of current resource level – change in available resource and its detection should occur promptly.
– If the change justifies a modification in the amount of resources consumed by an application, it should be executed promptly promptly.
• 3 adaptation strategies:
– A client guides the application to use less of a scarce resource – usually changes the user-perceived quality (fidelity). – A client requests a guaranteed amount of resources. Involves a reservation-based mechanism reservation based mechanism. – A client suggest that the user takes a corrective action. If the user agrees, resource supply may be adequate to meet the demand, but there is no guarantee.
• Requires the OS to play a role in managing resources, such as bandwidth and battery power, in addition the traditional resources such as CPU cycles and memory.
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• Which strategy to choose depends on the smart space environment.
– The system may need to switch between different strategies to guarantee a smooth and seamless transition.
• Defines 2 attributes to support adaptability: agility and fidelity. • Agility is the speed and accuracy with which an application detects and responds to changes in resource availability.
– I a complex attribute because different applications Is l tt ib t b diff t li ti may have different sensitivity to different resources. – For example, a mobile conferencing system is more sensitive to bandwidth fluctuation than in battery power level.
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• Has to balance between providing a proactive system and transparency. • Odyssey is a platform for mobile data access that supports application-aware adaptation that is targeted at applications that access rich, resourcedemanding data.
• Fidelity is the degree to which data presented to a mobile client matches the reference copy at the server.
– A reference copy is the most complete, current and detailed version of a data item.
• Examples of dissimilarity between data types.
– A video data packet received in error can be dropped instead of retransmitted – A database update must be received accurately and retransmitted if an error is detected. – The approach taken by Odyssey to support adaptability trades off data quality for resource consumption. – Hence, when bandwidth drops, a client playing fullcolour video data may switch to black and white video.
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• Factors that determine fidelity depends on the type of data and the applications using it. • In order to support adaptability for efficient resource usage, some form of awareness has to be incorporated into the system.
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• When implementing the strategies to support adaptability, keep in mind how fidelity for a broad range of applications can be lowered without causing too much inconvenience to the users. • Adaptation can be made the...