Transcript Chapter 1-2
Ch 1. Mobile Adaptive Computing Myungchul Kim [email protected] What is Mobile Computing 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 2 Adaptability – The key to Mobile Computing 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 3 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 – 4 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 5 Mechanisms for Adaptation What can be adapted? – How to adapt? – The functionality and the data 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 access 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. 6 – 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 the client matches the reference copy at the server. – QoS requirements – Video data – frame rate and image quality Spatial data – minimum feature size Telemetry data – sampling rate and timeliness 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. 7 How to develop or incorporate adaptations in applications 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 locally. Emulating: read and write access to local files Write-disconnected: whether to fetch files from the server or to allow local access Reintegration: the log of operation is used. 8 – Fig 1.3 9 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 10 – Fig. 1.4 11 WebExpress – Fig. 1.6 12 – – – – 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 13 Support for building adaptive mobile applications Odyssey [Noble 1997] – – – Between application and the operating system in handing the presence of wireless links Network bandwidth drops -> Odyssey informs the video, audio, and other applications of the change -> proper adaptations in network usage When network conditions change, the os notifies the application of what has happened. 14 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 the server to the client Queued Remote Procedure Call (QRPC): nonblocking rpc supporting split-phase operations Optimize use of expensive links Make use of asymmetric links Stage messages near the destination 15