Ch 1. Mobile Adaptive Computing Myungchul Kim

2 o Mobile computing? – Distributed system – Wireless communications – Mobility of communications devices o 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

3 o The vision of mobile computing – Roam seamlessly with your computing devices while continuing to perform computing and communication tasks uninterrupted. o 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

4 o 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. o Fig 1.1

5 o 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

6 o What can be adapted? – The functionality and the data o How to adapt? – Client-server (CS) model o 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. Mechanisms for Adaptation

7 – Impact of mobility on the CS model: a resource-poor mobile client = thin clients o Adapting data – Fidelity: the degree to which a copy of data presented for use at the 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.

8 o Software sensors – e.g., TCP’s congestion control o 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. How to develop or incorporate adaptations in applications

9 – Fig 1.3

10 o 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

11 – Fig. 1.4

12 o WebExpress – Fig. 1.6

13 – 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

14 o 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. Support for building adaptive mobile applications

15 o 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