Mobile Computing Models

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Mobile Computing Models

• Spring 2001 CIS6930 Term Paper
• University of Florida
• Mobile Computing Course
• A. Helal, Professor

Kevin Birkett
David Nordstedt
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Overview of Computing Models

• Traditional Client / Server Model
• Application Aware / System Unaware
• Application Unaware / System Aware
• Application Aware / System Aware

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Traditional Client / Server Model

• Neither the application (client / server) nor the
  system (middleware, OS) are aware of mobility
• Assumes the connectivity and availability of a
  fixed network workstation
• Cache invalidation does not work properly
• Possibility of inconsistent cache contents
  creates problems
• Working offline may not be possible

4
Application Aware / System Unaware Model

• The client and server have mobility awareness
• Application is responsible for adapting to
  mobility
• May require large changes to existing software
• A larger application is needed to deal with
  mobility
• No need to add hardware to the network, or add OS
  support

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Application Unaware / System Aware Model

• No changes required to the client or server
• Some middleware between the client and server
• Behaves like a server to the client
• Behaves like a client to the server
• Examples proxies, disconnected operation
• Legacy software can run unchanged

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Application Aware / System Aware

• Both application and system are aware of mobility
• System can be monitored by individual
  applications that can then adapt to those results
• System can make assumptions about the application

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Disconnected Operation Model

• Why do devices disconnect?
• Mandatory disconnections
• Not in range of a network access point
• Dead battery
• Voluntary disconnections
• Avoid connection charges
• Conserve battery power
• We need to allow mobile users to continue working
  even when disconnected

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Disconnected Operation Model (2)

• Goals
• Availability
• Provide access to information even during periods
  of disconnection
• Replication and hoarding
• Consistency
• Maintain a data consistency among shared files
• Coherency and reintegration

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Coda

• Availability
• Each client runs a process called Venus
• Handles file system request
• Uses local memory system as a cache
• With disconnections Coda strives for availability
• Causes possible version conflicts between files

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Coda (2)

• Hoarding
• Implicit caching
• Based on users recent file usage
• Most often a LRU replacement scheme is used
• Explicit caching
• Places certain files that are necessary in
  disconnected mode
• Selected by both the system and the user

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Coda (3)

• Consistency
• Callback mechanism used by the server
• Promise to notify a client before any other
  clients can modify the local file
• Coda assumes that in disconnected operation file
  conflicts will not occur
• If they do occur ask the users what to do

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Mobile Agents

• What is a mobile agent?
• Software that can move through a network under
  its own control
• Does computations on devices other than the local
  host
• Allows for work to be done on more powerful
  devices
• Limits network traffic

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Mobile Agents (2)

• Usefulness in weakly and disconnected modes
• Eliminates excess communication between client
  and server(s)
• Agents not affected by sudden loss of connection
• Calculations can still be performed on the remote
  host
• Agent returns upon client reconnection

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Thin Clients

• ComputerUser.com High-Tech Dictionary defines a
  thin client as
• A simple client machine or program that performs
  very little processing. In this client/server
  arrangement, most of the application processing
  is done in the (fat) server. For example, a thin
  client may supply only the graphical interface.
  The advantage of a thin client is simpler
  hardware and simpler maintenance the maintenance
  for applications is done on the server.

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Thin Clients

• Thin Clients initially created for use on NC
  (Network Computer)
• Network Computers like normal workstation except
  most storage, processing power, etc done on the
  network side
• But this is ideal for mobile computing
• Clients have little processing power
• Clients have little storage

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Thin Clients
Thin Client Server
Send Key Events and Mouse Movements
Thin Client
Receive Screen Updates
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Metaframe

• Commercial product sold by Citrix, Inc
• Windows server oriented
• Many client platform implementations
• Server-push technology
• Very popular because of large Windows base

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VNC

• Free open-source product by ATT Research at
  Cambridge
• Server binaries available for Solaris, Linux,
  Window, and even Java
• Client binaries available for Solaris, Linux,
  Window, Java, and web browsers (Java Applets)
• Client-pull technology

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VNC

• Open-source gives push for research and new ideas
  in mobile computing
• VNC servers can be built for very small devices
  in hardware to display a control console

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Proxies

• Webopedia defines a proxy server as
• A server that sits between a client application,
  such as a Web browser, and a real server. It
  intercepts all requests to the real server to see
  if it can fulfill the requests itself. If not, it
  forwards the request to the real server.
• Also - Proxy servers have two main purposes
• Improve Performance
• Filter Requests

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HTTP Proxy

• Devices may not understand complex HTML, XML,
  applets, graphics, etc
• Proxies can filter information
• HTML ? CHTML
• HTML text ? Compressed HTML text
• HTML ? Text only
• 24 bpp ? 2 bpp

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TCP Proxy

• Weak connection may be a problem in mobile
  environment when mobile host moves out of range
  temporarily or receives RF interference from
  surrounding transmissions
• TCP connection may be seen as
• Congestion
• Broken connection

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TCP Proxy

• From paper by Zenel and Duchamp
• The main purpose of the intermediary is to cache
  packets headed towards the Mobile Host and to
  perform local retransmissions when packet loss
  between the intermediary and Mobile Host is
  detected. This has the effect of speeding up the
  retransmission process and hiding packet loss
  from the sender side of the connection. The
  latter effect reduces the risk that TCP's
  congestion control algorithm will be activated
  accidentally.

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VNC Proxy

• A mobile host may not be able to communicate with
  a full VNC server
• Proxy can adapt raw encoding with 24 bpp to raw
  encoding with 2 bpp
• Proxy can check on updates for client and only
  pass updates back when needed (VNC protocol is
  client-pull and requires client to request
  information)

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Conclusion

• As prices drop, mobile computing models research
  will be tested in reality
• Security a big issue
• Computing world becoming more pervasive
• Mobile computing in infancy
• Much work to be done

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Future Work

• Bluetooth Scenarios
• Piconets, NAPs
• Cybiko lt 100.00
• Spreading mobile computing thru teenagers
• ATT Research and smart spaces
• Sentient Computing
• Active Badge System