Mobile computing
What is mobile computer?
• A computer which you can take with you all around.
• You can do all the things which can be done with a
desktop computer.
• You should be able to use same software, which you use
on a desktop computer.
Mobile computer - How?
• One possibility is to have a standalone computer capable of
• storing large amount of software and data files,
• processing power to support the required applications.
• Modern day laptop computer are something like this.
Whenever you are static, connect to internet through an
access point and you can do the file transfer, telnet, web
browsing etc..
• While on the move, connectivity is desired for using
software which require cooperation of at least two
machines.
Option for connectivity
• An easy option
• Use cellular mobile phone network to connect to some
Internet Service Provider and hence to internet.
• What you need - cellular phone/ corresponding modem.
• Not a good option for campus wide mobile network.
• dependent on the mobile telephone network operator
• cheaper wireless LAN options available
Other extreme of mobile computer
• Mobile computing device
• acts as a terminal
• have wireless connectivity to the network
• Whatever command or application you run is executed
on a remote server.
• Mobile computing device acts as remote terminal.
Advantages/ disadvantages of this extreme
• Very small, compact and light weight mobile computer
• Less power consumption and large battery life - this may
not be true, if radio communication requires more power
than to power up all necessary devices for a standalone
computer on average basis.
Higher reliability of data - theft of standalone computer
means everything is lost. But here all information is stored
on some static server.
• But data privacy while communicating to servers via
the network has to be ensured.
• The performance highly dependent upon network and
wireless channel conditions.
Issues in mobile computing networks
• Actual Mobile computer - somewhere in between the two
extremes.
• Issues due to
• nature of medium
• Mobility
• Portability
Wireless medium
• Disconnection
• Common in radio environments - due to noise.
• due to moving into dark areas
• Blocking by servers e.g., due to too many requests to a file
server.
• Standalone mobile computer can tolerate this kind of problem
better.
• Portable terminals will not function
Asynchronous operations - round trip latency and short
disconnections can be taken care of. (X11 window system
operates this way.)
• Synchronous remote procedure calls - After each request
wait for response.
• Asynchronous remote procedure calls - After several
requests have been sent then acknowledgement is asked
for.
• Pre-fetching / lazy write back decouples communication from
data usage/ generation.
• Decoupling allows the program to progress even during
disconnection.
Coda - distributed file system, developed for notebook computers
with less frequent disconnections
• on board cache
• users’ profile is used to keep best selection of files in the
cache
• Whole files are cached instead of fixed block of data.
After disconnection, the cache is automatically
synchronized with file server.
• File modification are allowed even during disconnection.
Probability of having more than two modified versions of a
file is less than 1% even when more than one user is given
write access to same file.
Summary
Mobile computing systems have different constraints and hence
different solutions. Main challenges are due to
Wireless - unreliable.
Mobility - dynamism of information.
Portability - limited resources
