Transcript Introduction, History, Overview of Wireless Systems

Challenges of Mobile and
Wireless Computing
CS 515
Mobile and Wireless Networking
İbrahim Körpeoğlu
Computer Engineering Department
Bilkent University, Ankara
CS 515
Ibrahim Korpeoglu
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Homework 2
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Read and digest the following papers
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H. Forman, J. Zahorjan, The Challenges of Mobile
Computing, IEEE Computer, V 27, N 4, (April 1994),
pp. 38-47.
T. La Porta et al., Challenges for Nomadic
Computing: Mobility Management and Wireless
Communication, ACM/Baltzer Journal of Mobile
Networking and Applications, Vol. 1, No. 1, 1996.
M. Satyanarayan, Fundamental Challenges in Mobile
Computing, M. Satyanarayan, Fifteen ACM Symposium
on Principles of Distributed Computing, 1996.
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Challenges of Mobile Computing and
Networking
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Challenges of Wireless/Mobile Network
Design
Challenges of Mobile System and Application
Design
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We will look to the problems more from Computer
Science point of view
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Enabling Developments for Mobile
Computing
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Two factors that enabled mobile and
ubiquitous computing (also called nomadic
computing)
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Advances in wireless communication systems (both
voice and data) and networks
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Advance in computer technology and development of
portable computers and devices
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Flexible communication
Less dependence on location for network access
You are not limited with length of cable
Wide use of laptop computers
Introduction of Palmtop and hand-help computers
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Factors challenging Mobile
Computing
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Wireless Communication
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Mobility
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Implications of using wireless communication for mobile
computing
The differences between wireless and wired media
Consequences of mobility on mobile application and
system design
Portability
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Pressures that portability places in the design of mobile
end-systems
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Wireless Communication
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Wireless network access is flexible and less
location dependent
Wireless communication is much more
difficult to achieve than wired communication
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Wireless signals are affected by surrounding
environment
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Blocking of the signals (walls etc.)
Interference from other signal sources
Reflections and fading
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Wireless Communication
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Wireless connections are of lower quality
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Lower bandwidths (bit-rates)
Higher error-rates and burst errors
More disconnections
These factors increase the communication
latency due to
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Losses and retransmissions
Retransmission timeout delays
Error control protocol processing
Short disconnections
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Wireless Communication
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Wireless connections can be lost due to
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Mobility that results out of coverage area roaming
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High interference at some locations
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Other devices around that use the same frequency band
High load on some cells
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Radio signal strengths drops with increasing distance
between a wireless transmitter and receiver
Lots of users who want to talk and access the network
at the same time
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Design Challenges
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Wireless Communication brings challenges to
mobile computing because of
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Disconnections
Low Bandwidth
High Bandwidth Variability
Heterogeneous Networks
Security Risks
Mobile Systems and Applications should consider
these issues for
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good operation/functionality
performance
availability
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Wireless Communication Disconnections
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Todays computers depend heavily on
network
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Network File Systems, ftp servers, telnet serves, Xservers, Web servers
Network failure will stall the applications and systems
Network failure is greater concern for mobile
computing
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Disconnections can be much more frequent
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Wireless Communication Disconnections
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There is trade-off between autonomy and
distributed computing
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The more autonomous the mobile computers, the better
they can tolerate to network disconnections
However, since mobile computer resources are scarce
and limited, it is preferable to use the network and
network services as much as possible to off-load
computation and storage to network
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For example using a network file system prevents
storing all the files in the local mobile computer
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Wireless Communication Disconnections
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Code File System is a good example of handling
network disconnections
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Designed as a file system for mobile computers like laptops
Information from user profiles is used to locally cache best
selection of files on the mobile computer
A whole file is cached (not only some blocks)
Optimistic caching scheme is used
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Users can update the cached copies
Studies show that only rarely (1%) are files actually shared
and written to in a distributed system
When network reconnects, the cache is automatically
reconciled with the master copy in the server.
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Wireless Communication Disconnections
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Hoarding: Periodically a good set of files are
copied from the master repository at the
server to the mobile computer cache.
The mobile users make their updates on the
files
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All events are logged into a log file.
When network reconnects, the log file is used
to merge the updates and to make the
caches consistent.
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Wireless Communication – Low
bandwidth
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Mobile computing designs need to be more
concerned about network bandwidth consumption
and constraints than designs for stationary
computing
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Wireless networks deliver lower bandwidth than wired
networks
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Wired networks
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1 Mbps Infrared communication
11 Mbps wireless local radio communications (shared)
9.6 Kbps for wide-area wireless communication
10-100 Mbps for Ethernet
100 Mbps for FDDI
155 Mbps for ATM
1 Gbps for Gigabit Ethernet
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Wireless Communication – Low
bandwidth
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To increase the system’s effective bandwidth
per user
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1) Use small cells with many base-stations
OR
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2) Use different frequencies with overlapping cells
Weiser defined the capacity of wireless network as
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There is a hardware tradeoff between bandwidth
and coverage area
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Bandwidth provided per cubic meter
Transmitters covering a smaller area achieve higher
bandwidths
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Wireless Communication – Low
bandwidth
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Some software techniques to cope with low
bandwidths
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Compress data that is to be transmitted
Log the data, and use bulk transfers
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Bulk transfers are more efficient than many individual transfers in
terms of bandwidth usage
Lazy-write back of local caches of mobile computers may also
reduce the network bandwidth demand
Pre-fetching allows transferring the data ahead of need and
thereby reduces the peek loads at time of many demands
Scheduling packets on the wireless channels is also
important.
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Priority should be given to packets that belong time-critical
applications
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Wireless Communication – High
bandwidth variability
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High Bandwidth Variability
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Mobile computers face much more variability in effective
bandwidth than stationary computers
Bandwidth can shift 1 to 4 orders of magnitude between
wired and wireless communication
A mobile application can cope with this bandwidth
variability in 3 ways
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Assuming availability of high bandwidth connections and
operating only on wired networks
Assuming low bandwidth connections and not taking advantage of
wired access and high bandwidths
Adapting to the currently available bandwidth: providing the user
with a variable level of quality and detail
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Wireless Communication –
Heterogeneous Networks
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Stationary computers access the network
over the same link for a long time
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No change in link characteristics: bandwidth, delay, lossrate
Mobile computers encounters heterogeneous
network connections
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Using different base stations
 Some have better quality and less number of users
Using different wireless technologies
 Indoor: infrared link; Outdoor: wide-area radio link
 Cities: cellular network; Rural areas: satellite network
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Wireless Communication – Security
Risks
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It is much easier to connect to a wireless link than to
connect to a wired link
Two kind of security concerns
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Access control to wireless network
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You may not want other un-authorized people to access your
wireless local area network at the company
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Use security protocols such as 802.1x that requires
authentication of users to the Wireless LAN before they can
transmit packets
Prevent others to sniff and read the data packets that are sent
over a wireless link
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Use encryption for data transmitted
Shared keys are used (manual or automatic key management)
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Mobility
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Mobility is ability to change locations while
connected to the network
This make the information more volatile
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A mobile computer may change the server that it is
using when it moves to a new location
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The server could be for example a print server or a
DNS server, etc.
Main problems introduced by mobility
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Address migration
Location Dependent Information
Migrating locality
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Mobility – Address Migration
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In networks designed for static stations (Internet for
example), an address has two functions
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It is used as the identity of the station
It is also related with location of the station, hence is used for
routing the packets to the station
For supporting mobile hosts, the two functions need
be separated
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We need a name for the mobile station that is independent of
the current location of the mobile station
We also need an address for the mobile station that shows the
current location where it resides
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Adress_of_mobile = f (current_location_of_mobile)
Hence the address changes when mobile station changes
location
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Mobility – Address Migration
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In order to communicate with a mobile
computers, one needs to find it current
location (address)
Some methods to find the most recent
address of a mobile computer are:
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Selective broadcast
Central services
Home bases
Forwarding pointers
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Mobility – Address Migration
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Selective Broadcast
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A message is sent to all cells in the networks
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Asking mobile computer to reply with its current address
Too expensive for frequent use and queries
Selectively directing the query to region or set of cells where
the mobile is expected to be in
Central Services
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The current address of each mobile computer is stored and
maintained in a logically centralized database
Each time mobile changes address, it updates the database
with the new address
The logically centralized database could be actually
implemented using various common techniques: distribution,
replication, and caching to improve both performance and
availability
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Mobility – Address Migration
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Home Bases
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A limited case of central service
A single server at the home location of a mobile computer
knows and maintains the current location of the mobile
The location queries, or packets are first directed to this server
at the home location
Home location could be for example, the subnet indicated by
the permanent IP address of a mobile computer
Forwarding Pointers
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A pointer (the new current address) is kept at every location
that mobile computers traverses.
Chain of pointers could be too long
No aging and removal (forgetting) mechanisms
Requires an agent or entity at the old location to forward the
packets to the new location
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Mobility – Location Dependent
Information
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For classical stationary computers, the information
that depends on the location is configured statically
and usually manually
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The information include
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The IP addresses of the primary and secondary local DNS
servers
Available printers
The time zone
Mobile computers need to access also more
location-dependent information
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Information about each room when you visit a museum
Information about the current town/area when you travel with
your car
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Mobility – Location Dependent
Information
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Privacy Concerns
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The location information of a mobile user should not be
revealed to everybody
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The location information can be revealed in a controlled
manner in some useful applications
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A burglar should not know the whereabouts of a homeowner
The location information of colleagues
Routing telephone calls to the current location of a
mobile user
Tailoring the content of electronic bulletin boards
depending on the mobile users that are roaming in the
vicinity
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Mobility – Migrating Locality
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When the mobile moves, the distance between the
mobile and services changes
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The physical distance is different then the network distance
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Change in network distance may mean longer paths
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With a small change in physical location, network
administrative domains could be crossed (from Bilkent
Network to METU Network)
Longer latency
Greater risk of disconnection
More consumption of overall network capacity
To avoid these, server connections may be
dynamically transferred to servers that are closer
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Portability
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Portability means that you can carry a computer or
device
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A mobile unit is not always portable
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It is designed so that it is feasible and practical to carry it with
you
A car is mobile but not portable (you can not carry it with you)
A portable unit does not have to always mobile
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You can use your laptop always at home
You can use your laptop at home or at school
You can use your laptop at home, at school, and also while
your are traveling at the campus ring bus or city bus.
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Portability
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Desktop computers are not intended to be carried
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Therefore their design is more liberal in their use of space,
form-factor, power, cabling, and heat dissipation
The design of a hand-held computer should strive
for the following features:
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Small size
Light-weight
Durable (against dropping, hitting, etc)
Water-resistant
Long battery life-time
Efficient in terms of screen use
Have easy to use input devices
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Portability
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Portability Constraints Include
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Low power consumption
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You would not want to carry a battery that is bigger than
your computer!
Increased risk of data loss
Small user-interfaces
Limited on-board storage
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Portability – Low power
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Batteries are the largest source of weigth in a
potable computer
Minimizing power consumption can improve
portability by reducign battery weight and
lengthening the life of a charge
Power consumption in a circuity
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P ~ CV2F
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P: power
C: capacitance
V: voltage (5V, 3V, etc.)
F: clock speed
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Portability – Low power
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Power can be saved by design
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Reduce capacitance by greater levels of VLSI
integration
Reduce voltage by redesigning chips that operate
at lower voltages
Reduce clock frequency
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Trade-off between computational speed and power
savings
Design processor that do more work per clock cycle.
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Portability – Low power
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Power can be saved by operation
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Use power management software
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That powers down individual components when they are
idle (disks, LCD screens for example)
Applications can conserve power by reducing their
appetite for computation, communication and memory
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Trading talking for listening
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Perform periodic operations more infrequently
Transmission consumes more power than receiption in a
communication device (10 times in cellular phones)
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Portability – Low power
Power consumption breakdown by subsystems of a portable computer
System
% Power
Display Edge-Light
35%
CPU/Memory
31%
Hard Disk
10%
Floppy Disk
8%
Display
5%
Keyboard
1%
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Portability – Risk to Data
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Making computers portable increases their risk of
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Physical damage
Unauthorized access
Loss and Theft
The risks can be reduced by minimizing essential
data that is kept on board
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Make backup copies
Prevent unauthorized disclosure of information
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Use encryption for the data that is stored on the disks and
memory
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Portability – Small User Interface
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Size contstaints on portable computers require small
user interface
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Requires a different windowing scheme (multpile windows
are not appropriate)
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Buttons versus Recognition
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There is not much space for a full keyboard
Trade buttons in favor of recognizing user’s intentions from
analog input devices
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Head-mounted virtual reality displays
Handwriting recognition (96-98% accuracy)
Voice recognition (96-98% accuracy)
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Storage and processing demand
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Disturbs others, compromise privacy
Gesture recognition
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Portability – Small User Interface
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Pointing Devices
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Mouse does not suite for mobile computers
Switch to Pens
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Requires change in user interface and also in software
interface
Pens can jump to any location
Pens can be used for writing besides pointing
Pen positioning resolution is several times that of screen
resolution
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Small Storage Capacity
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Storage capacity is limited because of physical size
and power requirements
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Portable devices do not use disks
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They consume too much power
They can not endure to the un-nice treartment that most portable
computers face
Coping with limited storage
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Copressing file systems
Accessing remote storage over the network
Sharing code libraries
Compressing virtual memory pages
Using interpreted languages instead of translated (compiled)
languages
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We don’t need object code in this case
Object code is many times larger than the source code
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Summary
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Wireless communication brings challenging network
conditions
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Slow and sometime disconnected communication
Mobility causes greater dynamicisim of information
Portability results limited resources to be available on
board
Mobile computing designers should consider these
issues in designing mobile systems, applications
and networks that are comparable with the
traditional stationary computing and communication
in terms of operation, performance, and availability
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General Techniques to face
Challenges
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Techniques to face Challenges
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Three general techniques that have been
applied in various systems for supporting
mobile/nomadic computing
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Asymmetric design of protocols and applications
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Use of Network based proxies
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To overcome the limitations of mobile devcices
To perform computing and communication functions on
behalf of mobile users
Use of intelligent caching and prefetching
techniques
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To imrove performance and availability
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Outline
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First discuss these general techniques
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Network bases proxies
Judicious acquisition and caching of Information
Asymmetric Protocols and Applications
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The solutions are not limited to these techniques
Then we will describe example systems that
make use of these techniques
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General Solution Techniques –
Network Based Proxies
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Many mobile systems make use of intelligent agents
that reside inside the wired network and perform
various functions on behalf of mobile users
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Cellular systems use intelligent switches and databases that
store user profiles to perform functions on behalf of these
users
Intelligent agents, called also proxies, can be used to process
control information (take part in connection
establishment/termination for the user)
Proxies are also used to manipulate user information that is
being exchanged between the mobile device and a networkbased server.
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General Solution Techniques –
Network Based Proxies
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General Benefits of Network-based Proxies
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Proxies may execute complex functions relieving
processing limited mobile devices.
Proxies may be used the reduce the amount of
communication required with the mobile device thus
reducing the amount of air interface bandwidth
consumed.
Proxies may account for mobile devices that are in
disconnected state.
Proxies may shield network-based applications from the
mobility of their clients.
Proxies may shield applications from the heterogeneity
of mobile devices.
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Example Functions of Proxies
Format Translation
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The information sent from servers to mobile devices is
reformatted
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A postscript file could be converted to an ascii text file at the
proxy if the mobile device display can only support text
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In this way:
1. The translation overhead (processing + storage) is relieved
from mobile device to the server
2. The bandwidth demand on the wireless link between mobile
device and proxy is reduced.
3. The web server need not to be modified to support different
kind of mobile devices (the proxy handles them)
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Example Functions of Proxies
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Control Functions
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Circuit-oriented communication requires
connection establishment (signaling) before
data/voice is transmitted.
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Signaling functions:
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Negotiation application and network capabilities
Allocating resources
Signaling involves a lot of message exchanges
Proxies may help in signaling and reduce the
signaling messages that the mobile device has to
send and receive
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Example Functions of Proxies
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Filter/Modify Application Information
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The images in a web page may be filtered out at the
proxy if the mobile device can only support text output
The frame quality of a video stream can be reduced so
that it is transferred with a lower bandwidth demand over
the wireless link
The video streams can be trans-coded into different
compression schemes in order to adapt the bandwidth
the video requires to the available bandwidth on the
wireless communication channel.
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Example Functions of Proxies
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Account for mobile devices
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Proxies can also account for disconnected or
powered-off mobile devices
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The SMS messages that are sent to cellular phone
subscriber can be stored in the network for later retrievel
if the subscriber can not be reached (either because of
powering-off the mobile unit or because of out-of-range
traveling).
This overcomes problems associated with limited
coverage to support reliable communication.
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Example Functions of Proxies
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Hiding Mobility
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The proxies can be used to hide the mobility of
the users from the correspondents (other users or
servers).
Proxy knows how to reach the mobile user
Disadvantage is that all information should be
intervened and processed at the proxy
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This could degrade the performance
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Example Functions of Proxies
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Hiding Heterogeneity
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Proxies perform conversion functions depending
on the capabilities of the mobile devices, the
standards they conform, etc.
This way the correspondent application need not
to be aware of the different characteristics of the
mobile end-devices
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General Solution Techniques – Pre-fetching
and Caching of Information
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These techniques are used in mobile
computing for:
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Limiting the communication caused by mobility
Improving performance and availability of services
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Example of Pre-fetching and Caching
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Location information
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The information about the current location of a mobile device
could be cached at a server
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Frequency of location updates
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This limits the amount of control traffic required in the network to
locate a mobile device
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Reduce signaling overhead
Accurate caching of location information reduces also the time
taken to locate a mobile device
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Reduce delay
If too frequent, control messages will occupy the network and will
be significant portion of the total traffic
If not frequent enough, the data cached could be stale and
performance may degrade
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Example of Pre-fetching and Caching
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Pre-fetching
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The mobile unit can pre-fetch items (files, etc) at
low-load network conditions
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The pre-fetching is done in the background for non-real
time applications
Prevent the network overload when a lot of requests on the
data items are close in the time.
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General Solution Techniques – Asymmetric
Protocols and Applications
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Asymmetric protocol and application design
helps to overcome the inherent imbalances
of:
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Processing power between the mobile wireless
end devices and network-based processors
Uplink and downlink bandwidth available due to
transmission power available from wireless mobile
devices
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Asymmetric Protocols and
Applications
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
Lower layer protocols can be developed that
place higher processing and memory
requirements on fixed servers
New applications can be developed for
mobile environment
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The applications will take into account the asymmetric
nature of wireless communication links and also
processing elements.
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General Solution Techniques System Examples
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
Application of Techniques


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On Error Control at the Data-Link Layer
On Routing at the Network Layer
On Some Applications
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Error Control

Wireless link experience much higher error
rates compared to wired links, because of


Wireless Channels characteristics such as fading,
attenuation, interference, etc.
Mobility



A user moving out of range of a base station or moving
into a crowded area
Data can be lost during handoff
Errors incurred on the wireless link have
dramatic effects on the performance of
reliable transport protocols.
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Error Control

Effect of errors on TCP

TCP assumes packet losses are due to congestion which
usually takes a long period of time



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Hence it triggers congestion recovery procedures when
packet losses occurs
This slows down TCP dramatically and takes quite a lot time
to speedup again
However, on a wireless link packet losses are due link
errors which take a short period of time
There is a need to know if the packet losses are due to
congestion or due to link error to react appropriately
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Error Control

Two approaches to recover from wireless link
errors

Use reliable link layer protocols



Let the TCP recover from packet losses to due link error
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More complex link layer
May interact with TCP reliability, timers, etc.
Simpler link layer
TCP source may need to distinguish packet losses due to
congestion and due to link layer errors
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Error Control – Link layer solutions

There are methods to alleviate the packet losses on
a wireless link due to high error rate

Use of FEC (Forward Error Correction) Schemes



Use of ARQ (Automatic Repeat Request) Schemes

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Increases the packet length with redundant bits to correct the bit
errors on the link
No retransmissions
Does not affect the original packet length
Retransmits the packets that are lost
Need feedback from the receiver if the packet(s) received
successfully or not
Need retransmission timers to detect losses if the receiver does
not send negative acknowledgements
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Error Control – Use of Asymmetry



AIRMAIL protocol designed at Lucent/Bell Labs,
Holmdel, New Jersey.
A link layer protocol designed to work over wireless
links
Two key ideas:



Provides reliable delivery over wireless link so that the TCP
like protocols are not affected by high link error rates
Support mobility so that reliable delivery is ensured without
a large performance degradation during handoffs.
Provides reliable delivery using a combination of
ARQ and FEC techniques.
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FEC technique adapts to the raw bit error rate that is
experienced over the wireless link
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Error Control – ARQ

Basics of ARQ
Mobile Device
Base Station
Packet N
Ack N+1
Packet N+1
lost
Retransmission
Timeout
Sequence Numbers are
used for:
- In order delivery of packets
- To eliminate duplicate
packets
Packet N+1
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Error Control – Asymmetric ARQ

AIRMAIL uses Asymmetric ARQ
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Place majority of processing complexity at the
protocol entity based inside the wired network at
the base-station
Place no-timers on the mobile device
Reduce the processing required due to
acknowledgments at the mobile device
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Error Control – AIRMAIL Asymmetric
ARQ

Communication from base station to mobile
 Base station maintains retransmission timers

Mobile device generates block acknowledgements unless specifically
requested by the base station

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
No timer required at the mobile
Number of ACKs generated is limited
Communication from mobile to base station
 When mobile transmits packets, it maintains a table where it
stores the time at which each packet is transmitted
 Base station periodically transmits the status of its receiver to the
mobile.
 The mobile checks the receive time of status message against
the transmission times stored in the packet records and
determines if the packet has not received an ACK in more than
one round-trip-time (RTT). If so, these packets are retransmitted.
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This way, mobile has no timers and it receives ACKs periodically.
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Error Control – AIRMAIL

AIRMAIL advantages

Compiled software size

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Base station: 150 Kbytes
Mobile unit: 100 Kbytes

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Processing time to transmit 200 Kbytes of data

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Base station: 0.7 seconds
Mobile unit: 0.23 seconds
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2/3 code reduction
1/3 reduction in processing time
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Error Control: Use of Proxies



Proxies can also be used at base station to
overcome errors on the wireless link
SNOOP protocol designed at UC, Berkeley is
an example that improves the performance of
TCP over wireless links
SNOOP acts as a proxy that is located on a
base station over the path of the TCP
connection between a mobile device and a
correspondent host (fixed host).
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SNOOP


SNOOP monitors every TCP segment that is
sent to and received from a mobile host.
It shields the TCP at the fixed host from
experiencing the effect of data loss on the
wireless link
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SNOOP
Internet
Fixed Host
Base Station
Mobile Host
TCP
TCP
SNOOP
CS 515
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SNOOP Operation

From Fixed Host to Mobile
 SNOOP intercepts and monitors at the Base Station every TCP
segment that is sent from Fixed Host to the Mobile Host
 Each segment is stored in a Cache.
 SNOOP also monitors all the ACKs that are sent from the Mobile
Host to the Fixed Host
 When a normal ACK is received at SNOOP, it removes all
segments up-to that ACK number and sends an ACK back to the
fixed host
 When a duplicate ACK is received at SNOOP (indication of
packet loss), SNOOP retransmit the segment from its cache.
 The duplicate ACK is not sent back to the fixed host
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Fixed host does not trigger congestion control procedures
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SNOOP Operation

From Mobile Host to Fixed Host



SNOOP also monitors all TCP segments that are sent
from mobile host to the fixed host.
If it detects that segments are lost on the wireless link, it
generates negative acknowledgments to the mobile host
and recovery starts at the mobile host.
For BER of 2x10-6 on wireless channel
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SNOOP provides around 67% improvement in
throughput over not using SNOOP.
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Routing


In static Internet and telephone network, the enddevice address designates its location and also its
identity.
For mobile network, this is no longer valid. Identity
and address needs to be seperated



Identity remains fixed wherever the mobile moves
Address changes depending on the current location of the
mobile
One problem is how to locate the mobile and how to
route the packets (or circuits) to it.

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Caching of location information and use of proxies can help
with this.
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Routing and Caching


Caching of location information is used in
varying degrees in telecommunication, PCS,
and packet networks
Cellular Telecom Network use of caches



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A connection is established before voice/data is
transmitted
A mobile device need to be located in order to route the
connection request to it and then establish the
connection
Cellular Networks use two-tier location database
structure for location discovery procedures
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GSM Location Tracking and Call
Setup
All subscriber info is kept at HLR
HLR keeps the mobile-VLR binding
HLR
5 – De-registration
3 - Registration
VLR keeps track of
the mobiles in its
Region
(mobile-address
2 – Learn about HLR
Old
VLR
Binding)
New
VLR
Two-tier hierarchy of
Databases:
HLR: Home Location Register
VLR: Visitor Location Register
1
4
Mobile Host
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GSM Location Tracking and Call
Setup
GMSC
1
1
HLR
2
VLR
2
1
1
3
MSC
3
Switches
Switches
The identity of the VLR that is serving the mobile
can be cached at the home switch (GMSC); In this way,
HLR can be by-passed.
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GSM Location Tracking and Call
Setup





Location information of mobile devices is stored in
VLRs.
A VLR serves a well-defined region.
Although it does have to be always true, we can say
that generally there is one VLR per MSC serving a
well-defined area.
HLR keeps all the subscriber information and also
the current VLR identity that is serving the mobile
device
HLR queries the VLR to find out the routable
address of the mobile device.
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Routing in Packet Networks – Mobile
IP



The location information of a mobile device is
cached at a home server, called home agent.
When a mobile moves into a foreign location,
it registers with a foreign agent.
The foreign agent then registers the mobile
device with the home agent
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Thereby, home agent knows which foreign agent serves
the mobile device at current time.
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Mobile IP – Routing Paths
Correspondent Host (A host that wants to communicate with
the Mobile Host)
Internet
Foreign Agent
Home Agent
(Knows the
current Location
of the Mobile Host)
Mobile Host
Route Optimization can be achieved by caching the current
location of the mobile device at the correspondent host
CS 515
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Applications

Use of Solution Techniques on the following
areas:


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File Systems
Multimedia Networking
Web Browsing
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Caching: File Systems

Coda is an example file system, which uses caching
for:

Supporting disconnected mode of operation over a network
file system designed for nomadic computers




Increases system availability
Increasing the performance of the file system
We described Coda briefly on earlier slides
A web browsing application (W4) uses pre-fetching
and caching



A mobile PDA is mated with a proxy in the network
Proxy caches the pages and sends them to PDA upon request
Subsequent pages are pre-fetched and cached at the PDA
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Cached access from PDA is around 1sec, proxy access is around
2-3sec.
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Summary

Characteristics of Nomadic Computing
environment is described



How does it affect the mobile system and application
design
General Solution Techniques to face the
challenges of mobile computing is described
Some example systems that use these
techniques are introduced.
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