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Challenges of Mobile and Wireless Computing

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Title: Challenges of Mobile and Wireless Computing


1
Challenges of Mobile and Wireless Computing
  • CS 515 Mobile and Wireless Networking
  • Ibrahim Körpeoglu
  • Computer Engineering Department
  • Bilkent University, Ankara

2
Homework 2
  • Read and digest the following papers
  • 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.

3
Challenges of Mobile Computing and Networking
  • Challenges of Wireless/Mobile Network Design
  • Challenges of Mobile System and Application
    Design
  • We will look to the problems more from Computer
    Science point of view

4
Enabling Developments for Mobile Computing
  • Two factors that enabled mobile and ubiquitous
    computing (also called nomadic computing)
  • Advances in wireless communication systems (both
    voice and data) and networks
  • Flexible communication
  • Less dependence on location for network access
  • You are not limited with length of cable
  • Advance in computer technology and development of
    portable computers and devices
  • Wide use of laptop computers
  • Introduction of Palmtop and hand-help computers

5
Factors challenging Mobile Computing
  • Wireless Communication
  • Implications of using wireless communication for
    mobile computing
  • The differences between wireless and wired media
  • Mobility
  • Consequences of mobility on mobile application
    and system design
  • Portability
  • Pressures that portability places in the design
    of mobile end-systems

6
Wireless Communication
  • Wireless network access is flexible and less
    location dependent
  • Wireless communication is much more difficult to
    achieve than wired communication
  • Wireless signals are affected by surrounding
    environment
  • Blocking of the signals (walls etc.)
  • Interference from other signal sources
  • Reflections and fading

7
Wireless Communication
  • Wireless connections are of lower quality
  • Lower bandwidths (bit-rates)
  • Higher error-rates and burst errors
  • More disconnections
  • These factors increase the communication latency
    due to
  • Losses and retransmissions
  • Retransmission timeout delays
  • Error control protocol processing
  • Short disconnections

8
Wireless Communication
  • Wireless connections can be lost due to
  • Mobility that results out of coverage area
    roaming
  • Radio signal strengths drops with increasing
    distance between a wireless transmitter and
    receiver
  • High interference at some locations
  • Other devices around that use the same frequency
    band
  • High load on some cells
  • Lots of users who want to talk and access the
    network at the same time

9
Design Challenges
  • Wireless Communication brings challenges to
    mobile computing because of
  • Disconnections
  • Low Bandwidth
  • High Bandwidth Variability
  • Heterogeneous Networks
  • Security Risks
  • Mobile Systems and Applications should consider
    these issues for
  • good operation/functionality
  • performance
  • availability

10
Wireless Communication - Disconnections
  • Todays computers depend heavily on network
  • Network File Systems, ftp servers, telnet serves,
    X-servers, Web servers
  • Network failure will stall the applications and
    systems
  • Network failure is greater concern for mobile
    computing
  • Disconnections can be much more frequent

11
Wireless Communication - Disconnections
  • There is trade-off between autonomy and
    distributed computing
  • 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
  • For example using a network file system prevents
    storing all the files in the local mobile computer

12
Wireless Communication - Disconnections
  • Code File System is a good example of handling
    network disconnections
  • 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
  • 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.

13
Wireless Communication - Disconnections
  • 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
  • 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.

14
Wireless Communication Low bandwidth
  • Mobile computing designs need to be more
    concerned about network bandwidth consumption and
    constraints than designs for stationary computing
  • Wireless networks deliver lower bandwidth than
    wired networks
  • 1 Mbps Infrared communication
  • 11 Mbps wireless local radio communications
    (shared)
  • 9.6 Kbps for wide-area wireless communication
  • Wired networks
  • 10-100 Mbps for Ethernet
  • 100 Mbps for FDDI
  • 155 Mbps for ATM
  • 1 Gbps for Gigabit Ethernet

15
Wireless Communication Low bandwidth
  • To increase the systems effective bandwidth per
    user
  • 1) Use small cells with many base-stations
  • OR
  • 2) Use different frequencies with overlapping
    cells
  • Weiser defined the capacity of wireless network
    as
  • Bandwidth provided per cubic meter
  • There is a hardware tradeoff between bandwidth
    and coverage area
  • Transmitters covering a smaller area achieve
    higher bandwidths

16
Wireless Communication Low bandwidth
  • Some software techniques to cope with low
    bandwidths
  • Compress data that is to be transmitted
  • Log the data, and use bulk transfers
  • 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.
  • Priority should be given to packets that belong
    time-critical applications

17
Wireless Communication High bandwidth
variability
  • High Bandwidth Variability
  • 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
  • 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

18
Wireless Communication Heterogeneous Networks
  • Stationary computers access the network over the
    same link for a long time
  • No change in link characteristics bandwidth,
    delay, loss-rate
  • Mobile computers encounters heterogeneous network
    connections
  • 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

19
Wireless Communication Security Risks
  • It is much easier to connect to a wireless link
    than to connect to a wired link
  • Two kind of security concerns
  • Access control to wireless network
  • You may not want other un-authorized people to
    access your wireless local area network at the
    company
  • 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
  • Use encryption for data transmitted
  • Shared keys are used (manual or automatic key
    management)

20
Mobility
  • Mobility is ability to change locations while
    connected to the network
  • This make the information more volatile
  • A mobile computer may change the server that it
    is using when it moves to a new location
  • The server could be for example a print server
    or a DNS server, etc.
  • Main problems introduced by mobility
  • Address migration
  • Location Dependent Information
  • Migrating locality

21
Mobility Address Migration
  • In networks designed for static stations
    (Internet for example), an address has two
    functions
  • 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
  • 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
  • Adress_of_mobile f (current_location_of_mobile)
  • Hence the address changes when mobile station
    changes location

22
Mobility Address Migration
  • 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
  • Selective broadcast
  • Central services
  • Home bases
  • Forwarding pointers

23
Mobility Address Migration
  • Selective Broadcast
  • A message is sent to all cells in the networks
  • 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
  • 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

24
Mobility Address Migration
  • Home Bases
  • 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
  • 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

25
Mobility Location Dependent Information
  • For classical stationary computers, the
    information that depends on the location is
    configured statically and usually manually
  • The information include
  • 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
  • Information about each room when you visit a
    museum
  • Information about the current town/area when you
    travel with your car

26
Mobility Location Dependent Information
  • Privacy Concerns
  • The location information of a mobile user should
    not be revealed to everybody
  • A burglar should not know the whereabouts of a
    home-owner
  • The location information can be revealed in a
    controlled manner in some useful applications
  • 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

27
Mobility Migrating Locality
  • When the mobile moves, the distance between the
    mobile and services changes
  • The physical distance is different then the
    network distance
  • With a small change in physical location, network
    administrative domains could be crossed (from
    Bilkent Network to METU Network)
  • Change in network distance may mean longer paths
  • 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

28
Portability
  • Portability means that you can carry a computer
    or device
  • It is designed so that it is feasible and
    practical to carry it with you
  • A mobile unit is not always portable
  • A car is mobile but not portable (you can not
    carry it with you)
  • A portable unit does not have to always mobile
  • 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.

29
Portability
  • Desktop computers are not intended to be carried
  • 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
  • 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

30
Portability
  • Portability Constraints Include
  • Low power consumption
  • 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

31
Portability Low power
  • 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
  • P CV2F
  • P power
  • C capacitance
  • V voltage (5V, 3V, etc.)
  • F clock speed

32
Portability Low power
  • Power can be saved by design
  • Reduce capacitance by greater levels of VLSI
    integration
  • Reduce voltage by redesigning chips that operate
    at lower voltages
  • Reduce clock frequency
  • Trade-off between computational speed and power
    savings
  • Design processor that do more work per clock
    cycle.

33
Portability Low power
  • Power can be saved by operation
  • Use power management software
  • 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
  • Perform periodic operations more infrequently
  • Trading talking for listening
  • Transmission consumes more power than receiption
    in a communication device (10 times in cellular
    phones)

34
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
35
Portability Risk to Data
  • Making computers portable increases their risk of
  • Physical damage
  • Unauthorized access
  • Loss and Theft
  • The risks can be reduced by minimizing essential
    data that is kept on board
  • Make backup copies
  • Prevent unauthorized disclosure of information
  • Use encryption for the data that is stored on the
    disks and memory

36
Portability Small User Interface
  • Size contstaints on portable computers require
    small user interface
  • Requires a different windowing scheme (multpile
    windows are not appropriate)
  • Head-mounted virtual reality displays
  • Buttons versus Recognition
  • There is not much space for a full keyboard
  • Trade buttons in favor of recognizing users
    intentions from analog input devices
  • Handwriting recognition (96-98 accuracy)
  • Voice recognition (96-98 accuracy)
  • Storage and processing demand
  • Disturbs others, compromise privacy
  • Gesture recognition

37
Portability Small User Interface
  • Pointing Devices
  • Mouse does not suite for mobile computers
  • Switch to Pens
  • 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

38
Small Storage Capacity
  • Storage capacity is limited because of physical
    size and power requirements
  • Portable devices do not use disks
  • They consume too much power
  • They can not endure to the un-nice treartment
    that most portable computers face
  • Coping with limited storage
  • Copressing file systems
  • Accessing remote storage over the network
  • Sharing code libraries
  • Compressing virtual memory pages
  • Using interpreted languages instead of translated
    (compiled) languages
  • We dont need object code in this case
  • Object code is many times larger than the source
    code

39
Summary
  • Wireless communication brings challenging network
    conditions
  • 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

40
General Techniques to face Challenges
41
Techniques to face Challenges
  • Three general techniques that have been applied
    in various systems for supporting mobile/nomadic
    computing
  • Asymmetric design of protocols and applications
  • To overcome the limitations of mobile devcices
  • Use of Network based proxies
  • To perform computing and communication functions
    on behalf of mobile users
  • Use of intelligent caching and prefetching
    techniques
  • To imrove performance and availability

42
Outline
  • First discuss these general techniques
  • Network bases proxies
  • Judicious acquisition and caching of Information
  • Asymmetric Protocols and Applications
  • The solutions are not limited to these techniques
  • Then we will describe example systems that make
    use of these techniques

43
General Solution Techniques Network Based
Proxies
  • Many mobile systems make use of intelligent
    agents that reside inside the wired network and
    perform various functions on behalf of mobile
    users
  • 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 network-based server.

44
General Solution Techniques Network Based
Proxies
  • General Benefits of Network-based Proxies
  • 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.

45
Example Functions of Proxies
  • Format Translation
  • The information sent from servers to mobile
    devices is reformatted
  • A postscript file could be converted to an ascii
    text file at the proxy if the mobile device
    display can only support text
  • In this way
  • The translation overhead (processing storage)
    is relieved from mobile device to the server
  • The bandwidth demand on the wireless link between
    mobile device and proxy is reduced.
  • The web server need not to be modified to support
    different kind of mobile devices (the proxy
    handles them)

46
Example Functions of Proxies
  • Control Functions
  • Circuit-oriented communication requires
    connection establishment (signaling) before
    data/voice is transmitted.
  • Signaling functions
  • 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

47
Example Functions of Proxies
  • Filter/Modify Application Information
  • 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.

48
Example Functions of Proxies
  • Account for mobile devices
  • Proxies can also account for disconnected or
    powered-off mobile devices
  • 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.

49
Example Functions of Proxies
  • Hiding Mobility
  • 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
  • This could degrade the performance

50
Example Functions of Proxies
  • Hiding Heterogeneity
  • 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

51
General Solution Techniques Pre-fetching and
Caching of Information
  • These techniques are used in mobile computing
    for
  • Limiting the communication caused by mobility
  • Improving performance and availability of
    services

52
Example of Pre-fetching and Caching
  • Location information
  • The information about the current location of a
    mobile device could be cached at a server
  • This limits the amount of control traffic
    required in the network to locate a mobile
    device
  • Reduce signaling overhead
  • Accurate caching of location information reduces
    also the time taken to locate a mobile device
  • Reduce delay
  • Frequency of location updates
  • 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

53
Example of Pre-fetching and Caching
  • Pre-fetching
  • The mobile unit can pre-fetch items (files, etc)
    at low-load network conditions
  • 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.

54
General Solution Techniques Asymmetric
Protocols and Applications
  • Asymmetric protocol and application design helps
    to overcome the inherent imbalances of
  • 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

55
Asymmetric Protocols and Applications
  • Lower layer protocols can be developed that place
    higher processing and memory requirements on
    fixed servers
  • New applications can be developed for mobile
    environment
  • The applications will take into account the
    asymmetric nature of wireless communication links
    and also processing elements.

56
General Solution Techniques -System Examples
57
  • Application of Techniques
  • On Error Control at the Data-Link Layer
  • On Routing at the Network Layer
  • On Some Applications

58
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.

59
Error Control
  • Effect of errors on TCP
  • TCP assumes packet losses are due to congestion
    which usually takes a long period of time
  • 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

60
Error Control
  • Two approaches to recover from wireless link
    errors
  • Use reliable link layer protocols
  • More complex link layer
  • May interact with TCP reliability, timers, etc.
  • Let the TCP recover from packet losses to due
    link error
  • Simpler link layer
  • TCP source may need to distinguish packet losses
    due to congestion and due to link layer errors

61
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
  • Increases the packet length with redundant bits
    to correct the bit errors on the link
  • No retransmissions
  • Use of ARQ (Automatic Repeat Request) Schemes
  • 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

62
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.
  • FEC technique adapts to the raw bit error rate
    that is experienced over the wireless link

63
Error Control ARQ
  • Basics of ARQ

Mobile Device
Base Station
Packet N
Ack N1
  • Sequence Numbers are
  • used for
  • In order delivery of packets
  • To eliminate duplicate packets

Packet N1
lost
Retransmission Timeout
Packet N1
64
Error Control Asymmetric ARQ
  • AIRMAIL uses Asymmetric ARQ
  • 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

65
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
  • 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.
  • This way, mobile has no timers and it receives
    ACKs periodically.

66
Error Control AIRMAIL
  • AIRMAIL advantages
  • Compiled software size
  • Base station 150 Kbytes
  • Mobile unit 100 Kbytes
  • 2/3 code reduction
  • Processing time to transmit 200 Kbytes of data
  • Base station 0.7 seconds
  • Mobile unit 0.23 seconds
  • 1/3 reduction in processing time

67
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).

68
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

69
SNOOP
Mobile Host
Fixed Host
Base Station
TCP
TCP
SNOOP
70
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
  • Fixed host does not trigger congestion control
    procedures

71
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
  • SNOOP provides around 67 improvement in
    throughput over not using SNOOP.

72
Routing
  • In static Internet and telephone network, the
    end-device 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.
  • Caching of location information and use of
    proxies can help with this.

73
Routing and Caching
  • Caching of location information is used in
    varying degrees in telecommunication, PCS, and
    packet networks
  • Cellular Telecom Network use of caches
  • 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

74
GSM Location Tracking and Call Setup
All subscriber info is kept at HLR
HLR keeps the mobile-VLR binding
HLR
3 - Registration
5 De-registration
VLR keeps track of the mobiles in its Region
(mobile-address Binding)
2 Learn about HLR
Old VLR
New VLR
Two-tier hierarchy of Databases HLR
Home Location Register VLR Visitor Location
Register
1
4
Mobile Host
75
GSM Location Tracking and Call Setup
GMSC
1
1
HLR
VLR
2
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.
76
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.

77
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
  • Thereby, home agent knows which foreign agent
    serves the mobile device at current time.

78
Mobile IP Routing Paths
Correspondent Host (A host that wants to
communicate with
the Mobile Host)
Foreign Agent
Internet
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
79
Applications
  • Use of Solution Techniques on the following
    areas
  • File Systems
  • Multimedia Networking
  • Web Browsing

80
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
  • Cached access from PDA is around 1sec, proxy
    access is around 2-3sec.

81
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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