Last Time

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Last Time

• Context Aware Applications
• Defining context
• Active vs. passive
• Survey of some applications
• No killer application yet but maybe in the
  future?
• Location (and time) most heavily used contexts
• Sensing context
• Location
• Outside GPS
• Inside Need a location traction mechanism

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Sensing other low level contexts

• Time?
• Physical sensors possible
• Light, acceleration, tilt, sound, temperature,
  pressure, proximity of humans
• Nearby objects?
• If system keeps track of location of objects,
  query to a database
• Bandwidth? Odyssey
• Orientation?
• Orientation sensor based on two mercury switches
  (Newton MessagePad)
• Directional antennas/directional transmitters?

3
High Level contexts

• This is by far the more difficult problem
• Not just an engineering challenge
• How does a node know whether this is a party or a
  wake?
• Check your calendar or detect the cake/coffin?

4
Sensing Context changes

• Publish subscribe model?
• Monitor polls the current context
• Notifies subscribers to the context
• Centralized or on a per-node?
• Polling rate is a function of rate of change of
  context

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Modeling Context Information
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Location Model

• How do we answer questions such as
• Given an object what is its location
• Given a location, return the set of objects there
• Determining paths between locations
• How does a vehicle navigation system do it?
• Geographical Information Systems/Spatial
  Databases
• Already pretty mature (part of Oracle, mysql,
  etc..)
• Model geometry and topology
• Typically locations are organized hierarchically
• Sonal Dedhias project

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Security and Privacy Issues

• I personally hate the idea that someone can know
  my location all the time
• I still refuse to get a cell phone, but it looks
  like I will finally cave in!
• Par for the course, many of these systems ignore
  privacy and security issues

8
Second Paper Anatomy of a Context Aware
Application
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Outline

• The Anatomy of a Context-Aware Application Andy
  Harter, Andy Hopper, Pete Steggles, Andy Ward and
  Paul Webster. ATT Labs, Cambridge, UK (cont)

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Overview

• Paper describes the system components necessary
  to support a context-aware application
• Location system
• Data model
• Distributed object model
• Resource Monitors
• Spatial monitors
• Working system, real details
• So what is the application? Teleporting/follow-me
  
• Application follows you as you move around a
  building
• Hmm!! Virtual dog?
• Definitely some gaming applications
• But what else? Virtual nanny?
• Framework not specific to this application

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Location System

• Would like
• Fine grained (accurate in space)
• High update rate (accurate in time)
• Inexpensive
• Works indoors
• Argue for ultrasonic
• Optical expensive detectors line of sight
  limitations
• GPS dies indoors
• RF multipath

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Bat Unit

• Radio transceiver, ultrasonic transducer and
  control logic
• Each bat has a GUID
• Base station transmits a periodic message with
  the GUID corresponding bat responds with
  ultrasound
• Use speed of sound in air (estimated from ambient
  temperature) to estimate location
• Ultrasonic receivers detect the delay and map it
  to distance
• Use multiple receivers to get 3D location using
  multilateration
• Reflections of ultrasonic waves statistical
  outlier elimination (can same techniques be
  applied to RF multipath?)

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Bat (contd)
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Bat Unit (cont.)

• It takes 20 ms between bat readings 50
  timeslots per base station per second
• Allow echos to die
• Location can be used to measure orientation
• Attach many bats to the same object. Use the
  measurements to infer the orientation
• If too cumbersome, can check shadow of a single
  bat
• Base station can provide Location
  Quality-of-service to allocate time slots to bats
  based on the expected update frequency
• Bats carried by people few times a second
• Bats attached to workstation once every few
  minutes

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Bat Unit (cont.)

• Bats perform handover when moving from one base
  station to another (similar to the cellular
  networks)
• Hand off decisions can also be made based on the
  Bat location
• Battery consumption is low, power consumed
  depends on the update frequency and power state
• Several updates a second several month lifetime
• 95 of samples within 9cms of actual location
• 95 of samples within 25 degrees with multiple
  bats within 70 degrees for a single bat
  (orientation)
• Bat is good enough to be used as a 3D mouse it
  has buttons and can communicate with basestation

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How well does it work?
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Modeling the environment

• Detailed model describing entities in the real
  world and their possible interactions
• Environment consists of real objects should use
  OO modeling (agree?)
• Modeling language based on entity relation
  diagrams and multiple inheritance
• Modeled people, computers, keyboards, monitors,
  networks, telephones and furniture
• Use CORBA and databases to implement persistent
  distributed objects

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3 Tier Architecture
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Populating and Updating the Model

• Some elements are static (e.g., furniture)
• but some are dynamic (e.g., is a keyboard in
  use)
• need to be updated automatically
• Centralized data repository (to optimize
  information access)
• Three classes of resource monitors
• Machine activity e.g. keyboard activity
• Machine resource e.g. CPU usage, memory usage
• Network point-to-point bandwidth and latency

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Client level event filters

• Update Frequency
• The frequency at which items are monitored is
  based on how quickly the item tends to change
• Relevancy
• If a value has not changed significantly, it is
  not sent. This value depends on the data being
  monitored
• Caching
• Caching improves performance at the cost of
  consistency

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Location Update

• Each Bat location is tracked
• Translated into object location in a type
  specific way (e.g., person different than
  workstation)

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API

• Absolute and relative spatial facts
• Person is at (x,y,z) facing in direction ? .vs.
  Person is standing in front of the monitor
• Geometric containment is used for relative
  spatial facts

monitor
person
Contained(person, screenspace)
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Reasoning about Space
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Scalability

• With many devices, containment is complex
• They use containment tree indexing system (a
  quad-tree based approach)

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Implementation

• BAT teleportation system
• With their earlier active badge based
  teleportation system, they only knew that a user
  was in the room and so they had to cycle between
  multiple displays in a room
• If a particular display was being use, they would
  still cycle that display because they did not
  monitor machines
• If a machine is dead, their system would still
  wait because they did not monitor machines
• With the BAT system, they have more accurate
  location information
• Event driven programming style

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Zones and Buttons
Action
monitor
Maintain

• Action zone triggers teleportation
• Within maintain zone, teleported desktops are
  maintained

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Active Names Flexible Location and Transport of
Wide-Area Resources

• Michael Dahlin, Thomas Anderson, and Amit
  Aggarwal. In Proceedings of the Second USENIX
  Symposium on Internet Technologies and Systems,
  October 1999

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Active Names

• Idea Intelligence in the network
• User specifies a name carrying some service
  intent
• Active name is mapped to a chain of mobile
  programs that can customize how the service is
  located and presented
• Each service owning a portion of the namespace
  has control over which protocols are used to
  access the service and where they run in the
  network
• Encapsulates Service location, customization and
  transport
• Can be used as an extensible general framework
  for deploying wide-area distributed services
• How does relate to mobility/context-aware
  applications?

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What exactly are they proposing?

• when a mobile user in Europe refers to cnn.com,
  the user probably wants to go to a different
  replica, fetch different data, and transform that
  data differently than a user in the US with a
  large screen and T3 connection

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Properties of the System

• Completeness it can support a wide variety of
  extensions
• Simple extensions can yield big performance wins
• End to end performance information can be
  gathered
• Programming model can support location
  independent program execution
• Programmable model supports extensibility

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Naming brief intro.

• DNS translates from machine names to IP addresses
• opal.cs.binghamton.edu ? 128.226.123.101
• static translation (usually valid for a few days)
• DNS round-robin used to dynamically match name to
  host (based on machine load, for example)
• www.cnn.com ? 207.25.71.23 or 207.25.71.24 or
• Network level switch (for e.g. CISCO director)
• Automatically forward network packets to some
  server

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Naming Intent

• A URL is a hint to get some value from some
  server that is appropriate (in terms of
  locality and access costs) and that fits in my
  display
• The exact host names and paths can be remapped to
  the appropriate forms.
• My view of cnn.com depends on my device, my
  advertisement preferences etc. Your view of USA
  Today may be different from mine

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Active Names Goals

• Server selection select appropriate server from
  replicas spread across the Internet
• Client customization Customize the page for the
  present client (e.g. Transcoding, customizable
  portals e.g. my.yahoo.com)
• Server customization Advertisements customized
  for the user, collecting statistics (e.g. hit
  counts, ad rotation etc.)

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Naming today
DNS Server
www.cnn.com
2. host
5. Name
1. Name
HTTP Server2
Proxy
Database Server
Client
4. URL
3. URL - redirect
HTTP Server1
RESULTS
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Active name system

• Clients generate active names (domainname) and
  name of a namespace program to resolve it.
• Clients hand them to a resolver.
• Name space program locates next program to run
  and then transports data to that program
• Each program acts as a filter that transports and
  transforms its input to its output.

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Active Name Architecture
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Active name system

• Active name resolver determines domain-specific
  program
• These programs are location independent and can
  run anywhere
• Application specific, the name is resolved in
  domain-specific manner
• Domain specific code (e.g. ad rotation)
• After methods are associated with each active
  name
• After methods are a list of programs guaranteed
  to be called
• They can perform client-specific transformation
  of data

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Multi-way RPC for efficiency
Proxy
Proxy
Client
Proxy

• Traditional RPC
• Results passed down
• the nodes.
• Adds latency

Server
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Multi-way RPC for efficiency
Proxy
Proxy
Client
Proxy

• Multi-way RPC
• Results sent directly
• Minimal latency

Server
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Multi-way RPC

• Security implications someone else responds to
  your queries
• Use capability certificates to authenticate
  response
• Resource consumption limit should be managed.
  Resolvers use compute resources on foreign hosts.
• Hierarchical namespaces.

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Applications

• Extensibility
• Compared DNS round robin (next server),
  distributed director (closest server as measured
  by hop count) and Active names (number of hops
  biased by a decaying histogram of previous
  performance)
• Average latency follows Distributed Director at
  low load and DNS round robin at high load

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Using Transcoding
Low JPEG Quality 10 KB
Grayscale 85KB
Original 116 KB
Crop 40KB
Foggy road with fall foliage canopy
Thumbnail 2KB
Transliterate few bytes
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Decision on where to transcode

• Can transcode either on the server or proxy
• Proxy is closer to client
• Wide area network from server to proxy could be
  congested. Tradeoff between sending a smaller
  image across the country vs performing
  transcoding at the proxy
• Active name migrates based on the current load

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Composability

• Use server-side include to update page based on
  the current request
• Banner ad rotation
• Logs cookies
• Implemented using server side and active
  namespaces.
• Active namespaces are shown to be composable