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Mobile and Ubiquitous Computing

Syllabus

  1. Introduction to Mobile and Ubiquitous Computing - definitions and challenges;
  2. Fundamentals of Android OS - Android architecture, activities, services, broadcast receivers, content providers, intents;
  3. Mobile UI Design;
  4. Location Mechanisms - GPS, Infrared/Ultrasound, 802.11 (WiFi), Cellular Networks;
  5. Privacy using Location-based Services - two-tier (PROBE) and three-tier spatial transformations (spatial k-anonymity);
  6. Context-Awareness - context definition, architecture, local vs distributed context-aware systems, context processing taxonomy (capture, infer, distribute, application);
  7. Energy in Mobile and Ubiquitous Computing;
  8. Replication and Consistency in Mobile Environments - consistency models and session consistency;
  9. Execution Offloading;
  10. Augmented Reality - overview, characteristics, marker-based and feature-based tracking, SLAM;

Key Concepts

Android

  • Activities - represent a single screen with a user interface;
  • View - a widget that the user can interact with (e.g., button, text field, etc.);
  • Intent - a message object that is used to request an action from another app component;
    • Start an activity - startActivity(intent);
    • Start a service - startService(intent);
    • Deliver a broadcast - sendBroadcast(intent);
    • Explicit Intents - specify the component to start by name (e.g., new Intent(this, MainActivity.class)) - Used to start a specific activity in the same application;
    • Implicit Intents - specify the action to perform and let the system determine the best component to handle the intent (e.g., new Intent(Intent.ACTION_VIEW));
    • It is possible to pass data between activities using extras (key-value pairs) in the intent.
    • You can also start activities for a result, using startActivityForResult(), and receive the result in the onActivityResult() method - you need to define a request code to identify the request.
  • An activity starts when an intent is received;
  • Activities are arranged in a stack (back stack);
  • Services - perform long-running operations in the background;
    • Started - started by calling startService(), and runs indefinitely, even if the component that started it is destroyed;
      • Foreground Service - a service that has a notification associated with it, to indicate that it is running in the foreground;
      • Background Service - a service that runs in the background without a notification;
    • Bound - bound to a component by calling bindService(), and runs only while the component is bound to it.
  • Broadcast Receivers - respond to system-wide broadcast announcements;
    • To broadcast a custom intent, you can use the sendBroadcast() method, and include the intent in the broadcast.
  • Content Providers - manage a shared set of application data.

Activity Lifecycle

Localization

  • GPS - outdoor location mechanism;

    • One-way: satellites send signals;

    • Preserves user privacy;

    • Ground stations monitor and control the satellites;

      • Clock corrections and orbit updates;
      • There are enough ground monitoring stations to ensure that each satellite is visible from at least two stations at all times;

    • GPS receivers receive the signals and calculate the location;

      • Determine the position using at least 4 satellites, but commonly use up to 12 for better accuracy - this can be augmented with other location mechanisms (accelerometers, gyroscopes, etc.).
      • The distance ($R_i$) from the satellite to the receiver is calculated using the time it takes for the signal to travel, multiplied by the speed of light;

      $$ R_i = \sqrt{(x - x_i)^2 + (y - y_i)^2 + (z - z_i)^2} + b $$

      • This way, four satellites are needed to calculate the four unknowns (x, y, z, b) - three satellites would be enough with perfect clocks;
      • Real-time differential GPS uses a network of ground stations to monitor the GPS signals and broadcast corrections to the GPS receivers;
        • Used to improve accuracy by correcting errors caused by the ionosphere and satellite clock drift

      GPS Comparison

  • Infrared and Ultrasonic - indoor location mechanisms;

    • Infrared proximity - active badges;

      • Badges emit infrared signals that are received by sensors placed in the room;

    • Ultrasound proximity - Cricket/Walrus;

      • Privacy-preserving - no centralized server;
      • Badges are the receivers and beacons are the infrastructure;
      • Beacons are placed on the ceiling and emit ultrasound signals, advertising their identity, using a combination of RF (radio frequency) and ultrasound;

    • Ultrasound TOF (Time of Flight) - Active Bat.

      • Similar to cricket, but bats (small pager-like devices) emit ultrasound signals and the receivers in the infrastructure calculate the distance to the bats;

      Indoor Location Mechanisms Comparison

  • 802.11 (Wi-Fi) - indoor/outdoor location mechanism;

    • Signal Strength Fingerprinting (indoor)

      • Mapping phase - collect signal strength measurements at known locations;
        • Distances need to be small to capture the variability of the signal strength;
      • Localization phase - estimate the location of the device based on the signal strength

    • Signal Strength Modeling (indoor/outdoor)

      • Based on the distance to the APs I am receiving;
      • War-driving - driving around with a laptop to collect signal strength measurements from APs;

      802.11 Comparison

  • Cellular-based Systems - outdoor location mechanism.

    Cellular-based Systems Comparison

Privacy in Location

  • Two-tier spatial transformation: query anonymization is performed by the mobile user itself - no trusted third party is required;

    • Dummy locations: generate a number of redundant queries, to hide the real query;
      • But, the the resulting trajectories need to mimic realistic movement patterns - e.g., in a sequence of queries, the user should not teleport from one location to another;
    • PROBE: an obfuscation system - it prevents the association between users and sensitive locations;
      • In an offline phase, an obfuscated map is constructed by partitioning the space into a set of disjoint regions such that the probability of associating each region with a certain feature type is bounded by a given threshold - obfuscation;
      • Obfuscation may require an additional trusted third party, but in the online phase, PROBE is a two-tier protocol;
    • ⚠️Limitation: if a user is situated in a remote location, and issues a query, an attacker who knows that the user is the only one in that area can associate the query with the user - three-tier protocols solve this issue.

  • Three-tier spatial transformation: presence of a trusted third party that performs the query anonymization.

    • Spatial k-anonymity: a cloaking region (CR) contains k-1 users in addition to the query source, and the LBS processes the query with respect to the CR;
    • Since all the k locations enclosed by the CR correspond to actual users, the probability of identifying the query source is at most 1/k;

Context-Awareness

  • Architecture:

    • End-user application: consumers of context information;

    • Middleware layer (optional): communication and coordination issues between distributed components;

    • Sensors layer: producers of context information;

      Context-Aware System Architecture

  • Context Processing:

    • Capture: context data is captured by sensors (Physical or Virtual sensors)

    • Infer: needed to obtain high-level context information from raw sensor data;

      Inference Layer

    • Distribute: context information is distributed to applications;

    • Consume: applications consume context information.

      • Pull-based (query): applications request context information when needed;
        • Polling: applications may need to poll the context server frequently to get the latest information - if the interval is too short, it may lead to high energy consumption, but if it is too long, the information may be outdated;
      • Push-based (subscription): applications subscribe to context information and are notified when it changes - more complex;

Energy

  • Energy-aware OSs: main idea is to reduce energy consumption by unifying resource and energy management, and by using collaboration between the OS and applications;

Consistency

  • Session consistency: provides a user with a view of the data that is consistent with the user's updates;
    • Read your writes: a user sees updates made by the user;
    • Monotonic reads: a user sees updates in the order they were made;
    • Writes follow reads: updates are made after reads on which they depend;
    • Monotonic writes: updates are made in the order they were requested;

Cyber Foraging

  • Cyber foraging: partition, migration and replication of application components across mobile devices and fixed infrastructure based on context (application state and environment);
    • Users are given the illusion that the application is running locally.

Augmented Reality

  • Identifying the location and position of the device to decide what to render and where;
  • Can be sensor-based or vision-based;
  • Virtual-based tracking can be marker-based or feature-based;
    • Marker-based: uses a predefined marker to identify the position and orientation of the device; markers are precise by impractical outdoors;
    • Feature-based: uses natural features in the environment to identify the position and orientation of the device; less precise but more versatile.