Context-aware Computing Context-aware Computing (Chapter 4)(Chapter 4)

2

Context-aware ComputingContext-aware Computing

Beyond application-aware adaptation Instead of adapting only to resource

levels, adapt to contextsContext:

– Enumeration-based (categories)– Role-based (roles of context in building

mobile applications)

3

Types of ContextTypes of Context

Computing context includes network connectivity, communication costs, communication bandwidth, and local resources, such as printers, displays, and workstations

User context includes user profiles, location, and people in the vicinity of the user

Physical context includes lighting and noise levels, traffic conditions, and temperature

Temporal context includes time of day, week, month, and season of the year

Context history is the recording of computing, user, and physical context over time

4

The 5 W’s…The 5 W’s…

Who is the user? Who are the people with which the user is interacting, or who is nearby?

What is the user doing? Where is the user? Home? Work?

Bathroom? Familiar coffee shop? When? What time is it? Why? Why is the user performing a certain

task? What is the task’s priority in the “grand scheme”?

Low-level vs. High-level details

5

Context OverviewContext Overview

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Context-aware RequirementsContext-aware Requirements

Contextual sensing– detection of environmental states

Contextual adaptation – capability of the system to adapt its behavior by using

contextual information Contextual resource discovery

– capability to discover available resources in an environment Contextual augmentation

– capability to associate contextual information with some digital data

– Example: association of a particular meeting place and attendees with a set of minutes

– Example: association of a digital photo with a specific location

7

Designing CA ApplicationsDesigning CA Applications

Build list of relevant contexts– e.g., “home”, “office”, “traveling”, “sleeping”, …

Specify context-aware behaviors– Presentation of context-sensitive information– Automatic discovery of relevant objects (e.g.,

nearby people for transmission of business cards)– Modification of the physical and digital

environments Integration of application with methods for

sensing context

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Simple Example: stick-e notesSimple Example: stick-e notes

Context-aware Post-it notes Build list of relevant contexts

– Based on location (latitude/longitude via GPS)– Temperature, whatever else can be sensed

Specify context-aware behaviors– stick-e notes “pop up” on a PDA when contextual info is

appropriate– Reminder to return a library book when near the library– Reminder to buy a new winter jacket when temperature

drops below 60F Integration of application with methods for sensing

context– Most difficult part– Ubiquitous sensing of environmental characteristics, such as

location, temperature, number of human beings nearby, the cat is near, not widespread

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Where Does Context Come From?Where Does Context Come From?

Returning to the “difficulty” point on the previous slide

Environmental sensors– Temperature, humidity, location, noise, motion– Cat sensor– Potential need for multiple types of sensors– GPS vs. indoor location sensors

History– Recording user actions and previous contexts

User’s personal computing environment– Schedules, notes, address books, financial info

Need real-time analysis to provide context

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Example: LocationExample: Location

Indoor locating systems – e.g., infrared or ultrasound

Wireless nanocell communication activity– Association with short-range base stations

GPS Associations with nearby computers Motion sensors and cameras, computer

vision Ask the user!

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Service-oriented ArchitectureService-oriented Architecture

Provide services to context-aware applications Context subscription and delivery service

– Delivers contextual information as available Context query service

– Delivers contextual information on-demand Context transformation and synthesis services

– Transforms low-level contextual information (location, temperature, lighting levels) into high-level (“YOU’RE IN THE CLOSET IN YOUR BEDROOM”)

Service discovery– Discovery of nearby services– We’ll examine service discovery protocols next!

Paper: Out of Context: computer Paper: Out of Context: computer systems that adapt to, and learn systems that adapt to, and learn

from, contextfrom, context(authors H. Lieberman & T. Selker)(authors H. Lieberman & T. Selker)

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Key PointsKey Points

Context-Sensitive (Aware) computing is antithetical to traditional computer science (search for context-independent abstractions)– Black box approach

Model of Context Aware application: Context as implicit input – System boundary decides what’s implicit and

what’s explicit

Context-Abstraction trade-off

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Key Points (Cont.)Key Points (Cont.)

Models of context:– System model: description of the system – User model: User’s state, history and

preferences– Task model: goals and actions intended to

be performed by the userTo create context-aware applications

these models should be dynamic and have ability to explain themselves.

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Context from other fieldsContext from other fields

Mathematics and AI– Statistical data mining techniques: knowledge

discovery by analyzing large amount of data HCI

– Ambient interfaces Sociology and Behavior Studies

– Situated action – stresses the effect of shared social context on human behavior

– Activity Theory: agent strategies for successful behaviour

– Nass and Reeves work on understanding how context affects human interaction with computers

Mobile Middleware (Chapter 5)Mobile Middleware (Chapter 5)

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What is Mobile Middleware?What is Mobile Middleware?

Software that supports mediation between other software components, fostering interoperability between those components across heterogeneous platforms and varying resource levels.

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Key AspectsKey Aspects

Adaptation (Chapter 1) Software Agents (Chapter 6):

– Put actions where data is!– Enable disconnected operations– Use infrastructure resources– Dynamic expansion of server/client functionality

Service Discovery (Chapter 7)– Dynamic discovery of services and enable

dynamic interaction between Clients and Services– Main issue is framework standardization to help

developers quickly build and deploy dynamic plug-and-play apps.

Mobile Agents (Chapter 6)Mobile Agents (Chapter 6)

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Mobile Agents: MotivationMobile Agents: Motivation

Mobile agent systems support dynamic migration of bits of software

These “bits of software” are agents, and perform tasks remotely in support of some local computation

Some advantages:– Disconnected operation– Customized, “do once” operations without

customizing server software– Attacks resource-poverty problem for mobile

computing• Less bandwidth, power, CPU needed

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Searching Tech Reports: No AgentsSearching Tech Reports: No Agents

Tech report server

Queries, responses, transfer of documents

MH

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Searching Tech Reports: AgentsSearching Tech Reports: Agents

Tech report server

MH

Mobile agent

localqueries

<<creation of document summaries orChanges in format, e.g. PDF TXT>>

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Mobile Agents: Another ExampleMobile Agents: Another Example

AGENT

Box office PLACE

MEETINGDrugstore PLACE

Music store PLACE

Shopping center server

Theater server

AUTHORITY/PERMIT evaluation

Tom

Daryl

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Mobile Agents: RoadblocksMobile Agents: Roadblocks

Security Users’ trust in mobile code has been badly

damaged– Applets, Javascript, other scripting languages are

far less sophisticated than agents, but…

Infrastructure Lack of a “killer” application Standardization (Security)

25Mobile Agents: Technical IssuesMobile Agents: Technical Issues

Security! Implementation language for agents

– Interpreted languages easier– Don’t want to insist on homogeneous architectures!

Migration strategy– Again, easier with languages like Java– Otherwise, becomes a process checkpointing problem

Communication strategy– Temporal locality: must communicate with other agents

while they’re around, or not?– Spatial locality: must be able to name and locate other

agents to communicate

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Agent Communication: TraditionalAgent Communication: Traditional

Sockets, RMI, …Strong temporal localityOther guy must be around to

communicateOther guy’s “name” reveals his location

27Agent Communication: Meeting PlacesAgent Communication: Meeting Places

A “meeting room” (aka service points) for agents

Agents must migrate to the meeting room to communicate

Temporal locality—other agents must be present to communicate

Spatial locality—communicate with other agents in the meeting place, can dynamically determine who’s there

28Agent Communication: Tuple SpacesAgent Communication: Tuple Spaces

Communicate by dropping objects into a shared space

Relaxes temporal and spatial locality Can drop objects for other agents whether

they’re around or not Don’t have to know their names or locations Simpler to use local tuple spaces (one per

place?) Can use reactions, which cause code to be

executed when a certain type of object (e.g., a “cash” object) is dropped into the space

Ad hoc / sensor networks (Ch 8Ad hoc / sensor networks (Ch 8))

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Mobile Ad Hoc NetworksMobile Ad Hoc Networks

May need to traverse multiple links to reach a destination

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Mobile Ad Hoc NetworksMobile Ad Hoc Networks

Mobility causes route changes

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Mobile Ad Hoc NetworksMobile Ad Hoc Networks

Formed by wireless hosts which may be mobile Don’t need a pre-existing infrastructure

– ie, don’t need a backbone network, routers, etc. Routes between nodes potentially contain

multiple hops Why MANET?

– Ease, speed of deployment– Decreased dependence on infrastructure– Can use in many scenarios where deployment of a wired

network is impractical or impossible– Lots of military applications, but there are others…

,

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Many ApplicationsMany Applications

Personal area networking– cell phone, laptop, ear phone, wrist watch

Civilian environments– meeting rooms– sports stadiums– groups of boats, small aircraft (wired REALLY impractical!!)

Emergency operations– search-and-rescue– policing and fire fighting

Sensor networks– Groups of sensors embedded in the environment or

scattered over a target area

34

Many VariationsMany Variations

Fully Symmetric Environment– all nodes have identical capabilities and responsibilities

Asymmetric Capabilities– transmission ranges and radios may differ – battery life at different nodes may differ– processing capacity may be different at different nodes– speed of movement different

Asymmetric Responsibilities– only some nodes may route packets – some nodes may act as leaders of nearby nodes (e.g.,

“cluster head”)

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Many VariationsMany Variations

Traffic characteristics may differ– bandwidth– timeliness constraints– reliability requirements– unicast / broadcast / multicast / geocast

May co-exist (and co-operate) with an infrastructure-based network

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Many VariationsMany Variations

Mobility patterns may be different– people sitting at an airport lounge (little mobility)– taxi cabs (highly mobile)– military movements (mostly clustered?)– personal area network (again, mostly clustered?)

Mobility characteristics– speed– predictability

• direction of movement• pattern of movement

– uniformity (or lack thereof) of mobility characteristics among different nodes

37

ChallengesChallenges

Limited wireless transmission range Broadcast nature of the wireless medium Packet losses due to transmission errors Environmental issues (“chop that tree!!”) Mobility-induced route changes Mobility-induced packet losses Battery constraints Potentially frequent network partitions Ease of snooping on wireless transmissions (security

hazard) Sensor networks: very resource-constrained!

38

Hidden Terminal ProblemHidden Terminal Problem

B CA

Nodes A and C cannot hear each other

Transmissions by nodes A and C can collide at node B

On collision, both transmissions are lost

Nodes A and C are hidden from each other

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First Issue: RoutingFirst Issue: Routing

Why is Ad hoc Routing Different? Host mobility

– link failure/repair due to mobility may have different characteristics than those due to other causes

– traditional routing algorithms assume relatively stable network topology, few router failures

Rate of link failure/repair may be high when nodes move fast

New performance criteria may be used– route stability despite mobility– energy consumption

40

Routing ProtocolsRouting Protocols

Proactive protocols– Determine routes independent of traffic

pattern– Traditional routing protocols for wired

networks are proactive

Reactive protocols– Discover/maintain routes only if needed

41

Trade-Off: Proactive vs. ReactiveTrade-Off: Proactive vs. Reactive

Latency of route discovery– Proactive protocols may have lower latency since routes are

maintained at all times– Reactive protocols may have higher latency because a route

from X to Y will be found only when X attempts to send to Y Overhead of route discovery/maintenance

– Reactive protocols may have lower overhead since routes are determined only if needed

– Proactive protocols can (but not necessarily) result in higher overhead due to continuous route updating

Which approach achieves a better tradeoff depends on the traffic and mobility patterns, but most ad hoc routing protocols are reactive