Challenges of Mobile Computing [Forman

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Challenges of Mobile ComputingForman
Zahorjan, 1994

• Wireless Communication
• Disconnection
• Low Bandwidth
• High bandwidth variability
• Heterogenous network
• Security

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• Mobility
• Address migration
• Location-dependent information
• Migrating locality
• Portability
• Power consumption (see Table 1)
• Form factor
• Small user interface and storage capacity
• Risks to data and physical damage

3
Xerox PARCTAB

• A first attempt at a thin handheld client for
  ubiquitous computing office environment

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Ubiquitous Computing Philosophy

• Demand less of our attention than current
  computers
• Advantages of an intelligently orchestrated and
  highly connected system.
• Context-aware features
• Users current location
• Identities of user and nearby people
• Identities and status of nearby resources
• Physical parameters (time, temperature, )

5
PARCTAB Design Goals

• Small, light and aesthetically pleasing
• Reliable wireless connectivity
• Tracking mechanism to know its location
  (resolution of a room)
• Run on batteries for atleast one day without
  recharging
• Allow casual interaction (even with one hand)
• Display graphics text, touch-screen
• Reasonable cost of hardware and network

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Hardware (1995!)

• 12 MHz, 8-bit Intel 87C524 Microcontroller (with
  low power modes)
• 128 KB memory
• 2.4in1.8in LCD (12864 monochrome pixels)
• Buttons Touch-screen
• 215g in weight (battery is 70g)
• Consumes 27mA at 5V (normal) and 30uA (low power
  mode) needs only around 1 charge per week
  (around 400 minutes)

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Communication

• Space and power constraints Infrared (IR)
• 9600/19200 baud
• IR signals are contained by walls
• One transceiver per room (cell)
• Transceiver is connected to RS-232 port of a
  workstation in the room (which is on a LAN)

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Transceiver

• Coverage around 20ft radius
• Performs transmission, receiving, coding,
  decoding, buffering, protocol checks.
• Transmission 2 dozen IR emitters placed at 15
  degree intervals
• Reception 2 detectors provide 360 degree coverage

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Transmission Control
2
1
1
4
4
3-247
Type Len. Dest. Src.
Payload
CS

• Time division multiplexing of medium
• CSMA protocol to provide access to IR channel
  with exponential backoff when busy.
• Ack needed bit in Type forcing explicit
  acknowledgements

10
User Interface

• Buttons Touch-screen
• Keyboard entry Unistrokes
• Display can only show 8 lines of 21 characters
• Elision and incremental searches

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Software Architecture
Video
Tab1 Agent
Shell
RS232
Tab1
Gateway
IR
Shell
Tab2 Agent
Tab4
Locate
Shell
Gateway
IR
Tab3 Agent
Tab3
Vote
Calendar
Tab4 Agent
Tab2
Gateway
IR
Shell
Transceiver
Mail
Ethernet
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• Tabs resemble dumb terminals
• Execute simple local functions in response to
  remote procedure calls (RPCs)
• There is one tab agent for each tab that
  maintains current info. (eg location)
• Request from application comes to agent, which
  forwards to appropriate gateway, which then sends
  it to transceiver on RS-232 to be broadcast using
  IR
• Events from tab are sent to the appropriate agent
  in the reverse direction

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• Simple tab functions (displaytext, displaybitmap,
  generatetones, wakeup, etc)
• Generates beacon events every 30 secs for
  location
• Gateway uses a name service to locate tab agent
• Appends a return address and location id to the
  agent
• A centralized location service keeps track of
  each tabs location that is updated by tab agent

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Example Applications

• Information access
• Weather forecast, dictionary, thesaurus, file
  browser, WWW, Diary (of days activities)
• Communication
• Mail, locator, pager
• Media applications
• Computer supported collaboration
• Group pointing, voting,
• Remote Control

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The InfoPad Mutimedia Terminal

• For wireless information access and display of
  multimedia data
• Again a very thin client does only wireless
  communication and I/O processing
• It is like a switch between the backend and the
  I/O devices

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InfoPad Hardware
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Power breakdown (9.6Watts)