Challenges of Mobile Computing

[Forman & Zahorjan, 1994]

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

• 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

Xerox PARCTAB

A first attempt at a thin handheld client for

ubiquitous computing – office environment

Ubiquitous Computing Philosophy

• Demand less of our attention than current computers

• Advantages of an intelligently orchestrated and highly connected system.

• Context-aware features– User’s current location– Identities of user and nearby people– Identities and status of nearby resources– Physical parameters (time, temperature, …)

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

Hardware (1995!)

• 12 MHz, 8-bit Intel 87C524 Microcontroller (with low power modes)

• 128 KB memory• 2.4in*1.8in LCD (128*64 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)

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)

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

Transmission Control

• 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

1 1 4 4 3-247 2

Type Len. Dest. Src. Payload CS

User Interface

• Buttons + Touch-screen• Keyboard entry + Unistrokes• Display can only show 8 lines of

21 characters• Elision and incremental searches

Software Architecture

Tab1

Tab4

Tab3

Tab2

Transceiver

Gateway

Gateway

Gateway

Tab1Agent

Tab2Agent

Tab3Agent

Tab4Agent

Video

Shell

Shell

Locate

Calendar

Shell

Mail

Vote

Shell

IR

IR

IR

Ethernet

RS232

• 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

• 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 tab’s location that is updated by tab agent

Example Applications

• Information access– Weather forecast, dictionary, thesaurus,

file browser, WWW, Diary (of day’s activities)

• Communication– Mail, locator, pager

• Media applications• Computer supported collaboration

– Group pointing, voting, …

• Remote Control

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

InfoPad Hardware

Wireless Network Interface (FPGA)

Speech Codec Display

VideoDecomp Keyboard,

Pointer

Other I/O

Processor ARM60 10MHz512K RAM, 128K ROM

Power breakdown (9.6Watts)

With Color Video

4%1%6%5%1%

18%

25%

40%

Processor

I /O Perip.

LCD

Misc I /O

Misc Sup.

Wireless

DC/DC

Video Disp

Without Color Video

6%

2%

10%

9%

1%

30%

42%