Weiser-Etal_2007_Some Computer Science Issues in Ubiquitous Computing

8/8/2019 Weiser-Etal_2007_Some Computer Science Issues in Ubiquitous Computing

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S Q M E O M PU T RS C | E N G G I S S U E S I N

m

U b i q u i t o u s c o m p u t i n g e n h a n c e s c o m p u t e r u s e b y

m a k i n g m a n y c o m p u t e r s a v a il a bl e t h r o u g h o u t t h e

p h y s i c a l e n v i r o n m e n t , w h i l e m a k i n g t h e m e f fe c t iv e l y i n v is i b le

t o t h e u s e r. T h i s a r t ic l e e x p l a i n s w h a t i s n e w a n d d i f f e r e n t

a b o u t t h e c o m p u t e r s c ie n c e in v o lv e d in u b i q u i t o u s c o m p u t i n g .

F i rs t, i t p r o v i d e s a b r i e f o v e r v i e w o f u b i q u i t o u s c o m p u t i n g ,

t h e n e l a b o r a t e s t h r o u g h a s e ri e s o f e x a m p l e s d r a w n f r o m

v a r i o u s s u b d i s c ip l i n e s o f c o m p u t e r s c i en c e : h a r d w a r e c o m -

p o n e n t s ( e .g . , c h i p s ) , n e t w o r k p r o t o c o l s , i n t e r a c t i o n s u b s t r a t e s

( e . g . , s o f t w a r e f o r s c r e e n s a n d p e n s ) , a p p l i c a t i o n s , p r i v a c y ,

a n d c o m p u t a t i o n a l m e t h o d s . U b i q u i t o u s c o m p u t i n g o ff er s a

f r a m e w o r k f o r n e w a n d e x c i t i n g r e s e a r c h a c r o s s t h e s p e c t r u m

o f c o m p u t e r s c ie n c e.

S i n c e w e s t a r t e d t h i s w o r k a t X e r o x P a l o A l t o R e s e a r c h

C e n t e r ( P A R C ) i n 1 98 8 a f e w p l a c e s h a v e b e g u n w o r k o n t h is

p o s s i b l e n e x t - g e n e r a t i o n c o m p u t i n g e n v i r o n m e n t i n w h i c h

e a c h p e r s o n is c o n ti n u a l ly i n t e r a c t i n g w i t h h u n d r e d s o f n e a r b yw i r e l es s ly i n t e r c o n n e c t e d c o m p u t e r s . T h e g o a l i s t o a c h i e v e t h e

m o s t e f f e c t iv e k i n d o f t e c h n o l o g y , t h a t w h i c h i s e s s e n t i a l l y

i n v i s i b l e t o t h e u s e r . T o b r i n g c o m p u t e r s t o t h i s p o i n t w h i l e

r e t a i n i n g t h e i r p o w e r w i l l r e q u i r e r a d i c a l l y n e w k i n d s o f c o m -

p u t e r s o f a l l s iz e s a n d s h a p e s t o b e a v a i l a b l e to e a c h p e r s o n . I

c al l t h is f u t u r e w o r l d " U b i q u i t o u s C o m p u t i n g " ( U b i c o m p )

[ 27 ]. T h e r e s e a r c h m e t h o d f o r u b i q u i t o u s c o m p u t i n g i s s t a n -

d a r d e x p e r i m e n t a l c o m p u t e r s c i e n c e : t h e c o n s t r u c t i o n o f

w o r k i n g p r o t o t y p e s o f t h e n e c e ss a i- y i n f r a s t r u c t u r e i n s u f f i c ie n t

q u a n t i t y t o d e b u g t h e v i a b i li t y o f th e s y s t e m s i n e v e r y d a y u s e ;

o u r s e l v e s a n d a f e w c o l l e a g u e s s e r v i n g a s g u i n e a p i g s . T h i s i s

Reprinted with permission. Copyright c) 1993 by The A ssociation orComputing Machinery, n c. All rights reserved. This article originallyap-peared in Communications f the ACM , Vol. 36, No. 7 (July 1993), Pages75-84.

12 Mobile Computing and Communications Review, Volume 3, Number 3



a n i m p o r t a n t s t e p t u w a r d e n ~ t a r i n g

t h a t o u r i n f i ' a s t r u c t u r e r e s e a r c h i s

r o b u s t a n d s c a l a b l e i ~ t h e f a c e o f t h e

de ta i l s o f the rea l wor ld .

" I' he i d e a o f u b i q u i t o u s c o m p u t i n g

f i r s t a r o s e t : ¥ o m c o n t e m p l a t i n g t h e

p l a c e o f t o d a y ' s c o m p u t e r i n a c t u a l

a c t i vi t i es o f e v e r y d a y l i f e , I n p a r t i c u -

l a r , a n t h r o p o l o g i c a l s t u d i e s o f w o r k

l i f e [14 , 22 ] teach u s tha t peop le p r i -

m a r i l y w o r k i n a w o r l d o f s h a r e d s i t-

u a t i o ns a n d u n e x a m i n e d t e ch n o l og i -

c a l s k i l l s . T h e c o m p u u e r t o d a y i s

i s o l a t e d f r o m t h e o v e r a l l s i t u a t i o n .

h o w e v e r , a n d f a i l s t o g e t o u t o f t h e

w a y o f t h e w o r k . I n o t h e r w o r d s ,

r a t h e r t h a n b e i n g a t o o l t h r o u g h

w h i c h w e w o r k , a n d t h u s d i s a p p e a r -

i n g f r o m o u r a w a r e n e s s , t h e c o m -

p u t e r t o o o f t e n r e m a i n s t h e g o c us o f

a t t e n t i o n . A n d t h i s is t r u e t h r o u g h -

o u t t h e d o m a i n o f p e r s o n a l c o m p u t -

i n g as c u r r e n t l y i m p l e m e n t e d a n d

d i s c u s se d i b r t h e f u t u r e , w h e t h e r

o n e t h i n k s o f p e r s o n a l c o m p u t e r s .p a h n t o p s , o r d y n a N ) o k s , T b e c h a r a c-

t e r i z a t i o n o f t h e f u t u r e c o m p u t e r a s

t h e " i n t i m a t e c o m p u t e r " [ t2 1 , o r

" r a t h e r l i k e a h u m a n a s s i s t a n t " [ 2 5 1

m a k e s t h i s a t t e n t i o n t o t h e m a c h i n e

i t se l f p a r t i c u la r l y a p p a r e n t .

G e t t i n g t h e c o m p u t e r o u t o f t h e

w a y i s n o t e a s y . T h i s i s n o t a g r a p h i -

c a l u s e r i n t e r f a c e ( G U I) p r o b l e m , b u t

i s a p r o p e r t y o f t h e w h o l e c o n t e x t o f

u s a g e o f t h e m a c h i n e a n d t h e a t t r i -

b u t e s o f i t s p h y s i c a l p r o p e r t i e s : t h e

k e y b o a r d , t h e w e i g h t a n d d e s k t o p

p o s i t i o n o f s c r e e n s , a n d s o o n . T h e

p r o b l e m i s n o t o n e o f " i n t e r f a c e . "F o r t i l e s a m e r e a s o n o f c o n t e x t , t h i s

i s n o t a m u l t i m e d i a p r o b l e m , r e s u l t -

i n g f r o m a n y p a r t i c u l a r d e f i c i e n c y i n

t h e a b i l i t y t o d i s p l a y c e r t a i n k i n d s o f

r e a l - t i m e d a t a o r i n t e g r a t e t h e m i n t o

a p p l i c a t i o n s . ( I n d e e d , m u l t i m e d i a

t r i e s t o g r a b a t t e n t i o n , t h e o p p o s i t e

o f t h e u b i q u i t o u s c o m p u t i n g i d ea l o f

i n v i s i b i li t y . ) T h e c h a l l e n g e i s t o c r e -

a t e a n e w k i n d o f r e l a t i o n s h i p o f p e o -

p l e t o c o m p u t e r s , o n e i n w h i c h t h e

c o m p u t e r w o u l d h a v e t o t a k e t h e

l e a d i n b e c o m i n g v a s tl y b e t t e r a t g e t -

t i n g o u t o f t h e w a y , a l l o w i n g p e o p l e

t o j u s t g o a b o u t d l e i r l iv e s .I n [ 9 8 8 , w h e n I s t a r t e d P A R C ' s

w o r k o n u b i q u i to u s c o m p u t i n g , v i r -

t u a l r e a l i t y ( V R ) c a m e t h e c l o s e s t t o

e n a c t i n g t h e p r i n c i p l e s w e b e l i e v e d

i m p o r t a n t . I n i t s u l t i m a t e e n v i s i o n -

m e n t , V R c a u s e s t h e c o m p u t e r t o

: | | J

b e c o m e e f f e c t i v e l y i n v i s ib l e by i n k i n g

o v e r t he h u m a n s e n s o r y a n d a i t e c to r

s y s t e m s [ 1 9 ] . V R i s e x t r e m e l y u s e f u l

i n s c i e n t i f i c v i s u a l i z a t i o n a n d e n t e r -

t a i n m e n t , a n d w i ll b e v e r y s i g n i f i c a n t

f i.~r those n iches . B u t a s a too l fo r

p r o d u c t i v e l y c h a n g i n g e v e r y o n e ' s

r e l a t i o n s h i p t o c o m p u t a t i o n , i t h a s

two c ruc ia l f l aws : f i r s t , a t the p re sen t

t i m e ( 1 9 9 2 ) , a n d p r o b a b l y f o r d e -c a d e s , it c a n n o t p r o d u c e a s i m u l a t i o n

o f s i g n i f i c a n t v e r i s i m i l i t u d e a t r e a -

s o n a b l e c o s t ( to d a y , a t a n y c o s t) . T h i s

m e a n s t h a t u s e r s w i l l n o t b e t o o l e d

a n d t h e c o m p u t e r w i l l n o t b e o u t o f

t h e w a y . S e c o n d , a n d m o s t i m p o r -

t a n t , i t h a s t h e g o a l o f f o o l i n g t h e

u s e r - - o f l e a v i n g t h e e v e r y d a y p h y s i-

c a l w o r l d b e h i n d . T h i s i s a t o d d s w i t h

t h e g o a l o f b e t t e r i n t e g r a t i n g t h e

c o m p u t e r i n t o h u m a n a c ti v it i es , s i n ce

h u m a n s a r e o f a n d i n t i l e e v e r y d a y

w o r l d .

U b i q u i t o u s c o m p u t i n g i s e x p l o f

i n g q u i t e d i f f e r e n t g r o u n d f r o m p e r -s o n a l d i g i t a l a s s i s t a n t s , o r t h e i d e a

t h a t c o m p u t e r s s h o u l d b e a u t o n o -

m o u s a g e n t s t h a t t a k e o n o u r g o a l s .

T h e d i f f e r e n c e c a n b e c h a r a c t e r i z e d

a s f o l lo w s : S u p p o s e y o u w a n t t o l i f t a

h e a v y o b j e c t . Y o u c a n c a l l i n y o u r

s t r o n g a s s i s t a n t t o l i f t i t t o t y o u , o r

y o u y o u r s e l f c a n b e m a d e e f f o rt -

l e s s l y , u n c o n s c i o u s l y , s t r o n g e r a n d

j u s t l i f t i t. T h e r e a r e t i m e s w h e n b o t h

a r e g o o d . M u c h o f t h e p a s t a n d c u r -

r e n t e f f o r t f b r [ l e t t e r c o m p u t e r s h a s

b e e n a i m e d a t th e f o r m e r ; u b i q u i t o u s

c o m p u t i n g a i m s a t t h e l a t t e r .

T h e a p p r o a c h I t o o k w a s t o a t -t e m p t t h e d e f i n i t i o n a n d c o n s t r u c -

t i o n o f n e w c o m p u t i n g a r t i f a c t s f o r

u s e i n e v e r y d a y l i f e. I t o o k m y i n s p i -

r a t i o n f r o m t h e e v e r y d a y o b j e c t s

f o u n d i n o f f i c e s a n d h o m e s , i n p a r -

t i t u l a r t h o s e o b j e c t s w h o s e p u r p o s e i s

t o c a p t u r e o r c o n v e y i n f o r m a t i o n .

T h e m o s t u b i qu i t o u s c u r r e n t i n t o r -

m a t i o n a l t e c h n o l o g y e m b o d i e d i n

a r t i f a c t s i s t h e u s e o f w r i t t e n s y m b o l s ,

p r i m a r i l y w o r d s , b u t i n c l u d i n g a ls o

p i c t o g r a p h s , c l o c k s , a n d o t h e r s o r t s

o f s y m b o l i c c o m m u n i c a t i o n . R a t h e r

t h a n a t t e m p t i n g t o r e p r o d u c e t h e s e

o b j e c t s i n s i d e t h e v i r t u a l c o m p u t e r : t

w o r l d , l e a d i n g t o a n o t h e r " d e s k t o p

m o d e l " [ 2 1 , I w a n t e d t o p u t t h e n e w

k i n d o f c o m p u t e r o u t i n i b i s w o r l d o f

c o n c r e t e i n f o r m a t i o n c o n v e y e r s .

S i n c e t h e s e w r i t t e n a r t i f a c t s o c c u r i n

m a n y d i f t e r e n t s i z es a n d s h a p e s , w i th

m a n y d i f f i e r e n t q u a l i t ie s , 1 w a n t e d

t h e c o m p u t e r e r n b o d i m e n t s u ) be o f

m a n y s iz es a n d s h a p e s , i n c l u d i n g t in y

i n e x p e n s i v e o n e s t h a t c o u l d b r i n gc o m p u t i n g t o e v e r y o n e .

T h e p h y s k a l w o r l d c o m e s i n a l l

s i z e s a n d s h a p e s . F o r p r a c t i c a l r e a -

s o ns o u r u b i q u i to u s c o m p u t i n g w o r k

t ~ g i n s w i t h j u s t t h r e e d i f f e r e n t s i z es

o f d e v ic e s : e n o u g h t o g i ve s o m e

s c op e , n o t e n o u g h t o d e t e r p r o g r e s s.

T he f i r s t s ize i s the wa ll -s~zed in t e rac -

t i v e s u r f h c e , a n a l o g o u s t o t h e o f f i c e

w h i t e b o a r d o r t h e h o m e m a g n e t -

c o v e r e d r e f r i g e r a t o r o r b u l l e t i n

t x ) a r d . T h e s e c o n d s i z e i s t h e n o t e -

p a d , e n v i s i o n e d n o t a s a p e r s o n a l

c o m p u t e r b u t a s a n a l o g o u s t o s c r a p

p a p e r t o b e g r a b b e d a n d u s e d e a s il y ,w i t h m a n y b e i n g u s e d b y a p e r s o n a t

o n e t i m e . T h e c l u t t e r e d o f f i c e d e s k

o r m e s s y t i ' o n t - h a l l t a b l e a r e r e a l - l i f e

e x a m p l e s . F i n a l l y , t h e t h i r d s i ze i s t h e

t i n y c o m p u t e r , a n a l o g o u s t o t i n y in -

d i v i d u a l n o t e s o r P o s t - i t n o t e s , a n d

a l s o s i m i l a r t o t h e t i n y l i tt l e d i s p l a y s

o f w o r d s f o u n d o n b o o k s p i n e s, l i g ht

s w i t c h e s , a n d h a l l w a y s . A g a i n , I s a w

t h i s n o t a s a p e r s o n a l c o m p u t e r , b u t .

a s a p e r v a s i v e p a r t o f e v e r y d a y l i f e ,

w i t h m a n y a c t i v e a t a ll ti m e s ; I c a l l e d

t h e s e t h r e e s i z e s o f c o m p u t e r s

I ~ ) a r d s , p a d s , a n d t a b s , a n d a d o p t e d

t h e s l o g a n t h a t , f o r e a c h p e r s o n i n a no f f i c e , t h e r e s h o u l d b e h u n d r e d s o f

t a b s , t e n s o f ' p a d s , a n d o n e o r t w o

b o a r d s . S p e c i f i c a t i o n s f o r s o m e p r o -

t o t y p e s o f t h e s e t h r e e s i z e s i n u s e a t

P A R C a r e s h o w n i n F i g u r e 1,

T h i s t h e n i s P ha s e I o f u b i q u i t o u s

c o m p u t i n g : t o c o n s t r u c t , d e p lo y , a n d

7G July 1~93!Va136, Nod ~, lm ~A 't "~ I : ~ l T J , m ~ , ~ m

Mobile Computing and Communications Review, Volume 3, Num ber 3 13



learn from a COl n p u t i r i g environ-

ment con sisting of tabs, pads, and

hoards. This is only Phase I, because

it is unlikely to achieve optimal invisb

bility. (Later phases are yet io be de-

termined.) tint it is a start down the

radical direction, for computer sci-

erlce, away from emphasis on the

macliine and back to the person and

his or laer life in the world of' work,

play, and home.

Hardware PrototypesNew hardware systems design tot

ubiquitous computing has been ori-

ented toward experimental plat-

forms for systems and ap plications of

invisibility. New chips have been less

important than combinations of ex-

isting contp onent s that create experi-

ntental opportunities. The first ubiq-

uitous computing technology to be

deployed was the Liveboard [6],

which is now a Xerox product, Two

otlter important pieces of prototype

hardware supportin g our research at

PAR(; are the Tab and the Pad.

Tats

The Parc'[hb is a tiny information

doorway. Fox user intera ction it has a

pressure-sensitive screen on top of

the display, three buttons positioned

underneath the natural finger post-

lions, and the ability to sense its posi-

tion within a building. The display

and touchpad it uses are standard

commercial units.

The key hardware design prob-

lems atfecting the pad are physicalsize and power consur nption. With

several dozens of these devices sitting

around the oflice, in briefi.'ases, in

pockets, one cannot change their bat-

|cries every week. The PARC design

uses the 8051 chip to control detailed

interactions, and includes soliware

that keeps power usage down. The

major outboard components arm a

srnall analog/digital converte r fo r the

pressure-sensitive screen, and analog

sense circuitry for the IR receiver.

Interestingly , aldaough we have been

approached by several chip manu-

facturers ab out our possiMe ne ed for

custo m chips t~)r the Ta b, the "l~tb is

not short of places to put chips. The

display size leaves plenty of room,

arid the display thickness dominates

total size. Off-the-shelf componentsare more than adequate for explor-

ing this design space, even with our

severe size, weight, and power con-

strain|s.

A key part of our design philoso-

phy is to put devices in everyday use,

not just demonstrate them. We can

only use techniques suitable for

quantity 100 replication. This ex-

cludes certain tedmiques that could

make a hugs difference, such as the

integration of components onto t h e

display surlace itself'. This technol-

ogy is being explored at PARC, aswell as other research organizations.

While it is very pr omis ing, it is not yet

ready ff)r replication.

"fire Tab architecture incorporates

a careful balance of' display size,

bandwidth, processing, and memory.

For instance , the small display means

that even the tiny processor is capa-

ble ot providing a four-frames-per-

second video rate, and the IR band-

width is capable of delivering this.

The bandwidth is also such that the

preKessor can actually time the pulse

widths in software timing loops. (.)ur

current design has insufticient stof

age, and we are increasing ~he

amount of nonvolatile RAM in fk~-

ture tabs from 8K to 128K. The tab's

goal of casual use similar to that oi

Post-it notes puts it into a design

space generally unexplored in the

commercial or research sector.

P a d

The pad is really a family of note-

t~ok-sized devices. Our initial pad,

the ScratchPad, plugged into a Sun

SBus card and provided an

X-Window-sysmm-com atible writ-

ing and display surt~ce. This same

design was used inside our first wall-

sized displays, the liveboards, as well.

Our later untethered pad devices,

the XPad and MPad, continued the

system design principles of

X-compatibility, ease of con struct ion,

and flexibility in software and hard-ware expansion.

A s l write this article, at the e n d o f

1992, commercial portable pen de-

vices have linen on the market fbr

two years, altltough most of the early

companies have now gone out of

business. Why should a pioneering

research lab build its own such de-

vice? Each year we ask ourselves the

sarne question, and so tar three

things always drive us to continue to

14 Mobi le Comput ing and Communicat ions Review, Volume 3 , Number 3



design om ~)wn pad hardware:

First, we need the correct balance

of ti:atures--this is the essence ot

systems design. 1he commercial de-

,ices all aim at particular niches, bal.

a~lcing their design to that niche. For

research we need a rather difti:rent

balance, particularly tbr ubiquitous

coinputmg, For instance, can tile

device communicate simuttaaeouslyalong multiple chaimets? Does the

operating system support muttipro-

cessing? What about the potential for

high-speed tethering? ts there a

high-quality pen? Is there a high-

speed expansion port sufficient for

video in and out? Is sound in/out and

ISDN commctivity available? Op-

tional keyboard? Any one commer-

cial device tends to satist}' some of

these needs, ignore other.s, and

choose a balance of the ones it does

satisfy, optimizing its niche, rather

than ubiquitous computing-style

scrap computing. The balance [or us

emphasizes communication, system

memory, muhimedia, and expansion

ports.

Second, apart from b alance of flea-

rares are the requirements for par-

ticular features. Key among these are

a pen emphasis, connection to re-

search envi ronm ents such as Unix,

and colnnmnication emphasis. A

high-speed (>64KB/sec) wireless

capability is not built into any com-

mercial devices, and they do not gen-

erally have a sufficiently high-speed

port to add such a radio. Conmmrcialdevices generaUy tonre with DOS or

Penpoint, and while we have devel-

oped in both, they are not our filvor-

ire research vehicles because they

lack full access and customizability.

The third factor driving out own

pad designs is ease of expansion and

modification. We need flail hardw are

specifications, complete operating

system sotuce code, and the ability to

remove and replace both hardware

and software components. Naturally

these goals are oppose d to best price

in a niche market, which orients the

documentation to the end user, andkeeps prices down by integrated

rather that) modular design.

We have built and used three gen-

erations of Pad designs. Six scratch-

pads were built, three XPads, and 13

MPads, the latest. The MPad uses an

FPGA for almost all random logic,

giving extreme flexibility. For in-

stance, changing tile power comrol

ftulctious and adding higtl-quatity

sound were relatively simple FPGA

changes. The MPad has both IR (tab

compatible) and radio communica-

tion built-in and indudes sufficient

uncommitted space tor adding new

circuit boards later. It can be used

with a tether that provides it withrecharging and operating power and

an Ethernet connection. The operat-

ing system is a st andalone version of

the public-do main Portable Common

Runtime developed at PAR('. [28].

1"he C o m pute r Sc ie nce o fU b l c o m plib construct and deploy tabs, pads,

and boards at PAR(:, we found our-

selves having to readdress some of

the well-worked areas of existing

computer science. The fluitfulness

of ubiquitous computing [br new

computer science problems justified

out belief in the ubiquitous comput-

ing framework.

The ibllowing subsections "as-

cend" the levels of organization of a

computer system, from hardware to

application. One or two examples of

computer science work required by

ubiquitous computin g are described

hn- each level. Ubicomp is not yet a

coherent body of work, but consists

of t few scattered communities. The

point of this article is to help others

understand some of the new re-

search challenges in ubiquitous com-puting, and inspire them to work on

them. This is more akin to a tutorial

than a survey, and necessarily selec-

tive, The areas included are hard-

ware components (e.g., chips), net-

work protoc,)ls, interaction sub-

strates (e.g., software for screens and

pens), applications, privacy, anti

computational methods.

I ss ue s o f H a rc lwa reC o m p o n e n t sI n a d d i t i o n t o t h e n e w s y s t e m s o f

t a b s , p a d s , a n d b o a r d s , u b i q u i t o u s

c o m p u t i n g n e c e s s i t a t e s s o m e n e wk i n d s o f d e v i c e s . E x a m p l e s o f t h r e e

n e w k i n d s o f h a r d w a r e d e v i c e s a r e

v e r y l o w - p o w e r c o m p u t i n g , l o w -

pow,e r h i g h - b i t s / c u b i c - m e t e r c o m m u -

n i c a t i o n , a n d p e n d e v i c e s .

LOW Pow erI n g e n e r a l t h e n e e d f o r h ig h p e r f o r-

mance has dominated the need t0r

low-power consm npti on in processor

design. However, r ecognizing the

new requirements of ubiquitous

computing, a numbe r of people have

begun work in using additional chip

area to reduce power rather than to

increase performance [16]. One key

approach is to reduce the clocking

frequeucy of their chips by increas-ing pipelining or parallelism. Then,

hy ru,min g the chips at reduc ed volt-

age, the effect is a net reduction in

power, because power falls off as the

square of the voltage, while only

about twice the area is neede d to ru n

at haft the clock speed.

Power = Cl*Vdd'~*]"

where Gt. is the gate capacitance,

Vda the supply voltage,

and f the clocking frequency.

This method of reducing power

leads to two new areas of chip design:

circuits that will run at low power,

and architectures that sacrifice area

lbr power over performance. The

second requires some additional

comment, because one might sup-

pose one would simply design the

fastest possible chip, and then run it

at reduced clock and voltage. How-

ever, as Lyon illustrates, circuits in

chips designed for high s p e e d g e n e r -

a l ly fail to work at low voltages. Fur-

thermore, attent ion to special circuits

may permit operation over a much

wider range o f vohage operation, or

achieve power savings via other spe-cial techniques, such as adiabatic

switching [ 16].

WirelessA w i r e le s s n e t w o r k c a p a b l e o f a c c o m -

m o d a t i n g h u n d r e d s o f h i g h - s p e e d

d e v i c e s f o r e v e r y p e r s o n i s w e l l b e -

y o n d t h e c o m m e r c i a l w i r e l e s s s y s -

t e m s p l a n n e d f o r t h e n e x t 1 0 y e a r s

[ 2 0] , w h ic h a r e a i m e d a t o n e l o w -

s p e e d ( 6 4 k b / s e c o r v o i c e ) d e v i c e p e t

p e r s o n . M o s t w i r e l e s s w o r k u s e s a

f i g u r e o f m e r i t o f b i t s /s e c x r a n g e ,

a n d s e e k s t ( ) i n c r e a s e t h i s p r o d u c t .

W e b e l i e v e t h a t a b e t t e r f i g u r e o tm e r i t i s b i t s / s e c / m e t e r ~ . T h i s f i g u r e

o f m e r i t c a u s e s t h e o p t i m i z a t i o n o f

t o t a l b a n d w i d t h t h r o u g h o u t a 3 D

s p a c e , l e a d i n g t o d e s i g n p o i n t s o f

v e r y t i n y c e l l u l a r s y s t e m s ,

B e c a u s e w e f e l t t h e c o m m e r c i a l

w o r l d w a s ig n o r i n g t h e p r o p e r f i g u r e

~ 8 J u i ~, 1 9 ~ .~ 3 /\ '~ L 3 6 , N o 7 C O ~ 4 M U l U C 4 ~ q O t 4 U s o l , ~ r m t a c ~ a

Mobile Computing and Communications Review, Volume 3, Number 3 15



, + k , ] o > i . L o ,

T comtruct and dephy tab~, pad5, and board5 at PA RC~ weJbuT zd oursdves

hav ing to readdress som e of the well-w orke d areas of existing co mp uter science.

of merit, we initiated our own small

radio program. In 1989 we built

spread-spectrum transceivers at900MHz, but fbund them difficult to

build and adjust, and pr one to noise

and multipath interference. In 1990

we built direct frequency-shift-keyed

transceivers also at 900MHz, using

very low power to be license-fi'ee.

While much simpler, these transceiv-

ers had unexpectedly and unpredict-

ably long range, causing mutual in-

terference and multipath problems.

In 1991 we designed and built our

current radios, which use the near-

field of" the electromagnetic spec-

trum. The near-field has an effective

fall-off of r 6 in power, instea d of t he

more u sual r 2, where r is the distance

from the transmitter. At the proper

levels this band does not require an

FCC license, permits reuse of the

same frequency over and over again

in a building, has virtually no muhi-

path or blocking effects, and permits

transceivers that use extremely low

power and low parts count. We have

deployed a number of near-field

radios within PARC.

P e n s

A third new hardware component isthe pen f2)r very large displays. We

needed pens that would work over a

large area (at least 60in x 40in), not

require a tether, and work with back

projection, "l'hese requirements are

generated from the particular needs

of large displays in ubiquitous com-

put ing -ca sua l use, no training, nat-

uralness, simultaneous muhiple use.

No existing pens or touchpads could

come close to these requirements.

Therefore members of the Electron-

its and Imag ing lab at PARC devised

a new infrared pen. A camera-like

device behind the screen senses thepen position, and information about

the p en state (e.g., but tons) is modu-

lated along the IR beam. The pens

need not touch the screen, but can

operat e f rom several feet away. Con-

siderable DSP and analog design

work underlies making these pens

effective components of the ubiqui-

tous coruputmg system [6].

N e t w o r k P r o t o c o l sUbicomp changes the emphasis m

networking ira at least four areas:

wireless media access, wide-band-

width range, real-time capabilities

for multimedia over- standard net-

works, and packet routing.

Wire less Media Access

A "media access" protocol provides

access to a physical medium. Com-

mon media access methods in wireddomains are collision detection and

token-passing. Thes e do n ot work

unchanged in a wireless domain be-

cause not every device is assured of

being able to hear every othe r device

(this is called the "hidden terminal"

problem). Furt her more , earlier- wire-

less work used assumptions of com-

plete autonomy, or a statically con-

figured network, while ubiquitous

computing requires a cellular topoi-

ogy, with mobile devices frequently

comin g on- and of f line. We have

adapted a media access protocol

called MACA, first described by

Karn [11], with some of our own

modifications for fairness arid eft]-

ciency.The key idea of MACA is for tire

two stations desiring to communica te

to first do a short handshake of Re-

quest-To-Send-N-bytes followed by

Clear-'lb-Send-N-bytes. This ex-

change allows all other stations to

hear that there is going to be traffic,

and for how long they should remain

quiet. Collisions, which are detected

by timeouts, occur only during the

short Request-To-Send packet.

Adapting MACA for ubiquitous

computi ng use required considerable

attention to fairness and real-time

requirements. MACA requires sta-tions whose packets collide to back

off" a ra ndo m time an d try again. If

all stations but one back-off, that one

can dominate the bandwidth. By re-

quir ing all stations m a dapt the back-

off para meter of their neighbors, we

create a much tairer allocation of

bandwidth.

Some applications l~¢.ed guaran-

teed bandwidth for voice or video.

We added a new packet type,NCTS(n) (Not Clear *lk) Send), to

suppress all other transmissions for

(n) bytes. This packet is sufticient for

a basestation to do effective band-

width allocation among its mobile

units. "l'he solution is robust, in the

sense that if the basestation stops

allocating bandwidth the system

reverts to norIIlill COIItCClltiOn.

When a number of mobile units

share a single basestation, that base-

station may be a bottleneck for com-

munication. For fhirness, a basesta-

tion with N > 1 nonemp ty ou tput

queues needs to contend for band-

width as though it were N stations.

We therefore make the basestatkm

cont end .just eno ugh more aggres-

sively that it is N times mor e likely to

win a contention tor media access.

O t h e r N e t w o r k I S S U E S

* [M:-o other areas of networking re-

search at PARC with ubiquitou s com-

puting implications are Gh networks

and real-time protocols. Gb-per.

second speeds are important because

of the increasing number of me-

dium-speed devices anticipated byubiquitous computing, and the grow-

ing importance of real-time (muhi-

media) data. One hun dre d 256kb/sec

portables p er office implies a Gb per*

grou p of 40 offices, with all of PARC

needin g an aggregate of some five

Gb/sec. This has led us to do research

into local-area NI'M switches, in as-

sociation with other Gb networking

projects [151.

Real-time protocols are a new area

of h)cus in packet-switched networks.

Although real-time delivery has al-

ways been important in telephony, a

few h undr ed milliseconds never mat-tered in typical packet-switched ap-

plicatinns such as telnet a nd file

transfer. With the ubiquitous use of

packet-switching, even ['or teleph ony

using ATM, the need for real-time

capable protocols has become urgent

if the packet networks are going to

support muhimedia applications.

16 Mobile Computing and Communications Review, Volume 3, Number 3



Again, in associatio~ with other

members of the research community,

PARC is exploring new protocols tb~

enabling multimedia on the packet-

swkched Internet [4].

The Inter net routing protocol, IP,

has been in use for over 10 years.

However, neither this protocol nor

its OSI equivalent, CLNP, providessufficient iniYastructure 1or highly

mobile devices. Both interpret fields

in the network names of devices m

order to route packets to the device.

For instance, the "13" in IP name

13.2.0.45 is interpreted to mean net

13, and network routers anywhere in

the world are expected to know how

to get a packet to net 13, and all de-

vices whose name starts with 13 are

expected to be on that network. This

assum ption fails as soon as a user of a

net t 3 mobile device takes her device

on a visit to net 36 (Stanford).

Changing the device name dynami-cally dep end ing on location is no so-

lution: higher-level protocols such as

TCP assume that underlying names

will not change during the life of a

connection, and a n ame change must

be accompanied by informing the

entire network of the change so that

existing services can fin d the device.

A nu mbe r of solutions have been

proposed to this problem, among

them Virtual IP from Sony [24], and

Mobile IP from Col umbia University

[ 10]. "l'hese solutions p erm it existing

IP networks to interoperate trans-

pare ntly with roam ing hosts. The key

idea of all approaches is to add a sec-

ond layer of IP address, the "real"

address indicating kKadon, to the

existing fixed-device address. Special

routing nodes that forward packets

to the correct real address, and keep

track of where this address is, are

required for all approaches.*

In teract ion S ubstratesUbicomp has led us to explore new

substrates for interaction. Four such

substrates are men tioned here, span-

ning the space from virtual key-boards to protocols for window sys-

tems.

Tabs have a very small interaction

area~too small for a keyboard, too

small even for standard handprint-

* T h e I n t e r n e t c o m m u n i t y h a s a w o r k i n g g r o u pc o n s i d e r i n g s t a n d a r d s f o r t h i s a r e a ( c o n t a c td e e r i n g @ x e r o x . c o m f o r m o r e i n f o r m a t i o n ) .

ing recognition. Handprmting has

the further problem of requiring

looking at what is written. Improve-

ments in voice recognition are no

panacea, because when other people

are present, voice will often be inap-

propriate. As one possible solution,

we developed a method of touch-

printing that uses only a tiny areaand does trot require looking. A

drawback of our method is it re-

quires a new printing alphabet to be

memorized, and reaches only half

the speed of a fast typist [8].

Liveboards have a high intera ction

area, 400 times that of the tab. Using

conventional pull-down or pop-up

menus might require walking across

the room to the appropriate button,

a serious problem. We have devel-

oped methods of location-indepen-

dent interaction by which even coin-

plex interactions can be popped up

at any location. [13].The X-Window system, although

designed for network use, makes it

difficult for windows to move once

instantiate d at a given X server. This

is because the server retains consid-

erable state about individual win-

dows, and does not provide conve-

nient ways to move that state. For

instance, context and window IDs are

dete rmin ed solely by the server, and

cannot be transferred m a new

server, so applications that depend

on knowing their value (almost all)

will break if a window changes serv-

ers. However, in the ubiquitous corn-

puling world a user may move fre-

quently from device to device, and

want to bring windows along.

Christian Jacobi at PAR(; has im-

plemented a new X toolkit that facili-

tates window migration. Applications

need not be aware that they have

moved from one screen to another;

or if desired, the user can be so in-

forme d with an upcall. We have writ-

ten a numb er of applications on top

of this toolkit, all of which can be

"whistled up" over the network to

follow the user from screen toscreen. Th e a uthor, for instance, fre-

quently keeps a single program de-

velopment and editing environment

ope n for days at a time, migrating its

windows back and forth from home

to work and back each day.

A f inal window system problem is

bandwidth. The bandwidth available

to devices in ubiquitous computing

can vary front Kb/sec to Gb/sec, and

with window migration a single ap-

plication may have to dynamically

adjust to bandwidth over time. The

X-Window system protocol was pri-

marily developed for Ethernet

speeds, and most of the applications

written in it were similarly tested at10Mb/sec. To solve the problem of

efficient X-Window use at lower

bandwidth, the X consortium is

sponsoring a "Low Bandwidth X"

(LBX) working group to investigate

new methods of lowering bandwidth.

[7].

ApplicationsApplications are o f course the whole

point of ubiquitous computing. Two

examples of applications are locating

people and shared drawing.

Ubicomp permits the location of

people and objects in an environ-ment. This was first pioneered by

Olivetti Research la bs in Cambridge,

England, in their Active Badge sys-

tem [26]. In ubiquitous comp utin g

we continued to extend this work,

using it for video annotation and

updating dynamic maps. For in-

stance, Figure 2 shows a portion of

CSL early one morning, and the in-

dividual laces are the locations of

people. This map is updated every

few seconds, permitting quick locat-

ing of people, as well as quickly notic-

ing a meeting one might want to go

to (or where one can find a fresh pot

of coffee).

Xerox PARC, EuroPARC, and tile

Olivetti Research Center have built

several different kinds of locauon

servers. Generally these have two

parts: a central database of informa-

tion about location that can be

quickly queried and dumped, and a

group of servers that collect infor ma-

tion about location and update the

database. Inf orma tion about location

can be deduced from logins, or col-

lected directly from an active badge

system. The location database may beorganized to dynamically notify cli-

ents, or simply to facilitate frequent

polling.

Some example uses of location in-

formation are automatic phone for-

warding, locating an individual for a

meeting, and watching general activ-

ity in a bu ildin g to feel in touc h with

8 0 , ] u b 1993/Vot.36.N od C O , I A ~ U m ~ . A ~ O N ' m O ~ T ~

Mobile Computing and Communications Review, Volume3, Number 3 17



i t s c y c l e s o f a c t i v i t y ( i m p o r t a m [ () ~

t e l e c o m r n u r in g ).

X e r o x P A R C h a s i n v e s t i g a t e d a

n u m | ) e r o f s h a r e d I I: le et it ~g t o o ls o v e r

t h e p a s t d e c a d e , s t a r t i n g w i t h t h e

( ; o l , a b w o r k [ 2 1 ] , a n d c o n t i n u i n g

w M / v i d e o d r a w a n d c o m m u n e [ 23 1.

T w o n e w t o ol s w e r e d e v e l o p e d for

i n v e s t i g a t i n g p r o b l e m s i n u b i q u i ~ o n s

c o m p u t i n g . T h e f i r s t is ' F i v o l i ( 18 1,

t h e s e c o r t d i s S l a t e , - - e a c h t o o l is

b a s e d o n d i f f e r e n t i m p l e m e t ~ t a t i o n

p a r a d i g m s . F i r s t t h e h ' s im i l a r i t i e s :

b o t h e n ' l p ha s i z e p e n - b a s e d d r a w i n g

o n a s u r f a c e ; b o t h a c c e p t s c a n n e d

i n p u t a n d c a n p r i n t t h e r e s u l t s ; b o t h

c a l l I t ~ l v e s e v e r a l t l s c r s s i ~ l l u | t a l l e -

o u s ly o p e r a t i n g i n d e p e n d e n d y o n

d i f f e r e n t o r t h e s a m e p a g e s ; b o t h

s u p p o r t m u h i p l e p a g e s . T i v o l i h a s a

s o p h i s t i c a t e d x m t i (m o f a s t r o k e a s

s p l in e , a n d h a s a m m l b e r o f f e a t u r e s

m a k i n g u s( : o f p r o c e s s i n g t h e c o n -

t e n t s a r i d r e l a t i o n s h i p s a m o n g

T a b l e 1 . S o m e h a r d w a r e p r o t o t y p e s in us e a t X e r o x P A RC

l.,HIIle IISIOnS: IU~,~- ,~ I*~- ~ ZdlI'CITI

Weight : 7 .2 ozScreen: 6 .2- x 4 .2cm, 128- x 64

m o n o c h r o m e]b u ch i n p u t : p a ss i v e p re ssu re sen s i n gS o u n d : P i ezo sp eak e rWireless in terfaces: IR at 850ran,

DEC/O l i v e tt i a c t iv e b ad g ecompa6ble , 19 .2k baud PWMb aseb an d m o d u l a t i o n , CS MA

Processor: In te l 8051-type, 8k (v l )t28k (v2) nvram

P o r t s : I~ C ex t e rn a l b u s , rech a rg e p o r tBat tery: 12 horn 's cont inuous or est . 2

w eek s n o rm a l u se , rech a rg eab l e

i lu r len sxon s: ZZ.Z- X Z~- X .%/SCgU

Weight : 511bs 4ozScreen: 640- × 480 LCD Display (3

levels of grey)P en : t e t h e red e l ec t ro m ag n e t i c sen s i n gS o u n d : B u i l t - i n m i c ro p h o n e , S p eak e r .

P i ezo B eep e rWi re l e ss In t e r | b ees : 2 5 0 Kb p s Rad i o ,

19.2Khps IRProcesm r: Motorola 68302, 4MB of

DRA M , 1 /2 MB o f V RA M , I /4 MBo f E P R

Ex t e rn a l P o r t s: S t y l u s /m i c ro p h o n e ,PCM CIA , IM B Seria l , RS232, I~C bus,Keylxaard

In t e rn a l P o r t s : S eco n d au d i o ch an n e l ,IS DN , Ex p an s i o n P o r t

B a t t e ry : rech a rg eab l e , 3 h o u rs

Ul[ l l e I lS lOt tS : t ~ 3 1 1 1 , 3XlI | an t i .~L l l f l

W e i g h t : 5 6 01 b ( 2 5 0 k g )S e r e e n : v e r y b r ig h t 4 5 - x 6 5 i n ,

1 0 2 4 - x 7 6 8 m o n o ch ro m epixels, 640- x 480 pixets color,a l so N TS C v i d eo

Pen: IR wirelessS t e reo so u n dN e t w o rk i n g , p ro cesso r , an d p o r t s

d e t e r m i n e d b y c h oi c e o f e m b e d d e dw o rk s t a t i o n , e i t h e r P C o r S u n

12 amps at 115 vol ts

a c t i v i t y f l ' o r nI o c a t o r s

: i

18

¢ O ' M l W U N I C A T I O N I O ~ T H I I ~ J ~ d ) 199~i\~A3b No? 8~

Mobile Computing and Communications Review, Volume 3, Number 3



s l r o k e s . T i v o l i a l s o u s e s g e s t u r e s a s

i n p u t c o n t r o l t o s e l e c t , m o v e , a n d

c h a n g e t h e p r o p e r t i e s o f o b j e c t s o n

t h e s c r e e n . W h e n s e v e r a l p e o p l e u s e

T i v o l i , e a c h m u s t tx : r u n n i n g a s e pa -

r a t e c o p y , a n d c o n n e c t t o t h e o t h e r s .

O n t h e o t h e r h a n d , S l a t e i s c o m -

p l e t e l y p i x e l - b a s e d , s i m p l y d r a w i n g

i n k o n t h e s c r e e n . S l a t e m a n a g e s a l lt h e s h a r e d w i n d o w s f b r a l l p a r t i c i -

p a n t s , a s lo n g a s th e y a r e r u n n i n g a n

X - W i n d o w s e r v e r , s o i t s a g g r e g a t e

r e s o u r c e u s e c a n b e m u c h l o w e r t h a n

T i w ) l i , a n d i t i s e a s i e r t o s e t u p w i t h

l a r g e n u m b e r s o f p a r t i c i p a n ts . I n

p r a c t i c e w e h a v e u s e d s l a t e f r o m a

S u n t o s u p p o r t s h a r e d d r a w i n g w i t h

u s e r s o n M a c s a n d P C s . B o t h S l a te

a n d T i v o l i h a v e r e c e iv e d r e g u l a r u s e

a t P A R C .S h a r e d d r a w i n g t o o l s a r e a t o p i c o f

r e s e a r c h a t m a n y p l a c e s . F o r i n -

s t a n c e , B e l l c o r e h a s a t o o l k i t f b r

b u i l d i n g s h a r e d t o o ls [ 9 ], a n d J a c o b -s e n a t L B L u s e s m u l t i c a s t p a c k e t s t o

r e d u c e b a n d w i d t h d u r i n g s h a r e d

t o ol u se . T h e r e a r e s o m e c o m m e r c i a l

p r o d u c t s [ 3 ] , b u t t h e s e a r e u s u a l l y

n o t m u h i p a g e a n d s o n o t r e a l l y s u i t -

a b l e t o r c r e a t i n g d o c u m e n t s o r i n t e r -

a c t i n g o v e r t h e c o u r s e o f a n e n t i r e

m e e t i n g . T h e o p t i m a l s h a r e d d r a w -

i n g t o o l h a s n o t b e e n b u i l t . F o r i t s

u s e r i n t e r f a c e , t h e r e r e m a i n i s s u e s

s u c h a s m u l t i p l e c u r s o r s o r ( )t ie , g e s -

t u r e s o r n o t , a n d u s i n g a n i n k o r a

c h a r a c t e r r e c o g n i t i o n m o d e l o f p e n

i n p u t . F o r i t s s u b s t r a t e , i s i t b e t t e r t o

h a v e a s i n g l e a p p l i c a t i o n w i t h m u l t i -

p l e w i n d o w s , o r m a n y a p p l i c a t i o n s

i n d e p e n d e n t l y c o n n e c t e d ? I s p a ck e t-

m u l t i c a s t a g o o d s u b s t r a t e t o u s e ?

W h a t w o u l d i t t a k e to s u p p o r t s h a r e d

d r a w i n g a m o n g 5 0 p e o p l e ; 5 , 0 0 0

p e o p l e ? T h e a n s w e r s a r e l i ke l y t o b e

I x> th t e c h n o l o g i c a l a n d s o c i al .

T h r e e n e w k i n d s o f a p p l ic a t io n s o f

u b i q u i t o u s c o m p u t i n g a r e b e g i n n i n g

t o b e e x p l o r e d a t X e r o x P A R C . O n e

i s t o t a k e a d v a n t a g e o f t r u e i n v i s i b i l -

i t y , l i t e r a l l y h i d i n g m a c h i n e s i n t h e

w a l ls . A n e x a m p l e i s t h e R e s p o n s i v e

E n v i r o n m e n t p r o je c t l e d b y S c o ttE h ' o d . " [ 'h i s a i m s t o n t a k e a b u i l d i n g ' s

h e a t , l i g h t , a n d p o w e r m o r e r e s p o n -

s i ve t o i n d i v i d u a l l y c u s t o m i z e d

n e e d s , s av i n g en e r g y a n d m a k i n g a

m o r e c o m t b r t a b l e e n v i r o m n e n t .

A s e c o n d n e w a p p r o a c h i s t o u s e

s o - c a l l e d " v i r t u a l c o m m u n i t i e s " v i a

t h e t e c h n o l o gT o f M U D s . A M U D , o r

" M u h i - U s e r D u n g e o n , " is a p r o g r a m

t h a t a c c e p t s n e t w o r k c o n n e c t i o n s

f r o m m u h i p l e s i m u h a n e o u s u s e r s

a n d p r o v i d e s a c c e s s t o a s h a r e d d a t a -

b a s e o f ' r o o m s ' , ' e x i t s ' , a n d o t h e r o b -

j e c t s . M U D s h a v e e x i s t e d f o r a b o u t

1 0 y e a r s , b e i n g u s e d a h n o s t e x c l u -

s i v e ly t b r r e c r e a t i o n a l p u r p o s e s .

H o w e v e r , t h e s i m p l e t e c h n o l o g y o fM U I ) s s h o u l d a l s o b e u s e f u l i n o t h e r ,

n o n r e c r e a t i o n a l a p p li c a t i o n s, p r o v i d -

i n g a c a s u al e n v i r o m n e n t i n t e g r a t i n g

v i r t u a l a n d r e a l w o r l d s [ 5 ] .

A t h i r d n e w a p p r o a c h i s t h e u s e o f

c o l l a b o r a t i o n t o s p e c i | } ' i n t o r m a t i o n

f i h e r i n g . D e s c r ib e d i n t h e D e c e m b e r

1992 i s sue o f Communications of the

A C M , t h i s w o r k b y D o u g " I ~e r ry e x -

t e n d s p r e v i o u s n o t i o n s o f i n t o r m a -

t i o n f i l t e r s b y p e r m i t t i n g f i l t e r s t o

r e f e r e n c e o t h e r f i l t e rs , o r t o d e p e n d

o n t h e v a l u e s o f m u h i p l e m e s s ag e s .

F o r i n s t a n c e , o n e c a n s e l e c t a l l m e s -

s a g e s t h a t h a v e b e e n r e p l i e d t o b yS m i t h ( t h e s e m e s s a g e s d o n o t e v e n

m e n t i o n S m i t h , o f c o ur s e ), o r a ll

m e s s a g e s t h a t t h r e e o t h e r p e o p l e

f o u n d i n t e r e s ti n g . I m p l e m e n t i n g t h is

r e q u i r e d i n v e n t i n g t h e i d ea o f a

" c o n t i n u o u s q u e r y , " w h i c h c a n e f fe c -

t i ve l y s a m p l e a c h a n g i n g d a t a b a s e a t

a l l p o i n t s i n t i m e . C a l l e d " T a p e s t r y , "

t h i s s y s t e m p r o v i d e s n e w w a y s f i n

p e o p l e t o i n v i s i b l y c o l l a b o r a t e .

Privacy of LocationC e l l u l a r s y s t e m s i n h e r e n t l y n e e d to

k n o w t h e l o c a t i o n o f d e v i c e s a n d

t h e i r u s e i n o r d e r t o p r o p e r l y r o u t ei n t b r m a t i o n . F o r i n s t a n c e , t h e t r av e l -

i n g p a t t e r n o f a f r e q u e n t c e l l u l a r

p h o n e u s er c a n b e d e d u c e d f r o m t h e

r o a m i n g d a t a o f c e l l u l a r s er v ic e p r o -

v i d e r s . T h i s p r o b l e m c o u l d b e m u c h

w o r s e i n u b i q u i t o u s c o m p u t i n g w i th

i t s m o r e e x t e n s i v e u s e o f c e l l u l a r

w i r e l e ss . So a k e y p r o b l e m w i t h u b i q -

u i t o u s c o m p u t i n g i s p r e s e r v i n g p r i -

v a c y o f l o c a t i o n . O n e s o l u t i o n , a c e n -

t r a l d a t a b a s e o f l o c a t i o n i n f o r m a t i o n ,

m e a n s t h e p r i v a c y c o n t r o l s c a n b e

c e n t r a l i z e d a n d p e r h a p s d o n e w e l l - -

o n t h e o t h e r h a n d o n e b r e a k - i n th e r e

r e v e a l s a l l , a n d c e n t r a l i t y i s u n l i k e l y

t o s c a l e w o r l d w i d e . A s e c o n d s o u r c e

o f i n s e c u r i t y i s t h e t r a n s m i s s i o n o f

t h e l o c a t i o n i n f o r m a t i o n t o a c e n t r a l

s i t e. T h i s s i t e is t h e o b v i o u s p l a c e t o

t r y t o s n o o p p a c k e t s , o r e v e n t o u s e

t r a f f i c a n a l y s i s o n s o u r c e a d d r e s s e s .O u r i n i t i a l d e s i g n s w e r e a l l c e n t r a l ,

i n i t ia l l y w i t h u n r e s t r i c t e d a c c e ss ,

g r a d u a l l y m o v i n g t o w a r d i n d i v i d u a l

u s e r s " c o n t r o l l i n g w h o c a n a c c e s s i n -

f o r m a t i o n a l : ~ ) u t t h e m . O u r p r e -

t i ~ rr e d d e s i g n a v o i d s a c e n t r a l r e p o s i -

t o r y , i n s t e a d s t o r i n g i n l o r m a t i o n

a t × ) u t e a c h p e r s o n a t t h a t p e r s o n ' s

P C o r w o r k s ta t i o n . P r o g r a m s t h a t

n e e d t o k n o w a p e r s o n ' s l o c a t i o nm u s t q u e r y t h e P C , a n d p r o c e e d

t h r o u g h w h a t e v e r s e c u r i t y m e a s u r e s

t h e u s e r h a s c h o s e n t o i n s t a l l .

E u r o P A R C u s e s a s y s t e m o f t h i s s o rt .

A c c u m u l a t i n g i n t o r m a t i o n a b o u t

i n d i v i d u a l s o v e r l o n g p e r i o d s i s o n e

o f t h e m o r e u s e f u l t h i n g s to d o , b u t

q u i c k l y r a is e s h a c k l e s . A k e y p r o b l e m

f o r l o c a t i o n i s h o w t o p r o v i d e o c c a -

s i o n a l l o c a t i o n i n l b r m a t i o n t o r c l i -

e n t s w h o n e e d i t , w h i l e s o m e h o w

p r e v e n t i n g t h e r e l i a b l e a c c u m u l a t i o t ~

o f l o n g - t e r m t r e n d s a b o u t a n i n d i v i d -

u a l . S o f a r a t P A R C w e h a v e e x p e r i -

m e n t e d o n l y w i th s h o r t - t e r m a c c u -n t u l a ti o n o f i n f o r m a t i o n t o p r o d u c e

a u t o m a t i c d a i l y d i a r i e s o f a c t i v i t y

[17].

I t is i m p o r t a n t t o r e a l i z e t h e r e c a n

n e v e r b e a p u r e l y t e c h n o l og i c a l s o lu -

t i o n t o p r i v a c y , a n d s o c ia l i ss u e s m u s t

b e c o n s i d e r e d i n t h e ir o w n r i g h t . I n

t h e c o m p u t e r s c i e nc e l a b w e a r e t r y -

i n g t o c o n s t r u c t s y s t e m s t h a t a r e p r i -

v a c y - e n a b l e d , g i v i n g p o w e r t o t h e

i n d i v i d u a l . B u t o n l y s o c i e ty c a n c a u s e

t h e r i g h t s y s t e m t o h e u s e d . T o h e l p

p r e v e n t f u t u r e o p p r e s s i v e e m p l o y e r s

o r g o v e r n m e n t s t ¥ o m t a k i n g t h i s

p o w e r a w a y , w e a r e a ls o e n c o u r a g i n gt h e w i d e d i s s e m i n a t i o n o f i n f o r m a -

t i o n a b o u t l o c a t i o n s y s t e n t s a n d t h e i r

p o t e n t i a l fo r h a r m , W e h a v e c o o p e r -

a t e d w i t h a n m n b e r o f a r t ic l es i n t h e

San Jose M ercury News, t h e Worthington

Post, a n d t h e New York T imes o n t h i s

t o p i c . T h e r e s u l t , w e h o p e , i s t e c h n o -

l o gi c al e n a b l e m e n t c o m b i n e d w i t h a n

i n f o r m e d p o p u l a c e t h a t c a n n o t b e

t r i c k e d i n t h e n a m e o f t e c h n o l o g y .

Computat ional MethodsA n e x a m p l e o f a n e w p r o b l e m i n th e -

o r e t i c a l c o m p u t e r s c i e n c e e m e r g i n g

f r o m u b i q u i t o u s c o m p u t i n g i s o p t i -

m a l c a c h e s h a r i n g . T h i s p r o b l e m

o r i g i n a l l y a r o s e i n d i s c u s s i o n s o f o p -

t i m a l d i s k c a c h e d e s i g n t b r p o r t a b l e

c o m p u t e r a r c h i t e c t u r e s . B a n d w i d t h

t o t h e p o r t a b l e m a c h i n e m a y b e q u i t e

l o w , w h i l e i t s p r o c e s s i n g p o w e r i s re l -

a t i v e l y h i g h . T h i s i n t r o d u c e s a s a

Mobile Compu ting and Communications Review, Volume 3, Num ber 319



2 0

p o s s i b l e d e s i g n p o i n t t h e c o m p r e s -

s i o n o f p a g e s i n a R A M : c a c h e , r a t h e r

t h a n w r i t i n g t h e m a l l t h e w a y b ac k

o v e r a s l o w l in k . T h e q u e s t i o n a r i se s

o f ~ h e o p t i m a l s t r a t e g y t 0 r p a r t i ti o n -

i n g m e m o r y b e tw e e n c o m p r e s s ed

a n d u n c o m p r e s s e d p a g e s .

T h i s p r o b l e m c a n b e g e n e r a li z e d

a s t o l l o w s [ 1 ] :

" I 'h e C a c h e S h a r i n g P r o b l e m . A

p r o b l e m i n s t a n c e i s g i v e n b y a s e -

q u e n c e o f p a g e r e q u e s ts . P a g e s a r e o f

t w o t y p e s , U a t td C ( f b r u n c o n t -

p r e s s e d a n d c o m p r e s s e d ) , a n d e a c h

p a g e i s e i t h e r I N o r O U T . A r e q u e s t

i s s e r v e d b y c h a n g i n g t h e r e q u e s t e d

p a g e t o I N i f i t i s c u r r e n t l y O U T . h f i -

t ia l ly al l p a g e s a r e O U T . T h e c o s t t o

c h a n g e a t y p e - U ( t y p e - C ) p a g e f r o m

O U T t o I N i s C u ( r e s p e c t i v e l y , C c ) .

W h e n a r e q u e s t e d p a g e i s O U T , w e

s a y t h a t t h e a l g o r i t h n t m i s s e d . R e -

m o v i n g a p a g e f r o m m e m o r y is f r e e .

L o w e r B o u n d T h e o r e m : N o de t er -

m i n i s fi c , o n - l i n e a l g o r i t h m f b r c a c h e

s h a r i n g c a n b e c - c o m p e t i t i v e f o r

c < M A X (1 + C t : / ( C t ~ + C c ) , 1 +

C c i ( C t ; + C c ) )

T h i s l o w e r b o u n d t o r c r a n g e s

f l - o m 1 . 5 t o 2 , a n d n o o n - l i n e a l g o -

r i t h m c a n a p p r o a c h c l o s e r t o th e o p -

t i m u m t h a n t h i s f a c t o r . B e r n e t a l . [ 11

a l s o c o n s t r u c t e d a n a l g o r i t h m t h a t

a c h i e v e s t hi s f a c to r , t h e r e t o r e p r o -

v i d i n g a n u p p e r b o u n d a s w el l. T h e y

f h r t h e r p r o p o s e a s et o f m o r e g e n -e r a l s y m b o li c p r o g r a m m i n g t o ol s f b r

s o l v i n g c o m p e t i t i v e a l g o r i t h m s o f th i s

s o r t .

Concluding R emarksA s w e s t a r t t o p u t t a b s , p a d s , a n d

b o a r d s i n t o u s e , t h e f i r s t p h a s e o f

u b i q u i t o u s c o m p u t i n g s h o u l d e n t e r

i ts m o s t p r o d u c t i v e p e r i o d . W i t h t h i s

s u b s t r a t e i n p l a c e w e c a n m a k e m u c h

m o r e p r o g r e s s b o t h i n e v a l u a t in g o u t

t e c h n o l o g i e s a n d i n c ht x > si n g o u r

n e x t s t e p s . A k e y p a r t o f t h i s e v a l u a -

t i o n i s u s i n g t h e a n a l y s e s o f p s y c h o l o -

g i st s , a n t h r o p o l o g i s t s , a p p l i c a t i o nw r i t e r s , a r t i s t s , m a r k e t e r s , a n d c u s -

t o m e r s . W e b e l i ev e t h e y w i l l f i n d

s o m e l i g a t u r e s w o r k w e l l ; w e k n o w

t h e y w i ll f i n d s o m e f e a t u r e s d o n o t

w o r k . T h u s w e w il l b e g i n a g a i n t h e

c y c l e o f c r o s s - d i s c i p l i n a r y f e r t i l iz a -

t i o n a n d l e a r n i n g . U b i c o m p i s l i ke l y

t o p r o v i d e a f r a n t e w o r k f o r i n te r e s t -

i n g a nd p r o d u c t i v e w o r k t b r m a n y

m o r e y e a r s o r d e c a d e s , b u t w e h a v e

m u c h t o l e a r n a b o u t t h e d e t a i ls .

A c k n o w l e d g m e n t s

T h i s w o r k w a s f u n d e d b y X e r o x

P A R C . P o r t i o n s o f " t h i s w o r k w e r e

s p o n s o re d u n d e r c o n t r ac t # D A B q

6 3 - 9 1 - 0 0 2 7 . U b i q u i t o u s c o m p u t i n g iso n l y a sm a l l p a r t o f t h e w o r k g o i n g

o n a t P A R C , a n d w e a r e g r a t e f t f l t b )

P A R C ' s r i c h , c o o p e r a t i v e , a n d f e r t i l e

e n v i r o m n e n t i n s u p p o r t o f th e d o c u -

m e n t c o m p a n y . []

R e f e r e n c e s

l . B e rn , M. , Green e , 1 ) . , Rag h u n a t b an .On - l i n e a l g o r i t h m s t o t cach e sh a r i n g .25th AC M Sym posium on Theory of Com-

puting (San Diego, Calif' ., 1993).2 . B u x t o n , W. S m o k e an d m i r ro rs . Byte

15 , 7 ( Ju ly 1 9 9 0 ) 2 0 5 -2 1 0 .3 . Ch a t t e r j ee , S . S u n en t e rs co m p u t e r

co n fe ren c i n g m ark e t . Sunworld 5, 10(Oct . 1992), In teg rated Media , SanFrancisco, 32-34.

4 . Clark , D.D., Shenker, S . , Zhang, I . .Support ing real -t ime appl icat ions inan in tegrated services packet net -w o rk : A rch i t ec t u re an d m ech an i sm .SIG CO M M "92 Con[krence Proceedings.

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9. Hi l l , R.D., Brinck, T. , Pat terson, J ,F . ,Rohal l , S .L. and Wilner, W.T. TheR E N D E Z V O U S l an g u a g e a n d a r ch i -t ec t u re : To o l s fo r co n s t ru c t i n g m u l t i -

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' 93 (Jan . 1993). "[b be publ ished.

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2 3 . l i m g , J .C . a n d M m n e m a n , & l . .

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2 4 . l e~ ao k a , I : . , l ' o k o t e , Y ., T o k o ro , M

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2 6 . W an t , R . , H o p p e r , A . , F a l cao , V. an d

G i b ~ m s , J , T h e a c t i v e b a d g e l o c a t io n

s y s t e m . ACM 7)'ans lnJl Syst. 10, 1

( J an . t 9 9 2 1 , 9 1 -1 0 2 .

2 7 , W e i s e r , M . T h e c o m p u t e s f o r t h e

t w e n t y - f i rs t c e n t u r y . Sci. Am. (S ep t .

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2 8 . W e i s e r , M . , D c m e r s , A . a n d H a u s e ~ ,

( ; . T h e p o r t a b l e c o m l n O | l r u n t i m c

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C R C a t e g o r i e s a n d S u b j e c t D e s c r i W

t o r s: C . 3 [ C o m p u t e r S y s t e m s O r g a n i z a -

t i o n] : S p e c i a l - p u r p o s e a n d a p p l i c a t i o n -

b as ed s y s t em s ; 1 " t . l . 2 [ In fo rm a t i o n S y s -

t e m s l : M o d e l s a n d P r i n c i p l e s - -

User~Machine . ~y st e,~' ; H A . I [ In fo rm a t i o n

S y s t e m s ] : I n f i ) r m a t i o n S y s t e m s A p p l i c a -

t i o n s - - O f f i c e Autmnatirm: J . 0 [ C o m p u t e r

A p p l i c a t i o n s ] : G e n e r a l

G e n e r a l T e r m s : D e s i g n, t l u m a n F a c-

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i n t e r n a l s , u b i q u i t o u s c o m p u t i n g a n d

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c o n t r o l [ 1 l, B y i n t e r c o n n e c t i n g P A R C 's

r ic h c o m p u t a t i o n a l i n f r a s t ru c t u r e w i t h a

c o m p u t e r i z e d b u i l d i n g m a n a g e m e n t s y s-

t e m ( BM S ) t h a t c o n t r o l s h e a t i n g , a ir c o n -

d i t i o n i n g , l i g h t i n g a n d d e s l d : o p a p p l l -

a n c e s , w e p l a n to e x p l o r e n e w s t r a te g i e s

f o r e n e r g y c o n s e r v a t io n a n d o f f i ce c o m -

f o r t c o n t r o l .

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t r o l a n d s i m p l e a n a l o g f e e d b a c k l o o p s

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a n d c o m p l e x c o n t r o l s t r a t e g i e s t h a t I n

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i n gs . F o r e x a m p l e , m o s t c o m p u t e r -

c o n t r o l l e d b u i ld i n g s o f f e r a u to m a t i c

t e m p e r a t u re s e t b a c ks t o r e d u c e e n e r g y

c o n s u m p t i o n a f t e r w o r k i n g h o u r s an d o n

w e e k e n d s . M a n y s y s t e m s a ls o c o n t r o l

l i g h t in g t o s a v e a d d i t i o n a l e n e r g y .

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u t e d c o m p u t in g , c o m m u n i c a ti o n s , a n d

s e n s i n g t e c h n o l o g i e s w i l l e n a b l e f u t u r e

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p r e f e r e n c e s a n d a c t iv i ti e s . T h e f o l l o w i n g

a r e e x a m p l e s o f t h e t y p e s o f e n h a n c e -m e n t s t h a t a r e p o s s i b l e :

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i n t e r f a c e t o a l l o w o f f ic e o c c u p a n t s a n d

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i s u c h a s pr e f e r r e d r a n g e s f o r t e m p e r a -

t u r e a n d l i g h t l e v e l) a n d r e c e i v e f e e d -

b a c k a b OU t c u r r e n t c o n d i t i o n s .

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P AR C t o a v o i d h e a t i n g o r c o o l i n g c o n f e r -

e n c e r o o m s w h e n t h e y a r e n o t i n us e .

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s e l e c t a b le s t r a t e g i e s f o r s w i t c h in g o f f

l i g h t s , c o m p u t e r d i s p l a y s , a n d o t h e r a p -

p l ia n c e s a n d f o r s e t t i n g b a c k t h e a i r c o n -

d i t i o n i n g w h e n o f f i c e s a r e u n o c c u p i e d .• o t h e r s e n s o r s c a n b e i n te g r a t e d i n t o

t h e s y s t e m t o p r o v i d e a d d i t i o n a l a u t o -

m a t i c f u n c t i o n a l i t y , s u c h a s a d j u s t i n g a r-

t i fi c i a l l i g h t s a n d / o r c l o s i n g b l in d s t o

c o m p e n s a t e f o r c h a n g i n g n a t u r a l l ig h t

l e ve l s, l o w e r i n g l i g h t l e v e l s w h i l e a w o r k -

s t a t i o n i s I n u s e , a n d a d j u s t i n g v e n t i l a -

t i o n d e p e n d i n g o n w h e t h e r d o o r s a re

o p e n e d o r c l o s e d .

• A c t iv e b a d g e s a n d o n - l i n e c a l e n d a r s

m a y b e u s e d t o i d e n t i f y pe r i o d s i n w h i c h

a n o f f ic e ' s o c c u p a n t w i l l b e a w a y f o r a ne x t e n d e d p e r i o d o f t im e ( e .g ., b e c a u s e

t h e o c c u p a n t IS a t a m e e t i n g , a t lu n c h ,

o r a w a y f r o m t h e b u il d in g ) .

TO q u a n t i f y t h e p o t e n t i a l f o r e n e r g y

s a v i n g s b y s u c h s t r a t e g i e s , w e c o n -

d u c t e d a d e t a il e d o n e - d a y e n e r g y a n d

o c c u p a n c y a u d i t o f P A R C 's C o m p u t e r S c i -

e n c e L a b o r a t o ry . T his s t u d y r e v e a l e d t h a t

o f f i c e s a r e t y p i c a l l y v a c a n t f o r 5 0% o f t h e

a v e r a g e 9 - h o u r w o r k d a y , I n i t i a l c a lc u l a -

t i o n s s u g g e s t t h a t o c c u p a n c y - ba s e d c o n -

t r o l o f l i gh t s a n d c o m p u t e r m o n i t o r s

a l o n e w o u l d s a v e $ 4 5 ,0 00 p e r y e a r a t

P AR C, a n d s i m i l a r c o n t r o l o f t h e a i r c o n -

d i t i o n i n g s y s t e m c o u l d s a v e as m u c h a s

$ 9 0 , 0 0 0 .E n c o u r a g e d b y t h e s e e s t im a t e s , w e

h a v e C O n S tr uc te d a f le x i b l e h a r d w a r e a n d

s o f t w a re t e s t b e d t o e x p e r i m e n t w i t h

n e w e n e r g y c o n t r o l s t r a t e g ie s . T h e

t e s t b e d c o n s i s ts o f 1 3 o f f i c e s a n d s o m e

a d j a c e n t p u b li c a r e a s . W i t h i n e a c h o f f i c e

w e h a v e i n s t a l le d t e m p e r a t u r e , l i g h t

l e v e l, o c c u p a n c y , a n d a c t i v e b a d g e s e n -

s o rs to g e t h e r w i t h c o m p u t e r - c o n t r o ll e d

v e n t i l a t i o n , h e a t i n g , e l e c t r i c o u t l e t s , a n d

o v e r h e a d l i g h t in g . T h e s e d e v i c e s a r e

c o n n e c t e d t o a c o n v e n t io n a l c o m p u t e r -

i z e d b u i l d i n g m a n a g e m e n t s y s t e m .

w h i c h i n t u r n i s c o n n e c t e d t o t h e P AR C

c o m p u t e r n e t w o r k t h r o u g h a g at e w a y,

u s i n g a c o n v e n t i o n a l b u il d i n g m a n a g e -m e n t s y s t e m a s t h e b a c k b o n e a ll o w s u s

t o e n s u r e t h a t t h e s y s t e m ' s ba s ic f u n c -

t i o n a l i t y i s h i g h l y r e l i a b le , w h i l e s t i l l a b

l o w i n g n e w c o n t r o l s t r a te g i e s a n d d a t a

a na ly s is t o b e r u n f r o m w o r k s t a t i o n s .

AS O f t h i s w r i t i n g ( M a r c h) t h e c o m p l e t e

s y s t e m i s J us t c o m i n g o n d l n e a n d w e a r e

b e g i n n i n g t o c o l l e c t t h e b a s e l in e d a t a

n e e d e d t o e v a l u at e f u t ur e e n e r g y s a v-

i n g s . A t p r e s e n t , t h e s y s t e m c o n t i n u -

o u s l y l o g s o v e r 8 0 0 v a ri a bl e s fr o m t h e

t e s t a r e a a n d f r o m o t h e r p a rt s o f t h e

b u il d i n g . O v e r t h e n e x t f e w m o n t h s w e

p l an t o e x p e r i m e n t w i t h a v a r ie t y o f c o n *

t r o l s t r a t e g i e s a n d u s e r i n t e r f a c e s ( UI s )

a n d m e a s u r e t l l e l r e f f e c t s .

In t h e d e v e l o p m e n t o f th i s s y s te m w e

e n c o u n t e r e d a n u m b e r o f d e s i g n is s ue s

c o m m o n t o m a n y u b i qu i to u s c o m p u t i n g

s y s t e m s :

• r e l i a b i l i t y a n d i n v i s i b i l i ty : C o m p u t e r s y s -

t e m s e n t r u s t e d w i t h s u c h b a s ic a s p e c t s

o f c o m f o r t a s t e m p e r a t u r e a n d l i g h t l e v-

e l s m u s t o b v i o u s l y b e h i g h l y r e l ia b le .

J~d? 1993d~kfl~(i. No.)~ l~(l~¢J(lult),ll(~i~lr)l)ll,Is F YllQllA~M

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