Multi Cluster

8/2/2019 Multi Cluster

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Multicluster, mobile, multimedia radio network*

M a r i o G e r l a a n d J a c k T z u - C h i e h T s a i

Computer Science Department, University o f California, Los Angeles, Los Angeles, CA 90024, USA

Abstract. A multi-cluster, multi-hop packet radio network architecture for wireless adaptive mobile

information systems is

presented. The proposed network supports multimedia traffic and relies on both time division and code

division access schemes.

This radio network is not supported by a wired infrastructure as conventional cellular systems are. Thus,

it can be instantly

deployed in areas with no infrastructure at all. By using a distributed clustering algorithm, nodes are

organized into clusters.

The clusterheads act as local coordinators to resolve channel scheduling, perform power

measurement/control, maintain time

division frame synchronization, and enhance the spatial reuse of time slots and codes. Moreover, to

guarantee bandwidth for

real time traffic, the architecture supports virtual circuits and allocates bandwidth to circuits at call

setup time. The network is

scalable to large numbers of nodes, and can handle mobility. Simulation experiments evaluate the

performance of the proposed

scheme in static and mobile environments.

1. I n t r o d u c t i o n

C u r r e n t wi r e l e s s s y s t e m s a r e c o n s t r a i n e d b y f i x e d

b a n d w i d t h a l l o c a t i o n ( o n a p e r c o n n e c t i o n b a s i s ) , f i x e d

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

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

t i o n s o r s e r v e r s t h a t a r e l i n k e d b y a w i r e l i n e n e t w o r k . I n

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

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

a v a i l a b l e , t h i s t y p e o f a r c h i t e c t u r e i s i n f e a s i b l e . I n a d d i -


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

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

n a r r o w b a n d i n n a t u r e . T o o v e r c o m e t h e s e c o n s t r a i n t s

a n d a d v a n c e t h e s t a t e o f t h e a r t i n W i r e l e s s A d a p t i v e

M o b i l e I n f o r m a t i o n S y s t e m s ( W A M I S ) , we p r o p o s e a

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

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

t i o n s . W e u s e S S - C D M A ( S p r e a d S p e c t r u m C o d e D i v i -

s i o n M u l t i p l e A c c e s s ) t o a l l o w f l e x i b l e s p e c t r u m s h a r i n g ,

a n d t o c o m b a t i n t e r f e r e n c e a n d m u l t i p a t h f a d i n g [1,2].

I n c o n s i d e r a t i o n o f S S - C D M A a n d e f f i c i e n t s p a t i a l r e u s e

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

m u l t i - h o p s u p p o r t b e c o m e c r i t i c a l a n d e s s e n t i a l [1].

T h u s , we d e v e l o p d i s t r i b u t e d a l g o r i t h m s f o r a d a p t i v e

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

A k e y r e q u i r e m e n t i n W A M I S is t h e s u p p o r t o f m u l t i -

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

( v o i c e , v i d e o ) a n d b u r s t y t r a f f i c ( d a t a , i m a g e ) . R e a l - t i m e

s e s s i o n s r e q u i r e b a n d w i d t h a n d d e l a y g u a r a n t e e , a n d

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

l i s h e d a t c a l l s e t u p t i m e . B u r s t y t r a f f i c , o n t h e o t h e r

* This work was supported by the U.S. Department of Justice/

Federal Bureau of Investigation, ARPA/CSTO under Contract

J-FBI-93-112 Computer Aided Design of High Performance Wireless

Networked Systems.


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h a n d , is c a r r i e d i n a c o n n e c t i o n l e s s , (i.e. d a t a g r a m )

m o d e . F o r c o n n e c t i o n o r i e n t e d t r a f f i c ( v o i c e , v i d e o ) ,

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

a c c e p t e d i n t h e n e t w o r k . A f t e r c a l l e s t a b l i s h m e n t , t h e

v i r t u a l c i r c u i t ( V C ) c o n n e c t i o n m u s t g u a r a n t e e b a n d -

w i d t h a n d q u a l i t y o f s e r v i c e ( Q o S ) . D a t a g r a m t r a f f i c is

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

t r a f f i c is. T h u s , i t m a y c r e a t e c o n g e s t i o n i n t h e n e t w o r k .

T o a v o i d c o n g e s t i o n , l i n k - b y - l i n k a n d e n d - t o - e n d f l o w

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

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

a c k n o w l e d g m e n t s a r e u s e d . N o t e t h a t t h e p a s s i v e

a c k n o w l e d g m e n t s c h e m e ( i . e . , t h e t r a n s m i s s i o n o n t h e

n e x t h o p a c t s a s a n A C K o n t h e p r e v i o u s h o p ) f i r s t i n t r o -

d u c e d i n t h e A R P A p a c k e t r a d i o n e t w o r k [3] wi l l n o t b e

s u i t a b l e h e r e s i n c e C D M A is u s e d , a n d t h e r e f o r e t h e f o r -

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

n a t i n g n o d e .

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

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

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

I n a m o b i l e , m u l t i - c h a n n e l ( c o d e ) , a n d m u l t i - h o p e n v i -

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

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

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


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r e q u i r e m e n t s . N o d e c l u s t e r i n g , V C s e t u p a n d c h a n n e l

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

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

p a p e r .

T h e r e s t o f t h e p a p e r is o r g a n i z e d a s f o l l ows : i n

s e c t i o n 2, t h e m u l t i - c l u s t e r n e t w o r k a r c h i t e c t u r e is p r e -

s e n t e d . C h a n n e l a c c e s s s c h e m e a n d V C e s t a b l i s h m e n t

a r e p r e s e n t e d i n s e c t i o n 3. P e r f o r m a n c e r e s u l t s a r e

s h o w n i n s e c t i o n 4. S e c t i o n 5 wi l l d i s c u s s t h e e x t e n s i o n s

o f t h e b a s i c a l g o r i t h m . F i n a l l y , c o n c l u s i o n s a r e g i v e n in

s e c t i o n 6.


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9 J . C . B a l t z e r A G , S c i e n c e P u b l i s h e r s 256 M. Gerla, J. T.-C. Tsai / Multicluster, mobile,

multimedia radio network

2. M u l t i - c l u s t e r n e t w o r k a r c h i t e c t u r e a n d c l u s t e r i n g

a l g o r i t h m

S e v e r a l n e t w o r k a l t e r n a t i v e s c a n b e c o n s i d e r e d f o r

W A M I S . I n o u r c a s e , b e c a u s e o f t h e r e q u i r e m e n t s o f

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

s u p p o r t a n d s u i t a b i l i t y t o C D M A , we h a v e s e l e c t e d a

d i s t r i b u t e d c l u s t e r a p p r o a c h [4]. I n f a c t , c l u s t e r i n g p r o -

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

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

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

c u i t s u p p o r t a n d b a n d w i d t h a l l o c a t i o n .

F o l l o w i n g t h e c l u s t e r i n g a p p r o a c h , t h e e n t i r e p o p u l a -

t i o n o f n o d e s is g r o u p e d i n t o c l u s t e r s . A c l u s t e r is a s u b -

s e t o f n o d e s w h i c h c a n ( t w o - w a y ) c o m m u n i c a t e w i t h a

c l u s t e r h e a d a n d ( p o s s i b l y ) w i t h e a c h o t h e r . I n F i g . 1,

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

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

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

c h a n n e l t h r o u g h p u t . W i t h i n a c l u s t e r , we c a n e a s i l y

e n f o r c e t i m e - d i v i s i o n s c h e d u l i n g . A c r o s s c l u s t e r s , we

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

T h e o b j e c t i v e o f t h e c l u s t e r i n g a l g o r i t h m is t o f i n d a

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

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


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

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

a r e m o v i n g a n d t h e t o p o l o g y is s l o w l y c h a n g i n g . O t h e r -

wise, t h e c l u s t e r h e a d s will n o t c o n t r o l t h e i r c l u s t e r s e f f i -

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

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

a r e c o n s i d e r e d . O n e is t h e l o w e s t - I D a l g o r i t h m [5]: t h e

l o w e s t - I D n o d e i n a n e i g h b o r h o o d is e l e c t e d a s t h e c l u s -

t e r h e a d . T h e o t h e r is t h e h i g h e s t - c o n n e c t i v i t y ( d e g r e e )

a l g o r i t h m , w h i c h is a m o d i f i e d v e r s i o n o f [6]. I n t h i s c a s e ,

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

c l u s t e r h e a d . T h e a l g o r i t h m s a r e d e s c r i b e d b e l ow:

/

E a c h n o d e is a s s i g n e d a d i s t i n c t I D . P e r i o d i c a l l y , t h e

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

i n g i t s e l f ) .

Fig. 1. Example of clustering. Fig. 2. Example of cluster formation (lowest-ID).


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9 A n o d e w h i c h o n l y h e a r s n o d e s w i t h I D h i g h e r t h a n

i t s e l f is a " c l u s t e r h e a d " ( C H ) .


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9 T h e l o w e s t - I D n o d e t h a t a n o d e h e a r s is i t s c l u s t e r -

h e a d , u n l e s s t h e l o w e s t - I D s p e c i f i c a l l y give s u p i t s

r o l e a s a c l u s t e r h e a d ( d e f e r r i n g t o a y e t l o w e r I D

n o d e ) .


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9 A n o d e w h i c h c a n h e a r t w o o r m o r e c l u s t e r h e a d s is

a " g a t e w a y " .


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9 O t h e r w i s e , a n o d e is a n o r d i n a r y n o d e .

F i g . 2 s h ows a 10 n o d e e x a m p l e , w h e r e n o d e s 1, 2 a n d 4

a r e c l u s t e r h e a d s ; n o d e s 8, 9 a r e g a t e w a y n o d e s .

II. H I G H E S T - C O N N E C T I V I T Y C LUS T E R

A L G O R I T H M :

E a c h n o d e b r o a d c a s t s t h e list o f n o d e s t h a t i t c a n

h e a r ( i n c l u d i n g i t s e l f ) .


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9 A n o d e is e l e c t e d a s a c l u s t e r h e a d i f i t is t h e m o s t

h i g h l y c o n n e c t e d n o d e o f all i t s " u n c o v e r e d " n e i g h -

b o r n o d e s ( i n c a s e o f a tie, l o w e s t I D p r e v a i l s ) .


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9 A n o d e w h i c h h a s n o t e l e c t e d its c l u s t e r h e a d y e t is

a n " u n c o v e r e d " n o d e , o t h e r w i s e i t is a " c o v e r e d "

n o d e .


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9 A n o d e w h i c h h a s a l r e a d y e l e c t e d a n o t h e r n o d e a s

its c l u s t e r h e a d gives u p i t s r o l e a s a c l u s t e r h e a d .

F i g . 3 s h ows t h e s a m e 10 n o d e e x a m p l e , w h e r e n o d e s

5,7 a n d 8 a r e c l u s t e r h e a d s ; n o d e s 2, 3, 9, 10 a r e g a t e w a y

n o d e s .

III. P ROP E R T I E S OF T H E TWO C LUS T E R

ALGOR I THMS :

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

I. LOWEST- ID C LUS T E R A L G O R I T H M : M. Gerla, J. 22-C. Tsai / Mul t i c lus t e r , mobile, mul t

imedia radio ne twor k 257

5 .~ ~ 2

Fig. 3. Example of cluster formation (highest-connectivity).

(b) I n a c l u s t e r , a n y t w o n o d e s a r e a t m o s t t w o - h o p s

a w a y , s inc e t h e c l u s t e r h e a d is d i r e c t l y l i n k e d t o e v e r y

o t h e r n o d e in t h e c lus t e r .

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

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

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

(a) a l l ows o n l y o n e c l u s t e r h e a d p e r c l u s t e r . T h e c l u s t e r -

i n g a l g o r i t h m m u s t be p e r f o r m e d a s r a p i d l y a s pos s ibl e ,

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

o f i t s m e m b e r s e f f i c i e n t l y .

W e h a v e s i m u l a t e d the s e t w o c l u s t e r i n g f o r m a t i o n

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

w o r k i n g p u r p o s e . W h e n t h e n o d e s a r e m o v i n g , a n d t h e

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


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t h e i r c l u s t e r h e a d s . T w o m e a s u r e s we r e m o n i t o r e d d u r -

i n g t h e m o v e m e n t o f n o d e s , n a m e l y : (a) t h e n u m b e r o f

n o d e s w h i c h c h a n g e t h e i r rol e s a s c l u s t e r h e a d s , a n d ; (b)

t h e n u m b e r o f n o d e s w h i c h swi t c h c lus t e r s . A t t h e begin-

n i n g o f s i m u l a t i o n , we r a n d o m l y ( u n i f o r m l y ) g e n e r a t e

N n o d e s ins ide a 100x 100 s q u a r e a s s h o w n in Fig. 4. W e

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

is w i t h i n t h e t r a n s m i s s i o n r a n g e . I n o u r s i m u l a t i o n , we

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

t y p e s o f m o v e m e n t s : (a) r a n d o m m o v e m e n t s : e a c h n o d e

m o v e s r a n d o m l y b y o n e u n i t in e a c h d i r e c t i o n (or s t a y s

wh e r e i t is) b e t w e e n t i m e t i cks ; (b) p r e d e f i n e d t r a j e c -

tor i e s : e a c h n o d e m o v e s a l o n g s t r a i g h t t r a j e c t o r i e s , a n d

b o u n c e s a g a i n s t t h e wa l l s o f t h e 1 0 0 x 1 0 0 s q u a r e . T h e

spe eds a r e t h e s a m e (1 u n i t / t i m e t i ck) in b o t h expe r i -

m e n t s . I n o u r e x p e r ime n t s , we m o n i t o r c l u s t e r c h a n g e s

p e r t i m e t i ck. T h e r e s u l t s a r e r e p o r t e d in Figs . 5(a) a n d

(b), a n d a r e a l m o s t i d e n t i c a l f o r t h e t w o t y p e s o f m o v e -

m e n t s . W e n o t i c e t h a t t h e l o w e s t - I D c l u s t e r i n g a l g o -

r i t h m y i e l d s t h e f ewe s t c h a n g e s in e i t h e r m e a s u r e (Fig. 5).

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

t h a n t h e h i g h e s t - c o n n e c t i v i t y one . T h i s is b e c a u s e in t h e

boundary

lOu

100


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N = total no. of nodes

.

/

tx_range ~ ) ) ,I

/

/

Fig. 4. Simulation boundary and txxange.

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

e v e n o n e l i n k d u e t o n o d e m o v e m e n t , i t m a y f a i l t o be re-

e l e c t e d a s a c l u s t e r h e a d . W h i l e t h e l o w e s t - I D n o d e c a n

still be a c l u s t e r h e a d . F o r the s e r e a s o n s , we will b a s e o u r

a l g o r i t h m o n t h e l o w e s t - I D c l u s t e r i n g f o r m a t i o n .

W e n o t e t h a t , a s r a d i o c o n n e c t i v i t y c h a n g e s , d i f f e r e n t

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

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

rol e s . P e r f o r m i n g t h e s e rol e s , h owe v e r , d o e s n o t r e q u i r e

a d d i t i o n a l r e s o u r c e s (e.g. b u f f e r s , p r o c e s s i n g p o w e r etc)

since p r o t o c o l s u p p o r t f u n c t i o n s a r e well d i s t r i b u t e d

a m o n g all n o d e s in t h e c l u s t e r , a s d i s c u s s e d i n s e c t i o n 3.

F o r e x amp l e , r o u t i n g w i t h i n a c l u s t e r is f u l l y d i s t r i b u t e d .

T h e r o u t e d o e s n o t n e e d t o go t h r o u g h a c l u s t e r h e a d .

F o r e x amp l e , in Fig. 2, a p a c k e t c a n be d i r e c t l y r o u t e d

f r o m n o d e 3 t o n o d e 6 w i t h o u t g o i n g t h r o u g h 1. A n i n t e r -

c l u s t e r r o u t e m u s t , h o w e v e r , g o t h r o u g h g a t e w a y s ( in

Fig. 2, t h e p a t h f r o m 7 t o 10 is {7,9,8,10}), since c lus t e r s


16/59

us e d i f f e r e n t code s a n d o n l y g a t e w a y s c a n l i s t e n t o b o t h

c o d e s a l t e r n a t i v e l y a s d i s c u s s e d in s e c t i o n 3.

R e q u i r i n g r o u t e s t o go t h r o u g h g a t e w a y s e l imi n a t e s

s o m e r o u t i n g o p t i o n s . I n Fig. 2, f o r e x amp l e , t h e p a t h

f r o m 7 t o 3 is {7,9,3}. T h e s e r e s t r i c t i o n s c a n be r e m o v e d

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

( D G ) . N a m e l y , a d i s t r i b u t e d g a t e w a y is a p a i r o f n o d e s ,

n e i t h e r o f wh i c h is a g a t e w a y , w i t h i n h e a r i n g r a n g e a n d

r e s i d i n g in d i f f e r e n t c lus t e r s . F o r e x amp l e , t h e p a i r (7,3)

is a D G . O n e c a n e a s i l y v e r i f y t h a t , b y a l l owi n g D G s

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

a c h i e v e d . A n i m p o r t a n t b e n e f i t o f D G s is t h a t o f m a i n -

t a i n i n g c o n n e c t i v i t y in p a t h o l o g i c a l s i t u a t i o n s w h e r e t h e

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

C o n s i d e r t h e e x a m p l e i n F i g . 6. L o w e s t I D c l u s t e r i n g a s

we l l a s h i g h e s t - c o n n e c t i v i t y c l u s t e r i n g l e a d t o t w o c lus - 2 5 8 M. Gerla, J. T.-C.

Tsai / Mul t i c lus t e r , mobile, mul t imedia radio n e t w o r k

( a )

E

"6

12

10

8

I i i i ! i I i I

lowest-D: CH changed

highest-conn: CH changed - x - - -


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I owe s t - ID: radio moved to other cluster - = . - -

, ~ / ~ " - " ~ ' ~ . h ~ h e s t - c o n n : radio moved to other cluster --~- ....

, , / . - ,

, / \

/ ' ,

I / ..x--.~ "~.,,

4 1 - I ,.~ . . . . ~ . .

2 I~ 7" . . . . . i . . . . i . . . . . . " " x . . ' i . . ~ ....

0 [ i I , - ~ ,~ r~ ~ ~ - ~ - . ~ - - - - ~ , - . . . i . . . . ~ . _ .

10 2 0 3 0 4 0 5 0 60 7 0 8 0 9 0 100

tx range

(b)

12

.=E 10

~, 8

4

o/

z

0

i l i i i i i i i

lowest-ID: C H c h a n g e d -e--

highest-conn: C H c h a n g e d -x---

lowest-ID: radio m o v e d to other cluster -A...

highest-conn: radio m o v e d to other cluster .-~ .....

i / i f / " ~ " ~ t ,


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, , % ,

/ .x----x...~ "~,.

/ ,,r ---~, , \

l i - ~ = ~, - . ~ , : ~ - - - . A . - . - I . . . . ~ _ . . . . . . . .

10 2 0 3 0 4 0 5 0 6 0 70 80 9 0

t x _ r a n g e

100

F i g . 5. C o m p a r i s o n s o f c l u s t e r i n g ( N = 30) . ( a ) R a n d o m m o v e m e n t s ; ( b ) p

r e d e f i n e d t r a j e c t o r i e s .

t e r s (wi t h n o d e 1 a n d 2 a s C H s , r e s p e c t i v e l y ) w h i c h a r e

d i s c o n n e c t e d , s inc e n e i t h e r 3 n o r 4 q u a l i f y a s g a t e w a y s .

W i t h t h e D G o p t i o n , t h e p a i r (3,4) b e c o m e s a d i s t r i b u t e d

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

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

d u c e s s o m e e x t r a o v e r h e a d , s inc e t w o r a d i o s m u s t

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

c a t e w i t h e a c h o t h e r . T h u s , i t m a y b e a d v i s a b l e t o e n a b l e

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

d i s c o n n e c t e d .

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

t i v e n e s s o f t h e D G t e c h n i q u e . T h e f i r s t s e t o f e x p e r i -

m e n t s wa s b a s e d o n 20 n o d e s , w i t h t r a n s m i s s i o n r a n g e

F i g . 6. E x a m p l e f o r d i s t r i b u t e d g a t e w a y s .

R = 50. F o r t h i s c a s e , we g e n e r a t e d 10,000 s a m p l e n o d e

d i s t r i b u t i o n s in t h e 100 x 100 s q u a r e . O n a v e r a g e , 2 D G

p a i r s w e r e f o u n d i n e a c h s amp l e . C o n n e c t i v i t y ( d e f i n e d


19/59

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

t h e D G s wa s 99.4%. W i t h t h e D G s , c o n n e c t i v i t y

i n c r e a s e d t o 1. I n a s e c o n d s e t o f e x p e r i m e n t s , t h e r a n g e

wa s r e d u c e d t o R = 40. I n t h i s c a s e , c o n n e c t i v i t y

w i t h o u t / w i t h D G s wa s 9 0% a n d 9 8 . 3% r e s p e c t i v e l y .

A g a i n , 2 D G p a i r s w e r e f o u n d o n a v e r a g e i n e a c h s a m -

pl e . T h e s e r e s u l t s s h o w t h a t w h e n c o n n e c t i v i t y is w e a k

(as in t h e R = 40 c a s e ) , t h e D G s t r a t e g y c a n b e e x t r e m e l y

e f f e c t i v e .

3. C h a n n e l a c c e s s s c h e m e

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

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

c l u s t e r h e a d s . N o w t h e p r o b l e m is h o w t o r u n t h e a l g o -

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

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

M o r e s p e c i f i c a l l y , h o w d o we s e l e c t t h e a p p r o p r i a t e

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

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

r e s o u r c e s . M. Gerla, J. T.-(2. Tsai / Multicluster, mobile, multimedia radio network 259

I n view o f the real time t r a f f i c c omp o n e n t (which

requires dedi c a t ed bandwidth) , we have chosen Time-

Divi s ion Mul t ipl e Access ( TDMA) wi thin a cluster.

Code Division Mul t ipl e Access ( CDMA) c an also be

ove r l ayed on top o f the T D M A inf r a s t ruc tur e ; name ly,

mul t ipl e sessions c an share the same T D M slot via


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C D M A . I n this case, the n e a r - f a r probl em a n d r e l a t ed

powe r cont rol a l g o r i t hm become critical to the efficiency

o f the channe l access [7]. In addi t ion, s epa r a t e codes are

assigned to di f f e r ent clusters in orde r to reduce the effect

o f int e r c lus t e r interference. Specifically, we a l low only

one t r ansmi s s ion (per code) wi thin e a ch cluster, a n d use

di f f e r ent sets o f codes in ne ighbor ing clusters.The cluster

solut ion wi th code s epa r a t ion reduces powe r interfer-

ence a n d ma i n t a i n s spatial f r equency reuse. As the net-

work grows large, the clusters m a y o u t n umb e r the

available codes. To overcome this probl em, codes c an be

reused in n o n - a d j a c e n t clusters. As suming a 4-cluster

code reuse, a n d a l loc a t ing two s epa r a t e codes to e a ch

cluster (for ove r l ayed C D M A / T D M A ) , plus one con-

trol code (which is c ommo n to all clusters), the t o t a l

n umb e r o f o r t h o g o n a l codes r equi r ed by this scheme

is9.

Wi t h i n the f r amewo r k o f the basic architecture, we

c an impl ement va r ious protocol s a n d func t ions . The fol-

lowing will describe the imp l eme n t a t i o n o f the clustering

a l g o r i t hm a n d o f the channe l access scheme.

3.1. S y n c h r o n o u s t i m e d i v i s i o n f r a m e

The t r ansmi s s ion time scale is organi z ed in frames,

each cont a ining a fixed n umb e r o f time slots. The entire

n e two r k is synchroni z ed on a f r ame and slot basis. The


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f r a m e / s l o t s y n c h r o n i z a t i o n me c h a n i sm is n o t described

here, b u t c an be impl ement ed wi th techniques similar to

thos e employed in the wired ne tworks , e.g. " f o l l ow the

slowest c l o c k " [8], prope r ly modi f i ed to ope r a t e in a

wireless mobile envi ronment . P r o p a g a t i o n delays will

cause imprecisions in slot synchroni z a t ion. However,

slot g u a r d times (fractions o f millisecond) will amp l y

absorb p r o p a g a t i o n de l ay effects (in the orde r o f micro-

seconds). A f r ame is divided into two phases, name ly,

cont rol pha s e a n d info pha s e as shown in Fig. 7. The fol-

lowing subsections will explain how these two phases

are used to p e r f o rm the desired func t ions .

3.2. C o n t r o l p h a s e

The cont rol pha s e is used to p e r f o rm all the cont rol

func t ions , such as slot a n d f r ame synchroni z a t ion, clus-

tering, rout ing, power me a sur ement , code assignment,

VC set-up, etc. By exchanging the conne c t ivi ty informa -

t ion amo n g the neighbors, each node selects its cluster-

h e a d a n d upda t e s its r o u t i n g tables. Clus t e rhe ads assign

the slot(s) a n d code to each VC request wi thin the i r cov-

ering area. The n umb e r o f slots per f r ame assigned to a

VC is de t e rmined b y the b a n dwi d t h r equi r ed by the VC.

< f r a m e >

I l '1 . . . . ] i I " ' l I

controlphase + infophase


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f i x e d T D M A

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

Fig. 7. Channel access frame.

An o t h e r f u n c t i o n o f the cont rol pha s e is the exchange

o f powe r gain lists amo n g neighbors. Powe r gain is actu-

ally de f ined as the power p r o p a g a t i o n loss f r om t r ans -

mi t t e r to receiver. As described below, the c lus t e rhe ad

(CH) c an ga the r all powe r gain lists f r om its members,

a n d ma i n t a i n the powe r gain ma t r ix. This ma t r i x is use-

ful to cont rol powe r a d j u s tme n t a n d code division inside

the cluster.

As depicted in Fig. 7, the c o n t r o l phase uses fixed

T D M A wi th full power t r ansmi s s ion on a c ommo n code.

T h a t is, each node t ake s turns to b r o a d c a s t its informa -

t i o n to all o f its neighbors in a pr ede f ined slot, such t h a t

the n e two r k cont rol func t ions c an be p e r f o rme d distri-

but edly. The following func t ions are pe r formed by each

node dur ing the cont rol phase.

Ea ch s t a t ion broadc a s t s :


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9 its up to da t e r o u t i n g list defining the next node on

the rout e to each de s t ina t ion a n d the n umb e r o f hops

(Be l lman-Ford R o u t i n g Alg.),


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9 ID o f i t s C H ,


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9 its powe r gain list wi th respect to its neighbors

(derived f r om received powe r me a sur ement s ) ,


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9 slot r e s e rva t ion s t a tus o f its i n f o rma t i o n subf r ame ,

indi c a t ing also whi ch code it will listen to in the free

slots,


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9 A C K s for l a s t i n f o rma t i o n subframe.

Ea ch s t a t ion pe r forms the fol lowing rout ing, a n d

powe r cont rol functions:


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9 u p d a t i n g the r o u t i n g table a c cording to the rout ing

lists t h a t i t has received f r om the ne ighbor s (Bellman-

F o r d R o u t i n g Alg.),


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9 me a sur ing powe r gains for a l l neighbors; u p d a t i n g

powe r ga in ma t r i x a c cording to powe r ga in lists

received f r om all its neighbors,


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9 me a sur ing SIR (Signal to Int e r f e r enc e Ra t io) for the

incoming links,


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9 selecting its clusterhead,


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9 r e cording the slot r e s e rva t ion s t a tus o f its neighbors,


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9 obt a ining A C K a n d reserving the slot. 260 M. Gerla, 3. T.-C. Tsai / Multicluster, mobile,

multimedia radio network

A t the end o f the cont rol phase, each node has l e a rned

( f rom the i n f o rma t i o n b r o a d c a s t by the clusterhead) the

channel r e s e rva t ion s t a tus o f the i n f o rma t i o n phase,

a n d the powe r me a sur ement ma t r i x to all its neighbors.

This i n f o rma t i o n will help it schedule free slots, verify

failure o f reserved slots, a n d drop expired real time

packets.

Since in a mobile n e two r k clusters are d y n ami c a l l y

r e conf igur ed ( and the r e for e CHs change), each node

mu s t be able to t ake on the role o f clusterhead. The pro-

posed scheme allows, however, the exclusion f r om C H

d u t y o f pr ede f ined sets o f nodes, for example nodes

which are moving too f a s t (airplanes), or which lack the

required resources (sensors).

F r o m Fig. 7, we not e t h a t the size o f the cont rol f r ame

(which uses a c ommo n code) grows l ine a r ly with the

n umb e r o f r adios in the ne twork. I f the n e two r k grows to

several h u n d r e d radios, the cont rol O / H becomes prohi-

bitive. The ove rhe ad c an be d r ama t i c a l l y reduced by

reusing the same cont rol slot f o r radios in n o n - a d j a c e n t

clusters (in the same way as codes were reused). Assum-

ing 4-cluster slot reuse a n d an average o f six nodes per

cluster, a 24 slot cont rol f r ame is required. This n umb e r

c an be f u r t h e r r educ ed by assigning to each cluster, dur-


34/59

ing n o rma l ope r a t ion, only two cont rol slots: one for

the CH, a n d a n o t h e r to be used by the othe r r adios in the

cluster in a r o u n d robin f a shion. Ful l channel access

f u n c t i o n a l i t y is ma i n t a i n e d , but pe r formanc e is

degr aded because o f slower rout e upda t ing, VC reserva-

tions, etc. Rega rding ACKs , one should not e t h a t the

CH c an A C K all successful d a t a t r ansmi s s ion in the clus-

ter, except for its own (recall t h a t voi c e /video packets

are n o t ACKe d ) . The r adio t r a n smi t t i n g in the " o t h e r "

cont rol slot will A C K the C H transmissions.

Fina l ly, one should not e t h a t the propos ed cont rol

s t ruc tur e c an be generalized to suppor t a n y di s t r ibut ed

rout ing scheme (either link s t a t e or distance ve c tor type).

In pa r t i cul a r , loop free, distance ve c tor schemes c an be

employed ( a l though, they have n o t ye t been implemen-

t ed in our simulator) [11]. F l o o d i n g can, o f course, also

be used.

3.3. I n f o ( v o i c e ~ v i d e o ~ d a t a ) p h a s e

The info pha s e mu s t suppor t b o t h vi r tua l circuit a n d

d a t a g r a m traffic. Since real time t r a f f i c (which is c a r r i ed

on a VC) needs g u a r a n t e e d b a n dwi d t h dur ing its active

period, b a n dwi d t h mu s t be pr e a l loc a t ed to the VC in the

info phase before a c tua l d a t a t r ansmi s s ion, as depicted

in Fig. 8. T h a t is, some slots in the info subf r ame are

reserved for VCs a t call set-up time. The r ema ining slots


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(free slots) o f each cluster c an be accessed by d a t a g r a m

t r a f f i c using an S -ALOHA scheme. F u r t h e rmo r e ,

C SMA probing o f VC slots by d a t a g r a m s t a t ions c an be

considered for reuse o f silent VC slots. Two or mor e

VCs c an share the same time slot using C D M A . In this

case, t r a n smi t powe r mu s t be adjus t ed in orde r to yield

I I

I I

CSMA CSMA

S - A L O H A

Fig. 8. Channel access ofinfo phase subframe.

acceptable signal to interference r a t io (SIR). A power

cont rol a l g o r i t hm is carried out which is based on the

c omp u t a t i o n , exchanging and u p d a t i n g o f SIR values a t

the nodes involved. These ratios, in turn, c an be com-

put ed f r om the powe r gain ma t r ix obt a ined f r om the

cont rol phase, or c an be obt a ined via a s epa r a t e distribu-

t ed proc edur e [7,9].

3.4. E x a m p l e o f V C s e t - u p

Le t us consider the t o p o l o g y in Fig. 2, and give an

example to i l lus t r a t e how a VC is set up. Suppose t h a t

node 7 want s to establish a conne c t ion to node 5. Fi r s t ,

node 7 broadc a s t s a VC request t h r o u g h the cont rol

phase. The clusterhead, node 4, is responsible for servi-

cing this request. Since the r e is no direct link between


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node 7 a n d 5, the c lus t e rhe ad selects the first leg o f the

p a t h t h r o u g h node 9 (by inspection o f its rout ing table),

a n d assign slot(s) a n d code to this link.

Next, a f t e r ge t t ing the slot a s s ignment f r om cluster-

he ad 4, node 9 inspects its r o u t i n g table and discovers

t h a t it has to ma k e a VC request aga in to get to next node

8. I t thus places a request to c lus t e rhe ad 1, which is com-

m o n to b o t h 9 a n d 8. Clus t e rhe ad 1 will assign a slot to

link 9-8. No d e 8 forwa rds the request a n d so on, unt i l the

p a t h is compl e t e ly t r a c ed to de s t ina t ion node 5 as

depicted in Fig. 9. The conne c t ion m a y be cha r a c t e r i z ed

by a given QoS, i.e., a c e r t a in b a n dwi d t h requirement,

which is t r a n s l a t e d into n umb e r o f slots per frame, or a

7?

~

2

Fig. 9. Example of VC set-up. M. Gerla, J. T.-C. Tsai / Multicluster, mobile, multimedia radio network 261

given channe l qua l i ty, r e l a t ed to SIR value or e r ror r a t e

on a link. These QoS r equi r ement s are checked a n d

enfor c ed dur ing the VC set-up phase.

3 . 5 . F a s t R e s e r v a t i o n V C

The VC set-up scheme described in the previous sec-

t i o n is suitable for a n e two r k wi th slow mobi l i ty. I n a

highly mobi l e e n v i r o nme n t i t m a y happen t h a t the time

r equi r ed to set up a new VC is compa r abl e to the int e rva l


37/59

between p a t h changes. Thus, the convent iona l VC

r e c o n f i g u r a t i o n scheme c a n n o t c a t ch up wi th s t a t ion

movement s . F o r highly mobile envi ronment s we pro-

pose an a l t e rna t e scheme ba s ed o n f a s t r e s e r v a t i o n s . We

a l r e ady showed t h a t the clustering a l g o r i t hm c an ade-

qua t e ly adjus t to s t a t ion movement s . We f u r t h e r assume

t h a t the d y n ami c r o u t i n g a l g o r i t hm c an efficiently t r a ck

the changes in topology. I n our experiments we have

used the basic B e l lma n - F o r d a l g o r i t hm which leads to

occasional loops dur ing t r ans i t ions . I n the futur e , we

pl an to impl ement a mor e robus t , loop free r o u t i n g algo-

r i t hm such as described in [ 11 ].

I n the F a s t R e s e r v a t i o n a l g o r i t hm each pa cke t in the

VC s t r e am is r o u t e d individually, ba s ed on the destina-

t i o n address, ve ry mu c h like a da t apa cke t . As a differ-

ence, however, the first pa cke t in the VC stream, upon

successfully c a p t u r i n g a slot in the info subf r ame , will

r e s e r v e it for all subsequent frames. I f the slot r ema ins

u n u s e d f o r a c e r t a in n umb e r o f frames, it is declared free

by the cluster cont rol l e r a n d i t is r e t u r n e d to the free

slot pool. This scheme, whi ch was inspired by the P R M A

p r o t o c o l [ 12], allows the VC s t r e am to d y n ami c a l l y select

a new p a t h to de s t ina t ion when the old p a t h fails. O f

course, each p a t h change will cause some di s rupt ion (i.e.

possible o u t o f sequencing; de l ay to acquire a slot on


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the new pa th; possible looping i f the r o u t i n g a l g o r i t hm is

n o t loop free, etc). Howeve r , the di s rupt ion is much less

severe t h a n i f a new p a t h had to be set up f r om source to

d e s t i n a t i o n using the convent iona l scheme. In the fol-

lowing section, expe r iment a l results show t h a t the F a s t

Re s e rva t ion scheme c an efficiently handl e a high degree

o f mobi l i ty.

Several r e f inement s o f the basic F a s t Re s e rva t ion

scheme are cur r ent ly explored. One int e r e s t ing f e a tur e ,

ma d e possible by C SMA probing, is the a s s ignment o f

p r i o r i t y to VC packets in c aptur ing free slots. Name l y ,

the VC pa cke t is t r a n smi t t e d immedi a t e ly, while the

d a t a g r a m pa cke t mu s t " p r o b e " the slot be for e transmis-

sion. Probing also allows the reuse o f unus ed VC slots

by d a t a g r a m t r a f f i c , thus alleviating the ove rhe ad

c aus ed by obsolete reservations left over a f t e r a p a t h

change. A n o t h e r opt ion is the dropping o f " l e a s t signifi-

c a n t " pa cke t s wi thin a hi e r a r chi c a l ly encoded video or

voice s t r e am following a rerouting. The dropping o f least

significant bits has been applied in the pa s t in several

c ommu n i c a t i o n systems either to increase channe l capa-

city (at the expense o f signal equality) or t o avoid conges-

t ion [13]. In our case, the d r o p p i n g o f low pr ior i ty

pa cke t s dur ing the r e rout ing pha s e will alleviate conges-

t ion a n d reduce r e s e rva t ion delay, a t the expense o f tem-


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p o r a r y signal qua l i ty degr ada t ion. Qua l i ty is r e s tor ed

a u t oma t i c a l l y a f t e r r e rout ing, to the degree t h a t the r e is

sufficient b a n dwi d t h o n the new pa th.

Ha v i n g defined b o t h a static a n d a " f a s t " VC set-up

procedure, we are now f a c ed wi th the di l emma o f decid-

ing which should be used in wh a t case. One wa y to avoid

this probl em is to combine the two schemes into a single,

f a s t r e s e rva t ion based scheme. Essentially, the call

request pa cke t o f the convent iona l scheme now becomes

a f a s t r e s e rva t ion packet. This packet, however, has

lower pr ior i ty t h a n the pa cke t o f an a l r e ady established

conne c t ion, so t h a t the new call is blocked f i r s t i f the r e is

insufficient b a n dwi d t h in the n e two r k to a c c ommo d a t e

it.

4. P e r f o r m a n c e e v a l u a t i o n

4 . 1 . L i n k t h r o u g h p u t

L i n k t h r o u g h p u t is de f ined as the sum o f the t h r o u g h -

puts on the links which are s imul t aneous ly active in the

mul t i c lus t e r ne twork. Since channe l r a t e is a s sumed uni-

f o rm over the entire ne twork, l ink t h r o u g h p u t is propor -

t iona l to the n umb e r o f s imul t aneous ly active links.

L i n k t h r o u g h p u t is a simple figure o f merit. I t allows to

derive pr e l imina ry t r adeof f s between system p a r am-

eters. I n orde r to comput e l ink t h r o u g h p u t , recall t h a t

powe r interference across clusters is reduced by code


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s epa r a t ion. Since powe r drops very r apidly wi th dis-

tance, in each time slot a t least one link per cluster c an be

active wi th tol e r abl e intercluster interference. Thus, the

average n umb e r o f s imul t aneous active links is approxi-

ma t e l y equa l to the average n umb e r o f clusters. As long

as a cluster ha s one or mor e links (i.e., it is n o t an iso-

l a t ed, single node cluster), the cluster ha s link through-

p u t equal to 1. He r e , we ma k e the optimistic a s s ump t i o n

t h a t the r e a l w a y s is o n e successful t r ansmi s s ion per clus-

t e r per slot. This a s sumpt ion is acceptable when compa r -

ing di f f e r ent strategies. However, it does n o t t ake into

a c c o u n t d a t a slot collisions, silence gaps, cont rol O / H

etc. F o r a mor e a c cur a t e eva lua t ion, s imul a t ion will be

used.

Fol lowing the same idea as in Fig. 4, we generate dif-

f e r ent topologies by placing N node s in the square wi th

variable t r ansmi s s ion power range. Mo r e precisely, we

generate a b o u t 100 r a n d o m placements o f the N nodes in

the 100x 100 square. We then step t h r o u g h p u t va r ious

values o f the t r ansmi s s ion r ange a n d for each value we

o b t a i n di f f e r ent topologi e s cor r e sponding t o the differ-

ent ( r andom) placements. We average our pe r formanc e

me a sur e s over the r a n d o m placements. The average

aggregate l ink t h r o u g h p u t versus t r ansmi s s ion r ange is

shown in Fig. 10. Also, the average n umb e r o f clusters 262 M . Gerla, aT. T . - C . T s a i / M u l t i

c l u s t e r , mobile, multimedia radio network


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E

g

s

L i i i i i i

10 N=20 - a ~

| N=25 -x--.

N = 3 0 -~,--

8 , , ~ ' ~

,,,' " ,

,' / "-Z~.

4

,' ] "J'~.

2

0

10 20 30 40 50 60 70 80 90 100

N _ r a n g e

F i g . 10. L i n k throughput.

and connectivity are moni tor ed whi l e generating those

t o p o l o g y (Fig. 11, Fig. 12). The "connectivity" here is

defined as the fraction o f node pairs whi ch can commu-

nicate (through single or multiple hops).

The results conf i rm our intuition that there exists a

t radeof f be twe en transmission range and throughput.

For relatively smaller transmission (tx) range, the graph

cons i s t s o f several i solat ed subgraphs, wi th g o o d spatial


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reuse but poor connectivity. T o o small a tx range, h ow-

ever, leads to a decrease in throughput since mo s t o f the

clusters contain only one node, (i.e. the CH), and no

links. As the tx range grows, we have better connectivity

but less efficient spatial reuse, and thus lowe r through-

put. Adapt ive p owe r adjustment can be used to optimize

the t radeof f between connectivity and spatial reuse.

Techniques similar to thos e reported in hierarchical

routing opt imi zat ion [14,15] can be employed for finding

optimal cluster size, although in this case nove l criteria

(e.g. spatial reuse) and constraints (e.g. distributed

implementation) must be accounted for.

In the previous experiment, we only have considered

T D M A . I f we adopt the p owe r adjustment algorithm

[9,10], then we can combine T D M A wi th C D M A . As

discussed in section 3, CH maintains the powe r gain

matrix for pairwise members. I f a call request come s in,

and there are no free slots, then the CH will search for a

0.8

0.6

8 0 . 4

0.2

' , . ~ . ; N _ ~ - " -~ = -T. = -7- - " ~ = ,,7

-EF.-- ,"X /

N=20 F


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N---25 -x--- , " /

N=30 -~.--. ,~ ,'

j

,'o 2'0 30 ' 40 ' 0 0 70 ' 80 90 '

tx range

100

F i g . 12. Connectivity.

feasible reserved VC slot so that two VCs can share the

slot in C D M A mode . "Feasible" here means that their

pairwise powe r gains satisfy a set o f condi t ions such that

they can adjust their tx powe r s and coexi s t wi th accepta-

ble signal quality.The C D M A sharing scheme leads to

approximately a 20% throughput improvement, as

shown in Fig. 13.

4.2. End- to- end throughput

In the previous experiments the throughput was

defined as the sum o f single link throughputs. This per-

formance measure, albeit simple to comput e , is howeve r

biased towards small transmission range, i.e. a large

number o f small clusters, many o f whi ch may actually be

disconnected. In practice, we are mor e interested in end

to end (rather than single link) throughput, and wo u l d

like to maintain connectivity among as many node pairs

as possible in the system. In order to reflect these require-

ments, we introduce a new throughput measure, whi ch


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we call the weighted end-to-end throughput, defined as

fol lows :

c s C i

i = 1

where:

"6

=le

3O

25

20

i

15

10

5

0

i

',,,, N=20

N=25 -x--.

, , ~ N = 3 0 - ~ - -

I i I I i i [ I I

10 20 30 40 50 60 70 80 90 100

N r a n g e

Multi Cluster

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