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End to

nd

Survivable Broadband

Networks

Within the EC-sponsored

RACE

program, the

I M M U N E

project

was established to analyze and specify appropriate strategies for

introducing end-to-end survivability into corporate and public

broadband networks

Leo Nederlof Kris Struyve Chris OShea Howard Misser Yonggang Du and

Braulio Tamayo

LEO NEDERLOF is with the

Aha tel Corporate Research

Centre in Antwerp.

K R S

S T R W E s wi th th e

Department of Information

Technologyat IMEClUniver-

sity of Ghent.

CHRIS O SHEA works in the

Broadband Multiservice Net-

works Unit

of

BT Research.

HOWARD MISSER is with

P TT

Telecom.

YONGGA NG DU s with

Philips Rese arch Lahorato-

lies.

BRAULIO TAMAYO is with

the Alcatel Corporate

Research Centrein Mudrid,

n recent years, a wide range of protec-

t i o n a n d r e s t o r a t i o n t e c h n i q u e s h a v e

b e e n d e v e l o p e d t o s u p p o r t t h e s u r vi v -

ability of todays and tom orrow s broa d-

b a n d n e t w o r k s . C o m p o n e n t r e d u n -

dancy, route diversity, self-healing rings

and dynamic restoration ar e among the solutions

to he lp specific parts of specific network s survive

fa i lu res o f one o r more o f the i r compr is ing e le -

ments . Som e o f these techn iques a re s t i ll under

s tudy , whi le o thers have been dem ons t ra ted to

work, and ar e already available to network oper-

a t o r s a n d p l a n n e r s . W h a t

is

s t i l l l a c k i n g i s a

coh eren t and in tegra ted assessment o f end- to -

end survivabili ty , consisting of the identif ication

and definit ion of requirements, m etrics and eval-

ua t ion m ethods , a s wel l a s the de f in i t ion o f the

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

applied in different network layers or parts , and

the role

of

the network managem ent system.

W i t h in t h e E C - s p o n s o r e d R A C E p r o g r a m ,

t h e I M M U N E p r o j e ct , b eg u n i n J a n u a r y

1994,

has se t a s i t s ob jec t ives to ana lyze a nd spec i fy

appropriate strategies for introducing end-to-end

s u r vi v a bi l it y i n t o c o r p o r a t e a n d p u b l i c b r o a d -

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

proper techn iques and eva lua t ion too ls , and to

d e m o n s t r a t e d i s t r ib u t e d r e s t o r a t i o n o n P S N

(pub l ic swi tched ne tworks) and C PN (cus tomer

premises netw orks) laboratory m odels. Six part-

ne rs f rom f ive European coun t r ies have jo ined

i n t h e I M M U N E c o ns o r ti u m : B T L a b s ( U n i t e d

K i n g d o m ) , t h e R e s e a r c h I n s t i t u t e I M E C ( B e l -

g ium) , PTT Researc h (The Neth er lands) , Alca -

tel Sta ndar d ElCctrica (Spain), Philips Research

Lab (Germany) , and Alca te l Be l l (Be lg ium) as

coordinating partner.

S ince su rv ivab i l ity has on ly recen t ly be come

an a rea of interest by i tself , no standardized d ef-

in i t ions o r normal ized qua n t i f ica t ions ex is t a s

ye t . The f i r st ob jec t ive was , the re fo re , to de f ine

a set of survivability requirements and metrics to

be used in the rest of the project. This has

led

to

the identification of a range of survivability strat-

e gy o p t i o n s a n d h o w t h ey c a n b e m a p p e d o n t o

u s e r , s e r v i c e p r o v i d e r a n d o p e r a t o r r e q u i r e -

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

g iven in the nex t sec t ion . Th e nex t s te p

on

t h e

r o a d t o i n t e g r a l s u r v iv a b il i ty i s d e s i g n i n g a n d

planning survivable networks, and th e evaluation

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

that will be applied in these networks. The third

section gives an overview of this part of the pro -

j e c t . M o s t p r o t e c t i o n a n d r e s t o r a t io n m e c h a -

nisms ope rat e within a single network layer and

n e t w o r k p a r t , a u t o n o m o u s f r o m n e t w o r k m a n -

agem ent. Th e interaction of mechanisms in dif-

f e r e n t n e tw o r k l a y e r s o r i n d i f f e r e n t n e t w o r k

par ts , and the ro le o f ne twork m anagement , a re

d i s cu s s ed i n t h e f o u r t h a n d f i ft h s e c t i o n s . F o r

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

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

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

A T M P S N , a n d a C P N A T M r i n g p r o t e c ti o n

s w i t ch i n g m e c h a n i s m . T h e s e t e c h n i q ue s a r e

described in the sixth section. Finally, in the last

s e c t i o n , a n o v e r v i e w i s g i ve n o f t h e o n g o i n g

a c t i v i t ie s w i t h in t h e I M M U N E p r o j e c t , w i t h a

summary of the status of the de mo models.

Requ i rements fo r Network

Surv ivabi l i ty

n or de r to dete rmi ne a networks survivability ,

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

w i t h m e t h o d s t o m e a s u r e a n d q u a n t if y t h e p e r -

fo rmance unambiguous ly . For the in te rp reta t ion

of the se metr ic s , a se t o f su rv ivab i l ity rcqu i re -

ments ha s been der ived f rom th e po in t o f view

of use rs, se rv ice p rov iders and ne tw ork ope ra -

tors. These requirements have been quantif ied in

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

app l ied to assess the pe r fo rm ance o f p roposed

end-to-end survivability strategies. In addition to

this, reference network configurations have been

presen ted which fo rm the bas is o f the ne twork

mode ls used fo r th e deve lopment o f res to ra tion

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

these techniques.

IEEE Communications Magazine September 1995

0163-6804/95/ 04.00 1995

EEE

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T

recovery time . poten tlal service class

~~ ~ ~~

W

Table

1.

Matrm of user and tewice

claJJet

W Figure 1.Siiwivabihty evaluation

framework.

Survivabi l ity can be m easu red f rom the con-

t ext of end- to-end p er formance of a par t i cular

deman d, in order to opt imize service to par t icu-

l a r cu st om er s . O n t he o t he r hand . a n o p e r a t o r

also n e e d s t o e n s u r e t h a t t h e w h o l e n e t w o r k

meet s so me survivabi li t y cr i t er i a . Theref ore , a

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

to

m e a s u r e

whole netw ork survivabi li t y as well . The re a r c

al so shor t t e rm and long t erm avai labi li t y mea-

sures for QoS. Long t erm measures , ca l cula t ed

f r om M T B F ( m ean t i m e be t w een f a i l u re ) and

MTTR (mean t ime to repair ) , are more appl ica-

b l e t o s u rv i v ab i li ty m e t r i a , w h i le s h o r t t e r m

measures

(c.g.,

bi t er ror rate or packet loss r a t e)

give a mea sure of t he qu al i t y of a connect ion ,

link or path, at a given point in time. Short term

performance measures can also be used to deter-

mine when a c i r cui t becomes unavai l able , and

trigger recovery systems.

T h e d e p l o y m e n t o f s u rv i v a b i l it y o p t i o n s

involves a choice betwccn s t r a t egies , i n which

many f actor s ar e involved, r c l a t ed to

cost,

ser -

v i ce t ype , ne t w or k t opo l ogy , e tc . T he r cqu i r e -

m e n t s n e e d

to

b e d r i v e n by t h e e n d u s e r a n d

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

t h r ough t he r ange and qua l i t y o f t he s e rv i ce s

provided via the transport network that is in turn

p r ov i ded by t he ne t w o r k ope r a t o r . ( I t is a l s o

possible that the service provider and the opera-

tor are one and the same.)

T h e m os t u se r - pe r ce i ved f ac t o r t ha t de t e r -

mines the

QoS

i s t e rmed an ou tagc. An outage

is defined as a significant degradation it7

the

ubili-

y o f u customer to establish and ma i i i tu i i~i chan

nel ofcot?znuti~icatiori.s s

U

result

of

fuilitr in uti

operators network 111. This in itself is not suffi-

cient because the term s ignificant degradat ion

needs to

be quant i f ied. Th e performance thresh-

olds for declar ing an outage wi ll depcn d

on

the

QoS

cr i ter ia agreed upon with the custom er and

the sensitivity of the service

to

dcgraded perfor-

m an cc [ 2 ] . F r o m t h e po i n t o f vi ew of s c r v ice

providers , current and new services can best be

classified according to their resilience to network

outages . The durat ion of a service outage can be

set out against the impact , def ined in terms of

call dropping, session time-outs, social and busi-

ne s s i m pac ts , e t c . O pe r a t o r r eq u i r em en t s a r e

der ived f rom users and service providers . How-

eve r , ne t w or k ope r a t o r r equ i r em en t s a r e also

dr i ven by t he f ac t t ha t t he ope r a t o r nee ds t o

m a k e a n o v e r a l l p r o f it w h i l e m a i n t a in i n g a n

a c c e p t a b l c l c v el o f s e r v i c e t o a l l c u s t o m e r s .

Table summarizes the user (ul-u4 ) and service

classes

( ~ 1 . ~ 6 )

ogether in a matrix. The opcrator

must then decide upon a res torat ion/protect ion

strategy to be applied 10 the cases i n quest ion.

0

h e o p e r a t o r C O n s id e r a i ons i nc 1u d e t h e

extent over which a disrup tionifailure occurs; the

range of potent ial fai lure scenar ios ; faul t propa-

ga t

o

n ; back - o n

o

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

rithm complexity, maintainability, reliability and

sensi t ivi ty. Al l of these issues are discussed in

In ord er t o provide an ac cura t e analys is of

network survivabi l ity s t rategics , informat ion on

cxist ing and planned network conf igurat ions has

b e e n g a t h e r e d f r o m a n u m b e r o f o p e r a t o r s .

Also, t he l ayering and p ar t i t i oning concept s as

described in [4] have been taken into account. A

survey has been carr ied out of var ious network

a r ch i t ec t u r e s i n t he U S and E u r ope , cove r i ng

S D H i S o n e t , p u r e A T M , A T M i S D H a n d

PDH iATM networks . From this s tudy, and f rom

litcrature, a library of network parts (e.g., mesh,

ring, star,

...)

has been established, from which a

generic set of reference netwo rk configura tions

has been derived. Thesc reference configurations

form the basis of the comparative evaluations of

survivability strateg ies.

PI.

Survivability Evaluation Model

etworks are m ade survivable by impleme nt-

N i n : r e st o ra t ion t echn iques on one hand , and

by providing spare capacity on the other. For the

evaluation of a certain survivability strategy as a

whole , tools are ne eded to val idat c thc r es tora-

tion techniques and to optimize the allocation of

spar e capaci ty . An as ses sment of per formance

v e r s u s c os t c a n t h u s b c m a d c

for

a n e t w o r k ,

64

IEEE Commuiiication~

aga7ine Scptember

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pr io r to ac tua l dep loym ent o f the s t ra tegy . Th is

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

part of the I M M U N E pro jec t , and a lso som e o f

the tools that were developed for the evaluation.

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

resource a l loca t ion m ethods i s shown in F ig .

1.

E a c h p a r t r e p r e s e n t s a p a r t i cu l a r a s p e c t o f t h e

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

capacity planning and recovery strategy parts ar e

i n p u t s t o t h e n e t w o r k e n v i r o n m e n t p a r t . T h e

ou tcom e o f ca lcu la t ions i s eva lua ted accord ing

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

different parts of the framew ork in Fig.

1

will

be

further described. Some techniques used for the

recovery s t ra teg ies a re d iscussed in the sec t ion

on res to ra t ion mechan isms fo r meshed and r ing

networks.

Fault Scenarios

Fai lu res occur r ing in rea l - l ife ne tworks may b e

g r o u p e d i n t o t w o m a j o r c l a ss e s: l i n k a n d n o d e

fa i lu res . As such , a l ink fa i lu re a t the phys ica l

layer might be u sed to m odel a f iber cable break,

while a l ink failure at the SDH VC-4 layer might

mode l a port failure. Multiple simu ltaneous fail-

ures also must b e considered, since a single fail-

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

netwo rk layers, as is illustrated by Fig.

2.

Also a

f i re in a hub bu i ld ing poss ib ly a f fec ts m ul t ip le

network elements.

Network Environment

Th e ne twork env i ronm ent compr ises th r ee lev -

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

func t iona l level and the management func t ional

leve l . The t ran spor t func t iona l level mode ls th e

t r a n s f e r o f c l i e n t i n f o r m a t i o n i n t h e n e t w o r k .

Th e control functional level realizes the transfer

of operations- and maintenance-related informa-

tion (e.g. , fault detection signals, ...). This level

a lso inc ludes the exchange o f res to ra t ion mes-

sages. The manag ement func t ional leve l , wh ich

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

operating survivabili ty mechanisms and T MN , is

further considered in the following sections.

ITU-T Recommenda t ion

(3.803

[4] provides a

bas ic

tool

t o m o d e l m u l t il a y er m u l t i p a r t n e t -

works which a re com posed o f l inks (e.g ., f ibe r

cables, coax cables, radio links, .

)

and e lements

e.g., M U X ,

ADM, CC, ...). The integration of

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

ments can be included in the model, especially if

the speed of restoration is assessed by mea ns of

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

time of an a lgorithm , it is necessary to use a rep-

r e s e n t at i v e m o d e l o f t h e n o d e s , t a k i n g i n t o

account the software and hardware architecture.

For in i t ia l eva lua t ion s tud ies , th e mode l ing o f

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

three parameters:

Cros s con nect delay:

t h e t i m e n e e d e d t o s e t

up a cross-connect point in a node.

Internal comm unication delay: the t ime need-

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

through the control architecture of a node.

Processing delay: the actual

PU

t ime needed

to process an algorithms code.

P e r f o r m a n c e b e n c h m a r k i n g of d i f f e r e n t

r e c ov e r y m e c h a n i s m s r e q u i r e s , a m o n g o t h e r s ,

a g r e e m e n t o n a r e f e r e n c e n e t w o r k . I t i s , ho w -

W Figure 2

Failure propag ation. A single SDH span failure causes multiple

A T M link failures.

W

Table

2 Evaluation criteria.

ever , no t imposs ib le tha t the pe r fo rmance o f a

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

ne twork e nv i ronm ent fo r which i t i s eva lua ted .

H e n c e , a m o r e g e n e r a l i z e d c o m p a r i s o n s h o u l d

inc lude a sens i t iv i ty ana lys is o f eac h recovery

mechan ism to pa ramete rs such as ne twork s ize ,

ne twor k connec t iv i ty , re la t ive am oun t of spar e

capacity and traffic load. For this analysis, a net-

work generation tool is developed that can gen-

e r a t e r e f e r e n c e n e t w o r k s a c c o r d i n g t o a s e t o f

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

tr ibuted restoration algorithms were applied to a

num ber of referen ce networks with varying con-

nectivity, while other p arameters were k ept con-

stant. Th e results of this study are given in [ 5 ]

Evaluation Criteria

To in te rp ret the ou tpu t of a performance evalua-

t i o n , a s e t o f c r i t e r i a i s n e e d e d . F o r r e c o v e r y

mecha n isms , two c lasses o f eva lua t ion c r i te r ia

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

direct output from the simulations; and advanced

criteria , which ar e calculated from th e basic cri-

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

pe r fo rmance of the recovery strategy. Examples

are given in Table 2.

Evaluation Tools

T h e c o m p le x i ty

of

t h e n e t w o r k m o d e l r e q u i r e s

t h e u s e

of

c o m p u t e r s t o s i m u l a t e s u r vi v a bi l it y

s t ra teg ies a nd

to

plan sp are capacity. Analytical

methods ten d to be use fu l fo r eva luat ing s imple

models, such as self healing protection rings, as

the n umb er of systems states is l imited. Analyti-

ca l methods can a lso be used to p rov ide upp er

a n d l o w e r b o u n d s o n p o s s ib l e p e r f o r m a n c e .

IEEE Commu nications Magazine Septcmhcr 1995

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The objective

o f a n

escalation

strategy is

to

optimize the

pegormanee

of the

network

under all cir-

cumstances

using

available

resources

and

implemented

mechanisms.

With in the RA CE I1 p ro jec t IM MU NE consor-

t ium d i f fe ren t com pute r simula t ion too ls have

b e e n a d a p t e d t o e v a l u a t e r e c o v er y a n d s p a r e

capacity planning algorithms.

The following tools have been used for evalu-

ation of recovery mechanisms.

Alcatel Standard Electricas INSE tool helps

evaluate survivability in a multilayered network.

Th e eva lua to r a l lows severa l su rv iva l a rch i tec -

t u r e s t o b e c r e a t e d i n t e r a c ti v e l y a n d a n a l y z e s

their effect on overall network survival perfor-

m a n c e . E v a l u a t i o n o f

loss

of connectivity and

loss of capacity metrics is currently supported to

de te rmine th e mos t vu lnerab le pa r t s of the ne t -

work for a variety of fault scenarios.

* T h e B T t o o l , c a ll e d T E N D R A . h a s b ee n

developed to investigate distr ibuted restoration

protocols in transport networks [6]. Restoration

protocols ar e enco ded as objects associated with

each nod e object, such that i t is possible to plug

d i f fe ren t res to ra t ion p ro toco ls in to a TEN DR A

simulation. Message-passing between restoration

p r o c e s s e s e x e c u t in g o n p r o c e s s o r s a t d i f f e r e n t

node sites is achieved using discrete-event simu-

lation techniques in which appropriate delays are

u s e d , m o d e l i n g f i n i t e l i n k tr a n s i t d e l a y s , a n d

nodal processing and crossconnection times.

I M E C h a s d e v e l o p e d t h r e e t o ol s , c al l e d

W D M S I M , S D H S I M a n d A T M S I M . T h e W D M -

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

m a n c e p a r a m e t e r s

of

c e n t r a l i z e d r e s t o r a t i o n

algorithms for multi layer networks. These multi-

layer ne twork m ode ls a re based on ITU -T Rec-

ommenda t ion G.803 . The A TMSIM s imulato r i s

s p e c i f i c a l ly d e v e l o p e d t o s t u d y d i s t r i b u t e d

res to ra t ion a lgor i thm s in s ing le layer ne tworks

using discrete- event simulation technique s.

Resource Allocation Tools

Plann ing too ls a re uscd to p rov ide the ne twork

env i ronm ent wi th su f f ic ien t spare resources to

p e r f o r m r e s t o r a t i o n . G i v e n a t r a ff i c d e m a n d

m a t r i x , a n d t h e t y p e o f r e c o v e ry t o b e i m p l e -

mented , the l inks and nodes in the ne twork can

be dimensioned and th e cost calculated. If those

planning tools are to be applied to existing nct-

works , ra the r than jus t fo r d imens ion ing re fe r -

e n c e n e t w o r k s f o r e v a l u a t i o n o f s u r v i v a b i li t y

strategies, real-life c onstraints can be taken into

account, e .g. , geographical a spccts and installed

base.

Th e fo l lowing two too ls a re examples o f the

resource allocation tools, and have been used in

the IMM UN E pro jec t.

* T h e A L C A L A t o o l d e v e l o p e d by A l c a t e l

Standard Electrica provides a means of planning

a n d d e s ig n i n g P D H a n d S D H n e t w o r k s w i t h

protection via link and path diversity with unidi-

r e c t i o n a l r i ng s a n d m e s h e s [ 7 ] he des ign i s

op t imized in te rms o f equ ipment d imens ion ing

and overall total network installation cost for a

given level of protection between any node pairs.

*BTs Res to ra tion Capac ity Heur ist ic ( RC H)

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

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

traffic in a fully or partly-meshed tran sport net-

work topology. Th e meth od used is applicable to

any meshed-based res to ra t ion s t ra tegy. and the

des igns p roduced have been ver i f ied us ing the

TE ND RA ne twork s imula tion too l .

I

I

vc 4 a Failure recovery in th e SDH layer

/

. /

/

O

End-to-endconn

vc-4

b

Escalation o the

ATM

layer

~.

Figure 3. Escalation between layers.

T h e t o o l s a v a il a b le w i t hi n t h e c o n s o r t i u m

need t o be fu r the r ex tended . The incorpora t ion

of d i f fe ren t recovery mechan isms in d i f fe ren t

parts and layers of the ne twork mode ls mus t be

improved , and dynamic scenar ios fo r the in te r -

work ing o f mechan ism s in d i f fe ren t layers and

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

t o o l s . T h i s i n t e r w o r k i n g , or e s c a l a t i o n , i s

described in the following section. The tools can

a lso bc adap te d to dea l d i rec t ly wi th a num ber

of the evaluation criteria that were mentioned in

th is sec tion . Fur the rmore , th e incorpora t ion o f

T M N a s p e c t s s h o u l d b e f u r t h e r i n v e s t i g a t ed .

M e t h od o og e s o p t i m i z i n g t h e i n t e r w o r k n g

between recovery mechanisms and spare capaci-

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

further.

Escalation Issues

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

D eployed in different network layers or sub-

networks. The interworking between these mech-

anisms is called escalation. T he objective of an

e s c a l a t i o n s t r a t e g y is t o o p t i m i z e t h e p e r f o r -

m a n c e o f t h e n e t w or k u n d e r

all

circumstances,

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

mechanisms. If no escalation strategy is provid-

ed, different recovery mechanisms may prevent

each other from acting in an efficient way, and

may even lock up the ne twork in an indef in i te

5 ta te . In an idea l s i tua t i on , d i f fe ren t recovery

sys tems ac t in a complem enta ry way and sh are

spare resources.

Since netw ork survivability is a relatively new

66

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subject , as was s a id in the in t roduc t ion t o th i s

a r t i c l e , t he i s s ue o f e s ca l a t i on i s even new er .

Spar se ins t ances of opera t iona l survivable net -

works are present ly implem ented, but escalat ion

is yet a purely theoret ical issue. No custom-ma de

s o l u t i o n s c a n th e r e f o r e b e p r e s e n t e d a s y e t .

Ins t ead, t he s t eps t aken o n th i s par t of t he road

t o e n d - t o - e n d s u r v i v ab i li t y a r e l i m i t e d t o a n

ident i f icat ion of the issues and the o pt ions , and

a qual i t a t ive deba te on some escala t ion s t r a t e-

gies.

Escalation Between Layers

A netwo rk can be modeled as consis t ing of net-

w or k l aye r s , w i t h a c l i en t / s e rve r r e l a t i ons h i p

between adjacent layers. A layer providing trans-

port is called a server, and the layer using trans-

port is called a client [4]. Th e fact that di f ferent

recovery mechanisms are im plemented in dif fer-

ent layers may have several causes: the natural

e v o l u t i o n o f c o m m u n i c a t i o n s n e t w o r k s m a y

r e s u l t i n add i ng new s u r v i vab l e l aye r s t o t he

exis t ing ones , o r th e dif fere nces in survivabi l ity

requirements between network layers can resul t

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

mechani sms . Wi thin th i s context , an escal a t ion

strategy between layers def ines the coordinat ion

of res torat ion m echanisms in dif ferent layers to

a v o id c o n t e n t io n , p r o m o t e c o o p e r a t i o n a n d

incr ease overa l l survivabi l i t y . I n the s im ples t

case, the responsibility t o restore services in case

of a fai lure is passed f rom o ne layer to anoth er ,

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

r e s t o r a t i on capab i l i ti e s

or

w h e n a p r e d e f i n e d

time interval has passed. Figure 3 gives an exam-

ple of escalat ion f rom an SD H VC4 server layer

to an A TM VP cl i ent l ayer . In Fig. 3a, a fai lure

i s r e c o v e r e d i n t h e S D H l a y er , a n d t h e A T M

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

When the SD H layer is not able to recover f rom

the fai lure, e.g. , due to lack of spa re resource s ,

t he f a i l u r e e s ca l a te s t o t he A T M l ayer , w he r e

res toration is performed using an al ternat ive VP

trai l. Not e that th e server layer providing t rans-

por t to the al ternat ive VP route does not neces-

sari ly have to be the s ame as the or iginal server

layer from which the failure esc alated.

Th e main issues related to escalat ion between

layers are defining in which layer the restoration

process s tar ts , when i t escalates to another layer

and to which layer it escalates. Additional issues

to be solved ar e def in ing the r e l a t ion between

the escalat ion s trategy and the network ma nage-

m e n t s y s t em a n d t h e f o r m o f s i g n a li n g t o b e

used in the escalation mechanism.

Escalation Between Subnetworks

Each l ayer network can be d iv ided in to subnet -

works in a way that reflects the internal structure

of that layer. From a survivability point of view,

a survivable subnetwo rk (SSN) i s def ined as a

set of network elements grou ped together by the

f ac t t ha t t hey a l l s ha r e one s i ng l e r e s t o r a t i on

mechanism, e.g., a self-healing ring

or

a meshed

network with back-up routes . When the res tora-

t ion mechanism in one SSN is not able to recov-

er fully from a failure, adjacent

SSNs

need to be

involved. Two types of escalat ion betwe en SSNs

will be illustrated by mea ns of examp les.

One type of interworking between SSNs con-

Table 3. Where to start restoration.

I

___

~ I

ore

mesh Feeder ring

__

- I

Figure

4 nterconnection between survivable

subnetworks.

c e n t r a t e s o n t h e i n t e r c o n n e c t i o n o r g a t e w a y

nodes . Figure

4

depicts the dual access between

t w o S S N s : a f eede r r i ng and a co r e m es h ne t -

work. Wh en any of t he l i nks or nodes ou t s i de

t he l i gh t blue s haded a r ea s f a i l s , e ach o f t he

S S N s i s c a p a b l e o f r e s t o r i n g t h e f a i l u r e

autonomous ly . When one of t he gat eway nodes

o r l i nks f a il s, bo t h S S N s have t o c oope r a t e i n

moving traffic from one gateway to the other.

A second type of escal a t ion can occur when

SSNs

are organized in a h i er arch ical way, i. e .,

when several SSNs together form a larger SSN.

This larger SSN can then use the spare resources

of its comprising SSNs after it has received con-

trol over the restoration, according to the escala-

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

mechanism on a wider scale.

Thi s l a t t er t ype of escal a t ion between SSNs

shows a great cor r espondence wi th escal a t ion

between layers, where a SSN in a client layer can

use t r anspo r t f ac i l it i es of s evera l conca tenated

server SSNs. If, for instance, the interconnection

between two SDH subnetworks fai ls , an over lay

ATM network may res tore i ts t raff ic on anoth er

VP trail, using resources in the same or in differ-

en t s e r ve r S S N s . N o t e t he d i f f e r ence t ha t i n

escalat ion between SSNs in one layer , the same

spare r esources can be r eused af t er escal a t ion ,

while in general sharing spare resources between

network layers is not a straightforward issue.

Escalation Strategies

T he s e t of r u l e s u s ed t o dec i de w h i ch m echa -

ni sms to ac t ivat e and when to hal t mechani sms

and to ac t ivat e o ther s , i s ca l l ed the escal a t ion

strategy. Two types of escalat ion s t rategies can

be ident i f ied: act ivat ion of mult iple res torat ion

mechanisms in parallel, and sequential activation

of restoration mechanisms.

In para l l e l s t r a t egies , d i f f er ent r es tora t ion

mechanisms are act ivated at the sam e t ime, as a

IEEE Communications Magazine Septem ber 1995

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Diagnostics

LTable

4

ion mechanism when a timer

active restoration mechanism

Using a diagnostics method requires more interaction between

th e escalation mechanism and th e restoration mechanisms, but

can reduce restoration time as compared to the use of a timer,

where it is possible that a restoration mechanism has already

given up

its

effortsbefore th e timer has expired. A diagnostics

method can detect when a mechanism has failed or deduce

tha t a mechanism will not be successful, and ha nd over restoration

control

to

th e next mechanism.

hen

to

escalate

result of a single failure event. When onc mcch-

anism succ eeds in restoring t he failure. all activi-

t i cs ar e s topped . Al though th is wil l achieve the

fastest result, the individual mechanisms must hc

condi t ioned careful ly so as no t to obstruct each

other or conten d for the sam e spare resources .

S eque n t i a l m echan i s m s m ay l ead to l o n g e r

overall restoration times than parallel activation.

but a r e eas i cr t o keep unde r cont rol . I ndividual

mechan isms can then be opt imized without r isk-

ing problems of content ion. A sequential cscal21-

t ion s t r a t egy detcrmines the orde r of ac t ivat ion

of the mechani sms and coordinates between the

mechanisms. Tw o var iables in scque nt ial escala-

t ion ar e the o rde r in which the mcchani sms are

a c t i v a t e d

(Table 3)

a n d t h e c r i t c ri a u s e d t o

decide when to escalate (Table

3 .

Management o f Restoration

ho ugh i t is a goa l

to

dep l oy au t onom ous l y

T

pera t ing survivabi li ty mech anisms, at som e

po i n t t he s c m echan i s m s w i ll i n t e r ac t w i t h t he

network managem ent , or TMN . This may r ange

f rom s imply informing thc T MN of the progress

of

t he r es tora t ion proces s, t o t he ac t ive par t i c i-

pat ion of TM N in the r es tora t ion proces s ( e .g ..

ins t igat ingi terminat ing res torat ion mechanisms

in other layers or par ts of the network. ...).

Th e r es tora t ion management funct ions in t er-

act wi th near ly a l l f ive ma nage ment funct ional

areas def ined in the TMN management concepts

[8],namely, conf igurat ion manage mcnt . per for -

mance management , faul t management . account-

ing manage ment a nd secur ity managem ent . Most

of t hem a re obviously r e l a t ed to thc f aul t man-

agem en t a r ea . H ow eve r , i m por t an t r e s t o ra t i on

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

on e o r m o r e o t he r f unc t i ona l a rea s , e . g ., t hose

funct ions deal ing with escalat ion. I n

the

follow-

ing paragraphs ,

the

ro l e of each funct ional ar ea

in the res torat ion ma nagem ent is dcxcr ibed, and

t he f unc t i ons a r e de f i ned t ha t a r e r equ i r ed i n

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

res toration managem ent system.

Th e f ault management ar ea conta ins

al l

func-

t ions re l a t ed t o the det ect ion an d r epor t ing of

the faul ts , including faul t diagnost ic and rccov-

ery funct ions . T h e alarm survei l lance activity is

an in t r insi c par t of any r es tora t ion n i cchani sni .

A

r e s t o r a t i on a l go r i t hm , e i t he r c en t r a l iz ed o r

d i s t ri bu t ed , needs s o m e au t om a t i c f au l t de t ec -

t ion mechan i sm to t r i gger i t. Then. o nce a f a il -

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

procedure mus t

be

invoked to local izc a nd an a-

lyze the faul t, e.g.. de termi ne whcthcr it is

a

link

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

occurs . Based

o n

this informat ion, thc faul t cor-

r ec t i

o

n fu n

c o

n w a c ti v a te t h e a p p r op r a t e

r es tora t ion mechani sm and cont rol .

i f

required,

any escala t ion proccdurc an d subscqucnt r epai r

act ions . Th e faul t m anagem ent i i rca also includes

tracking the s tatus of the dam aged network c om-

ponent , a nd the s t rategies for switching the n ct-

w or k back t o i t s no r m a l s t a t u s w hcn t hc f au l t

has bccn repaired (norni~ili7ation).

I n t h e c o n f i g u r a t i o n m a n a g e m e n t a r c a . a n

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

of

t h e

res tora t ion mechani sm in thc network. and the

related introduct ion s t rategy (e.g.. which node s

o f a m es hed ne t w or k u i l l

he

chosen f i rs t to b e

protected by a distributed restoration algorithm).

Th e provi s ioning

of

t hc r e s t o r a ti on da t a

t o

al l

conce r ned ne t w or k e l em en t s i s also r e l a t ed to

that funct ion. Another impor t ant conf iguration

funct ion is thc nioni tor ing of

the network s tatus

a n d t h e r e s u l ts

of

t he r e s t o r a t i on p r oces s . i n

o r d e r t o i n f o rm t h e n e t w o r k o p e r a t o r o n t h e

efficiency o f res torat ion, Beside these two main

funct ions , t

h c

norm a iz i

t o

n and t

h e

d ea d ock

processing fun ction s will also imply so me config-

r

a o

n m an agem

e

n s p

c

c fi

c ;I c o

n

s :

they ;ire

t he r e f o r e a l s o i nc l uded

i n

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

management ar ca .

Pcrforniancc managcnient of the rcs torat ion

process should include funct ions to modify any

parameter t hat i nf luences it s performance. wc h

s t imer values used as thresholds in escalat ion

s t r a t egic or de a d ock s i tu a t on d c c c o n . 0 ne

of thesc functions

is

the validation of th e rc5tora-

t ion mechan ism ( i .e. . the background tes t of the

d i s t r i bu t ed r e s t o r a t i on a l go r i t hm o r t he srlf-

heal ing r ing) in o rder t o get an es t ima te

of

t h e

res torat ion performance. Ano ther one is pre

ly the detection

of

I deadlock i n t hc r es tora t ion

proces s. Thi s l ast f unc t ion

is

more specif ic

t o

a

rcs tora t ion a lgor i thm, for which i t i s impor t ant

t o detect the col lapse through precise threshold

value5 ( t imers) . The opt imizat ion of th e network

i s a l so an impor t ant per formancc management

function that will clcan u p the network in

a

post-

restoration phase.

I n

t he a r ea of a ccoun ti ng m anagem en t w e r -

al aspects must be taken into account . When the

rcstoration process is offered as

;I

service to net-

work users. possibly including the subscription

to

ii higher pr ior i ty class , rules must

be

dcf incd

to

determine the corresponding subscr ipt ion rates .

A r e l a ted cha r g i ng adap t a t i on m us t be consi d -

e r ed t o t ake in to account t he impact

of

t he nct -

work faul t on th e user . Th e s i tuat ion in which

it

i is e r w ou l d be cha r ged n i o r c bccaus c t he con -

nect ion is us ing it l onger path af t er r es tora t ion

must be avoided.

Th e r estora tion man agement does not i n t ro-

duce any ncw speci f i c funct ions in the s ecur i ty

managemcnt ar ea . The exi s t ing wcur i ty proce-

du r e s o f o t he r ne t w ork m anagem e n t f unc t ions

c a n

bc

c x t c n d e d f o r t h e r e s t o ra t i o n n i a n a g e -

inen

T h e m a p p i n g

of

r c \ t o r a t i o n m a n a g e m c n t

funct ions wi th the TMN management concept s

provides

R

c lea r pic ture

of

t he specif i c manage-

ment issues that arc addressed within the R A C E

I1 I M MU NE p roject . Tackl ing these i s sues wil l

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H Figure

5.

The threephases of theflood ing algo-

rithm.

lead to practical res torat ion m anage men t appl i -

ca t ions in the two t es tbeds to be s e t up toward

the end of the project .

Restoration Mechanisms for

Meshed and Ring Networks

ar t of the object ives of the IM MU NE project

P s to demonstrate survivability in a laboratory

envi ronment. T o th i s end, two t es tbeds a r e cur -

r ent ly being se t up: a f iv e - n od e m e s h e d A T M

P S N m o d e l a t t h e A l c a t e l r e s e a r c h l a b i n

A n t w e r p , and a b i -d i r ec t iona l A T M r i ng C P N

model a t t he Phi lips r esearch l ab in Aachen. In

Antwerp, a distributed restoration algorithm will

be d em on s t r a t ed ; in A ach en , a r i ng p r o t ec ti on

swi tching mechani sm, based o n a new hardware

concept, will be demonstrated.

For meshed r es tora t ion , cent r a li zed sys t ems

have been implemented as network management

applications. Distributed re storation is still a new

area unde r invest igat ion; so far only s imulat ions

h a v e v a l i d a t e d s e v e r a l a l g o r it h m s . F o r t h e

t e s t bed i n A n t w er p , a d i s t r i bu t ed r e s t o r a t i on

algor ithm has bee n developed

[9]

and val idated

by simulations. This algorithm is currently being

integrated in the control sof tware of a commer-

cial ATM cross-connect . Th e algor i thm is based

on a previously publ ished two-prong algor i thm

[lo]

a n d s o m e e x t e n s io n s h av e b e e n a d d e d t o

also

cover multiple link and node failures, and to

make i t more robust . Figure

5

explains the three

phases of the algorithm.

When a fai lure is detected, nodes adjacent to

the fai lure ( referred to as request source, or

RS,

nodes) broadcast request messages , containing a

s ignature , t he

R S

nodes own ID, t he r eques ted

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

used to distinguish between multiple requests on

t h e s a m e l i nk . A n y i n t e r m e d i a t e n o d e t h a t

r eceives a r eques t message ( t an dem, or T node)

s tores the informat ion of t he message, updates

t h e m e s s a g e c o n t e n t s w h e r e n e c e s s a r y , e .g .,

when t he r eques t ed bandwidth i s not avai lable ,

and broadcasts i t fur ther af ter increment ing the

hop count . Whenever the hop count in a request

m es s age r eaches a p r e s e t m ax i m um va l ue , t he

message is no longer forwarded.

Eventual ly, two branches of th e request t rees

will meet in a tande m nod e, that is now referred

t o a s c o n f ir m n o d e ( C F ) . T h e c o n f i r m n o d e

sends a confirm m es s age t o t he RS nodes w i th

lowes t

ID,

w hi ch i s now a s s i gned t he r o l e o f

c h o o s e r ( C R ) n o d e ; t h e

RS

n o d e w i l l be t h e

chosen (CN ). The conf i rm messages contain the

am oun t o f bandw i d t h t ha t i s ava i l ab l e on t he

newly found route. In general , n ot al l bandwidth

that was on t he f a i l ed l i nk wil l be avai l able o n

on e al ternat ive route, an d th e los t t raff ic will be

r e s t o r ed a l ong s eve r a l o t he r r ou t e s . T hus , t he

chooser nod e wi ll r eceive s evera l conf i rm mes -

sages, indicat ing which ro utes are avai lable, and

how much bandwidth there is on that route. Th e

chooser nod e now sends connect messages along

the al ternat ive routes , containing informat ion for

t h e t a n d e m n o d e s a n d t h e c h o s en n o d e a b o u t

the new configuration.

Wherea s in the core of publ i c networks , t he

D C S - b a s e d m e s h e d n e t w o r k t o p o lo g i e s a r e

promis ing s t ruct ures , f or survivabil i t y mecha-

ni sms in the cu s tomer -or i ented pa r t s of corpo-

r a t e n e t w o r k s , d i s t ri b u t e d A T M - s w i t c h i n g

s ys t em s bas ed on r i ng t opo l ogy have r ecen t l y

been p r opos ed [11].These swi tches ar e a l r eady

based on d is t r ibuted control and , therefore, dis-

t r i bu t ed s u rv i vab i li ty m ech an i s m s a r e l og i ca l

extensions of these control s t ructures . However ,

not much practical experience has been achieved

yet.

Th e second demon s t r a tor wi ll be based on a

dis t r ibuted mult i - r ing ATM switching network,

which is described in detail in another paper [12]

i n t h is i s s ue . T h e r e s t o r a t i o n m e c h a n i s m f o r

AT M mul t i -r ing networks i s based on a unique

AT M swi t ching e l eme nt t hat i s ca l l ed a duplex

ATM transceiver . These duplex t ransceivers are

connected in a bidirect ional r ing us ing UT P. T he

duplex t ransceiver works l ike an A TM add-drop

mult iplexer . This add s the necessary redundancy

i n t o t h e r i n g c o n n e c t i v i ty t o p r o t e c t s e r v i c e s

f r o m r i n g br e a k - d o w n . T h e d u p l e x A T M

transceiver can be prog ramed ei ther as a normal

user acces s nod e or as a br idge to in t er connect

two di f f er ent r i ngs. In th i s projec t a prototype

duplex ATM transceiver is constructed by three

simplex AT M transceivers.

B as ed on

t he dup l ex A T M t r ans ce i ve r con -

cep t , f a s t s el f hea l i ng and du a l hom i ng a l go -

r i thm s a r e deve l oped f o r t he m u l t i - ri ng bas ed

ATM CPNs. The multi-ring restoration system is

then inc orporat ed in to the exi s ting d i s t r ibuted

switch control sof tware. I ts interact ion with the

dis t r ibuted conf igurat ion and resource manage-

m en t f unc t i ons is s t ud i ed , and t h e r e s t o r a t i on

performance is evaluated.

Distributed

restoration is

still a new

area under

investigation;

so

far only

simulations

have

validated

several

algorithms.

IEEE

Communications Magazine Septem ber

1995

69

5/17/2018 00408427

8/8

The

evolution

of

this study

field from

theory to

practical

networks will

inevitably

lead to a

need

for

standards

regarding

suw iva bility.

Conclusions and Targets

of

th e Project

l though th e e f fo r t s in study ing res to ra t ion

A echn iques , eva lua t ion m ethods , e sca la t ion

s t r a t e g ie s a n d m a n a g e m e n t i n t e r a c t i o n f o r m a

s u b st a n ti a l p a r t o f t h e I M M U N E p r o j e c t , t h e

most tang ib le ou tpu t wil l be th e demons t ra t ion

of res to ra t ion mechan isms on the two separa te

laboratory testbeds. Currently, Alcatel Bell and

PKI are work ing on the ha rdware a nd so f tware

configuration and the integration of thc restora-

tion algorithm s. In order to extrapo late the labo-

ratory results to real-life networks, the simul ation

and planning tools described in the third section

wi ll be app l ied . Par t icu la r ly fo r t he d is t r ibu ted

restoration algorithm, extensive simulations will

be requ i red t o va l ida te the resu l t s ob ta ined on

the five-node testbed.

The performance evaluation tools will also be

app l ied to va l ida te the p rop osed s t ra teg ie s fo r

escalation across network layers and parts . The

evolution of this study field from theory to prac-

tical netwo rks will inevitably lead t o a need f or

standards regarding survivability, considering the

multi-operator and m ulti-vendor environment of

today's telecommunication networks. Following

an incentive exerted by the R AC E program, i t is

t h e i n t e n ti o n o f t h e I M M U N E p r o j e ct

to

c o n -

tribute to standardization within the area of net-

work survivability.

References

[ l ]M. Daneshmand et al., Measuring Outages i n Telecom-

munications Switched Networks , /E Commun Mag,

vol. 31, no. 6, June 1993.

121

K

Glossbrenner, Availabil ity and Reliability of Switched

Services, /E Commun. Mag, vol. 31, no. 6, June 1993

131 RACE

II

- IMMUNE, Requirements & Reference Configu-

rations for Survivability, R21Ol/IMMUNE/BT/D&P/DS/P/

002ib0, 31 Aug. 1994.

[4] TU-T Recommendation

G .8 0 3 ,

Architectures of Trans-

port Networks based on the Synchronous Digital Hier-

archy, July 1992.

151

RACE

II -

IMMUNE, Performance repor t on netw ork

restorat ion algorithms, Deliverable D7, Sept. 1995

161 D. Johnson et al., Distributed restoration in telecom-

munications networks, BT Technology Journal, vol. 12,

no.

2,

April

1994

[7] E. Lafuente and

C.

Alcazar, Planning of High Capacity

Transmission Networks wit h Flexibility, Proc. Sixth lnt?

Network Planning Symposium, Budapest, Sept. 1994.

[8] ITU-T Recommendation M.3010. Principles for a

Telecommunications Management Network , Oct. 1992

[9] L. Nederlof, H. Vanderstr aeten, and P. Vankwikelberge,

A New Distributed Restoration Algor ithm t o Protect

ATM Meshed Networks against Link and Node Failures,

Proc.

55 95,

April 24-28, 1995, Berlin, pp. 398-402.

[ l o ] Chow, J . Bicknell, and S McCaughey, A F a s t Dis-

tributed Net work Restoration Algorithm. Proc.

lnt' l

Phoenix Conf. on Computer a nd Communications,

March 22-26. 1993, Tempe, AZ.

[ l l ]

Y

Du, H. J. Reumerman, and

R.

Kraemer, A Distribut-

ed Architect ure for Switched ATMLAN, Proc.

SS 95,

April 24-28, 1995, Berlin, Germany, pp. 484-488.

[12] K. P May et al., Self-Healing ATM Rings for Local

Area Networks, to appear in this issue.

Biographies

LEO

NEDERLOFeceived an M.Sc.E.E. fr om t he Delft Universi-

ty of Technology in the Netherlands in 1992, and joined

the Alcatel Corporate Research Centre in Antwerp , Bel-

gium, in that same year. He has been working on network

survivability and self-healing networks, and contributed to

th e RACE

1022 project and the RACE 2101 project

IMMUNE. He

is

currently working on the implementation

of a distr ibuted restoration algori thm on a laboratory

testbed of 5 ATM cross-connects.

KRISSTRUWE received an MSc. degree in electrical engineer-

ing from the University of Ghent, Belgium, in 1992. In

1993 he joined th e Broadband Communications Networ k

group at the Department of Information Technology at

IMEC/University of Ghent. His activities are focused on

research of distributed restoration techniques for ATM

networks.

CHRIS

O'SHEA raduated from the University of Salford with

a B.Sc. degree in electronic engineering in 1981 and was

subsequent ly employed by BT Research wit hin th e Subma-

rine Optical Systems Division. From 1986 t o 1988 he suc-

cessfully completed a part-time MSc in Telecommunications

at the University of Essex. From 1992 to 1993 he support-

ed the design of a standardized enviro nment fo r element

managers using a UNlX platform. He currently works with-

in the Broadband Multiservice Networks Unit on advanced

network restoration strategies for SDH/ATM technology

development.

HOWARDE W B E R A T HMISSEReceived an M.Sc. in electrical

engineering from the Delft University of Technology (The

Netherlands) in 1990 In 1990 he worked at the same uni-

versity as a researcher in the field of telecommunication

and traffic control systems. From 1991 until the first half

of 1995 he worked at KPN Research, the research depart-

ment of t he K oninklijke PTT Nederland (Royal Dutc h PTT).

During this period he contribut ed to research in the field

of broadband communications and reliability engineering.

He recently joined the Tactical Planning department of the

network division of

PTT

Telecom, The Netherlands.

YONGGANGu received a diploma and Ph.D. in electrical

engineering fr om the Aachen University of Technology in

1985 and 1991, respectively. From 1986 to 1991 he was

involved in lo w bit rate sti l l /motion image coding and

image statistical modeling. In 1992 he joined t he Philips

Research Laboratories i n Aachen. He has been active in the

evaluation of ATM system perf ormance wit hin t he RACE

1022 project, and in the development of distributed ATM

switch architecture for CPN. He

i s

currently responsible for

the survivability research work f or t he Philips distribut ed

ATM switch with in the framework of th e CEC project

IMMUNE.

BRAULIOAMAYO was graduated in 1968 from the Polytech-

nic University of Madri d as an electronic engineer. He

joined ITT Standard Electrica in 1969 and was appointed

project leader of PCM systems. In 1982 he moved to I n

ITC steering center for 5-1 2 switching development pro-

grams, where he was later involved in the development of

defense projects, Currently he i s a member of t he Alcatel

SESA Network Architecture team at the Alcatel Corporate

Research Centre in Madrid.

7

IEEE

Communication\ Magazine September

1995