00408427
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00408427TRANSCRIPT
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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
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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
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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
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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
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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