ESTELLE: A Me hod o Analyze Au oma ically
he Pe o mance o Telecon ol P o ocols
in SCADA Sys ems
Ve ónica Medina, Isabel Gómez, Joaquín Luque, Membe , IEEE, and Se gio Ma ín
Abs ac —This pape p esen s he use o ESTELLE, a o mal
desc ip ion echnique, as a me hod o calcula e au oma ically he
pe o manceo elecon olp o ocolsinSCADAsys ems.Somespe-
ci ic p imi i esa e added o he ESTELLE desc ip ion language in
o de o achie e ha goal. As an example, we analyze he pe o -
mance o a elecon ol p o ocol. The esul s om his me hod a e
compa ed o pe o mance measu emen s ob ained om analy ical
and simula ed solu ions
Index Te ms—Pe o mance analysis, pe o mance modeling,
p o ocol speci ica ion, SCADA sys ems, elecon ol p o ocols,
h oughpu , ansmission delay.
I. INTRODUCTION
IN THE las ew yea s, he ole o powe u ili ies in he wo ld
o elecommunica ions has unde gone apid changes which
a ec no jus echnological aspec s, bu also issues o egula-
ion, access o new ma ke s, he c ea ion o new se ices, e c.
This si ua ion c ea es u gen echnological demands ha ha e o
be sa is ied o be able o keep on compe ing success ully [1]–[9].
One o he mos impo an aspec s o upda ing echnology
is o manage he ope a ion o a powe u ili y ne wo k. Tha is
why se e al elecon ol sys ems ha e been applied o ope a e
on such a ne wo k in a sa e and economical way since he 1960s
[10]–[12].
These elecon ol sys ems a e based on p ocesso a chi ec u e
ins alled h oughou he ha dwa e o he powe u ili y ne wo k.
The p ocesso s a e mainly loca ed in bo h he ene gy gene a ion
plan and he ene gy dis ibu ion and ans o ma ion s a ions.
These plan p ocesso s a e called “ emo es” [ hey a e also called
emo e e minal uni s (RTUs)] and make i possible o wo k on
he powe ne wo k. Remo es communica e wi h one o se e al
cen e s (also called con ol cen e s), sending he ne wo k s a us
in o ma ion and ecei ing commands. Communica ion p o o-
cols, which a e applied o con ol hese powe ne wo ks, should
be s udied ho oughly, because he expense o se ing up such
sys ems can be educed jus by op imizing hem. These p o o-
cols a e called elecon ol p o ocols.
A o mal desc ip ion echnique (FDT) is chosen when a p o-
ocol has o be speci ied in a o mal way. Al hough he e a e
many FDTs, h ee o hem a e he mos popula . These a e ex-
ended s a e ansi ion language (ESTELLE) [13] and speci i-
ca ion and desc ip ion language (SDL) [14], based on ex ended
ini e s a e machines, and language o empo al o de ing spec-
i ica ion (LOTOS) [15], based on p ocess algeb a. All h ee a e
in e na ional s anda ds and he selec ion o one o he o he de-
pends on he speci ic needs o he use o designe . No single
FDT sa is ac o ily ul ills all o he equi emen s.
Se e al s udies [4], [16] ha e shown ha ESTELLE adap s
be e o he elec ical sec o han he o he speci ica ion
languages, since i does no equi e much lea ning o special-
iza ion e o . A ool, called a uni e sal p o ocol con e e o
con e ido uni e sal de p o ocolos (CUP), was designed o
implemen and in eg a e elecon ol p o ocol om i s speci i-
ca ion in ESTELLE [17].
Ou ecen wo k has ocused on adding pe o mance mea-
su emen capabili y o he CUP ool. This way, he pe o mance
o elecon ol p o ocols can hus be analyzed om i s speci ica-
ion in ESTELLE, and an e icien implemen a ion can be made
o hem. To achie e his, some speci ic p imi i es a e added o
he speci ica ion in ESTELLE o a elecon ol p o ocol [18],
[19].
In his pape , we p esen an al e na i e me hod o s udying
he pe o mance o a elecon ol p o ocol wi hou using analy -
ical o simula ed solu ions. In ESTELLE, we speci y he pe -
o mance analysis model o he elecon ol p o ocol (exis ing
o new) including he pe o mance p imi i es. The pe o mance
measu es a e hen au oma ically calcula ed. The pe o mance
analysis model is needed because i is possible o s udy only
one pa (a laye ) o a whole p o ocol.
Ino de o alida e hisme hod,wes udy he same elecon ol
p o ocol analyzed in [20] and [21], whe e he pe o mance was
calcula ed by bo h an analy ical and a simula ed solu ion. We
show all he s eps ha a e necessa y o ob ain he pe o mance
o such a p o ocol au oma ically and he esul s o ha analysis
a e compa ed o hose ob ained in he a o emen ioned wo k.
II. PERFORMANCE ANALYSIS
Pe o manceanalysis(anac i i yincludedin op o ocolengi-
nee ing, i.e., o he se o ac i i ies which, based on some com-
munica ion equi emen s, a e able o gene a e a p o ocol exe-
cu able code in an e icien and eliable way), is used o analyze
ap o ocolino de op edic andop imizei sbeha io .The ea e
di e en echniques o measu e pe o mance ha a e no only
applied op o ocolsbu also osys emsingene al.Mos sys ems
[22] a e s udied by making models o he samesys ems in e ms
o logical and quan i a i e ela ionships. They a e hen manipu-
la ed and changed o see how he model eac s, and hus how he
sys em would eac , ob iously, i he model we e a alid one. I
he model is simple enough, i may be possible o wo k wi h i s
ela ionships and quan i ies o ge an exac , analy ical solu ion.
Howe e , many sys ems a e so complex ha hei alid ma h-
ema ical models a e also e y complex, making i necessa y o
eso o o he s udy echniques, such as simula ion; ha is, nu-
me ically exe cising he model o he inpu s in ques ion o see
how hey a ec he ou pu measu es o pe o mance. Al hough
simula ions a e o en e e ed o as a “me hod o las eso ,”
hey a e in ac almos always he only e ec i e way o measu e
sys em pe o mance. This is due o he shee complexi y o he
sys ems in ol ed and o he models necessa y o ep esen hem
in a alid way.
We p opose ano he solu ion o de e mine he pe o mance
o a elecon ol p o ocol (ou sys ems) by using FDTs. Fi s ,
he elecon ol p o ocol, which could be a new o an al eady
exis ing one, is speci ied in ESTELLE, including he sui able
pe o mance p imi i es. Then, an execu able code is gene a ed,
called he simula ing code, which, when i is un, calcula es he
pe o mance o such a p o ocol. The ad an age o his p oposal
is ha he pe o mance o he elecon ol p o ocol is au oma i-
cally de e mined while i s speci ica ion in ESTELLE is made.
This way, pe o mance can be p edic ed and some pa ame e s
can be se up.
The di e ence be ween a simula ed solu ion and ou s is ha
he simula ed one simula es a model o he p o ocol and we only
simula e op edic pe o manceau oma ically, becauseweac u-
ally speci y in ESTELLE he e y p o ocol (wi h some speci ic
p imi i es o he pu pose o pe o mance analysis). The e o e,
we bo h de e mine he pe o mance and speci y in ESTELLE
he elecon ol p o ocol, and also, he execu able code o such
a p o ocol can au oma ically be gene a ed using he app op ia e
ools.
III. ESTELLE PERFORMANCE MODEL
The pe o mance o a laye (see Fig. 1), in line wi h he
open sys em in e connec ion model (OSI) [23] o a simila
one, is going o be s udied using wo pa ame e s, namely, he
h oughpu and ansmission delay. The h oughpu is de ined
as he numbe o success ully ansmi ed messages pe mean
ansmission ime o a message, and he ansmission delay, is
de ined as he ime in e al, in uni s o he a e age ansmission
ime o a message, om he momen a message is gene a ed o
he ins an i is co ec ly ecei ed.
Fo he pu pose o analysis, he beha io o he and lowe
laye s is ypi ied, om he poin o iew o he laye , as
a i ual channel (see Fig. 2). Tha channel sends a se o
bi s a a a e o and he e is a delayed due o he laye
p opaga ion ime. The h oughpu o he laye is hen de ined
as
(1)
Fig. 1. Laye ed model.
Fig. 2. Lowe laye s model.
Le be he ime in which he laye COMPUTER A(see
Fig. 2) ecei es a message om he laye o be sen .
Fu he mo e, le be he ime in which he COMPUTER B
laye ecei es such a message om he laye . Then,
he ansmission delay is gi en as
(2)
When a elecon ol p o ocol is speci ied in ESTELLE, he e
is no in o ma ion abou which modules belong o he same
laye . The messages a e sen using in e ac ion poin s among
modules (di e en laye s) ha a e connec ed o a ached, bu
he e is no possible knowledge abou he pee en i ies (o
modules) ha a e eally exchanging messages. Thus, some
addi ional in o ma ion has o be added o he speci ica ion in
ESTELLE o ob ain he ansmission delay and he h oughpu
in o de o use he p e ious pe o mance analysis model in he
ESTELLE simula ing model, as de ined in (1) and (2). O he s
pa ame e s, such as he medium, minimum, and maximum
message wai ing ime o a queue in a module, he numbe o
messages ecei ed, e c., a e au oma ically calcula ed wi hou
adding ex a in o ma ion.
Th ee kinds o p imi i es ha e been added o he ESTELLE
speci ica ions, which a e explained in de ail in [18]. The i s
wo a e o he h oughpu , and he hi d one is o he message
delays. These p imi i es a e lis ed as ollows.
1) CUP_PETICION_TRANSMISION: This p imi i e is
used when he laye ecei es a eques om he
laye o send a se o bi s.
2) CUP_TRANSMISION_VALIDA: This p imi i e is
used when he laye sends a se o bi s using he
se ices p o ided by he laye . This se o bi s
consis s o wo pa s— he se o bi s o he laye
and he ex a bi s o he laye .
3) CUP_CATEGORIA:The messages o a laye a e classi-
ied o ha e hei own ansmission delay. This p imi i e
is used o his pu pose.
A p o ocol designe who wan s o measu e he pe o mance
o a elecon ol p o ocol has o wo k in he ollowing way.
a) Fi s , he speci ies he elecon ol p o ocol in ESTELLE,
adding he a o emen ioned p imi i es o his speci ica ion.
The p imi i es a e included in he body de ini ion o a
module a he adequa e ansi ion, depending on he mea-
su emen he designe needs. As he ime is simula ed, he
ESTELLE TIMESCALE clause is used o de e mine i s
uni .
b) Once he designe has speci ied he p o ocol in ESTELLE
wi h he pe o mance p imi i es in he app op ia e place,
he can compile i and make i s execu able code. Some
iles a e sa ed wi h he pe o mance in o ma ion when
he execu able code is execu ed ( he designe can se he
execu ion ime) o analyze esul s.
IV. APPLYING THE MODEL
In o de o alida e ou me hod o calcula ing he pe o -
mance, we s udy he same elecon ol p o ocol analyzed in
[20]. Tha pape p esen ed a me hod o calcula ing he ca-
paci y o a mul ipoin communica ions channel when a polling
p o ocol is used. An exac solu ion, an app oxima e bu easie
o use solu ion, and simula ed solu ion we e ob ained o e-
spond o he ollowing ques ion: “How many emo es (RTUs)
can sha e a link wi hou deg ading sys em pe o mance?” Ob-
iously, he answe would depend upon a ious pa ame e s
such as link eloci y, message leng h, he amoun o in o ma-
ion gene a ed by each RTU, e c. Howe e , i also would de-
pend signi ican ly upon he communica ions p o ocol, called
medium access con ol (MAC) p o ocol, used o communica e
be ween he con ol cen e and he emo es [24]. This p o ocol
akes a ques ion-answe o m in many con ol cen e s, also
Fig. 3. Laye ed medium access p o ocol.
called polling p o ocol. The con ol cen e polls he i s e-
mo e on he link; i he emo e has messages o send, i does
so, and i no , i sends a null message. The con ol cen e goes
on o he poll, he second emo e, and so on, successi ely un il
i has con ac ed all he emo es on he link, a which ime i
s a s o e again wi h he i s RTU. This is he ypical p o-
ocol used in he MAC laye in elecon ol p o ocols.
Le us analyze such a MAC p o ocol (polling p o ocol) o
measu e i s pe o mance using i s speci ica ion in ESTELLE
and compa e esul s. As desc ibed in he p e ious pa ag aph,
he e a e wo kinds o s a ions, namely, a p ima y s a ion (con-
ol cen e ) and one o mo e seconda y s a ions ( emo es). The
seconda y s a ions ha e o be polled by he p ima y s a ion be-
o e acqui ing he channel, ha is o say, he p ima y s a ion
con ols he communica ion channel. Fig. 3 illus a es he p o-
ocol communica ion model. This p o ocol is di ided in o h ee
laye s and he e is an addi ional one o join all he communi-
ca ing en i ies o he pu pose o pe o mance analysis. We a e
only in e es ed in he MAC laye so he uppe and he lowe
laye s a e speci ied o beha e as he eal ones ( he ones used in
he elecon ol p o ocol s ack).
The e a e only se en di e en modules in ESTELLE o
speci y his p o ocol: a p ima y USER module, a p ima y
MAC module, a p ima y PHYSICAL module, a COMMU-
NICATION CHANNEL module o in e connec he p ima y
s a ion o he seconda y s a ions, a seconda y USER module, a
seconda y MAC module, and a seconda y PHYSICAL module.
Each module in e changes in e ac ions only wi h he modules
o which i is connec ed by means o in e ac ion poin s. The
in e ac ion poin s a e de ined in he module heade de ini ion
and i s desc ip ion is associa ed wi h he channel de ini ion.
Each module also needs a body de ini ion o desc ibe i s
beha io . The beha io o a module in ESTELLE is desc ibed
by an ex ended ini e s a e machine; he e is a speci ic syn ax
o do so.
The s a ions exchange ou kinds o messages: 1) e en ;
2) measu emen ; 3) command; and 4) command acknowl-
edgmen . The e a e ou possible ansmissions delayed; one
o each class. The ansmission delayed is measu ed a he
USER laye and he h oughpu is measu ed a he MAC
laye .
Fig. 4. P o ocol e iciency.
Fig. 5. Measu emen ansmission delay.
Once he speci ica ion o he MAC p o ocol1is compiled and
e o ee, he execu able code is gene a ed. A ile is sa ed wi h
he pe o mance in o ma ion when he execu able code is exe-
cu ed; he execu ion ime can also be se up. Sec ion V consis s
o he analysis o esul s.
V. ANALYZING RESULTS
We make he same conjec u e as in [20], i.e., each seconda y
s a ion gene a es messages exponen ially (a Poisson p ocess)
wi h an a e age ime o 4 s, and he p ima y s a ion gene a es
command messages exponen ially wi h an a e age ime o 8 s.
A seconda y s a ion is less likely o gene a e an e en message
han a measu emen message. The swi ching ime is 10 ms. The
leng h o measu emen and e en messages is 380 b and he
leng h o command messages is 60 b. The simula ing ime is 1 h
and he maximum numbe o RTUs (seconda y s a ions) is 28.
The ansmission delay is au oma ically calcula ed using (2)
o each kind o message sen . Equa ion (3) shows he e iciency
( h oughpu ) o he MAC laye , i.e., he bi - a e o he MAC
laye pe ansmission bi - a e (bi - a e o he physical
laye ) only subs i u ing o MAC in o (1). Equa ion (4) shows
, which is ela ed o he o al bi s sen om he USER
laye and he simula ing ime. The e iciency o he MAC laye
is hen au oma ically calcula ed using (5) by only subs i u ing
(4) in o (3). The h oughpu is a numbe be ween one and ze o
bi - a es (3)
o al numbe o bi sen om use laye
simula ing ime (4)
1The speci ica ion in ESTELLE is no shown o b e i y.
Fig. 6. E en ansmission delay.
Fig. 7. Command ansmission delay.
Fig. 8. P o ocol e iciency.
Fig. 9. E en ansmission delay.
o al numbe o bi sen om use laye
simula ing ime bi - a e (5)
Figs. 4–7 compa e ou esul s o e iciency, measu emen
ansmission delay, e en ansmission delay, and command
ansmission delay wi h hose ob ained in [19] by means o a
simula ed solu ion o a bi - a e o 1200 b/s.2The ESTELLE
esul s a e labeled “Es -speed,” being speed he numbe o bi s
sends pe second (b/s). The e iciency esul s in a simula ed so-
lu ion a e he same as in he ESTELLE solu ion o all bi - a es.
Howe e , he e a e di e en esul s o he ansmission delay.
The eason o his beha io is ha messages a e gene a ed
andomly so ha ansmission delay can luc ua e be ween
an e o gap. Fo ins ance, le us ake he ansmission delay
o he command message o 4 RTUs. The e is a di e ence
o 0,2 s be ween ou esul and a simula ed one because he
ansmission delay o he command message always luc ua es
be ween 0,4 s and 0,8 s. This depends on whe he he command
message is sen as soon as i is gene a ed o a e he a i al o
he answe o he p e ious que ying message.
Finally, he h oughpu and e en ansmission delays calcu-
la ed by he ESTELLE solu ion a e compa ed o bo h he ana-
ly ical solu ion (exac and app oxima e) and he simula ed solu-
ion, as shown in Figs. 8 and 9.
VI. CONCLUSIONS
The op imiza ion o elecon ol p o ocols can educe he
ins alla ion cos s o elecon ol sys ems in powe u ili ies.
FDTs a e used o speci y p o ocols, o which ESTELLE
is mo e sui able han o he me hods in he elec ical sec o .
P imi i es added o ESTELLE make i possible o measu e he
pe o mance o such p o ocols; he e o e, elecon ol p o ocols
can be imp o ed. These imp o emen s could simply consis
o se ing up some new pa ame e s o an exis ing elecon ol
p o ocol o eplacing hem wi h be e ones ( o ins ance,
s anda dized ones).
In his pape , his al e na i e me hod o s udying he pe o -
mance o a elecon ol p o ocol has been p esen ed. Fu he -
mo e, in o de o alida e i , we ha e s udied success ully he
same elecon ol p o ocol analyzed in [20] and [21], whe e he
pe o mance was calcula ed by bo h an analy ical and a simu-
la ed solu ion.
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