hsn - unit 2 question answers -
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VII Sem ECE CS2060 High Speed Networks
DHANALAKSHMI SRINIVASAN COLLEGE OF ENGINEERING
(An ISO 9001:2008 Certified Institution)
Coimbatore-641 1!
"#E#$"#%e&'# DEGREE EAMINA%ION
VII Semeter
E*e&tro+i& a+, Comm+i&atio+ E+.i+eeri+.
CS/6 0 HIGH SEED NE%2ORKS
QUESTION & ANSWER BANK
/13-14
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+it-/
AR% A (/ Mar5)
1# State t'e 5e &'ara&teriti& to be &o+i,ere, 7or ,eri8i+. t'e a+a*ti& e9atio+ 7or t'e
9ei+. mo,e*# No8$De& /11
Key characteristics for deriving the analytic e!ations for the !e!ing "odel are#
Ite" $o$!lation
Q!e!e si%e
is$atching disci$line
/# 2'at i mea+t b &'o5e :a&5et; No8$De& /11
A cho'e $ac'et is a control $ac'et generated at a congested node and trans"itted (ac' to aso!rce node to restrict traffic flo)*
E+a"$le# Internet ,ontrol -essage .rotocol So!rce Q!ench $ac'et*
3# 2'at are t'e a,8a+ta.e o7 :a&5et o8er &ir&it :*ai+ t'e :arameter 7or a i+.*e er8er 9ee#No8$De& /1/
arameter E>:*a+atio+
Arrival Rate Ite"s arriving $er second
Waiting ti"e A certain n!"(er of ite"s )ill (e )aiting in the )aiting line
Ti"e interval (et)een the dis$atching of an ite" to the
Service ti"e server and the de$art!re of that ite" fro" the server*
Utili%ation 2raction of ti"e that the server is (!sy*
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arameter E>:*a+atio+
The average no of ite"s resident in the syste"0 incl!ding the
Ite"s resident in !e!ing syste" ite" (eing served & the ite"s )aiting
Residence ti"e The average ti"e that an ite" s$ends in the syste"*
6# 2'at i mea+t b "ECN; No8$De& /1/
Bac')ard E+$licit ,ongestion Notification 3BE,N4 notifies the !ser that
congestion avoidance $roced!res sho!ld (e initiated )here traffic in the o$$osite
direction of the received fra"e*
It indicates that the fra"es that the !ser trans"its on this logical connection "ay
enco!nter congested reso!rces*
?# 2'at i mea+t b t'e term @Co+.etio+ i+ +et
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11# 2'at i t'e ,ra
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AR% " (16 Mar5)
1# (a) De&ribe t'e i+.*e er8er 9ei+. mo,e* :*ai+ Si+.*e Ser8er ei+. mo,e* i+ ,etai*# (1 Mar5) Ma$=+e /13
The si"$lest !e!ing syste" is de$icted in a(ove 2ig!re* The central ele"ent of the syste"
is a server0 )hich $rovides so"e service to ite"s* Ite"s fro" so"e $o$!lation of ite"s
arrive at the syste" to (e served* If the server is idle0 an ite" is served i""ediately*
Other)ise0 an arriving ite" 6oins a )aiting line7* When the server has co"$leted serving
an ite"0 the ite" de$arts*
If there are ite"s )aiting in the !e!e0 one is i""ediately dis$atched to the server* The
server in this "odel can re$resent anything that $erfor"s so"e f!nction or service for a
collection of ite"s*
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E+a"$les# a $rocessor $rovides service to $rocesses8 a trans"ission line $rovides a trans"ission service to$ac'ets or fra"es of data8 an I9O device $rovides a read or )rite service for I9O re!ests*
ee arameter
The a(ove 2ig!re also ill!strates so"e i"$ortant $ara"eters associated )ith a !e!ing
"odel* Ite"s arrive at the facility at so"e average rate 3ite"s arriving $er second4* At any
given ti"e0 a certain n!"(er of ite"s )ill (e )aiting in the !e!e 3%ero or "ore48 the
average n!"(er )aiting is w0 and the "ean ti"e that an ite" "!st )ait is Tw*
Tw is averaged over all inco"ing ite"s0 incl!ding those that do not )ait at all* The server
handles inco"ing ite"s )ith an average service ti"e Ts8 this is the ti"e interval (et)een
the dis$atching of an ite" to the server and the de$art!re of that ite" fro" the server*
Utili%ation is the fraction of ti"e that the server is (!sy0 "eas!red over so"e interval of
ti"e*
2inally0 t)o $ara"eters a$$ly to the syste" as a )hole* The average n!"(er of ite"s
resident in the syste"0 incl!ding the ite" (eing served 3if any4 and the ite"s )aiting 3if
any40 is r8 and the average ti"e that an ite" s$ends in the syste"0 )aiting and (eing served0
is Tr8 )e refer to this as the "ean residence ti"e* If )e ass!"e that the ca$acity of the
!e!e is infinite0 then no ite"s are ever lost fro" the syste"8 they are 6!st delayed !ntil
they can (e served* Under these circ!"stances0 the de$art!re rate e!als the arrival rate* As
the arrival rate0 )hich is the rate of traffic $assing thro!gh the syste"0 Th!s0 the theoretical
"a+i"!" in$!t rate that can (e handled (y the syste" is#
To $roceed0 )e need to "a'e so"e ass!"$tion a(o!t this "odel#
Item :o:*atio+ Ty$ically0 )e ass!"e an infinite $o$!lation* This "eans that the arrival
rate is not altered (y the loss of $o$!lation* If the $o$!lation is finite0 then the $o$!lation
availa(le for arrival is red!ced (y the n!"(er of ite"s c!rrently in the syste"8 this )o!ld
ty$ically red!ce the arrival rate $ro$ortionally*
ee ie Ty$ically0 )e ass!"e an infinite !e!e si%e* Th!s0 the )aiting line can gro)
)itho!t (o!nd* With a finite !e!e0 it is $ossi(le for ite"s to (e lost fro" the syste"* In
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$ractice0 any !e!e is finite* In "any cases0 this )ill "a'e no s!(stantive difference to the analysis* Weaddress this iss!e (riefly0 (elo)*
Di:at&'i+. ,i&i:*i+e When the server (eco"es free0 and if there is "ore than one ite"
)aiting0 a decision "!st (e "ade as to )hich ite" to dis$atch ne+t* The si"$lest a$$roach
is first1in0 first1o!t8 this disci$line is )hat is nor"ally i"$lied )hen the ter" queueis !sed*
Another $ossi(ility is last1in0 first1o!t* One that yo! "ight enco!nter in $ractice is a
dis$atching disci$line (ased on service ti"e* 2or e+a"$le0 a $ac'et1s)itching node "ay
choose to dis$atch $ac'ets on the (asis of shortest first 3to generate the "ost o!tgoing
$ac'ets4 or longest first 3to "ini"i%e $rocessing ti"e relative to trans"ission ti"e4*
Unfort!nately0 a disci$line (ased on service ti"e is very diffic!lt to "odel analytically*
1# (b)E>:*ai+ t'e Ke+,a** +otatio+ a+, t'e &ommo+ ,itribtio+ 7or t'e 9ei+. mo,e*#
(6 Mar5) No8$De& /11
Ke+,a** Notatio+
Kendall $ro$osed a set of notations for !e!ing "odels* This is )idely !sed in literat!re*
The co""on $atter of notations of a !e!ing "odel is given (y#
3a9(9c4 # 3d9e4
)here#
a # .ro(a(ility distri(!tion of the inter1arrival ti"e
( # .ro(a(ility distri(!tion of the service ti"e
c # N!"(er of servers in the !e!ing "odel
d # -a+i"!" allo)ed c!sto"ers in the syste"
e # Q!e!e disci$line*
Commo+ ,itribtio+ 7or t'e 9ei+. mo,e*
Smbo* Name De&ri:tio+
- -ar'ovian E+$onential service ti"e
egenerate distri(!tion A deter"inistic or fi+ed service ti"e
E' Erlang distri(!tion An Erlang distri(!tion )ith ' as the sha$e $ara"eter
: :eneral distri(!tion It refers to inde$endent service ti"e
.; .hase1ty$e distri(!tion So"e of the a(ove distri(!tions are s$ecial cases of the $hase1ty$e0
often !sed in $lace of a general distri(!tion
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/# (a) E>:*ai+ i+ ,etai* t'e e>:*i&it a+, im:*i&it &o+.etio+ i.+a*i+.# (B Mar5) No8$De& /11
E>:*i&it Co+.etio+ Si.+a*i+.
E+$licit congestion control techni!es o$erate over connection oriented net)or's and
control the flo) of $ac'ets over individ!al connections* E+$licit congestion signaling
a$$roaches can )or' in one of t)o directions
"a&5
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Im:*i&it Co+.etio+ Si.+a*i+.
I"$licit signaling is an effective congestion control techni!e in connectionless or
datagra" config!rations s!ch as I.1(ased internets* In s!ch cases there are no logical
connections thro!gh the internet on )hich flo) can (e reg!lated*
;o)ever0 (et)een the t)o end syste"s0 logical connections are esta(lished at the T,.
level* T,. incl!des "echanis"s for ac'no)ledging recei$t of T,. seg"ents and for
reg!lating the flo) of data (et)een so!rce and destination on a T,. connection* T,.
congestion control techni!es (ased on the a(ility to detect increased delay and seg"ent
loss*
I"$licit signaling can also (e !sed in connection1oriented net)or's* 2or e+a"$le0 in fra"e
relay net)or's* The /A.2 control $rotocol incl!des facilities si"ilar to those of T,. for
flo) and error control* /.A2 control is ca$a(le of detecting lost fra"es and ad6!sting the
flo) of data accordingly*
/# (b) Lit a+, e>:*ai+ t'e 7rame re*a &o+.etio+ &o+tro* te&'+i9e# (B Mar5) No8$De& /11
Or
3# (a) E>:*ai+ i+ ,etai* abot 7rame re*a &o+.etio+ &o+tro* te&'+i9e# (B Mar5)
Ma$=+e /1/ Mo,e*-/
Frame Re*a Co+.etio+ Co+tro* %e&'+i9e
%e&'+i9e %:e F+&tio+ Ke e*eme+t
iscard control iscard .rovides g!idance to net)or'
strategy concerning )hich fra"es to discard E (it
Bac')ard e+$licit ,ongestion .rovides g!idance to end syste"s
congestion avoidance a(o!t congestion in net)or' BE,N (it
notification
2or)ard e+$licit ,ongestion .rovides g!idance to end syste"s
congestion avoidance a(o!t congestion in net)or' 2E,N (it
notification
I"$licit congestion ,ongestion End syste" infers congestion fro" Se!ence n!"(ers in
notification recovery fra"e loss higher1layer .U
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The a(ove ta(le lists the congestion control techni!es defined in vario!s ANSI doc!"ents*
iscard Strategy deals )ith the "ost f!nda"ental res$onse to congestion* When congestion
(eco"es severe eno!gh0 the net)or' is forced to discard fra"es*
,ongestion avoidance $roced!res are !sed at the onset of congestion to "ini"i%e the effect
on the net)or'* Th!s0 there "!st (e so"e e+$licit signaling "echanis" fro" the net)or'
that )ill trigger the congestion avoidance*
,ongestion recovery $roced!res are !sed to $revent net)or' colla$se in the face of severe
congestion* These $roced!res are ty$ically initiated )hen the net)or' has (eg!n to dro$
fra"es d!e to congestion* S!ch dro$$ed fra"es )ill (e re$orted (y so"e higher layer of
soft)are and severe as an i"$licit "echanis"*
O(6ectives of fra"e relay congestion control are# -ini"i%e fra"e discard
-aintain0 )ith high $ro(a(ility and "ini"!" variance0 an agreed !ality ofservice*
-ini"i%e the $ossi(ility that one end !ser can "ono$oli%e net)or'reso!rces at the e+$ense of other end !sers*
Be si"$le to i"$le"ent0 and $lace little overhead on either end !ser ornet)or'
3# (b) E>:*ai+ abot tra77i& ma+a.eme+t i+ :a&5et
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If all of the !e!e (!ffers are of e!al length0 then the !e!es )ith the highest traffic load
)ill s!ffer discards "ore often0 allo)ing lo)er1traffic connections a fair share of the
ca$acity*
a*it o7 Ser8i&e
ifferent traffic flo)s have different $riorities8 for e+a"$le0 net)or' "anage"ent traffic0
$artic!larly d!ring ti"es of congestion or fail!re0 is "ore i"$ortant than a$$lication traffic*
It is $artic!larly i"$ortant d!ring $eriods of congestion that traffic flo)s )ith different
re!ire"ents (e treated differently and $rovided a different !ality of service 3QoS4*
2or e+a"$le0 a node "ight trans"it higher1$riority $ac'ets ahead of lo)er1$riority $ac'ets
in the sa"e !e!e* Or a node "ight "aintain different !e!es for different QoS levels and
give $referential treat"ent to the higher levels*
Reer8atio+
On the )ay to avoid congestion and also to $rovide ass!red service to a$$lication is to !se
a reservation sche"e* S!ch a sche"e is an integral $art of AT- net)or's* When a logical
connection is esta(lished0 the net)or' and the !ser enter into a traffic contract0 )hich
s$ecifies a data rate and other characteristics of the traffic flo)*
The net)or' agrees to give a defined QoS so long as the traffic flo) is )ithin contract
$ara"eters8 e+cess traffic is either discarded or handled on a (est1effort (asis0 s!(6ect to
discard*
If the c!rrent o!tstanding reservations are s!ch that the net)or' reso!rces are inade!ate to
"eet the ne) reservation0 then the ne) reservation is denied*
One as$ect of a reservation sche"e is traffic $olicing* A node in the net)or'0 the node to
)hich the end syste" attaches0 "onitors the traffic flo) and co"$ares it to the traffic
contract* E+cess traffic is either discarded or "ar'ed to indicate that it is lia(le to discard or
delay*
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4# (b) De&ribe abot e77e&t o7 &o+.etio+# (B Mar5) Ma$=+e /1/ Mo,e*-/
Or
?# (b) Di& brie7* t'e e77e&t o7 &o+.etio+ i+ +et
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The de$art!re of thro!gh$!t fro" the ideal is acco!nted for (y a n!"(er of factors* The
load is !nli'ely to (e !nifor"ly s$read thro!gho!t the net)or'* Therefore0 )hile so"e
nodes "ay e+$erience "oderate congestion0 others "ay (e e+$eriencing severe congestion
and "ay need to discard traffic*
In addition0 as the load increases0 the net)or' )ill atte"$t to (alance the load (y ro!ting
$ac'ets thro!gh areas of lo)er congestion*
2or the ro!ting f!nction to )or'0 an increased n!"(er of ro!ting "essages "!st (e
e+changed (et)een nodes to alert each other to areas of congestion8 this overhead red!ces
the ca$acity availa(le for data $ac'ets*
As the load on the net)or' contin!es to increase0 the !e!e lengths of the vario!s nodes
contin!e to gro)* Event!ally0 a $oint is reached (eyond )hich thro!gh$!t act!ally dro$s
)ith increased offered load*
The reason for this is that the (!ffers at each node are of finite si%e* When the (!ffers at a
node (eco"e f!ll8 it "!st discard $ac'ets* So as "ore and "ore $ac'ets are retrans"itted0
the load on the syste" gro)s0 and "ore (!ffers (eco"e sat!rated*
While the syste" is trying des$erately to clear the (ac'log0 end syste"s are $!"$ing old
and ne) $ac'ets into the syste"* Even s!ccessf!lly delivered $ac'ets "ay (e retrans"itted
(eca!se it ta'e too long0 at a higher layer to ac'no)ledge the"* Under these circ!"stances0
the effective ca$acity of the syste" is virt!ally %ero*
!# (a) E>:*ai+ i+ ,etai* t'e 7o**o:*i&it &o+.etio+ i.+a*i+.# (4 Mar5)
(1) "a&5 :rere
Bac'$ress!re can (e selectively a$$lied to logical connections0 so that the flo) fro"one node to the ne+t is only restricted or halted on so"e connections0 generally the ones)ith the "ost traffic* In this case0 the restriction $ro$agates (ac' along the connectionto the so!rce*
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Bac'$ress!re is of li"ited !tility* It can (e !sed in a connection1oriented net)or' that
allo)s ho$1(y1ho$ flo) control*
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Bac')ard infor"ation is trans"itted either (y altering (its in a data $ac'et headed for the
so!rce to (e controlled or (y trans"itting se$arate control $ac'ets to the so!rce*
For
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Some ta+,ar, +otatio+ 7or ,itribtio+ i
- for a -ar'ovian 3e+$onential4 distri(!tion
E> for an Erlang distri(!tion )ith > $hases
for eter"inistic 3constant4
: for :eneral distri(!tion
.; for a .hase1ty$e distri(!tion
6# (b) E>:*ai+ abot tra77i& rate ma+a.eme+t i+ 7rame re*a +et
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CIR Access Rate6
Where0
,IRi06is the co""itted infor"ation rate for connection i on channel 6
Access Rate6is a data rate of !ser access channel 6*
The ,IR (y itself does not $rovide "!ch fle+i(ility in dealing )ith traffic rates* So
t)o additional $ara"eters assigned on $er"anent connections and negotiated on
s)itched connections0 are needed#
Committe, brt ie ("&)
The "a+i"!" a"o!nt data that the net)or' agrees to transfer0 !nder nor"al conditions0
over a "eas!re"ent interval T* These data "ay not (e contig!o!s that is they "ay a$$earin one fra"e or in several fra"es*
E>&e brt ie ("e)
The "a+i"!" a"o!nt of data in e+cess of Bcthat the net)or' )ill atte"$t to transfer0
!nder nor"al conditions0 over a "eas!re"ent interval T* These data are !nco""itted in
the sense that the net)or' does not co""it to delivery !nder nor"al conditions* .!t
another )ay0 the data that re$resent Beare delivered )ith lo)er $ro(a(ility than the
data )ithin Bc*
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B# 2rite +ote o+ &o+.etio+ &o+tro* te&'+i9e e, i+
(i) a&5et S
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A,8a+ta.e o8er &ir&it
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I"$le"ented (y end syste"(!t not net)or'
/A.2 control
/A.2 core
I"$le"ented (y end syste" .hysical
/ayerand net)or'
2ra"e relay involves the $hysical layer and a data lin' control $rotocol 'no)n as /.A2
3/in' Access .roced!re for 2ra"e -ode Bearer Services4* There are t)o versions of /A.2
defined* All fra"e relay net)or's involve the i"$le"entation of the /A.2 core $rotocol
on all s!(scri(er syste"s and on all fra"e relay nodes* /A.2 core $rovides a "ini"al set
of data lin' control f!nctions0 consisting of the follo)ing#
2ra"e deli"iting0 align"ent and trans$arency
2ra"e "!lti$le+ing9de"!lti$le+ing !sing the address field
Ins$ection of the fra"e to ens!re that it consists of an integer n!"(er ofoctets $rior to %ero (it insertion or follo)ing %ero (it e+traction
Ins$ection of the fra"e to ens!re that it is either too long or too short
etection of trans"ission errors ,ongestion control f!nctions
A(ove this0 the !ser "ay choose to select additional data lin' or net)or'1layer end1to1end
f!nctions* One $ossi(ility is 'no)n as the /A.2 control $rotocol* /A.2 control is not $art
of the fra"e relay service (!t "ay (e i"$le"ented only in the end syste"s to $rovide flo)
and error control*
The fra"e relay service !sing /A.2 core has the follo)ing $ro$erties for the trans"ission
of data*
.reservation of the order of fra"e transfer fro" one edge to the net)or' to the other A s"all $ro(a(ility of fra"e loss
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# (a) E>:*ai+ t'e +ee, 7or ei+. A+a*i (1 Mar5) Mo,e*-1
In !e!eing theory0 a !e!eing "odel is !sed to a$$ro+i"ate a real !e!eing sit!ation or
syste"0 so the !e!eing (ehavio!r can (e analysed "athe"atically* Q!e!eing "odels allo)a n!"(er of !sef!l steady state $erfor"ance "eas!res to (e deter"ined0 incl!ding#
The average n!"(er in the !e!e0 or the syste"0 the average ti"e s$ent in the !e!e0 or the
syste"0 the statistical distri(!tion of those n!"(ers or ti"es0 the $ro(a(ility the !e!e is
f!ll0 or e"$ty0 and the $ro(a(ility of finding the syste" in a $artic!lar state*
These $erfor"ance "eas!res are i"$ortant as iss!es or $ro(le"s ca!sed (y !e!eing
sit!ations are often related to c!sto"er dissatisfaction )ith service or "ay (e the root ca!se
of econo"ic losses in a (!siness* Analysis of the relevant !e!eing "odels allo)s the
ca!se of !e!eing iss!es to (e identified and the i"$act of any changes that "ight (e
)anted to (e assessed*
Notatio+
Q!e!eing "odels can (e re$resented !sing KendallCs notation#
A9B9S9K9N9isc
)here#
A is the interarrival ti"e distri(!tion
B is the service ti"e distri(!tionS is the n!"(er of servers
K is the syste" ca$acity
N is the calling $o$!lation
isc is the service disci$line ass!"ed
Some ta+,ar, +otatio+ 7or ,itribtio+ (A or ") i
- for a -ar'ovian 3e+$onential4 distri(!tion
E> for an Erlang distri(!tion )ith > $hases
for eter"inistic 3constant4
: for :eneral distri(!tion
.; for a .hase1ty$e distri(!tion
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Co+tr&tio+ a+, a+a*i
Q!e!eing "odels are generally constr!cted to re$resent the steady state of a !e!eing
syste"0 that is0 the ty$ical0 long r!n or average state of the syste"*
As a conse!ence0 these are stochastic "odels that re$resent the $ro(a(ility that a !e!eing
syste" )ill (e fo!nd in a $artic!lar config!ration or state*
A general $roced!re for constr!cting and analy%ing s!ch !e!ing "odels is#
Identify the $ara"eters of the syste"0 s!ch as the arrival rate0 service ti"e0 Q!e!eca$acity0 and $erha$s dra) a diagra" of the syste"*
Identify the syste" states* 3A state )ill generally re$resent the integer n!"(er ofc!sto"ers0 $eo$le0 6o(s0 calls0 "essages0 etc* in the syste" and "ay or "ay not (e
li"ited*4
ra) a state transition diagra" that re$resents the $ossi(le syste" states andidentify the rates to enter and leave each state* This diagra" is a re$resentation of a
-ar'ov chain*
Beca!se the state transition diagra" re$resents the steady state sit!ation (et)eenstates there is a (alanced flo) (et)een states so the $ro(a(ilities of (eing in
ad6acent states can (e related "athe"atically in ter"s of the arrival and service
rates and state $ro(a(ilities*
E+$ress all the state $ro(a(ilities in ter"s of the e"$ty state $ro(a(ility0 !sing theinter1state transition relationshi$s*
eter"ine the e"$ty state $ro(a(ility (y !sing the fact that all state $ro(a(ilitiesal)ays s!" to ?*
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# (b) E>:*ai+ M*tier8er ee i+ ,etai* (6 mar5) Mo,e*-1
The a(ove fig!re sho)s a generali%ation of the si"$le "odel )e have (een disc!ssing for
"!lti$le servers0 all sharing a co""on !e!e* If an ite" arrives and at least one server is
availa(le0 then the ite" is i""ediately dis$atched to that server*
It is ass!"ed that all servers are identical8 th!s0 if "ore than one server is availa(le0 it
"a'es no difference )hich server is chosen for the ite"*
If all servers are (!sy0 a !e!e (egins to for"* As soon as one server (eco"es free0 an ite"
is dis$atched fro" the !e!e !sing the dis$atching disci$line in force*
If )e haveNidentical servers0 then r is the !tili%ation of each server0 and )e can consider
Nr to (e the !tili%ation of the entire syste"8 this latter ter" is often referred to as the traffic
intensity0 u*
Th!s0 the theoretical "a+i"!" !tili%ation isND ?@@0 and the theoretical "a+i"!" in$!t
rate is#
The 'ey characteristics ty$ically chosen for the "!ltiserver !e!e corres$ond to those for thesingle1server !e!e* That is0 )e ass!"e an infinite $o$!lation and an infinite !e!e si%e0 )ith asingle infinite !e!e shared a"ong all servers* Unless other)ise stated0 the dis$atching disci$lineis 2I2O*
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1/# (a) (i) E>:*ai+
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9ee# (B)
(ii) E>:*ai+ i+ ,etai* abot t'e =a&5o+ t'eorem# (B)
(Or)
(b) (i) E>:*ai+ t'e e77e&t o7 &o+.etio+ i+ :a&5et