Switching NetworksSingle Stage NetworkThe fig shows M inlets and
N outlets,consisting of matrix of crosspoints ,these may be
separate relays or electronics devices or contacts of
crosspoints.This switch gives full availability , no call lost
unless outgoing trunks are congested.The system contains M x N
crosspoints . If MN then number of crosspoints is !"#N$The cost
increased as the s%uare of the si&e of the switch .The
efficiency 'N(N$I(N ) decreases inversely with N. It is
uneconomical to use single stage network for large number of inlets
and outlets.132546879101234 5678 9 10M inletsN outletsCrossbar
Switch (also called Crosspoint switch)Switching NetworksSingle
Stage Network*or example a switch with #++ inlets and #++ outlets
re%uires #+, +++ crosspoints and only #, of these can be used at
any time.*or making connections between large number of trunks are
constructed as network containing several stages of
switches.Switching NetworksTwo Stage NetworknnnnN inletsN outletsIf
the two stage network shown above has N incoming and N outgoing
trunks and contained primary switches having n inlets and secondary
switches having n outlets , then number of primary switches gnumber
of secondary switchesnumber of outlets per primary switchnumber of
inlets per secondary switch.n x gn x g g 2n x gn x gTwo Stage
Switching NetworksgN(nThe No. of crosspoints per primary switchNo.
of crosspoints per secondary switchg nN The total No. of
crosspoints "-in the network No. of switches x crosspoints per
switch . "- - g n-N$(n'#)Since there is one link from each primary
switch to each secondary switch , the No. of links is e%ual to No.
of primary switch x No. of secondary switches g$'N(n )$'-)The No.
of crosspoints varies as #(n and No. of links varies as #(n$ , if n
is made large to reduce the No. of crosspoints . There will be few
links to carry traffic ..et the No. of links be e%ual to No. of
incoming and outgoing trunks then !g-N,substituting in e%uation '-)
NN-(n-n-N-(Nn / N '0)Two Stage Switching NetworksThe total No. of
crosspoints from e%uation'#) is !"--gn-N$(nputting value of n
N"--N$( N "-- ' N )0(-'1)Select one of nearest integer to n that is
factor of N. "rossbar switch may be of si&e #+ x #+ or #+ x -+
.It is economic to use the network with more than two stages.Two
Stage Switching Networks2xample ! 3esign a two stage network for
connecting -++ incoming trunks to -++ outgoing trunks..et
n/-++#1.#1 ,however n must be factor of -++, so nearest possible
values aren#+ and n-+ . Two possible networks are shown
below!nnnn20x2010x1010x10 20x20-+ switches #+ switches200 incoming
trunks200 outgoing trunksFig (a)Two Stage Switching
Networks20x2010x1020x20 10x10#+ switchesTwo possible networks are
shown above , each containing 4+++ trunks . The network of 'a)
suitable for -+ outgoing routes each having #+ trunks and fig 'b)
is suitable for #+ outgoing routes each having -+ trunks.10x10-+
switchesnnnn200 incoming trunks200 outgoing trunksFig
(b)20x2020x2010x10-+ switchesnnnn200 incoming trunks200 outgoing
trunksFig (b)20x2020x2010x10-+ switchesnnnn200 incoming trunks200
outgoing trunksFig (b)20x2020x2010x1010 x10Two Stage Switching
NetworksThe fig 'a) have same No. of outgoing and incoming trunks.
5owever concentrator have more incoming trunks than outgoing trunks
and expander have more outgoing trunks than incoming trunks
."onsider the concentrator with M incoming trunks and N outgoing
trunks'M 6N) , let primary switches have m inlets and each
secondary switch has n outlets then.No. of primary switchesM(mNo.
of secondary switchesN(nNo. of crosspoints per primary switchm N(n
No. of crosspoints per secondary switchn M(mTotal No. of
crosspoints are! "-M(m x mN(n7 N(n x n M(m"-MN 8 #(n 7 #(m 9 '#)Two
Stage Switching NetworksNo. of linksNo. of primary switchesx No. of
secondary switchesMN( mn. Since traffic capacity is limited by No.
of outgoing trunks , there is not feasible to provide more than
this number of links . So No. of links be N .MN(mnN
nM(m'-)substitutingvalue of n from e%uation '-) to e%uation '#)"-
MN8#(M(m 7 #(m9MN8 m (M 7 #(m9'0)In order to minimi&e "- ,
treat m as if were a continuous variable and
differentiatingw.r.t.mdc-(dmMN 8I(M : #(m-9 + , when m/M*rom
e%uation '-)mn/MTwo Stage Switching Networks ;hen m/M 2%uation '-)
mn/M '1) The No. of crosspoints are minimum if No. of inlets per
primary switch No. of outlets per secondary switch , from e%uation
'0)"-MN 8#(/M7 #( / M 9 MN 8#7 # 9
"-- 'M) < N '=)m and n are integer and factor of M and N
respectively. If M 6 Ne%uation '1) gives larger and fewer secondary
switches thanif n/N were chosen . To obtained an expander , M is
exchanged with N and m with n./ M Three Stage Switching NetworksA
LinksB Linksg2 = Sec. Si!c"esg1 =#$n%&i'a&(
Si!c"esnnnnnxg2nxg2#$n x #$ng2x ng2 x n#$n x #$nN inletsN outletsg3
= #$n )e&!ia&( si!c"es*ig shows three stage switchThree
Stage Switching NetworksThere is one link from each primary switch
to each secondary switch and one link from each secondary switch to
each tertiary switch > connectionfrom given inlet on primary
switch to a selected outlet tertiary switch may be made via
secondary switch , unless its link to primary or it link to
secondary switch is busy . If the three stage network has N
incoming trunks and N outgoing trunks and primary switch with n
inlets and secondary switch with n outlets.The No. of primary
switches 'g#)No. of tertiary switches'g0)N(n.The primary switches
have N(n inlets and outlets, if the No. of primary ?secondary links
'> links) and secondary ?tertiary links '@ links )are each N ,
then No. of secondary switches is !g-N A N(nn No. of outlets per
primary switch No. of inlets per tertiary switch.No. of crosspoints
in primary stagen$ N(nNn.No. of crosspoints in secondary stagen '
N(n)$N$( n.No. of crosspoints in tertiary stagen$ N(nNn. The total
No. of crosspoints are !"0N'-n 7 N(n) '#)Three Stage Switching
Networks3ifferentiating e%uation '#) w.r.t. n and e%uating to
&ero. No. of crosspoints are minimum whenn/N(- '-)"0- /- ' N )
3/2C3 =2 X C2C3 = (2) 3/2 (N) - C1 (3) If a three stage
concentrator has M incoming trunks an N outgoing trunks (M ! N) "
#he $rimar% s&itches ha'e m in(ets an tertiar% s&itches
ha'e n out(ets then )No" of $rimar% s&itches =M/m No" of
tertiar% s&itches =N/nIf there are g2 seconar% s&itches
Cross$oints $er $rimar% s&itch = mg2Cross$oints $er seconar%
s&itch = M/m * N/nCross$oints $er tertiar% s&itch =
N/nThree Stage Switching NetworksThe total No. of crosspoints is
!"0M(m x mg- 7 M(m x N(n x g-7 N(n x ng-."0Mg- 7 MN(mn g- 7
Ng-"0g-8M 7 N 7 MN(mn9 '1)Since M6N , No. of > .inksNo. of @
.inksNg- M(mg- N(n 5ence g-n and mn M(N Substituting e%uation '1)
"0'M 7 N ) n 7 N-(n 3ifferentiating w.r.t. n to find minimum mM( /M
7 N, nN( /M 7 N"0-N/N 7 M '=)To obtain an expander M exchanged with
N and m with n.*+''+n *+n!&+, S(s!e'Brinciple of "ommon
"ontrol"ommon control system was first introduced in crossbar
exchanges. The common control can be traced in director system
facilitate the uniform numbering of subscribers in multi:exchange
area.Cniform numbering is that to call a particular subscriber ,the
same number is dialed ,no matter from which exchange the call is
originated."onsider multi:exchange network shown in fig 0.# , it is
not fully connected network . If a subscriber in exchange >
wants to call a subscriber * , the call is routed at least three
exchanges. Two routes are possible > ? @ ? " ? D : * and > ?
I ? 5 ? E : **+''+n *+n!&+, S(s!e'*-. F/01BA0*-. F/21*ig
0.#Multi:exchange network*+''+n *+n!&+, S(s!e'The ten levels
can be connected to #+ different exchanges.+rom ,*change-ut(et #o
,*change. /1 0. /2 I0 /1 CC /3 2I /3 45 /2 +2 /1 +*+''+n
*+n!&+, S(s!e'.et #1=F be subscriber to be called in exchange *
from exchange > . The called subs can be reached by dialing
either of following se%uence .*or route > ? @: ": D: *
+#:+1:+0:+# #1=F*or route >:I:5:E:* +-:+=:+#:+- #1=FIf routing
is done by exchange and uniform numbering scheme is presented as
for as user is concerned , the numbering may consist of #. >n
exchange identifier.-. Subscriber line identifier within the
exchange.The exchange must have capable of receiving and storing
the dialed digits ,translating the exchange identifier in the
routing digits and transmitting the routing and subs line
identifier digits to switching network. This function is perform by
director system in Strowger 2xchange.*+''+n *+n!&+, S(s!e'>s
soon as the translated digits are transmitted , the director is
free to process an other call and is not involve in maintaining the
circuit for conversation."all processing is independent of
switching network.*unctional diagram of common control system is
shown in fig.0.- , the control functions in switching system placed
in four broad categories.#. 2vent Monitoring -. "all Brocessing 0.
"harging1. Gperation and Maintenance.*+''+n *+n!&+,
S(s!e'3egis!e& Sen4e&Fina, )&ans,a!+&Line
5ni!Si!c"ing #e!+&k3egis!e& Fin4e&-igi! 3ecei6e&
an4 S!+&age 3egis!e&0ni!ia, )&ans,a!+&*"a&ging
*i&c7i!8!ce. *i&c7i!9:e&a!i+n *+n!&+,.6en!
8+ni!+&"alling Susbs"alled Susb-a!a +& 0n;+&'a!i+n
%a!"*+n!&+,%a!"Line 7ni!*+''+n c+n!&+, s(s!e'*a,,
:&+cessing s7b s(s!e'*+''+n *+n!&+, S(s!e'2vent occurring
outside the exchange at the line units ,trunks , Hunctors and inter
exchange signaling sender(receiver units are all monitoring by the
control system.Typical events include call re%uest and call release
signals at line units.The occurrence of events are signaled by
operating relays which initiate control action. The control
subsystem may operates relays in the Hunctors senders (receivers
and line units and command these units to perform certain
functions.2vent monitoring may be distributed . The line unit may
initiate control action on occurrence of certain line events. ;hen
subs goes off hook .the event is sensed, the calling location is
determined and dial tone is extended and register finder is
activated to find free register .2vent Monitoring*+''+n *+n!&+,
S(s!e'Identity of calling line is used to determine the category
and class of service to which subs belongs ."all Brocessing >
register is chosen which send out dial tone to concerned subscriber
.>s soon as initial digits 'usually - to = ) which identify the
exchange are received in the register , they are passed on to
initial translator for processing ,similarly the register continue
to receive the remaining digits .The initial translator determine
the route for call through network and decide whether call should
put through or not. It also determine charging method and rates
applicable to subscriber.*+''+n *+n!&+, S(s!e'3ecision such as
service information of subscriber as follow !#. "all barring ! >
subs may be barred from making certain calls e.g. ST3 or IS3
barring.-. "all Briority! ;hen exchange or network is overloaded
only calls from subscribers identified as priority calls may be put
through.0. "all "harging ! It is possible to define different
charging rules for different subscribers in same exchange.1. Grigin
based "harging ! Iouting or destination of certain calls may depend
on geographical location calling subscribers.=. No. dialing calls!
These calls are routed to predetermined number without calling
party to dial e.g. hot line connection.*+''+n *+n!&+, S(s!e'If
the call is destined to a subs within the same exchange,the digits
are processed by final translator .The translation to directory
number to e%uipment numbertake place at that stage .The final
translator may determine line unit to which call must be connected
and category of called line, for example there may be no charge for
emergency numbers or fault repair service line."ontrolling the
Gperation of switching network is an important function of common
control subsystem.This is done by making the switching elements and
different stages in the set of binary data defining paths and then
connecting the actual connection path.*+''+n *+n!&+,
S(s!e'>dministration of Telephone 2xchange involves activities
such as new subs line and trunk into service .Modifying subscriber
service entitlement and changing routing plans based on network
status. "ontrol subsystem may facilitates such administrative
functions.Maintenance >ctivities May includes supervision and
proper functioning of exchange e%uipment , subscribers lines and
trunks.It should be possible for maintenance personnel to access
any line or trunk for performing tests and making measurements of
different line parameters. 2lectronics Switching SystemIn
electromechanical exchanges common control mainly used switches and
relays which were originally designed for switching system.In
common control they are used more fre%uently and wear out earlier
.In contrast the life of electronics devices is almost independent
of its fre%uency of operation.This gave an incentive for
development of electronic common control.>dvances made in
computer technology were incorporated and led to the development of
stored program control 'SB") . This enables a digital computer to
be used as central control and perform different functions through
same hardware by executing different programs. >s a result
digital exchange can offer vide variety of facilities than earlier
systems. 2lectronics Switching SystemThe facilities provided to the
individual customer can be readily altered by changing customer
class of service data stored in a central electronic memory, some
of the facilities can be controlled by customers are as below !#.
"all barring 'incoming or outgoing )! The customer can prevent
unauthori&ed calls being made and prevent incoming calls when
wishing to be left in peace.-. Iepeat last call !If call line is
engaged the caller can try again with having to redial the full
number.0. Iemainder "alls !The exchange can be re%uested to call
the customer at prearranged time'e.g. for wakeup calls).1. "all
3iversion !The exchange can be instructed to connect calls to a
different number when customer goes away.=. Three ;ay "all ! The
customer can be instruct the exchange to connect third party to a
call that is already in progress. 4. "harge >dvice ! >s a
result of caller sending the appropriate instruction when starting
the call , the exchange call back at the end of call to indicate
the call duration and charge.Stored Brogram "ontrolModern digital
computersuse the stored program control concept. > program or
set of instructions to the computer are stored in memoryand are
executed automatically one by one by processor."arrying out the
exchange control functions through program stored in the memory of
computer led to the nomenclature stored program control."ommon
channel signaling '""S ), centrali&ed maintenance and automatic
fault diagnosis and interactive human machine interface are some of
features that have become possible due to application of SB" to
telephone switching .> telephone exchange must operate without
interruption -1 hours a day ,04= days a year and for 0+ to 1+ years
. This means that computer controlling the exchange must be highly
tolerant to faultStored Brogram "ontrolThe #st electronics
switching system known as No. # 2SS was installed in New Dersey in
#J4=. Since then electronics switching system and stored program
control rapid growth in range of services.The two type of
electronics switching systems 'space division) , one using
electromechanical and other using electronics switching system.
;ith evolution of time division switching ,which is done in
electronics domain .Modern exchanges are fully
electronics..,ec!&+'ec"anica,Si!c"ing n$S!+&e4
:&+g&a' c+n!&+,.,ec!&+nics si!c"ing n$S!+&e4
:&+g&a' c+n!&+,(a). .,ec!&+'ec"anica, Si!c"ing
s(s!e'(b). .,ec!&+nics Si!c"ing s(s!e'Fig (a)Fig ( b )There are
two approaches to organi&e stored program control#.
"entrali&ed SB" -. 3istributed SB""entrali&ed SB"!In
centrali&ed control , all the e%uipment replaced by single
processor which must be %uite powerful.It must be capable of
processing #+ to #++ calls per second. 3epending on load of system
and simultaneously performing many other tasks.> typical
configuration of an 2SS is shown in next slide using
centrali&ed SB" .> centrali&ed configuration may use
more than one processor for redundancy purposes.2ach processor have
accessto all exchange resources like scanner and distribution
points and is capable of executing all control functions Stored
Brogram "ontrolTypical "entrali&ed SB" organi&ationSignal
3istributorScannersMemoryBrocessorMaintenance consoleSecondary
storage! "all recording, program storage etcTo lines*rom
linesStored Brogram "ontrol> redundant centrali&ed structure
is shown in fig .> redundancyis alsoprovided at the level of
exchange resources and function program.In actual implementation
the exchange resources and memory modules containing program for
carrying out thevarious functions may be shared by processors.Gr
each processor may have its own dedicated access path to exchange
resources and its own copy of program and data in dedicated memory
modules.In almost all the present day electronic switching systems
using centrali&ed control , only two processors configuration
is used .Stored Brogram "ontrol> dual processor configuration
architecture may be configured to operate in one of three modes.#.
Standby mode.-. Synchronous duplex mode. 0. .oad sharing mode.#
Standby ModeStandby mode of operation issimplest of the dual
processor configuration operation .Normally one processor is active
and other is in standby mode, both hardware and software wise.The
standby processor is brought into service only when active
processor fails. >n important re%uirement of thisconfiguration
is the ability of standby processor to reconstitute the state of
exchange system, when it takeover the load i.e. to determine which
of the subscriber and trunk is busy, which of path is connected to
switching network.Stored Brogram "ontrolThe active processor copies
the status system periodically ,say every = seconds into a
secondary storage.;hen switchover occurs , the online processor
loads the most recent update of system status from secondary
storage and continue system operation. In this case only the calls
which changed status between last update and failure of active
processor are disturbed..xc"ange en6i&+n'en!%1
%&+cess+&%2 %&+cess+&Sec+n4a&( s!+&age%1 =
Ac!i6e :&+cess+&%2 = S!an4b( :&+cess+&Fig 4.4Stored
Brogram "ontrolSynchronous 3uplex Mode !In this configuration
hardware coupling is provided between two processors which execute
same set of instructions and compare the results continuously .If a
mismatch occurs, the faulty processor is identified and taken out
of service within few milliseconds . ;hen system is operating
normally, the two processor have same data in their memories all
time and simultaneously receive all information from exchange
environment .Gne of the processor actually control the exchange
while other is synchroni&ed with former but does not
participate in the control of exchange.The synchroni&ed
configuration is shown in fig 1.= .Stored Brogram "ontrol.xc"ange
.n6i&+n'en!%1*%28281*= *+':a&a!+&vailability of
BrocessorsCnavailability of dual processor!C3 -'MTTI)-'MT@*)-
-'1)-'-+++)- K x #+:4*or #+ years ! -1 hrs x 04= days x #+ x K x
#+:4 +.F++K hrs 1-.+1 minutes.
*or 0+ years ! -1 hrs x 04= days x 0+x K x #+:4
-.# hours.
Time Switch> time slot in conventional B"M contains K bits
and a basic frame is #-= Msecond in duration . *or the North
>merica 3S# format , the basic contains -1 time slots and for
the 2uropean 2# has 0- time slots . The time duration of an eight
bitstime slot is #-=(-1=.-+K0 Msec for 3S# and #-=(0- 0.J+4 Msec
for 2# .The time slot interchanging involves moving the data
contained in each time slot from the incoming bit stream to an
outgoing bit stream but with different time slot arrangement in
accordance with the destination of each time slot .To accomplish
this at least one time slotmust be stored in memory and then called
out of memory in changed position . The operation must be
controlled in some manner and some of these control actions must be
kept together with the software managing such action.Time
SwitchTime SwitchTypical control actions are time slot idle or busy
.There are three basic blocks of time switch.#. Memory for speech-.
Memory for control0. Time slot counter or processor.These three
blocks are shown in figure.In the first case se%uential write , the
time slots are written into the speech memory as they appear in the
incoming bit stream .*or the -nd case the random write, the random
write , the incoming time slots are written into memory in the
order of appearance in the outgoing bit stream . This means that
the incoming time slots are written into memory in the desired
output order.The writing of incoming time slots are controlled by a
simple time slots counter and can be se%uential. Time SwitchTime
Switchand can be se%uential 'e.g. in order in which they appear in
the incoming bit stream) .The readout of speech memory is
controlled by control memory.In this case readout is random where
the time slots are readout in the desired output order. The memory
has as many cells as there are time slots.*or the 3S# for example
are -1 cells . The time switch, as shown work well for a single
inlet:outlet switch . ;ith Hust -1 cells it could handle -0
stations .5ow can we increase a switch capacity N 2nter the space
switchas shown in fig.J.0 affords a simple example of this concept.
*or example time lot @# on the @ trunk is moved to the O trunk into
the in to time slot O# and timeslot "n is moved to the trunk ; into
time slot ;n , we observe that there is no change in time slot
position.6$ace 6&itch Connects #ime 6(otsSpace Switch>
typical time division switch is shown in fig. J.1 It consist of
cross point matrix made up of logic gates that allow the switching
of time slots in the spatial domain.The B"M time slot bit streams
are organi&ed by the switch into the pattern determined the
network connectivity . The matrix consist of a number of input
hori&ontals and a number of output verticals with a logic gate
at each cross point.The arrayas shown in fig. J.1 , has M inlets
and N outlets and we call it as M x N array. If MN the switch is
non blocking If M 6Nthe switch concentrates and if MPN the switch
expands.*or a given time slot , the appropriate logic gate is
enabled and the time slot passes from input hori&ontal to
desired output vertical . The other hori&ontal each serving a
different serial stream of time slots ,can have same time slot 'a
time slot from time slot number #:-1,#:0+ ,#:n , for instance time
slot F on each stream) switched into verticals enabling their
gates.#ime 7i'ision 6$ace 6&itchSpace SwitchIn the next time
slot position'time slot K),a completely different path
configuration could occur, allowing time slots from hori&ontal
to be switched to selected vertical.The space array 'cross points
matrix ) does not switch time slots but as does a time switch .
This is because the occurrence of time slots are identical on the
hori&ontal and vertical . It switches in space domain not in
the time domain .The control memory in the fig J.1 enables gates in
accordance with its stored information.If an array has M inputs and
N outputs , M and N may be e%ual or une%ual depending on function
of switch . *or a tandem or transit switch web expect MN . *or
local switch re%uiring concentration and expansion , M and N would
be une%ualSpace SwitchIf it is desired to transmit a signal from
input # 'hori&ontal) to output - 'vertical) the gate at the
intersection may be activated by placing enable signal on S#-
during desired time slot period.Then the K bits of that time slot
would pass through the logic gate onto vertical. In the same
time*or example if array is -+ x -+ and time slot interchanger is
placed on each input'hori&ontal ) and interchanger handles 0+
time slots , the array then can serve -+ x 0+4++ different time
slots Connectivity of Space Switch