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