saudi arabia-the largest telephone project in the world a new ...

ERICSSON REVIEWNUMBER 2 1978 VOLUME 55Copyright Telefonaktiebolaget LM EricssonPrinted in Sweden, Stockholm 1978RESPONSIBLE PUBLISHER DR. TECHN. CHRISTIANJACOB/EUSEDITOR GUSTAF O. DOUGLASEDITORIAL STAFF FOLKEEDITOR'S OFFICE S-12625BERGSTOCKHOLMSUBSCRIPTION ONE YEAR $6.00 ONE COPY $1.70PUBLISHED IN SWEDISH, ENGLISH, FRENCH ANDSPANISHContents42 • Saudi Arabia-the Largest Telephone Project in the World46 A New Generation of Power Supply Equipment, Type BZD 11258 • Operational Experience from the Mollison International SwitchingCentre in London66 • Laser Activities at LM Ericsson76 • Digital Multiplex Equipment for 8 and 34 Mbit/s Line SystemsCOVERTransistors with cooling flanges in the boosterconverter that forms an essential part of LM Ericsson'snew power supply system. The oscilloscopecurves represent control pulses in the convertercontrol system.

Saudi Arabia—the LargestTelephone Project in the WorldThe Kingdom of Saudi Arabia has been experiencing very radical changes —social, economicand technological-in the past two decades. The rapid development is seen in the technicallyqualified projects and in the enormous expansion of general construction work, roadnetworks and air lines, harbours, and industry. Telecommunications are, of course, animportant element in these projects.LM Ericsson has supplied and installedtelephone exchanges, outside plant andcable, telephone sets and transmissionequipment in Saudi Arabia since 1964.There are now in operation and under installationin the country local exchangesfor about 200000 subscriber lines, of LMEricsson's ARF system (register- andmarker-controlled systems with crossbarswitches). The transit exchanges, includingthe three international exchanges, areof type ARM (likewise register- andmarker-controlled systems with crossbarswitches). The national transit exchangesare now being converted to the AREsystem (SPC). LM Ericsson's cablenetwork material has been used for thelocal outside plant, while the long distancenetwork has been installed by, among others,Philips, SIRTI (Italy), LM Ericssonand, for the present major microwave ex-Fig. I. Extension works in the harbour ofJeddahtension. Western Electric. The telephonesets are of LM Ericsson's DIALOG type.In May 1977 the Ministry of Post, Telegraphand Telephone published a FunctionalSpecification for the AutomaticTelephone Project for the Kingdom ofSaudi Arabia and a number of telecommunicationmanufacturers were invited tosubmit tenders by September 27. 1977.On December 13. 1977, His Excellencythe Minister of Post, Telegraph and Telephoneannounced that the responsibilityfor the telephone project had beenawarded to the consortium LM Ericsson,Philips and Bell Canada. The final contractwas signed on January 25, 1978. The projectis the largest of its kind ever to be undertakenand has aroused much attention.Scope of the projectTelephone exchange equipmentLocal exchanges for altogether 476000subscriber lines are to be installed, as wellas the associated subscriber and junctionline cable networks; also tandem and transitexchanges, the latter both for nationaland international traffic.All ARF exchanges supplied under earliercontracts, totalling 198 800 lines, are to bemodernized to ARE by introduction ofANA 30 processors.The diagram, fig. 2, shows the plan for installationof the 476 000 new lines duringthe 3-year period up to December 13, 1980.The project is to be completed in threestages: 12, 18 and 36 months as from December13, 1977. Table 1 shows the threestages with the various exchange systemsassigned to each.

Tabell I shows the dividing of the projectbetween LM Ericsson and Philips, and thetime scheduleNumberof subscriberlinesextensionsFig. 2. Growth of the number of subscriberlines 1978-1980The national transit exchanges ARE Bareto be connected to operators' consoles ofav new type, ANE 30, supplemented bytoll ticketing equipment. The modernizedcrossbar exchanges, ARE 11, will offer anumber of new facilities such as connectionof push-button telephones andabbreviated dialling. The present subscribermeters will be replaced byelectronic metering.Exchange buildingsTwenty-eight buildings are to be constructedfor the new exchanges. A series ofstandard buildings, adapted to differentsizes of exchanges and extendable, havebeen designed for the purpose. Owing tothe often exacting climate, great attentionhas been paid to the design of a reliable airconditioning system.A number of the tabulated PRX exchangesare transportable units of 500 or 1000 linesdelivered in special containers, they thusdo not require permanent exchange buildings.The project also includes renovation of existingexchange buildings and overhaul oftheir air conditioning and power plants.Telephone setsAbout 500 000 telephone sets of LM Ericsson'snew model Dl A VOX are to be delivered.This delivery to Saudi Arabia marksthe introduction of this set on the worldmarket.A number of coin-box telephones are alsoto be supplied. These will come from LMEricsson's Danish associated company.Telefonfabrik Automatik A/S.Cable, PCM equipment, outside plantLarge quantities of cable and outside plantare to be delivered for new subscriberFig. 3. Map of Saudi ArabiaFigures in black: existing number of subscriberlinesFigures in red: total number of subscriberlines after project implementation• Cities and places included in the project

Fig. 4. Telephone set D1AVOXnetworks and for the junction linenetworks in the larger towns. The totallenght of cable is estimated at some 2 millionpair-kilometres.The junction line network is to a large extentto be constructed with PCM. About2200 PCM system ends are to be deliveredfor this purpose, apart from the PCMterminals integrated in the digital exchanges.The project-a turn-key undertakingThe project has many interesting aspectsapart from its size. It is a turn-key undertaking,which implies in this case that theequipment will be delivered, installed andput into operation by the supplier, who isthereafter responsible for its technicalmaintenance during a guarantee period ofone year.At the end of the guarantee year the responsibilityfor maintenance will be handedover to the telephone operating organization,which is being built up by BellCanada and which will operate the telecommunicationsin the entire country for 5years.The elaboration of the project required intimatetechnical cooperation between thethree tendering companies. The conditionsfor this cooperation were created at an earlystage, in that the three companies jointlydrew up a series of national basic technicalplans for signalling, transmission, charging,numbering, and operation, and preciselyspecified all technical parameters. Itwas thus possible lo make the assignmentbetween AXE and PRX exchanges (PRX205 is the designation for Philips' SPC exchanges)on an economic basis without regardto their geographical location.Operation and maintenanceThe SPC technique offers a very largenumber of administrative facilities for operationand maintenance. This had beentaken into account in the specification offunctional requirements, and an operationand maintenance organization based onfar-reaching centralization had been outlined.On this foundation LM Ericsson,Philips and Bell Canada designed an overalladministrative organization for the entirecountry. This organization is served bya special operation and maintenanceFig. 5. Network operation centresTypes of centreNNCC National Network Control Centre 1 )TMC Transmission Maintenance Centre 2 )TAC Traffic Administration Centre 2 )SAC Service Assessment Centre 2 )DMC District Maintenance Centre')RSC Repair Service Centre 4 )ACC Assignment and Control Centre 5 )Man-Machine communication in thedifferent centres') Network Status Panel. Test Positions,Printers2 ) Video Displays, Printers-') Video Displays. Colour monitor. MobileUnits4 )Test Positions, Printers•) Video Displays. Printers«— Data links

ig. 6. Measuring on cables Damman-Kuwaitnetwork based on LM Ericsson's SystemAOM 101.AOM 101 consists of computer centreswhich are connected both to exchange andtransmission equipment and to the variousadministrative centres specified.AOM 101 is designed for connection of allthe SPC exchanges to be installed in SaudiArabia, i.e. ARE, PRX and AXE. Themap, fig. 5, shows the operation andmaintenance network with the locations ofthe various centres.Important centres which constitute separateunits in the operation and maintenancepart of the project are also- the repair centre (for PC boards, certainpower equipment and telephone sets)- spare parts depots, central and regional- the centre for software production- the training centre (training installationsfor ARE, AXE and PRX).A consistent theme in the project is"Saudization", i.e. that the contractor undertakesto train Saudi Arabian staff tosuch an extent that the greater part of theoperation and maintenance organization isstaffed by nationals at the end of the 5-yearperiod. Training is to be given not only totechnical staff but for all functions in atelephone administration such as economy,administration, planning, inventorymanagement, documentation, etc.Some detailsAn interesting detail in this extensive projectis that Saudi Arabia will have its first'telephone directory. It will be bilingual, inArabic and English.The trunk network will be constructed to alarge extent with PCM transmissionsystems, both on existing and new cableruns. Since the new trunk and tandem exchangesare entirely digital, as also are thegroup selectors in the larger local exchanges(AXE), it has been possible tomake use of the economic advantages offeredby integrated digital switching andtransmission-the first stage towards thedigital network.To sum up, the Kingdom of Saudi Arabiawill acquire a telephone network characterizedby- SPC in all permanently installed exchanges- digital switching in major traffic centresand high penetration of PCM transmission- new electronic telephone sets with improvedtransmission characteristics- a very advanced system for centralizedoperation and maintenanceIn other words, one of the absolutely mostmodern and sophisticated telephonesystems in the world.John M curling

A New Generation of Power SupplyEquipment, Type BZD 112Svein Michelsen, Folke Ekelund and Kjell RundkvistA new generation of power supply equipment for telecommunications has beendeveloped by LM Ericsson. The design is based on the booster converter principlelong used by LM Ericsson. The article describes the new booster converter systemwith newly developed thyristor rectifiers and high-frequency booster converters. Anew type of distribution and battery charging equipment is also described. A newpackaging structure considerably simplifies installation and maintenance. Theequipment is well suited for the powering of modern electronic switching systemssuch as AXE, AXB etc.UDC 621.311.4:621 3957Fig. 1All power supply equipment is type tested inLM Ericsson's new power supply laboratory inStockholm. Here equipment of type BZD 112 isbeing carefully tested before being delivered toSaudi Arabia. In the background can be seenthe distribution panel that is used for testingcomplete power supply systems for up to 6000 AFifteen years ago LM Ericsson introduceda power supply system in whichthe previously common cell switchingdevice was replaced by an electronicdc/dc converter, a so-called boosterconverter. Since then, this system hasbeen used for different types of telecommunicationequipment and is atpresent in use in approximately 2000plants. The system is noted for its extremelygood operational characteristics,the most important of which is constantdistribution voltage, which ismaintained not only during normal operationbut also during mains failures.The system is also characterized by fullyautomatic operation, good economyand high reliability.LM Ericsson are now introducing a newgeneration of power supply equipment,BZD 112, which is in accordance withthe booster converter principle. The designis based on the latest developmentin converter and regulation techniquesas well as long experience of installationand operation of the system. The mostprominent features of the equipment are• new conversion technique for boosterconverters• new component technology-powertransistors, Schottky diodes, ferritecores, integrated circuits for regulationand control• new charging principle• new packaging structure• simple handling during installation,operation and maintenanceDa high degree of personnel safetyPower supply equipment BZD 112 hasbeen designed in close collaborationwith the designer groups that developedthe latest electronic switching systemsin LM Ericsson. The equipment is thusparticularly well suited for the poweringof the modern AX systems for telephonyand telex.The new equipment can also be used inother system configurations than thosewith booster converters, for example inordinary full float systems and dividedbattery systems. In this article, however,only the booster converter system willbe described.Description of the systemBZD 112 is a converter system intendedfor supplying uninterrupted DC powerto various types of modern telecommunicationequipment. The main unitsare thyristor rectifiers, high-frequencybooster converters, distribution equipmentfor high or low-ohmic distributionand storage batteries with automaticcharging equipment.The system, which is wholly automatic,keeps the distribution voltage withinclose limits under all operating conditions,and also ensures that the storagebatteries are utilized in the mosteconomical manner. The standard versionof BZD 112 is designed for a nominalvoltage of 48 V. All voltages givenbelow raiate tnJbisvxdtaoe^^

SVEIN MICHELSENFOLKE EKELUNDKJELLRUNDKVISTPower Supply DepartmentTelefonaktiebolaget LM EricssonFunctionThe function of the booster convertersystem has been described in a previousissue of Ericsson Review 1 , and thus onlya brief description will be given here.Fig. 2 shows a simplified block diagram.The whole sequence during a mainsfailure and the subsequent rechargingof the battery or batteries is illustrated infig. 3.During normal operation the output voltageof the system is kept at the prescribedlevel by the rectifiers, which arefed from the public AC mains network.The value of the regulated voltage,which is the same as the battery voltage(51 V±0.5%), was chosen in order toprovide suitable floating for the battery.In this operational mode the boosterconverters are in what is known as passiveoperation, which means that theygive no output voltage but are alwaysready to be activated if additional voltageshould be required. In passive operationthe booster converters cause avoltage drop of approximately 1 V, andhence the system output voltage U D isapproximately 50 V.If a mains failure occurs, the batterytakes over the power feeding and as aresult its voltage falls. The booster converters,which are fed from the battery,are then activated, and their rising outputvoltage is added to the successivelydecreasing battery voltage, so that theoutput voltage of the system remainsconstant.When the mains voltage is restored therectifiers automatically resume thepower feeding and also recharge thebattery. The output voltage from thebooster converters decreases graduallyand when the normal voltage level hasagain been reached the converters returnto passive operation. So-calledoperating charge is then started in orderto charge the battery quickly and completely.This is done by sending a signalfrom the charging control system to therectifiers, which causes them to increasetheir output voltage to the charginglevel (normally 2.35 V/cell) of 54 V.When the battery is fully charged thecharging signal ceases and the rectifiersreturn to the normal level.Fig. 2Power supply system BZD 112 with highfrequencybooster convertersFig. 3Booster converter system 48 V with 23 batterycells. Working cycle during a mains failure andthe subsequent recharging

48Fig. 4Thyristor rectifier, BMT 313, seen from the frontwith the front plates removedThyristor rectifierThe rectifier is a three-phase unit withthyristor control, designed for use in alltypes of power supply systems, includingsystems without battery, fig. 4. It isavailable in two sizes, BMT 303, 50 A/48V, and BMT 313,100 A/48 V. The rectifieris well suited for feeding from standbypower plants and from mains networksof poor quality, such as networks withlarge voltage and frequency variations,frequent mains failures or large distortion.The connection is not sensitive tophase sequence. The rectifier is equippedwith protective and control equipmentthat permits fully automatic operationand operation in unattended exchanges.Main circuitThe secondary side of the rectifier mainstransformer is connected as a six-phasestar with an interphase transformer, fig.5. The six thyristors of the thyristorbridge regulate the rectifier output voltagethrough phase shift control. Therectifier connection gives a d.c. voltagewith a ripple frequency that is six timesthe mains frequency. An LCL filter attenuatesthe ripple so that the rectifieroutput voltage is well smoothed andsuitable for telephone exchanges andsystems.ControlThe control system of the rectifier maintainsthe output voltage at a constantlevel. When the current exceeds therated value the rectifier changes over toproviding constant current in order toavoid overloading. The most notablecharacteristics of the control systemare:D correct function even with large distortionof the mains voltage supplyDequal shareing of the load betweenthe thyristors in the rectifierDthe regulation level is not sensitive totemperature variationsProtection circuitsThe rectifier contains a number ofcircuits for protection against abnormaloperating conditions:— Current limiting protects the rectifieragainst overloading and in mostcases also against external shortcircuits— Additional short-circuit protection isprovided in the form of two fast actingsemiconductor fuses— A fuse at the output provides protectionagainst short circuits inside therectifier— Phase failure protection blocks thetrigger pulses to the thyristors if oneor more phases of the mains supplynetwork fail.— The overvoltage protection is activatedif the voltage on the rectifieroutput rises because of a fault in therectifier. The protection deviceblocks the trigger pulses to thethyristors and realeases the mainssupply contactorThe rectifier restarts automatically whenan alarm condition ceases, for exampleFig. 5The three-phase voltage is connected to thetransformer via a contactor. The thyristors rectifyand regulate the voltage which is smoothed bythe LCL filter. The control unit gives triggerpulses to the thyristors with the right timing, sothat the output voltage is kept constant

49when a blown fuse has been replaced. Inthe case of both automatic and manualstart the rectifier voltage successivelyincreases to the set regulation level.This smooth start prevents unnecessarycurrent surges and component stresses.Parallel operationAny number of rectifiers can work inparallel, and they will then share theload equally.Remote control facilitiesThe rectifier can operate as an entirelyautonomous unit. It can also be remotelycontrolled from a central equipmentvia a cable and plug. By means of remotecontrol it is then possible to— raise the rectifier voltage to a presetvalue for charging of the batteries— control the rectifier output voltage sothat it follows the voltage of anotherpower plant— block the trigger pulses to thethyristors— open and close the rectifier mainscontactorIn addition the rectifier can send alarmsto central equipment.Mechanical constructionThe rectifier consists of two plug-in units.The upper one, fig. 6, consists of d.c.filter, thyristor bridge and control unitwhile the lower unit contains the transformerand operating device with anammeter, operating buttons and lightemitting diodes.Technical data for thyristorrectifiersBMT313 BMT303System voltage V 48 48Output dataCurrent A 100 50Static regulationaccuracy mV 50 50Dynamic regulation:Response time withwise load change25%ofl ma), ms 150 150Noise voltage:— psophometric mV

50Fig. 7Booster converter, BMR 263, seen from the frontwith the front plate removedBooster converterThe latest development in convertertechnique —conversion at high frequency(20 kHz) —has been applied inthe design of the booster converter, fig.7. It has the following advantages:DSmall dimensions and low weight• Noiseless operationDFast regulationThe booster converter type BMR 263 isa d.c. converter with an input voltage of48 V and an output of 0-8 V 100 A. Itadds its output voltage to the batteryvoltage and regulates the output voltageof the system to the desired level, forexample when the battery becomes dischargedbecause of a mains failure.Main circuitFig. 8 is a simplified block diagram of thebooster converter. The converter containsan inverter circuit with transistors(T1, T2). It is fed with battery voltage viathe input filter (L1, C1) and provides thetransformer (TU) with a 20 kHz rectangulara.c. voltage having variable pulsewidth. The a.c. voltage is transformeddown (in TU) and rectified (in RU). Thewidth of the rectified voltage pulse determinesthe magnitude of the outputvoltage. The output filter (L2, C2) attenuatesthe a.c. voltage component sothat the rectified mean value is obtainedat the output.High-frequency conversion requires theuse of new components with advancedcharacteristics: fast power transistorsthat can withstand current surges,Schottky power-type diodes with lowvoltage drop, transformers with ferritecores and copper tape windings.Regulation and monitoringThe output voltage of the converter isregulated by means of pulse width modulation.The control unit generatespulses with a frequency of 20 kHz andvariable pulse width (0 — 25 fis), fig. 9.Feedback circuits control the pulsewidth in such a way that the convertermaintains constant voltage (on the distributionbars) as long as the output currentof the unit does not exceed therated value of 100 A. If the current tendsto exceed this value the regulationchanges over to constant current (currentlimiting).The use in the control unit of operationalamplifiers and digital logic circuits hasgiven function blocks with well definedcharacteristics.The converter works either in the passiveor the active mode. Passive operationmeans that the regulation circuitshave set the pulse width to zero. No controlpulses are sent to the transistors inthe main circuit, and thus no power istransformed over to the output side. Thedistribution current, which flowsthrough the output circuit, causes a voltagedrop of 0.3-0.8 V.Active operation means that controlpulses are sent to the transistors in themain circuit and that voltage is transformedto the output side. Passive operationtakes place when the distributionvoltage is higher than the regulationlevel (preset value) of the converter. Activeoperation is initiated the momentthe level of the distribution voltage fallsbelow the regulation level. The changeoverfrom passive to active operationuFig. 8Functional block diagram for the boosterconverter 100 A, 1 kWIF Input filterIII Inverter unit 20 kHz with power transistorsTU TransformerRU Rectifier unit with fast power diodes of theSchottky typeOF Output filterBD Base drive circuits, patented circuit designCU Control unitSH Shunt for measuring the output current

Fig. 9Control unit for booster converter BMR 263The picture shows the control unit connected to theconverter via an extension board. In this position themeasuring points on the printed board assemblies areeasily accessibleTechnical data for the boosterconverterSystem voltage48 VOutput dataCurrent100 AVoltage— max. value with U, N=40 V,IOUT=100A8 V— variation during any operatingcondition0-10 VStatic regulation accuracy ±0.5 %Dynamic regulation:Response time with step loadchangeAI=25A5 msRipple voltage, psophometricalvalue

Fig. 11Distribution equipment BMG 650. seen from theside. The outgoing distribution cables can be runupwards or downwards as desired. The cablerunning is very easily carried out with the aid ofspecially designed cable clamps.Parallel operationAny number of converters can work inparallel, and they then share the loadequally.Mechanical constructionThe converter consists of a plug-in unit,fig. 10. Its fuses are placed so that theyare easily accessible after the front plateof the unit has been removed from therack. The control unit, which consists oftwo printed board assemblies mountedin a cassette, contains all the controland supervisory circuits. Operating buttonsand indicating LEDs are mountedon the front of the control unit. Anammeter is mounted in a front plate nextto the control unit.DistributionThere are two variants of the distributionequipment, one for low-ohmic distributionand the other for high-ohmic transientlimiting distribution, see fig. 11.By low-ohmic distribution 2 , fig. 12, ismeant the conventional distributionmethod, where current feeding from thedistribution rack is carried out by meansof large cables that are branched in theswitching room to smaller cable areas.Each distribution branch then has a relativelylow resistance, comparable withthe internal resistance of the powersupply equipment. This means that if ashort circuit occurs in the distributionnetwork the whole of the telecommunicationequipment will be subjected to alarge voltage transient. The low-ohmicdistribution is used in electromechanicalexchanges, where short-lived transientsdo not affect the operation.For the powering of electronic telecommunicationequipment I_M Ericsson hasFig. 12 (left)Distribution equipment for low-ohmic distribution.The fuse holders used have a rated current of 125A. The fuses come in sizes from 6.3 A to 125 A.The fuse unit contains a built-in alarm device.Each fuse has visual alarm indicationFig. 13Distribution equipment for transient limitingdistribution. The equipment consists of specialfuse strips for fuses rated max. 35 A. Resistorstrips have been inserted in series with each fusefor adjustment of the conductor resistance whenthe distribution distance varies. The equipment isprovided with a protective plate in order toprevent the staff from accidentally touching liveparts when changing fuses.

53Fig. 14Battery fuses with equipment for automaticcharging of the batterydeveloped a transient limiting distributionsystem. The current from the distributionrack is fed via individual cablesdirectly (i.e. without branching) to thefunctional units in the telecommunicationequipment, fig. 13. A characteristicfeature of the system is that the resistancein each of the distribution branchesis approximately 10 times the totalinternal resistance of the power supplyplant. A special two-conductor cable,with an area of twice 2.5, 6, 10 or 16mm 2 , is used in order to ensure lowinductance between the conductors.This distribution system effectively eliminatesdisturbing transients that mayarise as a result of short circuits in thedistribution network.Equipment for automaticbattery chargingUninterrupted d.c. power supplysystems usually contain storage batteriesthat serve as a standby energysource and filter. The function and lifetimeof the batteries are dependent ontheir being kept fully charged at alltimes. Thus the system contains equipmentfor automatic charging in order toprovide rapid and complete rechargingafter each discharge. The equipment includedin the standard version of BZD112 functions in accordance with a simpleprinciple based on the measurementof voltage and time. The time measurementis carried out with the aid of digitalcircuits.The function, in a 48 V plant with 23 batterycells, is as follows (fig. 15):The battery voltage falls during discharge(mains failure t, —1 3 ). When it hasfallen to 47 V (2.05 V/cell) (t 2 ) the measurementof the discharge time starts.The time measurement is interrupted (t 3 )when the mains voltage is obtainedagain. The rectifiers start to charge thebattery (t 3 -t 4 ) and the battery voltagerises. The measuring of the chargingtime does not start until the voltage hasrisen to 52 V (2.25 V/cell) (t 4 ). The charginggoes on until the measured time(t 4 -t 5 ) is 8 times the measured dischargetime, but not more than 15 hours.The charging is carried out with the currentavailable from the rectifiers until thevoltage has risen to 54 V (2.35 V/cell)and thereafter at constant voltage.It can be shown with the aid of charge/time characteristics that this type of automaticcharging gives a very highcharging efficiency regardless of theamount of charging current availableand the degree of discharge of the battery.At the same time overcharging,which reduces the life of the battery, isavoided.In addition to the charging process describedabove, periodic equalizationcharging is also carried out. The lengthof the periods can be 10, 20, 40 or 80days. The normal setting is 40 days andthe charging time 15 hours. Fifteenhourcharging can also be initiatedmanually. Functional tests of the chargingequipment can easily be carried out,and it takes only about a minute to runthrough a complete function cycle.The equipment consists of a cassetteFig. 15Charging and discharging sequences controlledwith the aid of voltage and time measurement.The upper curve shows the battery voltage andthe lower the d»»grw "' charge of the battery

55Fig. 18Parallel cooling. This method provides uniformcooling conditions for all units and their loadcapability will be independent of their position inthe rackFig. 19Booster converter system BZD 112. 48 V for amaximum current output of 400 Ain the rack. This disadvantage is avoidedby the use of parallel cooling. Parallelcooling means that each active unit hasits own air intake at the bottom of thefront panel. A guide plate ensures thatthe air flows through the unit. The air isthen let out at the rear top edge of theunit into a common cooling, channel,where a good chimney effect isobtained. The principle is illustrated infig. 18.All units in system BZD 112 can operatein the temperature range 0 to +45°Cwithout loss of performance and in therange -10 to +55°C without beingdamaged.A power supply plant with amodular structureThe mechanical design of racks and unitsgives a power supply plant which hasa high degree of flexibility and which iseasy to adapt for different types of telecommunicationsystems. The flexibilityis obtained through the double modularstructure. This means that standardizedfunctional units, such as rectifiers andconverters, have been combined to formplant modules. The plant modules intheir turn constitute either complete andindependent power supply plants orconstruction modules that are combinedto form larger plants of what is inprinciple unlimited size, fig. 19. Thesmallest standardized plant moduleconsists of a full-sized rack built togetherwith a half-sized rack and with acommon top, fig. 20. The overall dimensionsof the module are: width 900 mm,height 2200 mm and depth 600 mm. Thefull-sized rack is intended forthe mountingof active units, such as rectifiers andbooster converters, while the half-sizedrack is intended for the distribution andbattery connection equipment.The plant modules are equipped withstandardized sets of units as follows:a) 1 -2 rectifiers 50 or 100 A+ 1-2 booster converters 100 Ab) 1 -3 rectifiers 50 or 100 Ac) 1 — 6 booster converters 100 AEach of these alternatives can be equippedwith distribution fuses for lowohmicor high-ohmic distribution.The equipment alternatives have beenchosen so that all power that is convertedin the plant module is distributedfrom the same module via its own distributionfuses. The advantages of thismethod are that— only equalization currents flow in theconnection bus bars between the variousmodules— extension of the distribution and batteryconnection equipment is carriedout at the same rate as the rectifierequipmentThe unit and rack construction permitsgreat freedom as regards room layout.The plant modules can thus be installedin single rows, double rows or alongwalls. There are no requirements as re-

56gards the order within a row. Extensionscan be made both to the right and to theleft. This simplifies considerably theplanning of new plant and the space inexisting premises can be utilized rationallyand without difficulty.Master slave control-a newmethod for extending andsectionalizing power supplyplantMaster slave control, MSC, is a new andvaluable feature of booster convertersystem BZD 112. Briefly this methodmeans that the output voltage (distributionvoltage) of a power supply plant canbe made to follow an external variablereference voltage with a high degree ofaccuracy, for example the output voltageof another plant. MSC can be usedanywhere where voltage equality is requiredbetween different power supplyplants. Consequently system BZD 112 iswell suited for the extension of olderpower supply plants, irrespective of thesystem principle of the old plant. Forexample, the earlier cell switch plantscan be extended even if the cell switch isfully loaded, a problem which has hithertobeen difficult to solve. Anotherapplication where this method is usefulis in sectionalized plants. The differentsections can be placed at a great distancefrom each other and yet maintainthe same voltage without having to beconnected by large cables or bars. Asignalling connection via small-diameterwire is all that is necessary. In thisway great savings in the distribution cablesare made.Fig. 20A complete plant module for booster convertersystem BZD 112. The module is equipped with twohigh-frequency booster converters 100 A, twothyristor rectifiers 100 A, 56 fuses for transientlimitingdistribution and also battery fuses andequipment for automatic charging. The modulecan either operate as an independent plant orform a part of a plant of what is in principleunlimited size.

57^complete plan, modu le with the tron, pla.es inplaceInstallation, operation andmaintenanceSimple and fast installation was one ofthe most important aims when designingthe units and the packagingstructure. The equipment is deliveredfrom the factory as easy-to-handle,tested units, which are very easily connectedto the rack busbars by means ofdisconnectors. The installation of theequipment is carried out using simplehand tools, and the installation testing islimited to functional testing and a visualinspection in order to detect any transportdamage.The booster converter system is whollyautomatic, that is to say all operationalchanges are carried out without anymanual intevention. Rectifiers and convertersare loaded and unloaded accordingto the actual demand of the switchingequipment and the battery. The automaticcharging equipment ensuresthat the batteries are fully charged andin peak condition. All that is normallyrequired in the way of maintenance is totoo up the batteries with water and tocarry out certain checking once or twicea year.The repair freguency has been calculatedto be in the order of 0.05 per unitand year.Live parts are equipped with protectivecovers in order to reduce the risk ofpersonnel injuries during work on theplant. Units can be removed and replacedduring operation without hestaff having to work close to live parts.ReliabilityThe power supply equipment in a telecommunicationsplant must have h ghreliability. This has been achievedthrough the following measures:Unit design- only high quality c°mP°^nts a *selected and these are subjected toextensive testing before being- dKSoning rules with wide safetymargins are appliedSystem design-consistent use of redundancy, i.e. aspare for each unit and importantfunction- each of the rectifiers and convertersconstitutes independent units- no central devices are used that canaffect the operation of theplantwholeThe calculated value for the expectedmean time between system failures(MTBSF) is 3000 years or more, on conditionthat the normal number of spareunits (20 % but at least one unit) is used.A system fault is assumed to have occurredwhen the number of faulty units exceedsthe number of spares. Normallythis only means a deterioration in theoperation of the plant and not a completebreakdown. The life of the equipmentis calculated to be 40 years. Theprobability of a system fault during thistime is less than 0.01.References:1 Wolpert, T. and Bjork, D.: PowerSuDpIv System with Booster Converters-Viewpointsafter 10 Yearsin Operation. Ericsson Rev. 52(1975):1, PP- 14-23.2 Orevik A.: D.C. Distribution forPower Supply of TelecommunicationEquipments. Ericsson Rev. 49(1972):1, pp. 14-28.3 Orevik A.: Power Supplies for' Electronic Telephone Exchanges.Ericsson Rev.57 (1974):4, pp. 120-127.

Operational Experience from theMollison International SwitchingCentre in LondonRowland W. ButtonThe first stage of the Mollison International Switching Centre, (ISC) in London wasopened in October 1974. This ISC is of LM Ericsson's transit exchange system ARM20 with crossbar switches and currently it carries about 50% of all internationaltraffic load in the United Kingdom. It has been described in a previous issue ofEricsson Review\ In this article the good operational experience from more thanthree years service will be illustrated, covering the incoming and the outgoingexchange units ARM 20, the international maintenance centre, the automatic transmissionmeasuring equipment, the accounting equipment, the training of maintenancestaff and the repair of printed circuit boards.UDC621 395 722Fig. 1Mollison exchange with switches for outgoingtrafficMollison International Switching Centre(ISC) is divided into two twin units accordingto fig. 2. One twin unit is intendedfor incoming and the other foroutgoing international traffic and theyare completely independent of eachother.The outgoing unit caters solely for outgoingInternational Subscriber Dialledtraffic (ISD) and the incoming unit forincoming international traffic not requiringassistance from any internationaloperator in the United Kingdom. Furthermorethe ISC switches traffic over asmall number of large routes only, representingcountries with whom the UKhas substantial mutual traffic interest.These limitations of the ISC were plannedwith the objectives of cost reduction,short installation period and, mostimportant, high operational efficiencyand these objectives have been met.When all international circuits are inservice Mollison ISC has a total internationalswitching capacity of 8 000Erlangs for outgoing and incoming trafficat a load of about 0,75 Erlangs percircuit and a grade of service of 2% 0 . Aswill be seen, all terminations are currentlynot in service.

59ROWLAND W BUTTONHead of Network Control DivisionPost Office External TelecommunicationsExecutive. LondonOutgoing twin unit ARM 202 equipped Incoming twin unit ARM 201 equippedas follows:as follows:5420 (2177)* international terminations 5420 (2710) international terminations6200 (3453) national terminations 5820 (3385) national terminations612 registers 424 registers'The figures within brackets represent the numbernow in use while the other figures show the totalnumber deliveredFig. 2Separate twin units for incoming and outgoinginternational trafficFig. 3Intornatinnal ac-cnuTTtlmreuntDmenl

Table 1.Fault statistics from the 6-months period1.9.1976-28.2.1977Table 2.Fault analysesFig. 5 shows a rather normal developmentof the growth of traffic in theswitching centre, while fig. 6 illustratesthe corresponding call attempt characteristics.Finally the trend of outgoingpaid minutes per month is indicated infig. 7.Number of faults belowstandard valuesThe Mollison ISC is maintained undercontrolled corrective maintenancephilosophy, which is described in anearlier issue of Ericsson Review 2 . As willbe seen from table 1 above the numberof faults per relay and crossbar rack andyear are much below the standard valuesgiven by LM Ericsson.Fig. 4Daily traffic distributiona. In the outgoing unit as number of register seizuresper hour.b. In the incoming unit as Erlangs per hour.Fig. 5Growth of traffic in Erlangs in the outgoing andincoming units____Outgoing International trafficIncoming international trafficFig. 6. to the rightNumber of call attempts per month in the outgoingand incoming units^^ Outgoing unit^^_ Incoming unitThedaily profileof theoutgoing ISD trafficdemand is shown in fig. 4a. The afternoonpeak in this profile occurs whenEuropean and North American routebusy hours coincide. The profile for incominginternational traffic in Erlangs isshown in fig. 4b. As will be seen the exchangeis rather heavily loaded for morethan 12 hours a day which is a result ofthe composition of the internationallines.Several of these 553 faults were due towrongly jacked-in relay sets and incorrectlypivoted relay armatures. The majorityof the remaining faults were wiringfaults as will be seen from the faultanalyses in table 2.The jacking-in and the relay armaturefaults are mainly attributed to human errorsand recent results indicate thatthese type of faults have been eliminatedto a great extent. Any fault directlyattributable to manufacture or installationprocesses have been remedied byLM Ericsson with minimal delay.The use of plug and socket connectionon rack installations is advantageous in

Table 3Service observations of international traffic duringa period of three monthsFig. 7Number of paid minutes per month for the totaloutgoing international trafficFig. 8, to the rightNumber of faults in the outgoing unit per outgoingcircuitMillions/monthrespect of cabling and flexibility andthere has been no evidence of jacking-inproblems in this connection.From the International TransmissionMaintenance Centre, ITMC, reports aresent regarding operational difficultieson identified circuits. For identificationof faults these reports constitute an excellentsupplement to the alarm andmonitoring devices controlled by the InternationalSwitching MaintenanceCentre ISMC. The number of faults reportedby ITMC may also be related tothe number of international and nationalcircuits in service. This is illustrated infig. 8, showing that the trend since openingto the current achievement is 0.03reported faults per month per circuit inservice.Good service quality of ISCBy use of the statistical equipment, theswitching loss within the Mollison ISChas been calculated and the result isshown in fig. 9. As this loss has beenbelow 1 % for a considerable period andis now running at about 0.3 %, the servicequality of the switching centre isvery good.The overall service performance for internationalcalls from subscriber to subscriberis observed by supervisors in theservice observation equipment. Over arecent three month period the results intable 3 were applicable.In connection with the 21.8 % ineffectiveoutgoing calls the congestion withinISC is very small and may be neglected,especially as the exchange is not fullyloaded.The CCITT recommendation standard isthe objective in all international routedimensioning and this is achieved in themajority of cases. However, there aresignificant departures from thisstandard on certain routes with high callingrate. This fact, coupled with cableand other transmission failures whichalso result in international route busyconditions, leads to the conclusion that,on average, about 8% of call attemptsfail due to both primary and alternativeinternational route choices being fullyengaged or unavailable due to transmissionmedia breakdowns. As indicatedin fig. 10 this average is subject toconsiderable variations. Some 300system breakdowns are reported eachmonth on international routes radiatingfrom UK and the annotations in fig. 10indicate some failures where the effecton performance is significant.Our effective call rate to some countriesis much lower than we would wish andadditional international links do not resultin significant improvement. Thisleads to the conclusion that on averagesome 11 % of the calls fail duetod/sfanffailures in engineering performance. Ashas been indicated previously the loss inMollison ISC is less than 0.3 %.Finally, miscellaneous losses and lossesdifficult to categorise, including a substantialnumber of engineering testcalls, are of the order of 2.5 %.

62Fig. 9Percentage switching attempt loss in outgoingand incoming units^—~ Outgoing unit—• Incoming unitFig. 10, to the rightNumber of call attempts which meet internationalcircuit congestion (weekday average)Cable faults©Cables to the Netherlands and USA(2) Cable to Denmark(3) Cable to SpainThe 7.1 % ineffective incoming calls dueto failures in the UK network canapproximately be broken down into 2%no tones, 3% number unobtainable tones,less than 1 % switching loss in MollisonISC and just over 1 % miscellaneousloss. There is insignificant congestionon national routes outgoing fromMollison ISC, the majority of nationalgroup switching centres being servedby circuits dedicated to international incomingtraffic.International MaintenanceCentre (IMC)This centre consists of one transmissionmaintenance centre (ITMC) and oneswitching maintenance centre (ISMC),each serving both the outgoing and theincoming units.ITMC is equipped with 24 test consolesfig. 11 with adequate facilities asspecified by the Post Office. They havebeen most satisfactory in practice with aminimum of design faults.The consoles are used for both mainte-Call attemptsnance and circuit provision. The testpoints on all national and internationallines are reached automatically as wellas via associated test jacks.Owing to the organization of the exchange— dealing solely with selectedroutes carrying ISD traffic only and havinga relatively low fault incidence - thedaily working load in ITMC is limited,which may even lead to a certain degreeof boredom. The route supervisionfacilities within ITMC are, however, veryvaluable as they permit quick and accurateassessment of the route status interms of utilisation, blockage and congestion.They also offer a measure offault identification. It seems highly likelythat the ITMC staff should be given moredirect responsibility for the totalperformance of the ISC within thenetwork, thus relieving them from theboredom.ISMC is equipped with LM Ericsson'sstandard centralographs and othersupervision facilities. To date it hasfunctioned well without any significantproblems.

63For an ISC as large as Mollison theelectromechanical centralographs may,however, be supplemented by computertype equipment in the future.The electronic traffic recording equipmentis functioning well after the clearanceof some difficulties in the initialstage.Automatic transmissionmeasuring equipment,ATME2The electronic measuring equipmentATME 2 in Mollison was one of the firstinstalled plants of this type. It is workingmost satisfactorily and to date only twoprinted circuit boards have beenfaulted.It supervises some six routes on whichother administrations at present canprovide comparable facilities. Initiallythere were some teething difficulties inthe day to day operation of the system.There is now however, little doubt thatsubstantial improvement in internationaltransmission performance standardswill be achieved and maintained withminimal delay and resources, whenmore administrations are also equippedwith this device.Service observationequipment (SOE)One service observation position hasbeen provided for the outgoing andanother for the incoming internationaltraffic, containing facilities specified bythe Post Office and specially developedby LM Ericsson. The positions are functioningwell and provide valuable informationto assess and improve performance.The facility of the outgoing positionto select calls in particular routes isa notable asset of great value. The trafficsupervisors who operate the positionsare also most satisfied with the facilitiesand layout.International accountingequipmentThis equipment consists of two parts,the duplicated computers, UAC 1610,and the interfaces containing bothFig. 11International transmission maintenance centre,ITMC

64electromechanical and electronic devices.The performance of the computers havebeen of a very high order, only ninefaults having occured over a 3 1 M yearperiod. They are maintained by a staff atMollison ISC with advisory help fromother Post Office or LM Ericsson resources.Now that the staff have become familiarwith the interfaces and the procedureshave been more clearly indicated thenumber of error reports regarding thisequipment is kept within acceptablelevels. Initially the number of such faultswas, however, too high mainly due to installationand circuit commissioningactivities. It should be mentioned that anew design of interface with electronicequipment only has been installed in ourmost recent ARM 20, Thames ISC.Training of maintenance staffThe training of the maintenance staffwas based on courses run by the PostOffice and LM Ericsson covering allaspects. Additionally the staff was involvedin the installation and maintenanceof the ISC. A training ARM 20 exchangehas also been installed in thetraining school where courses continueto be run on all aspects to maintain theexpertise of the staff. The ISC is nowmaintained by the staff alone with littleneed for any recourse to other maintenancesupport functions, which is an indicationof the good training efficiency.Spares and repair of printedcircuit boardsNow that the ISC has settled down into anormal operational mode it is possibleto assess the spare parts and repair requirementsreadily.It is planned that the majority of theboards should be repaired on site or atleast within a Post Office repair centre.The fault rate is, however, low (fiveboards per month on average) and thereare a number of boards, eg in the computers,for which provision of spares orrepair facilities is uneconomic. Hencesome dependence on the manufacturerwill be essential at least for a period andto date urgent needs in this respect havealways been met.Fig. 12International switching maintenance centre,ISMC

65Some other observationsIn an 8 000 Erlang ISC there are areaswhere operational problems may arise,which are not evident from statistics andother data.It is difficult to make route and othernetwork changes quickly enough in anon-SPC-exchange such as MollisonISC, to meet the very dynamic internationalnetwork needs now experienced.This problem should, however, find itssolution in the future when SPC-exchangesare introduced.During the period since opening in 1974there have also been some faults inequipment specially designed for Mollison,but the effect on the switchingperformance has been marginal. LMEricsson have cooperated with the PostOffice in the analysis and clearance ofthese faults.It has been found that the traffic loadbetween the twins especially in the outgoingunit is rather high. This is mainlydue to too high a level of route busycondition on certain outgoing routes.This should be considered when dimensioningthe number of lines between thetwins.General conclusionsMollison ISC was installed ahead of thecontracted date to meet an urgent internationalswitching capacity requirement.The performance, which is continuallymonitored, is fully meeting ourrequirements. The maintenance staffhave pride in the ISC, it is kept extremelyclean and the environment is good.Shortly Mollison ISC will be complementedby Thames ISC now underconstruction. This ISC will initially alsobe a limited facility switching centrecatering for the same kind of traffic asMollison and it will be a 10,000 Erlangunit with 5,000 Erlang in ARM 20 and5,000 Erlang in LM Ericsson's SPCsystemAKE 13. The Thames ISC will bedescribed in a future issue of the EricssonReview.Mollison and Thames ISC's will thusswitch much of UK's international directlydialled traffic on a limited facilitybasis into the mid 1980's.These two exchanges together with anew full facility ISC in Mondial HouseinPlessey 5005 T practice —will form acomplement to the existing ISC's in UK.If the operational performance so farachieved within Mollison ISC is maintainedthere, and attained in ThamesISC, the situation will be most satisfactory.If also the switching performanceof the ISC's can be matched by an improvementin overall international circuitavailability, customers should finda marked improvement in internationaltelephone services to and from the UK.References1 Button, R. W. and Buchmayer, M.:The Mollison International SwitchingCentre. Ericsson Rev. 52 (1975):2, pp. 46-60.2. Eriksson, V.: CCM-A Well-triedand Economic Maintenance System.Ericsson Rev. 53 (1976): 3 pp.134-137.3. Soderberg, A.: Automatic TransmissionMeasuring Equipment,ATME 2. Ericsson Rev. 57 (1974):1, pp. 21-28.

Laser Activities at LM EricssonGoran DahlFor some twenty years LM Ericsson's work on design and construction of electronicequipment for defence applications has been centred on the radar field.When laser technology was developed at the beginning of the 1960s, this type ofequipment was also included in the product range. The article is a survey of thelaser equipment range. The article is a revised version of a paper presented at theMilitary Electronics Defense Expo held in Wiesbaden in 1977.UDC621 375.826After the first laser light source had beenmade to work in 1960, one of the earliersuggested applications was rangefindingutilizing laser light pulses, i.e. opticalradar. Two features of the laserformed the basis for this suggestion.One was that the laser light is concentratedwithin extremely narrow andintense beams. The divergence of suchbeams can easily be reduced below 1milliradian. Because of this the laserbeam can be concentrated on small targetsalso at long ranges. The other thingwas that the laser energy also easily wasconcentrated in high peak power pulsesof a duration of c. 20 ns. Development oflaser rangefinder systems for fire controluse started as early as 1962 at ourplant in Molndal near Gothenburg. Thisdevelopment constituted a natural expansionof our activities since the majorpart of our business consists of radarsystems for military search and fire controlapplications. The first rangefinderprototype was field tested during 1965.In the spring of 1968 we obtained ourfirst large laser rangefinder productioncontract. This system was a long distancelaser rangefinder for coast artilleryfire control and was based on rubylaser technology. To the best of ourknowledge this was one of the very firstproduction contracts placed for a laserrangefinder intended for operativemilitary use. Fig. 1 shows a picture of thetransceiver unit mounted on a platform,on top of a TV camera.Since then LM Ericsson have obtainedFig. 1Laser rangefinder for coast artillery mounted ona platform and on top of a TV camera

67GORAN DAHLorders for more than 1000 laser rangefinders^ _ Missile, target area 0.5 m 2for such applications as artillery,Ml DivisionTelefonaktiebolaget LM Ericssoncoast artillery, anti-aircraft artillery andtank fire control. Of the 1000 lasers soldabout 250 are of the high repetition rateanti-aircraft artillery type. Ever since thestart in 1962 we have been workingindependently with laser rangefindertechnology without obtaining any licenceagreement from any other organization.Present production anddevelopmentThe present large scale production oflaser rangefinders comprises long distanceequipment for artillery and coastartillery and high repetition rate systemsfor anti-aircraft fire control. Fig. 2 showsFig. 2Laser rangefinder for anti-aircraft fire controlFig. 3a picture of the anti-aircraft laser rangefinder.The system consists of a transceiverunit and a power module.Anti-aircraft laser range capabilitymmm_ Transport aircraft, target area 10 m 2The transceiver unit is mounted on a__ Strike aircraft, strike helicopter, target area 2 m 2servo-positioned director and the angulartracking is performed either automaticallyby a TV or infrared sensor, ormanually with an optical sight. The laseris based on neodymium-YAG technology(wavelength 1.06/;m) and the repetitionrate is 10 pulses/sec. the rangecapability against aircrafts is typically8 — 10 km. This is shown in greater detailas a function of the optical visibility infig. 3. Range accuracy is ±4 m (1 a value).The transmitter is cooled by forcedair circulation within the transceiverunit. We have five different productioncontracts for this system and a numberof prototypes are also under evaluation.In a couple of these prototypes thetransmitted laser beam width can becontrolled and set to either 1.5 or 3mrad. Anti-aircraft fire control utilizingelectro-optical sensors is a rapidyadvancing technology.The new generation of the LM Ericssoncoast artillery lasers typically have range

Fig. 5Artillery laser rangefinderThe laser is mounted on a tripod. Azimuth and elevationangles are obtained with an accuracy ot 1 milliradlancoverages against ship targets out to 20km. The repetition rate is 2 Hz. This alsomakes possible quick ranging againstwater splashes from gun shells. Rangeseparation between target and shellscan be measured and used for fire controlcorrections.Fig. 4 shows the transceiver unit and anelectronic unit comprising controls,range displays, range counters, interfacecircuitry to adjacent fire controlequipment and power converters. Wenow have over 100 equipments of thiskind under production.LM Ericsson now use only neodymium-YAGtechnology for both productionand the development of prototypes.The development of this technologynow makes it possible to design smallsize rangefinders for Army use. MostArmy applications demand small andlight-weight equipment. At present wehave four laser rangefinder developmentcontracts at LM Ericsson for theArmy.These equipments include a smalltripod mounted rangefinder intendedfor field artillery forward observers. Thisrangefinder, which is now being fieldtested by the Swedish Army, is shown infig. 5. The laser head, which weighsabout4 kg including the optical sight, ismounted on a 2-axis goniometer fromwhich target angles can be obtainedwith an accuracy of 1 mrad. The totalweight is 11 kg with the tripod and asmall size NiCd battery. The rangecapability is 10 km and measurementscan be performed every 2 secondsAbout 500 measurements can be madeon one battery charge.A still smaller equipment under developmentis a hand-held version of theartillery laser. Fig. 6 gives an idea of thedimensions. The total weight includingbattery is approx. 2.5 kg. It is intendedfor artillery forward observers, infantryuse, mortar fire control, etc., and forranges up to 5 km. The equipment canbe mounted on a small size goniometer,also being developed, which gives anangular accuracy of 3 mrad. The totalweight including laser, battery andgoniometer with tripod is 7 kg. Field testingof a number of prototypes will startearly 1978.A third equipment, also intended forArmy use, is a laser rangefinder integratedinto a tank gunner's sight, fig. 7.Laser rangefinding will considerably increasethe range within which an acceptablehit probability is obtained. Thefirst application is a laser sight for theFig. 4Coast artillery laser rangefinderThe transceiver unit to the left and the electronic unitto the right

69improvement of the old Centurion tanks.Among the features of this sight is anoptical system, with a dual field of view,the wider field for surveillance, the narrowerhas a magnifying power of about 8times and is used for gun-laying. In thislatter telescope is integrated a spot injectionsystem. An electrically steerablepoint gives an "independent" aimingmark after the range measurement hasbeen performed and the gun has beenpointed in elevation and azimuth. Whilethe original line of sight follows the gunthe aiming mark remains on the targetand is used for further target tracking.The injection system is based on a linearLED array which is driven by a servomechanism in the azimuth direction.Elevation is chosen by switching on oneof the 100 elements of the array. The deflectionof this aiming mark is controlledby a fire control computer cooperatingwith the laser rangefinder system. Theequipment can be installed without anymodifications to the Centurion tank.Boresighting to the gun is made by twothumb wheels on the sight. The lasersight can of course easily be interfacedto suit also other armoured vehicles.Field testing of the system has beenmade this year.Finally we are developing a laser rangefindersight for anti-tank weapons. Thisequipment includes circuits that computesuperelevation of the weapon fromdata such as target range, gun powdertemperature and air temperature. Amoving horizontal bar in the sight givesan aiming mark for the gunner.All these four Army applications utilizethe same "common modules" fortransmitter, receiver, range countercircuitry, etc. The transmitter module isbased on a passively Q-switched Nd-YAG rod.We expect series production of lightweightArmy rangefinders to start during1978.Transmitter and receivertechnologySince laser rangefinder developmentstarted at LM Ericsson in 1962 the availablebasic laser material technologyhas improved very much. This has led tothe efficiency of the Q-switched lasertransmitter being increased substantially.Data given in table 1 illustrate thisclearly. Because of this improvement ithas been possible to reduce dimensionsand weights for rangefinders very significantly.When we built our first "portable"laser rangefinder in the beginningof the sixties you could say that the opti-Fig. 6 to the leftHand-held rangefinderRange coverage c. 5 kmFig. 7 to the rightLaser sight for armoured vehicles

70Table 1Laser transmitter dataIn rangefinding laser equipment the emitted laserenergy must be concentrated in one single highpeak power-pulse. The method to achieve this iscalled Q-switching Axial laser oscillations will startonly when the excitation of the laser material is atmaximum. There are various ways to proceed Ahigh speed rotating and retro-reflecting 90 degreeprism will together with a fix mounted mirror duringshort time intervals form the laser cavity. Only duringthose time intervals laser oscillations will begenerated. The position of the prism and the"pumping" of the laser rod are synchronized in away so that the cavity is formed at that momentwhen the excitation is at maximum In anothermethod an optical filter that absorbs the laser radiationis introduced into the cavity During initial laserradiation this filter is saturated and becomes transparentAfter this strong laser oscillations will bedeveloped between two mirrors fix mounted onboth sides of the laser rod. The absorbtion coefficientis selected in a way that saturation is achievedonly when the excitation on the laser material hasreached its maximum. This method is called "passive"Q-switching.cal sight for aiming was mounted on thelaser. Today it would be more true (seefigs. 5, 6 and 7) to say that the laser isadded to an optical sight. Through theyears the ruby crystal was succeededfirst by neodymium-glass and later byneodymium-YAG. Since about 2 yearsago all development and productionutilize the Nd-YAG laser.Until the development of the abovementioned Army applications started weused a rotating prism for Q-switching.Contrary to what has often been saidabout high speed rotating Q-switcheswe have found their reliability to bewholly adequate for our applications.Our transmitter "minimodule", however,works with a dye or "passive" Q-switch. The use of this considerably reducesthe weight, volume and complexityof the transmitter and also gives alower overall power consumption forthe complete system. Moreover, theequipment noise level is very much reduced,which is of importance in certainapplications. Fig.8 shows a comparisonbetween an older Nd-glass transmitterwith its rotating prism Q-switch and thenew transmitter module.Fig. 9 shows input-output energydiagrams for the module and an earlierNd-YAG transmitter of the same outputlevel and working with a rotating prism.Pulse lengths are 12 and 20 ns respectively.There is an interesting differencebetween the two curves in the diagramof fig. 9. In the case of the rotating Q-switch the output above the thresholdincreases gradually with the input to theflashtube that pumps the laser rod. At aFig. 8Earlier and new neodymium laser transmitters

71certain level the threshold for doublepulsingis reached. By then the energythat remains in the flashtube pulse, afteremission of the first laser pulse, is sufficientto create a second pulse.In the case of the laser transmitter utilizinga passive Q-switch the pulse growsup to a top amplitude value immediatelyafter the threshold is reached and staysconstant even if more pumping energyis supplied. The reason for this is thatonce the laser rod has reached a level ofexcitation where "prelasing" radiationis strong enough to saturate and openthe passive Q-switch dye, all the energystored is released in a short high peakpower pulse. The only thing that happenswhen more pumping energy issupplied is that the laser pulse is generatedearlier during the pumping interval.If the pumping level is high enough thethreshold level of the rod will be reacheda second time and another laser pulse ofthe same amplitude will be emitted. Ascompared to the rotating Q-switch thisis a very favourable situation. The levelof the input energy is much less critical.Fig. 10 gives an idea of the dimensionsand the design of the passive Q-switchtransmitter. The size of the laser rod is4x63 mm and a krypton flashtube isused for pumping.The Q-switch consists of an optical filterthat absorbs laser radiation at 1.06«mThere is an upper limit for the absorptionability of the filter. This is reachedwhen all absorbing molecules havebeen excited by the laser radiation. Thefilter material has been produced byKodak. Typically an output of 25 mJ isreached at a pumping level of 5 J. Sincethe half width of the pulse is approx. 12ns this means a peak power of 2 MW.Beam divergence out of the transmitteris 3 mrad and can be reduced by opticsto its desired value. Without any specialcooling laser pulses can be repeatedwithin 2 seconds. This is wholly adequatein equipment where the range ismanually read from a display.Over an ambient temperature intervalfrom -35°C to +55°C the thresholdchanges less than 5 % in energy. As canbe seen from the diagram of figure 9 nosteps therefore have to be taken to adjustthe pumping level. Output energyvariations are within ±15% over thetemperature interval. Lifetests done onthe transmitter show that more than100 000 pulses can be fired without anydegradation.Higher peak power transmitter levels, orcirca 5 MW, are used for coast artilleryand anti-aircraft artillery lasers. Fig. 11Fig. 9Laser transmitter characteristicsX Double pulse ,

72Fig. 115 MW laser transmitter energy characteristicsFig. 10Transmitter moduleshows a typical input-output energydiagram. As can be seen from thisdiagram an efficiency of about 1 % isobtained. The rod size is 1/4x3 inchesand a rotating prism Q-switch (24.000rpm and optical doubling) is used. Thebeam divergence from the transmitter isapprox. 2 mrad.When developing ruby laser transmittersabout 10 years ago we spent a considerableamount of time studying howto cool the rod and flashtube assemblyby a forced air stream. The methods weused in our first ruby laser rangefinderwere found to be applicable also forhigh repetition rate Nd-YAG transmitters.A high speed blower delivering airat the rate of 0.5 m 3 /minute provides thecooling. The air is forced through therod and flashtube assembly and thenpasses over both Xe-flashtube holdersand is cooled by the walls of the transceiverunit. Compared to other methodsused, such as liquid cooling or highpressure gas cooling, cooling by forcedair offers a simple and inexpensive solution.Moreover it has turned out to bevery reliable and is easy to maintain.The high repetition lasers now beingproduced operate continously at 10 Hzwithout any practical limitation of time.The flashtube lamp load is circa 100 W.In fact over shorter intervals of the orderof a few minutes, the repetition rate maybe increased to 15-20 Hz.A high repetition rate laser transmitter isshown in fig. 12. A fairly long cavity isneeded in order to obtain an energy ofover 100 mJ in one single pulse and witha rotating Q-switch. In this case we use alength of approximately 30 cm. Thelonger the cavity, the shorter is that timeinterval during which laser oscillationscan be generated. Outside this time intervallaser radiation will be lost quickeras the cavity becomes longer. In fig. 12the laser transmitter is the second unitfrom the right.Our life tests on high repetition ratelaser transmitters show that a couple ofmillion shots can be fired without anyserious reduction in output energy andwithout any necessary service to bedone.A silicon avalanche photodiode is usedas detector for the rangefinder receiversystem. To obtain maximum signalsensitivity the detector bias must be setto an accuracy of approx. 5 V out of 300

73V. Furthermore this optimum bias valueis dependent on ambient temperatureand background illumination. It can beshown that this bias is reached when thedetector noise is about equal to thenoise generated by the preamplifier. Inorder to be able to control the detectorat various ambient temperatures andbackgrounds the total receiver noise istherefore measured and made to definethe level of the bias voltage source.With the latest avalanche photodiodes,utilizing a so-called "reach through"structure, a signal sensitivity ofapproximately 10 nW is obtained for a 20ns pulse, at a wavelength of 1.06fi. Thisis at a signal-to-noise ratio of about 7times. The detector signal gain is bythen 80— 100 times. As a preamplifier weuse a FET cascode design. The equivalentinput current noise is 8-10 nAr.m.s. at a bandwidth of 25 MHz. To avoidbackscatter signals at short rangesfrom the atmosphere and from small"targets" of no concern, thesignal amplifieris provided with a time variablegain function. During time intervals correspondingto short ranges the gain isreduced. At minimum range this reductionis 30 — 40 dB. The gain is graduallyincreased and reaches its full and finalvalue at a time corresponding to 1 to 2km.1.06/< is very close to the absorptionedge of silicon and the quantum efficiencyof the detector is only about20%. Furthermore this quantum efficiencyis slightly dependent on theambient temperature. Fig. 13 shows thetemperature dependence which wehave measured for the signal sensitivity.The quantum efficiency is reduced by afactor of two at —30°C. In the region of+ 70 to +90°C the sensitivity is reducedrather quickly as the dark current becomesthe limiting factor. This receivertemperature dependence must be takeninto account when calculating rangeperformances for military environments.The subunit to the extreme right in fig.12 is an example of a receiver module.Our latest module is only about a fourthof this size.The receiver sensitivity for 1.06 u laserrangefinders migh be improved in thenot too distant future. It has been reportedthat GaAsT-xSbx avalanchephotodiodes with a quantum efficiencyof over 96 % at 1.06/

74Fig. 13Signal sensitivity temperature dependence'Equipment for generation of imagery based ontemperature radiationOther laser activities atLM EricssonIn addition to what has been mentionedabove we are also working on a coupleof other laser developments, which areworth mentioning here:Within an R & D programme we have developedlaser designators and functionalmodels of laser seeker heads forsemiactive guidance systems. The Nd-YAG designators work with repetitionfrequencies of up to 25 Hz and pulsepowers of approximately 3 MW. Laserseekers utilize PIN photodiode quadrantdetectors to generate an angular errorsignal. With the laser designator on theground, flight testing in a slow transportaircraft of the gimballed laser seeker hasbeen carried out. Acquisition rangesand tracking performances were in accordancewith the theoretical calculations.We are now also carrying outstudies of counter-measure methodsagainst such semiactive guidancesystems.Within an experimental programme weare adding an angular tracking functionto our anti-aircraft artillery rangefinder.For this a receiver utilizing four detectorsmounted in a quadrant arrangementis built into a transceiver unitof the same kind as we are now producing(fig. 2). The same detectors will alsobe used for rangefinding. The repetitionfrequency will be increased to 20 Hz.Field testing of this experimental systemwill start during the spring of 1978. Thesystem will then be mounted on LMEricsson's general purpose sensorplatform.For some years we have been participatingin evaluation of laser rangefindersfor air-to-ground ranging from attackaircrafts. It has been proven that a laserrangefinder is extremely useful for improvingthe precision of bomb tossing,of rocket delivery, etc.ConclusionsLaser rangefinders for use in military firecontrol systems are now fully developedproducts and their usefulness in varioussuch applications has been well proven.Since a number of years these devicesare produced in large quantities.For example, laser rangefinders areused in armoured vehicles where they,compared with the old optical rangefinders,provide an exact target rangevery quickly. Radar is a far less attractivealternative for such applications as tankfire control and forward artilleryobservers. Because of the much widerbeams used by radar equipment individualtargets cannot be pointed outwith sufficient unambiguity. The laser ishere the solution to a problem. In otherapplications, such as for example antiaircraftfire control, laser rangefinderswill either be used instead of a radarsystem or as a complementing back-upoptical alternative.Compared with the corresponding radarsystems, an electro-optical fire controlequipment, which includes a laserrangefinder and either an optical sight,TV tracker or FLIR* equipment for angulartracking, is less vulnerable to activecounter-measures. Angular tracking isperformed by passive equipment andtransmitted laser signals are confined tovery narrow zones. Moreover the dimensionsand weights of electro-opticalequipment are generally considerablysmaller.To continue the comparison with radarsystems, 1 ,w laser rangefinders ofcourse have a handicap because of theirlimited performance during reducedoptical visibility. However, our experienceis that "when the target is opticallyvisible, it can also be ranged with thelaser". This is a general rule from whichthere are exceptions, for example sometimesduring heavy rain and heavysnow-fall. Because of their longerwavelength Nd-lasers are less dependenton the weather than the older rubyrangefinders. In the future when FLIRtechnology will be taken into operativeuse it might be of interest to use C0 2laser technology for rangefinding. TheC0 2 laser wavelength of 10.6// will bettermatch the ability of the FLIR to seethrough haze, light fog, smoke, etc. Weare now studying this subject.It is well-known that the radiation fromhigh peak power lasers is hazardous totheeye. Maximum permissibleexposurelevels have now been recommended forvarious types of lasers. (See for exampleANSI 136J^J3Z6AJivthe case of direct

75viewing into the beam of an Nd-laserrangefinder this level is 5/

Digital Multiplex Equipment for 8and 34 Mbit/s Line SystemsStig Karlsson and Walter WidlThe future development of digital networks includes different kinds of pulse transmissionsystems, which constitute the p.cm. system hierarchy. Digital multiplexersare employed for the conversion between the different hierarchical levels.As early as 1974 LM Ericsson had developed a 120-channel digital multiplexer inthe M4 design^ The introduction of the M5 design 2 and new components made itpossible to miniaturize and modernize the digital multiplex equipment. The articledescribes multiplex equipment for 8 and 34 Mbit/s line systems (i.e. for 120 and 480telephone channels respectively). The multiplexers use positive justification asrecommended by CCITT. Different branching options and extensive supervisioncan be obtained.The same shelf can accommodate units for 120-channel or 480-channel multiplexers.This considerably simplifies maintenance and planning.UDC 621 376.56621.395.43Fig. 1Digital multiplex interfacesLIU Line interlace unitBU Buffer unitCUT Transmit control unitCUR Receive control unit2 Corresponds to 2048 kbit/s8 Corresponds to 8448 kbit/s34 Corresponds to 34368 kbit/sAB CD Tributary pulse MowsThe basic task of a digital multiplexer isto combine a number of incoming pulseflows (tributaries) into one outgoingpulse flow with a gross digit rate whichis somewhat higher than the sum of thetributary bit rates (and vice versa). Differentmultiplexing methods, based onthe 24- and 30-channel p.cm. multiplexers,synchronous or asynchronousworking, positive, positive-negative orpositive-zero-negative justification andbit or time-slot interleaving, have beendiscussed internationally to a great extentby administrations and manufacturers.As a result a number of CCITT recommendationshave been issued coveringhigher orderment.digitalmultiplexequip-The described digital multiplex equipmentswork asynchronously with positivejustification and bit interleaving ascovered by CCITT recommendations G742 (120 channels) and G 751 (480channels).The interfaces are defined by recommendationG 703.The second order digital multiplexequipment ZAK 30/120-2 designed byLM Ericsson combines four 2048 kbit/stributaries into one 8448 kbit/s signal.Correspondingly the third order digitalmultiplex equipment ZAK 120/480 combinesfour 8448 kbit/s tributaries intoone signal with a 34368 kbit/s gross digitrate. Both multiplexers are built in thenew M5 design.Due to similarities in basic principlesand standardization, as well as to suitableblock configurations, both kinds ofmultiplexers use the same shelf constructionwith several common units.Fig. 1 indicates interfaces and units, fig.2 the frame structures for both multiplexers.Owing to the similarities of themultiplexers it is sufficient to describeonly one of them in detail—the second

77STIG KARLSSONWALTER WIDLTransmission DivisionTelefonaktiebolaget LM Ericssonordermultiplexer-and just indicate thedifferences between the two.Multiplexing principleUp to four independent primary 2048kbit/s digital signals (tributaries A, B, Cand D) are combined to form one secondary8448 kbit/s digital signal. Thebits from the different tributaries aretransmitted bit interleaved. Additionalbits for frame alignment, alarm transmissionand tributary timing transmissionare arranged in the second orderframe structure.Since no synchronism is required betweenthe tributaries and the 8448 kbit/spulse flow, provisions have to be madeto inform the tributary receivers aboutthe gross digit rates generated by thetributary transmitters. The process involved,pulse justification, is based onthe use of additional timing informationrelated to each tributary.The multiplexing and demultiplexingprocess is explained in more detail forone tributary. The continuous 2048kbit/s pulse flow from the tributary iswritten into a bit rate conversion memory.The reading out from this memorytakes place discontinuously at 2112 kHz(8448:4 = 2112 kHz) due to pause intervalsin the second order frame structure.The majority of the pause intervals arefixed, i.e. the reading out is interruptedduring the transmission of the framealignment and justification service bits.The basic principle is explained in fig. 3where the memory bit positions areshown as a function of time during theperiod of a secondary frame (read clockphase relative to write clock). The sequencestarts with memory bit position 0point X. After the duration of a secondaryframe without use of positivejustification, memory position Y isobtained. When using positive justification,however, memory position Z isreached.Positive justification (connected with acertain triburary pulse flow) means thatone particular bit position in the secondorder frame (bit position 155 in ac-Fig. 2Second and third order digital multiplex framestructures, _„, h 848 blt/s (1536 bit/s)l'° me e " 9 bu, arv 206 bit/s (378 bit/s)Max Slicatlon rate per tributary 10 kbit/s (22375 bit/s)figures in brackets are related tothird order multiplex.• Bits available for justification

Fig. 3Justification processThe incoming tributary signal is fed into a buffer memorycontinuously and read out discontinuously at a somewhathigher clock frequency in order to maintain a bufferequilibrium. During the interruptions in the reading out,service Information for the multiplexing is inserted intothe pulse flow. This Information Is removed in thedemultiplexing process.Just servicebits 1Just.servicebits 2Just servicebits 3Fig. 4Generation of waiting time jitterMemory underflow leads to justification request. Justificationoccurs after a certain waiting time when thejustifiable bit position In the frame Is available. Thewaiting time variations f are responsible for low-frequencyjitterFig. 5Digital multiplexer jitter modelThe incoming jitter and the muldex Internal jitter areattenuated by the jitter transfer function|H(1 Jitter transfer functionpr] Multiplex internal jitter generator[J] AdderBitpositionscordance with CCITT recommendations)is not used for tributary transmissionand a dummy bit is inserted. Therefore,a pause interval of one bit length isadded to the fixed pause intervals duringone secondary frame and the bit flowrelated to the tributary in question is delayedone bit. In order to inform the receiverthat bit position 155 (connectedwith a certain tributary) should beignored, a three-bit signal consisting ofjustification service bits 1, 2 and 3 isgenerated e.g. A1, A2 and A3 for tributaryA. Positive justification is indicatedby the signal 111, no justification by thesignal 000. The detection of this signal inthe demultiplexer is based on majoritydecision. The loading of the memory ischecked by a comparator circuit. Secondaryframes without any justificationbits alternate with frames with a justificationbit per tributary in order toguarantee a loading equilibrium in themultiplexer bit rate conversion memory.The alternating sequence depends onthe precise relationship between writeinand read-out frequencies as indicatedin fig. 4 for a number of sequences.In the demultiplexer part of the equipmentthe inverse operation takes place.After separation into four tributary signalsthe incoming data is written intoa demultiplexer bit rate conversionmemory under the control of a timingsignal derived from the incoming 8448kbit/s stream. The timing works discontinuouslyin order to prevent framealignment digits, justification controldigits and justification digits from beingwritten into the bit rate conversionmemory.The read clock rate should be the averageof the write clock rate and this isachieved by deriving the read clock froma phase-controlled oscillator which iscontrolled using a phase comparatormonitoring the read and write clocks.The phase difference of the clocks has acharacteristic similar to that shown infig-4.Jitter influenceIn many cases the tributary pulse flowsto digital multiplexers are influenced bytransmission disturbances. The resultingshort term variations of the pulsedurations lead to pulse flow jitter.

Fig. 7Typical values for tolerable input jitterThe jitter amplitudes are related to nominal signalelement length. The amplitudes accepted by the multiplexerfor errorfree transmission depend on the frequencyof the litter.(T) High frequency fitter behaviour, the multiplexertolerates jitter amplitudes up to the nominal pulsewidth(5) The timing recovery circuit at the tributary Input isable to follow the Input jitter. Simultaneously the jitteris absorbed by the multiplexer bit rate conversionmemory3 The justification system is able to transmit the inputlitter from the tributary Input to the demultiplexerbit rate conversion memory.(4) The jitter is absorbed by the demultiplexer bit rateconversion memory(5) The timing recovery circuit at the tributary output isable to follow the input jitterFig. 6Jitter transfer functionThe jitter amplitudes are attenuated above the cut-offfrequency of the clock regeneratorT0 nominal pulse width = 1/2 unit intervall p.t.p.TIr input jitter amplitudeTout output jitter amplitudef, jitter frequencyF, jitter gain:20 log z-^-As shown by the simplified model in fig.5 multiplexer internal jitter is addedto the incoming tributary jitter. Theperformance of a digital multiplexer isdetermined by its ability both to tolerateand attenuate the jitter at the tributaryinputs and to limit the internal generatedjitter components.Fig. 6 shows the jitter gain for the multiplexequipment ZAK 30/120-2 as functionof sinusoidal jitter frequencies.The jitter input tolerance mask indicatesthe amount of input jitter which can behandled by a digital multiplexer. Thisparameter is of importance for the functionof a digital multiplexer as part of atransmission path and is treated extensivelyin CCITT. In fig. 7 both the tolerancemask provisionally discussed byCCITT and the performance of themultiplexer ZAK 30/120-2 are shown.The different sections of the jitter inputperformance can be derived in the followingway:LHigh frequency jitter behaviour, themultiplexer tolerates jitter amplitudesup to the nominal pulse width.2.The timing recovery circuit at thetributary input is able to follow the inputjitter. Simultaneously the jitter isabsorbed by the multiplexer bit rateconversion memory.3. The justification system is able totransmit the input jitter from thetributary input to the demultiplexerbit rate conversion memory.4. The jitter is absorbed by the demultiplexerbit rate conversion memory.5. The timing recovery circuit at thetributary output is able to follow theinput jitter.The multiplexing and demultiplexingroutines described above lead to thegeneration of a number of jitter componentswith different jitter frequencies.The jitter generated by the digital multiplexingand demultiplexing consists of:— Systematic jitter caused by the removalof frame alignment and justificationcontrol bits. The jitter containsfrequency components above 10 kHz.— Justification jitter caused by the insertionof justification bits at thenominal justification rate of 4.23 kHz.Fig. 8Block schematic for digital multiplex ZAK 30/120-2In order to obtain the digital multiplex ZAK 120/480 theunits LIU 2, BU 2 and LIU 8 are replaced by the units LIU 8,BU 8 and LIU 34. In the case of branching the buffer unitBU 2 (resp. BU 8) of the D-tributary is replaced by thebranching unitLIU 2 2 Mbit/s line interface unitHDBD HDB 3 decoderHDBC HDB 3 coderCR 2 2 MHz clock regeneratorBU 2 2 Mbit/s buffer unitMEM Bit rate conversion memoryCOM Phase comparatorAL.IND Alarm IndicationVCO Voltage controlled oscillatorJCR Justification control receiverCUT8/34 Transmit control unitJCT Justification control transmitterFAL Frame alignment logicFTL Frame & timing logicRTG Pseudo random bit sequence generatorCUR 8/34 Receive control unitTS Tributary selection circuitFTR Frame & timing receiverAL.DET. Alarm detector circuitsLIU8 8 Mblt/s line interface unitPS Parallel-serial converterSP Serial-parallel converterCR8 8 MHz clock regeneratorA B C D Tributariesni ' ' 2 Mblt/s digital link Interface02 8 Mbit/s digital link interface

80- Waiting time jitter caused by the factthat a justification request can not becarried out immediately. The durationof the waiting time varies and resultsin waiting time jitter which can havespectral components at very low frequencies.The effect of digital multiplexing anddemultiplexing on a tributary is increasedjitter. However, with proper designthe generated jitter can be kept wellbelow the levels where it wouldjeopardize the function of subsequentsystems. The values shown in figs. 6 and7 lie well within the limits discussed internationally.Digital multiplexer functionThe function of the multiplexer is illustratedby the block schematic in fig.8. The 8448 kbit/s gross digit transmitrate and the tributary read-out frequency2112 kHz are derived from a 8448 kHztiming generator containing a crystalcontrolledoscillator and a phase comparatorcircuit (VCO loop). The phasecomparator is located on the transmitcontrol unit (CUT 8/34) and the oscillatoron the line interface unit (LIU 8).The 8 Mbit/s VCO loop can also be controlledby an external 2112 kHz clocksignal. This frequency has been chosenin order to simplify the clock signal distribution.The oscillator frequency dividedby 4 controls the timing logic(FTL) which generates secondary framesignals such as frame alignment andalarm bits and also the justification bitand justification control bits for eachtributary.The HDB 3 coded tributary signals comingfrom the D1 -interface are convertedinto binary signals and simultaneouslythe tributary gross digit rate is retrievedin the line interface unit. (LIU 2). Thesubsequent buffer unit (BU 2) contains abit rate conversion memory and a comparator,which interwork with the justificationcontrol transmitter (JCT) on thetransmit control unit (CUT 8/34). Thetributary signals and their bits for timingtransmission, the frame alignment bitsand alarm bits are assembled in theframe assemble logic (FAL) andgrouped in a 4-bit parallel format for thetransmission to the 8 Mbit/s line interfaceunit (LIU 8). In the line interfaceunit serial representation is once againobtained, coded into HDB 3 signals andfed to the D2 digital interface.In the reverse direction the incoming 8Mbit/s HDB 3 signals are decoded withthe help of the regenerated 8 MHz clockand the serial representation is con-Fig. 9BranchingFig. 9a shows the traditional branching principle withback-to-back connected multiplexersFig. 9a and 9c indicate branching with LM Ericsson'sunique branching principle, permitting branching withonly one multiplexerButler unitBranching unitLine interface unitTransm.justif.unit +Receive justif.unit +Line interface unit

81Fig. 11Spectrum distribution of 8448 kbit/s pulse flowThe connection of unused tributary inputs to an internalpseudo-random pulse generator permits spectrumshaping suitable for line transmission systems. A correspondingfacility exists for the multiplexer ZAK 120/480.verted into a 4-bit parallel format. Theframe and timing regenerator (FTR) inthe receive control unit (CUR 8/34)performs frame alignment and generationof tributary bit positions, justificationbit positions and justification controlbit positions for each tributary. Inaddition, a tributary selection signal ispassed to the tributary selector (TS) andthe information related to the tributariesis obtained on four separate tributarybuses. Each bufferunit(BU2)containsabit rate conversion memory for the outgoingtributary signals. The informationis written in discontinuously with 2112kHz clock signals and read out with acontinuous 2048 kHz timing signal. Thistiming signal is regenerated by a timinggenerator which is controlled by thememory loading. The low cut-off frequencyof the LP-filter in the VCOfeedbackloop decreases the jitter generatedin the framing process. Owing tothe jitter suppression the 2 Mbit/s linesystem can be connected to the multiplexertributary outputs.Branching principlesThe introduction of digital multiplexingin multi-point networks often requiresbranching of tributaries at differentmultiplexing levels. Two back-to-backcoupled digital multiplexersare normallyrequired in order to obtain access tothe tributaries. An example is shown infig. 9a, where tributaries C and D arethrough-connected from Terminal I to IIIvia a branching Terminal II. TributariesA, A', B and B 1 are terminated in II. Thiscase requires two fully-equipped multiplexersin Terminal II. The8 Mbit/s linesbetween Terminals I and II, and II and IIIrespectively are not synchronizedHowever, LM Ericsson's digital multiplexershave been equipped withfacilities which simplify branching, asindicated in the following examples ofleak and stop branching.The principles used for simplified leakbranching are explained in fig. 9b. The 8Mbit/s lines between I and II and II and IIIrespectively are syncronized. This permitsthrough-connection of tributariesC and D through the use of an 8 Mbit/sbranching unit, a unit which is commonfor all through-connected tributaries.The 8 Mbit/s signal from Terminal IIIsynchronizes Terminal II. The secondaryframe with the C and D tributary bits isdetected in the branching unit (whichrecognizes the secondary frame alignmentword) and the bits are distributedto the proper positions in the secondaryframe transmitted from Terminal II toTerminal I. In the reverse direction thecomplete secondary frame received atTerminal II is distributed to Terminal IIIfor selection of the C and D tributarybits. The link between Terminals II andIII is synchronized by the link between Iand II. The arrangement in the branchingstation is simple: The D-tributary 2Fig. 10Alarm relations, second order digital multiplexThe nroarammlng facilities permit simple adaptation ton»«onal maintenance principles. Corresponding relations"isifor the multiplexer ZAK 120/480

82Mbit/s line interface unit is replaced byan 8 Mbit/s line interface unit and thecorresponding 2 Mbit/s buffer unit isreplaced by the branching unit.The stop branching, see fig. 9c, is basedon leak branching. The connection betweenTerminal III and Terminal II ismade via Terminal I by looping of thetributary interfaces in Terminal I. Theexamples in figs. 9b and 9c show thatone fully-equipped digital multiplexercan be saved in certain branching configurations.Alarm functions andmaintenanceThe digital multiplexer complies withLM Ericsson's general alarm philosophy,i.e. alarms from equipment placedin shelves cause bay frame alarms andcentralized station alarms. In addition,visual alarm indications are provided foreach individual equipment. Primaryalarms are generated automatically ormanually in the equipment itself (nearend) or are received from the far end.The alarms result in secondary alarmswhich are processed and indicated visuallyat the near end, transferred to thebay frame and station alarm units andtransmitted to the far end. The relationshipbetween primary and resulting(secondary) alarms is determined mostlyby PROMs (programmable read onlymemories) and in some cases by wirestrappings.Fig. 10 shows the alarms provided in thedigital multiplexer, ZAK 30/120, that arecommon for all tributaries. In addition,an LED indication of failure of the 2Mbit/s tributary timing is provided foreach 2 Mbit/s buffer unit and detectionof branching timing and framing failurefor each branching unit. The providedalarm functions and LED indications(situated on the transmit control unitCUT 8/34) permit easy location ofequipment failures and simplify equipmentmaintenance. In fig. 10 the correspondingalarm function for the thirdorder digital multiplexer is indicated inbrackets.The transmit control unit of the multiplexercontains a pseudo random bitsequence generator RTG. The signalsfrom this generator can be used eitherinstead of incoming tributary signals orfor measuring purposes. For partlyequipped multiplexers the empty tributaryinputs can be connected to thepseudo random signal source in orderto shape the spectrum of the higher ordersignal.Fig. 12Shelf for two digital multiplexersThe same shelf houses either two multiplexers ZAK30/120-2 or two multiplexers ZAK 120/480

83Technical dataZAK 30/120-2 ZAK 120/480Nominal bit rate 8448 kbit/s 34368 kbit/sTolerance ±30x10~ 6 ±20x10" 6Frame structure See fig. 2Frame alignment routinessee CCITT Recommendations G 742 G751JitterTributary input mask See fig. 6Jitter gain See fig. 7Digital link interfaces D1, D2 and D375 L> unbalSee CCITT Recommendation G 703 G 703Permitted attenuation of the input signal 0-6 dB 0-12 dBExternal and reference clock frequency 2112 kHz 8592 kHz(1/4 of gross digit clock rate.)Alarm functions See fig 10Criteria See CCITT Rec. G742 G 751Alarm indication signals (AIS):Transmitted on tributary outputs when themultiplexer is in the alarm state256 onesThe alarm indication state is activated atIthe reception of256 onesat the higher order signal inputSignalling channel bit 12E- and M-type interface, gross digit rate of 9.96 kbit/s 22.375 kbit/sPower supplyBattery 24, 36, 48, 60 V d.c. ( + 20%-15%)Mains 110, 127, 220 Va.c. ( + 10% —10%)Power consumption 22 W 48 WDimensionsBay frameMultiplex shelfHeight 2743 mmWidth 600 mmDepth 225 mmHeight 244 mmWidth 473 mmDepth 225 mmFig. 11 shows the spectrum of a multiplexerworking with only one tributarywith and without transmission ofrandomized signals in the empty tributarybit position. The smooth spectraldensity of the randomized line signal reducesthe risk of crosstalk.Mechanical designThe digital multiplexers are built in theM5 design. A standard M5 shelf accomodatestwo second order digitalmultiplexers, ZAK 30/120-2 (see fig. 12)or two third order digital multiplexersZAK 120/480. The same multiplexershelf is used for different branchingalternatives.The second order digital multiplexer isequipped with a special interface betweenthe 2 Mbit/s line interface unitand the 2 Mbit/s buffer unit (Ta interface).The same interlace exists in the30-channel p.cm. terminal as an option,i.e. when combining both 30-channelp.cm. terminals and a second orderdigital multiplex equipment within thesame bay frame. This makes possible adirect interconnection based on the Tainterface which saves two 2 Mbit/s lineinterface units per tributary. The digitallink interface resp. external clock interfaceconsist of easily accessiblecoaxial contacts situated on the left sideof the shelf.References1. Fremstad, P., Karl, H. and Karlsson,S.: Multiplex and Radio-RelayEquipment for a 120-channel PCMsystem Ericsson Rev. 52 (1975):2,pp. 76-89.2. Axelson, K., Harris, P.-O. and Storesund,E.: M5 Construction Practicefor Transmission Equipment.Ericsson Rev. 52 (1975):3/4, pp.94-105.

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