remote axe 10 subscriber switch in a container an optical fibre line ...

ERICSSONREVIEW41982REMOTE AXE 10 SUBSCRIBER SWITCH IN A CONTAINERAN OPTICAL FIBRE LINE SYSTEM FOR 34MBIT/STRANSMULTIPLEXERSERITEX 10 FOR TELETEX AND WORD PROCESSINGLOCAL AREA RADIO SYSTEM-LARSLOAD STUDY OF THE AXE 10 CONTROL SYSTEM

ERICSSON REVIEWNUMBER 4 • 1982 • VOLUME 59Printed in Sweden, Stockholm 1982Copyright Telefonaktiebolaget LM Ericsson, StockholmRESPONSIBLE PUBLISHER GOSTA LINDBERGEDITOR GOSTA NEOVIUSEDITORIAL STAFF MARTTI VIITANIEMIDISTRIBUTION GUSTAF 0 DOUGLASADDRESS S-12625 STOCKHOLM, SWEDENSUBSCRIPTION ONE YEAR $12,000 ONE COPY $3.00PUBLISHED IN SWEDISH, ENGLISH, FRENCHAND SPANISH WITH FOUR ISSUES PER YEARTHE ARTICLES MAY BE REPRODUCEDAFTER CONSULTATION WITH THE EDITORContents174178185194203208Remote AXE 10 Subscriber Switch in a ContainerAn Optical Fibre Line System for 34 Mbit/sTransmultiplexersERITEX 10 for Teletex and Word ProcessingLocal Area Radio System-LARSLoad Study of the AXE 10 Control SystemCOVERERITEX 10, electronic typewriter for teletex communicationand word processing

;2Remote AXE 10 Subscriber Switchin a ContainerBjorn NorevikThe digital subscriber switch in the AXE 10 system can be placed either in theparent exchange or remotely. In the case of remote subscriber switches it canoften be an advantage to have the equipment installed in a container, not only asa temporary solution but also as a more permanent arrangement. A productpackage has therefore been prepared which can be used for remote units ofdifferent sizes up to 2048 subscribers. The product package also includescooling equipment, which can be of the conventional type or a system wherewater is used to remove the surplus heat. The latter system offers standbycooling if there is a mains failure.The author discusses the motives for this type of equipment and what demandscan be made on it, and also describes the equipment included in the productpackage.UDC 621 395 3When planning a new telephone networkor extending an existing one it isnecessary to assess many uncertain factors.However, some basic facts are wellknown. The subscriber network is responsiblefor the greater part of theoverall cost of the telephone network.Thus an extension often requires largeinvestments in the network, particularlyif existing cable routes are already fullyutilized.Consideration must also be paid to increasesin the building costs. Uncertainfactors, e.g. the rate of growth and thedemand for new telephone facilities,necessitate flexibility in the choice ofboth temporary and permanent solutions.A digital remote subscriberswitch-an economicsolutionThe digital subscriber switch in AXE 10has been described in detail in a previousarticle 1 . The article also describedhow the digital subscriber switch couldbe installed remotely from the parent exchange.A remote digital subscriber switch, RSS,constitutes an integral part of an AXE 10exchange, and subscribers connectedto RSS are thus offered exactly the samefacilities as other AXE 10 subscribers.It is economical to connect remote subscriberswitches to their parent exchangesby means of PCM links bothwhen building up new subscriber networksand when extending existingones. In the case of new networks theFig. 1A container in place and connected to the networkin Saudi-Arabia

175BJORN NOREVIKPublic Telecommunications DivisionTeletonaktiebolaget LM Ericssonneed for cables in the primary networkwill be considerably less than if the subscribersare connected directly to theparent exchange, and in the case of extensionslarge investments can also beavoided. This applies especially if thecable routes are already fully utilized,since every PCM link installed frees severalcable pairs.Remote subscriber switchesin containersThe relatively limited physical size ofRSS also makes it possible to install it ina container, fig. 1. This method can beused for subscriber switches with bothtemporary and permanent positions.The equipment can be moved accordingto need.A container is also considerably cheaperthan a conventional exchange building.ApplicationsThe reasons given above make RSS installedin a container particularly suitablefor the following applications.- as a permanent extension to an alreadyfully developed exchangebuilding- as a permanent extension in an existingnetwork, particularly when the existingcable routes are fully exploited- as a semi-permanent or permanentsolution when building up a new subscribernetwork and there is some uncertaintyregarding future developments- as a temporary solution when quickservice is required, i.e. before the exchangebuilding has been completedor in emergency situations when theordinary exchange is out of operationbecause of a fire or for some otherreason.Product packageThe type of container chosen for RSS isan insulated refrigeration container,built to ISO standard dimensions 2 (10 or20 feet long, 9 feet high, 8 feet wide). Theshorter container is used for an RSS forup to 512 subscribers, and the 20-footcontainer for the full 2048-group, fig. 2.Since the container is insulated, thesame type can be used for all installationsregardless of the widely differingenvironments that will be encounteredaround the world, and adaptations tosuit different markets are avoided. Anefficient and reliable cooling systemensures a suitable working environmentfor the telephone equipment inside thecontainer.The container is built to ISO standard 2as regards dimensions and handling facilities.The handling requirements relateto, for example its strength and devicesfor lifting the container. The standarddimensions also facilitate transport(by train, lorry, boat or aricraft),since standardized handling equipment,such as cranes and trucks, can beused. The easy handling and the standardizedconstruction also contributeto speedy installation on site, so that theequipment can be put into operation almostimmediately. If necessary, RSScan also just as easily be moved to a newsite later on, to meet new demands.Since the version of RSS intended forcontainer installation is standardized,all cabling within and between magazinegroups, as well as to the otherFig. 2The layout of a 20-foot container with a remotedigital AXE 10 subscriber switch for 2048 subscribersLSM Line switching modulesLT Line terminal for PCMR Power supply equipmentSTB Signalling terminal equipmentSE Special subscriber equipment tor coin box sets,subscribers' private meters etc.MDF Main distribution trameAC Alternating current connector

Fig. 3The container is brought out from one ot Ericsson'sfactories near Stockholm, SwedenFig. 4The container en routeFig. 5TransshipmentFig. 6Jacks are used at the installation site to lower thecontainer onto its baseequipment in the container, can be prefabricatedand installed before delivery.This means a considerable reduction inthe amount of installation work requiredon site.A complete RSS in a container, i.e. theproduct package, includes power feeding,cooling, main distribution frameetc., and the equipment in the containercan therefore be tested as a completeunit before delivery. This means that theinstallation testing on site is reduced toa simple verification test before theequipment is taken into service.LayoutFig. 2 shows the layout of a containerwith a full 2048-group RSS. The necessarytelephone equipment, line modulesLSMO-15 for the 128-groups and theequipment common for the whole 2048-group, are placed in a single row againstthe wall and one double row. This layoututilizes the available space efficientlyand gives two aisles with sufficientspace for installation and maintenancework. The cooling equipment has beenplaced at one end so that installation aswell as any maintenance can be carriedout via the doors at that end, withoutaccess to the telephone equipment. Thisalso applies for the battery area, whichfor safety reasons is completely separate,with access via a separate door.The main distribution frame can also beseparated from the telephone equipment,by means of a folding inner door,thereby creating a dust lock. The maindistribution frame is also placed againstthe wall in order to minimize the amountof space required and to ensure sufficientworking space for installation andany alterations that might be necessaryduring operation. The container alsohas a certain amount of space for transmissionequipment, temporarily connectedI/O devices, fire extinguishersetc.Container constructionThe container is made of a special steelwith the walls welded to a framework sothat an airtight and robust constructionis obtained. It is then given a finish thatprovides long-term protection againstcorrosion even in extreme environments.Ventilation and dehumidificationtakes place via an air inlet at one end andthrough an outlet at the other end viathebattery area.The container is equipped with four adjustablefeet so that it can be alignedhorizontally on a concrete foundation orconcrete plinths, fig. 1.Cooling systemIn order to ensure a suitable environmentfor the telephone equipment, regardlessof the external environment,RSS in a container can be equipped witha conventional air-conditioning systemor Ericsson's new cooling system whichuses water to remove the surplus heat.This system has been described in detailin a previous issue of Ericsson Review 3 .Ericsson's new cooling system is particularlysuitable for unmanned equipmentof this type, since it has high reliabilityand requires only a minimum ofmaintenance and, above all, since it isequipped with standby cooling in theform of a tank of cold water. This tank isdimensioned so that if a mains failureshould occur the standby tank will lastas long as the batteries of the powerequipment. This ensures reliable operationthroughout the specified standbytime.Power equipmentThe power equipment is of the typeBZA106, a type which has been developedfor use in small equipments likeRSS and which meets Ericsson's standardrequirements for power distributionto electronic exchange systems.The power equipment has a modularstructure and its capacity can be adaptedfor RSS units of different size.The batteries are placed in a separatebattery area, mounted on one to threeshelves. There is sufficient space for thelarge battery capacity required in orderto provide the long standby time, whichin many situations is needed for a remotesubscriber switch in a container.Main distribution frameThe main distribution frame is of thesame type as Ericsson's miniature MDF.BAB 340 with slot connectors, but withthe construction practice modified forsingle-sided installation (BAB345).fig. 7. This layout provides ample workingspace for installation and for alterationsriurinn nneratinn

177Fig. 7Interior view ot a container with equipment for2048 subscribers. The main distribution framecan be seen in the foregroundSummaryThe advantages of placing the AXE 10remote digital subscriber switch in acontainer can be summarized as follows:- In all applications there is a considerablereduction of the installationtime, sincetheequipment can be testedas a complete unit at the factorybefore delivery.- The chosen container types are builtto ISO standard sizes, which simplifieshandling and transport,figs.3-6.- The exchange environment for thetelephone equipment is maintainedwith the aid of a conventional air-conditioningsystem or Ericsson's newcooling system with stanby coolingduring mains failures.- The power distribution meets Ericsson'sstandard requirements for electronicexchanges.- The batteries are accessible from outsidethrough a separate door, and themain distribution frame can also beseparated from the telephone equipment.- Two sizes of RSS in container areavailable:• up to 512 subscribers in a 10-footcontainer• up to 2048 subscribers in a 20-footcontainerOther AXE 10 applicationsThe digital remote subscriber switch isalso available in a smaller size for therange 64-128 subscribers. This equipmentmakes it possible to introduceAXE 10 with its many facilities in the outermostparts of the subscriber network.The volume of this equipment is so smallthat it has been mounted in a cabinet forinstallation either indoors or outdoors.Other AXE 10 products, mainly small exchanges,are also available in standardizedcontainer versions. They have thesame general structure as described forRSS in this article, and thus also thesame specific properties and advantages.References1. Persson, K. and Sundstrom, S.: DigitalLocal Exchanges AXE 10. EricssonRev. 58(1981):3, pp. 102-110.2. ISO Standard TC104. Freight Containers.3. Almquist, R.: A Cooling System forElectronic Telephone Exchanges.Ericsson Rev. 58 (1981):4. pp. 188-195.

An Optical Fibre Line Systemfor 34 Mbit/sTommy Jansson and Bo StjernlofThis article introduces Ericsson s digital line system for optical fibre cable.ZAM34-2. The transmission rate of the system is 34 Mbit s. which corresponds to480 PCM coded telephone channels. The system meets all relevant CCITT andCEPT recommendations. It is primarily intended for use in urban networks, e.g.as the transmission link between exchanges or between an exchange and anumber of remote concentrators or as an entrance link to a radio relay link. Thesystem is available in two versions, one with a laser transmitter and the otherwith a light emitting diode (LED) transmitter. The laser permits a repeaterspacing of 12km as against 5 km for the LED. However, the LED version offersgreater reliability.fibres having an attenuation of 3-4dBper km and a bandwidth of 300 MHz • kmat a wavelength of 850 nm.The small spectral width of the laser,together with the large bandwidth of thegraded index fibre, result in negligiblematerial and mode dispersions in thelaser system, e.g. the system is limitedby the attenuation. Because of the relativelylarge spectral width of the LED,the limitations of this system, is set bythe material dispersion of the fibre.UDC 621.315:535 394621.391.63Fig. 1The equipment in the 34 Mbit/s systemLDM Line terminating magazineFDU Fault detector unitFDS Fault detection shelfSCS Service circuit shelfST Service telephoneCTB Cable terminating boxD3 34 Mbit/s interface, CCITT G.703 8F3 41 MBaud optical fibre interfaceAL Alarm interfaceDuring the years 1977-1979 a 34 Mbit/sline system for optical fibre, ZAM34-1,was developed for field trials. The purposeof this system was to give Ericssonand the telephone administrations theopportunity to gain experience of digitalline systems for optical fibre cable. Anumber of such systems have been deliveredto various users, and today severalare in regular operation. The newline system, ZAM34-2, is to a great extentbased on the experience gainedduring the design and installation ofZAM34-1 field trial systems 45 .Line system ZAM34-2 with a transmissionrate of 34.368 Mbit/s is intended fordigital transmission of 480 PCM codedtelephone channels over fibre cable.The light source used is either a laserdiode or a light emitting diode (LED),and the photo detector is an avalanchephoto diode (APD). The transmissionmedium is a cable with graded indexZAM34-2 offers the following advantages:- laser or LED transmitter. The latter,which could be used for repeaterspacings less than 5 km, offers higherreliability and lower costs- large repeater spacings, 12 km withlaser transmitters- thermoelectrical cooling of the laserdiode for maximum reliability- modular structure. The system caneasily be converted from a shortwavesystem for 850 nm to a longwave systemfor 1300 nm by changing thetransmitter and receiver units- simple connection to the fibre viaplug-in optical connectors which donot require any subsequent adjustment- flexible mechanical constructionusing Ericsson's BYB constructionpractice, which simplifies installationand handling. All connections aremade with connectors on the fronts ofthe units

TOMMY JANSSONBO STJERNLOFFibre optics andline transmissionTelefonaktiebolaget LM Ericsson- the same magazine is used for the lineterminal and the intermediate repeater,which makes it easy to change theequipment over from termination tothrough-connection- fault location function and facilitiesfor connection to Transmission MaintenanceSystem ZAN 101System characteristicsThe interface for the 34 Mbit/s line systemis D3. and is in accordance withCCITT Rec. G.703. An 8/34 Mbit/s or2/34 Mbit/s multiplexer, e.g. Ericsson'sZAK 120/480' orZAK30/480'.oradigitalradio relay link can be connected to thisinterface.- fault detection equipment for remotesupervision of line terminals and intermediaterepeaters- two or four-wire service circuit equipmentin the terminal bay, and connectionpoints in the intermediate repeatersfor a service telephoneThe incoming line cable isterminated ina cable terminating box, where thefibres are distributed to the different systems.In the box the fibres of the linecable are welded to the fibres in the baycables. The other ends of the bay cablesare fitted with plug-in optical connectors,for connection to the transmitterand receiver units of the system inquestion.Fig. 2Block diagram of the line terminating magazineAISDCLCELCEILSRLSTLDDPFLD3Alarm indication signalData channelLine code error detectionLine code error injectionLoss of signal, receive directionLoss of signal, transmit directionLaser diode degradationFailure of local power supplyLine system ZAM34-2, fig. 1, comprisesline terminals and two-way intermediaterepeaters for transmission over opticalfibre cableThe equipment is normally mounted inM5/BYB bays and consists of- line terminating magazines, LTM,where the signal is recoded, the jitteris reduced and the line is supervised- locally powered two-way intermediaterepeaters that regenerate the signalat regular intervals along the fibrecable- a cable terminating box (CTB) forconnecting the fibres in the line cableto the fibres in the bay cablesAny copper pairs in the fibre cable canalso be terminated in the cable terminatingbox.The equipment is easy to install and haswell defined internal interfaces TheBYB magazines are delivered with theunits in place and with all external connectionson the front of the units. Theequipment is strapped for the most commonapplication, and only a minimumnumber of straps have to be made duringinstallation.Terminal equipmentLine terminating magazineThe main functions of the line terminatingmagazine, LTM, fig. 2, are to- adapt the signal in the send and receivedirections between the D3 interfacestandardized by CCITT for34 Mbit/s and the optical fibre interfaceF3- detect and indicate alarms.The basic version of the line terminalconsists of seven printed board assemblies:- interface unit- line coder- laser or LED transmitter- APD receiver- line decoder- alarm unit- d.c./d.c. converterfor±5Vand ±12V.In the receive direction of the interfaceunit the HDB3 signal from the D3 interfaceis equalized for the attenuation ofthe connecting cable (max. 12dB at

Fig. 3Repeater spacings and hardware complexity withdifferent types of codes. The choice of the 5B6Bcode gives a good compromise between hardwarecomplexity and dispersion limiting17 MHz) and regenerated. The signal isthen recoded to a binary serial bitstream and passed to the line coder Inthe case of loss of input signal an alarmis given to the alarm unit. Violations ofthe HDB3 coding law result in an errorpulse which is available at a test pointThe line coder carries out the code conversionto adapt the data bit stream for alaser or LED transmitter. The chosencode is of type 5B6B, i.e. five binary symbolsare recoded as six binary symbols.This gives a symbol rate on the fibrecable of 6 345 = 41 MBaud Since thenumber of incoming binary states.2 5 = 32, is less than the number of outgoingbinary states. 2 6 = 64, the codehas built-in redundancy This redundancyis used to create a code spectrumwith a constant d.c voltage componentand a spectral energy distribution that issuited for optical transmission. The redundancyis also used for error supervisionof the signal and to get alow-capacityasynchronous data channelThe choice of code is by necessity acompromise. Other codes, i.e. 3B4Bwhich is used in ZAM 34-1. give a greaterdispersion penalty in LED systems.Higher order codes would require toocomplex equipment, fig.3.The bit rate conversion that is neededfor the coding is carried out with the aidof a crystal-controlled phase lockedloop, PLL. The loop bandwidth has beenchosen so that the jitter on the incomingsignal is reduced, resulting in a decreasedalignment jitter requirement inthe receivers of the repeater.Line code errors can be injected in theline coder at an optional rate in order totest the fault location system and alarmunit. The coding ensures that none ofthese bit errors remain after the decodingof the line signal.The laser transmitter receives the coded41 MBaud NRZ (Non Return to Zero) bitstream and the clock signal from the linecoder and converts them to a 41 MBaudRZ (Return to Zero) bit stream This isdone in order to reduce the problems ofinter-symbol interference in the receiver.The RZ signal modulates a laserwhich provides an optical signal on thefibre cable The operating point of thelaser is mean value regulated by meansof optical feedback with a photo diode atthe rear mirror of the laser. This stabilizesthe laser operation and compensatesfor temperature variations andageing effects.The laser is cooled by a thermoelectricelement to a constant temperature inorder to obtain maximum reliability. Aspecial laser unit has been developedwhich meets the system requirements.fig. 4.The laser temperature can be checkedby measuring a voltage in a test point onthe front of the unit.The operating life of the laser systemdepends mainly on the life of the laser,and the system is therefore equippedwith a circuit which supervises the conditionof the laser. An alarm is givenwhen the laser has degraded. The alarmis given well in advance, before the systemperformance is affected.Fig. 4The laser transmitter with the laser unit.The laser is cooled to a constant temperature inorder to obtain the highest possible reliability.The laser is stabilized by means of mean valueregulation of the operating pointAn alternative to the laser transmitterhas been developed, namely an LEDtransmitter which is intended for shorttransmission distances. It is fully compatiblewith the laser transmitter as regardsits connection. The LED transmittergives the system even higher reliabilitythan the laser transmitter. Likethe latter, the LED transmitter convertsthe incoming 41 MBaud NRZ flow to anoutgoing optical 41 MBaud RZ flow. TheLED is mounted on a cooling flange inorder to reduce its operating temperature.The receiver detects the incoming opticalsianal and converts it tn a 41 MRaurl

181Fig. 6Oscilloscope picture of the signal at the detectionpoint in the form of an eye diagram tor a lasersystem. Cable length: 10kmFig. 5aAPD receiver.The phase-locked loop is built up around avoltage controlled crystal oscillator, which ensureslow output jitterNRZ electrical bit stream. An avalanchephoto diode, APD, is used as the photodetector in order to obtain the best possiblereceiver sensitivity and dynamicrange. After the conversion in the APDthe signal is amplified in an input stage.This is followed by an amplifier with automaticgain control, AGC. The APD ismounted direct on the hybrid-type inputcircuit, fig.5. This minimizes the inputcapacitance, which would otherwise reducethe sensitivity of the input stageThe use of hybrid technology also minimizesthe effects of any electromagneticdisturbances. The sensitivity and dynamicrange are further improved byregulation of the APD reverse voltage.When an LED transmitter is used, a dispersionequalizer is automatically connectedin after the AGC amplifier Theamplified signal is filtered in a low-passfilter so that the highest possible signalto-noiseratio is obtained at the detectionpoint. Fig. 6 shows the signal at thedetection point in the form of an eyediagram for a laser system with 10 km offibre cable.The timing recovery from the signal flowtakes place in a PLL containing a voltagecontrolled crystal oscillator, whichprovides the necessary compensationfor temperature variations and ageingeffects. The relatively high Q value of theoscillator and its insensitivity to disturbancesensure low output jitter. The signalis regenerated with the aid of therecovered timing. The binary regenerated41 MBaud signal and the clock signalare fed to the decoder. These signalsare also accessible for an optional errordetector, ED The voltage across theAPD can be measured at a test point onthe front of the unit. For reasons of safetythe voltage is divided down to a hundredthof the actual valueThe binary input bit stream to the decoderis series/parallel converted andcode converted in a 5B6B decoder. Thedata signal and timing signal are thenfed to the previously described interfaceunit.The redundant words that are not usedin the normal encoding process causeerror pulses These pulses are fed to aresynchronization circuit and also to analarm unit. The resynchronization logicensures that the system is resynchronizedif the error rate of the incoming bitstream is too high. The logic is fittedwith error burst blocking in order to preventresynchronization for high errorrates of very short duration. The informationtransmitted over the data channelis also accessible in the decoderThe timing conversion required for thecode conversion is carried out with theaid of a crystal-controlled PLL. The loopbandwidth has been chosen so that theline signal jitter is reduced The jitter outtowards the multiplex equipment isthereby minimized. The serial datastream from the line decoder is HDB3coded in the send side of the interfaceunit.Fig. 5bA part of the APD receiver with the lid removedfrom the unit containing the hybrid circuit withthe avalanche photo diode. The APD is mounteddirect on a hybrid circuit in order to ensure thebest possible receiver sensitivity. A wide dynamicrange is obtained through regulation of the APDvoltaae combined with an AGC amplifier

182The primary alarms are those recommendedby CCITT. They are concentratedin the alarm unit of the magazine.Alarm concentration and alarm transmissionare carried out in accordancewith the same principles that apply forother equipment in the BYB constructionpractice. An alarm indication signal(AIS) is inserted in the send or receivedirection if there is a loss of input signalat the D3 interface or too high error rateon the line.The equipment is mounted in a BYBmagazine, fig. 7, and is powered from abattery via the d.C./d.c. converter in themagazine.It has been possible to use standard wirewrapping methods for the wiring of theequipment, since the internal transmissionsymbol rate between the printedboard assemblies is limited to41 MBaud.Intermediate repeatersThe 34 Mbit/s line system is intended primarilyfor urban networks. This meansthat the repeater spacings are such thatpoint-to-point circuits or circuits containingonly a few intermediate repeatersare most likely to be used. In thosecases where intermediate repeatershave to be used it is therefore also possibleto use local power supply.The two-way intermediate repeater withlocal power supply is mounted in a BYBmagazine which is an equipment versionof the line terminating magazine.The intermediate repeater conists of- two laser transmitters- two APD receivers- one d.c./d.c. converter.Identical types of units are used in theintermediate repeaterand the line terminatingmagazine.Intermediate repeaters which can bepower fed via a separate copper pair inthe fibre cable will be available later. Theonly modification required for these willbe to change the d.c./d.c. converters.These intermediate repeaters can thenbe installed in buried containers if required.Fig. 7The line terminating magazine is built up in a BYBmagazine, with the connectors at the front of theunits

183The line terminals and intermediate repeaterscan be equipped with a unit forsupervising bit errors in the transmission.This error detector unit. ED, containstwo bit error detectors, one foreach direction of transmission, whichmonitor the variations in the runningdigital sum, and which send error pulsesto the fault detector in the fault detectionsystem if bit errors are detected. EDalso contains a function for monitoringlaser alarms from the laser transmittersin the intermediate repeaters and lineterminals. Any laser alarm causes shortcircuiting of an extra copper pair, whichis connected to ED. The short circuit isdetected by the ED in the line terminal ofthe supervising station which is connectedto the terminal alarm unit viaconnectors on the fronts of both units.The extra copper pair is power fed fromthe supervising line terminal via a separateremote power feeding unit.BayA bay holds terminal equipment for amaximum of ten systems. The capacityis reduced to six systems if a fault detectionshelf and rectifier are also mountedin the bay. A board holder unit ismounted at the bottom of the bay. It cancontain an alarm concentrator for bayalarms, a service telephone connector,and also a d.c./d.c. converter for poweringthe fault detection shelf, if provided.Fault locationRepeater fault locationFaulty line repeaters are located with theaid of a fault detection system which iscommon for several line systems. Thesystem consists of a fault detection shelf(FDS), placed in the station from whichfault location is to be carried out, and afault detector unit (FDU), placed togetherwith the intermediate repeaters.The fault detection shelf is identical tothe one used for Ericssons 2, 8 and140Mbit/s line systems, which meansthat it can be connected to the TransmissionMaintenance System ZAN 101The fault detection system makes it possibleto detect bit errors in the line signalduring traffic.Each repeater contains an ED, which isconnected to a common FDU. The latter,in its turn, is connected to the fault detectorshelf via a separate metallic wirepair.The fault detector unit is identical to thatused for the 140Mbit/s coaxial line systemZAY140-1 6 .Cable terminating boxLine terminating magazineLine terminating magazineInformation concerning the bit errorrate on a certain repeater output can betransmitted to the fault detection shelffor analysis by means of a simple addressingprocedure. It is possible to locatea fault by addressing consecutiveintermediate repeaters and comparingthe bit error rates The system can betested by injecting code errors in the lineterminating magazine.Fig. 8A line terminal bay with six line terminals, cableterminating box, fault detection shelf and mainsrectifierFault detection shellLine terminating magazinewith front coverRectifierService circuitA two-wire or four-wire service telephonecircuit is available in systemZAM34-2, for example at fault location.The fault detector units in repeater stationshave access to a service telephone.At the ends of the route the twoor four-wire service circuit equipment isconnected to the ordinary two-wire servicetelephone in the M5/BYB bay.

184Technical dataDigital interface (D3)Bit rateLine codeImpedancePulse amplitudePermissible cableattenuation at 17 MHzOptical interface (F3)Symbol rateLine codeOutput powerWavelength (typical)Spectral bandwidth (3 dB)Input sensitivityDynamic rangeTransmission mediumFibre typePermissible dispersionCore diameterCladding diameterNumerical aperturePower supplyNominal battery voltageMains voltageToleranceMains frequencyPower consumptionLine terminalIntermediate repeater(two-way)Fault locationLocating faulty repeatersMaximum number ofrepeater stationsService telephone circuitAmbient temperatureLine terminalIntermediate repeaterDimensionsLine terminal andintermediate repeater34.368 Mbit/sHDB375 ohms, unbalanced1 V12 dBLASER LED41.2416 MBaud5B6B-2 dBm -24 dBm830 nm 880 nmS3nm 5=50nm-51 dB -47 dB30 dBLow loss, gradedindex9 ns 18 ns50 |im125 |im0.21 ±0.0236, 48, 60 V110, 127. 220 V10%45-65 Hz45 W16 WVia a metallic pair32Two-wire orfour-wire0 to +45"-20 to +55 : CH, W, D244x244x225 mmThe permissible attenuation over theloaded service circuit pair is 25dB fortwo-wire and 40dB for four-wire circuits.In order to avoid extra copper pairs inthe fibre cable, a new fault detection systemis being developed which utilizesthe possiblitity of transmitting the servicechannel and fault detection signalsover the fibre used for the normal transmission.SummaryLine system ZAM 34-2 for optical fibre isa product with good technical performanceand high reliability. It has a modularstructure, is robust and easy to handleand install, features which are all ofgreat importance to the user. Theseproperties have been achieved by- drawing on the experience gainedfrom the design and installation ofother digital line systems, such asZAM 34-1 and ZAY 140-1- applying standardized constructionpractice, alarm and supervision methods- using plug-in optical fibre connectors- simulating and optimizing the systemperformance and the functions of thecircuits with the aid of computer programs- using active cooling and continuousalarm monitoring of the laser- using LED for distances of less than5km.References1. Hamacher, H.-H. and Karlsson. S :Higher-Order Digital multiplexers.Ericsson Rev. 58 (1981):4, pp. 196-200.2. Hallberg. P.-A and Viklund, B :Construction Practice BYB forTransmission equipments. EricssonRev. 57(1980):4, pp. 124-128.3. Arras, J. and Mattsson, O.: Digital LineEquipments for 8 Mbits and 2Mbit/s.Ericsson Rev. 54 (1977):3. pp. 114-124.4. Giertz, H. and Vucins, V.: 34MbitsOptical Fibre Line System ZAM 34-1Ericsson Rev. 57 (1980):3, pp. 104-108.5. Gobi, G. and Hogberg, S.: Field Trialwith Optical Communication. EricssonRev. 57(1980):3, pp. 109-116.6. Eneborg, M. and Mattsson. 0..140 Mbit s Line System. Ericsson Rev.59 (1982):2, pp. 91-99.

TransmultiplexersSixten EkelundAnalog transmission systems are distinguished by their high capacity, whichmakes for good economy in the long-distance network. Since digital systems arebeing introduced in various parts of the network, it is likely that both FDM andTDM signals will be used in the telecommunication network in the foreseeablefuture. The transmultiplexer carries out the conversion between the twomultiplex structuresEricsson has developed transmultiplexers for different purposes, all of which arein accordance with applicable CCITT recommendations. In this article thetransmultiplexer applications, properties and structures are described Theequipment is designed using conventional analog digital conversion per speechchannel in the baseband, which has technical and economical advantagesUDC 621.376621 395 43During recent years there has been anincreasing demand for analog multiplexequipment (FDM). FDM technology issuperior in the long-distance network asregards capacity and economy.The transmission media used are traditionalcoaxial cable systems having acapacity of up to 10800 channels, aswell as radio relay link and satellite systems.Recently developed radio systems,which permit single sidebandtransmission, have a capacity of up to6000 channels.Fig 1 shows the current hierarchy ofanalog multiplex systems and the positionsof the transmultiplexers in it.The hierarchy comprises not only thesystems standardized by CCITT butSIXTEN EKELUNDPublic Telecommunications DivisionTelefonaktiebolaget LM Ericssonalso systems in accordance with theAmerican L-plan, ZAG 60/600 andZAG 600/2400, which have recently beendeveloped by Ericsson.Digital multiplex equipment (TDM) isbeing introduced on a large scale at thelower levels of the telecommunicationnetwork. This means that the number ofcircuits in existing cables can be increasedand at the same time the transmissioncharacteristics can be improved.Demands for new services, which aremost easily realized by means of digitaltechnology, expedite the introductionof digital switching and transmissionequipment at different levels in the network'.The digital networks that arebeing built up in many countries areoften countrywide but initially they havesmall routes and an open structureThe above-mentioned factors mean thatthere is a need for through-connectionbetween analog and digital networksThe through-connection can be carriedout by means of transmultiplexers, withthe task of converting an FDM signal to aFig. 1The analog multiplex hierarchy (FDM), and thepositions of the transmultiplexers, ZAJ2x30/60,•7A IWJvH anri 7A.I 5 xJl/JxBO. in it

Fig. 2, leftThe transmultiplexer as through-connectionequipment between the digital and the analognetworkFig. 3, rightA digital exchange connected to an analog longdistancenetwork via a transmultiplexerTDM signal and vice versa. Transmultiplexershave very well defined interfacesand can therefore be utilized in asimple and flexible manner, regardlessof the degree of digitalization.Transmultiplexers will be required forinternational traffic for a very long time,since there is an efficient analog networkin existence, which will be retainedfor many years in parallel with the digitalnetworks that are now being built up inmany countries.System applicationsDifferent strategies are used when extendingand modernizing telecommunicationnetworks, and the need fortransmultiplexers can therefore vary.However, there are always problemswhen two networks with different structureshave to interwork, and these problemsare easily solved with the aid oftransmultiplexers, which are primarilyused for:- through-connection between theanalog and the digital network, fig. 2- connecting digital switching systemsto the analog long-distance network,fig. 3.Through-connectionThe hierarchic structure of both the analogand the digital multiplex equipmentmakes it possible to inter-connect standardizedgroups of each type. Transmissionnetworks are built up in such a waythat only a proportion of the circuitsprovided by the line systems are terminatedin each terminal, the remainderbeing connected through to other lines.When the network contains both analogand digital line systems the transmultiplexeroffers a means of simple throughconnectionbetween the systems, fig. 4.Similar problems with through-connectionin digital satellite transmission(TDMA) can also be solved with the aidof the transmultiplexer. Earth stationsusually have analog connections to thenational network, fig. 5.Many networks are built up with standbyroutes. For example, an analog radio relaylink route can have a standby route,although with a reduced capacity, via adigital cable system, fig. 6.Exchange connectionNew exchange systems with digitalthrough-connection are becoming increasinglyimportant in the networks.When digital switching is introduced,particularly at higher levels in the network,connection to the analog longdistancenetwork is necessary. Thetransmultiplexer allows the long-distancenetwork to be connected direct tothe digital standard interface of the exchange.From the point of view of theexchange the digital standard interfaceis preferable to analog speech and signallinginterfaces. The transmultiplexersolution offers economic advantagesand requires very little space.It is often desirable to divide the trafficbetween two independent routes, fig. 7.This means that the transmultiplexerequipment will be required for a longtime, since the introduction of indepen-Fig. 4, leftThroug-connection of routes between differentline systems^—^~Analog lineDigital lineFig. 5, rightAn earth station for satellite transmission connectedup via a transmultiplexer

Table 1Comparison of certain parameters for the transmultiplexerand separate FDM and PCM equipmentsrespectivelyParameterLMEFDMCCITTG.232G.233LMEPCMCCITTG.712TRALMENSMUXCCITTG.792Group delayDelay distortion,1000-2600 HzAttenuation distortion,600-2400 HzmsmsdB10.25+ 0.50.525+ 0.90.450.17±0.30.6025±0.51.50.42±0.430.5±0.6Fig. 6A digital line used as the standby route for ananalog radio relay link routedent digital circuits to each digital exchangeat this level will not take place inthe foreseeable future.System characteristicsCCITT has standardized a transmultiplexerfor 60 channels'. Standards for a24-channel transmultiplexer are beingprepared. Ericssons transmultiplexersZAJ 2* 30/60, ZAJ 24/2x12 andZAJ5x24/2x60 are intended for networksbuilt up with 30 and 24 channelPCM systems respectively. The transmultiplexerdescribed here is the firstmentionedone, which converts 60channels.ZAJ2x30/60 converts two 30 channel2048kbit/s PCM bit streams to a standardizedsupergroup in the frequencyband 312-552kHz and vice versa Thetransmultiplexer is a mechanical andfuctional unit with a performance in accordancewith relevant CCITT recommendations,which ensures safe and rationaloperation.In order to illustrate the performance requirements,a comparison has beenmade between the requirements for thetransmultiplexer and the appropriateFDM and PCM equipment, with regardto certain essential parameters. In table1 the values recommended by CCITTare compared with the values for theEricsson equipment. Note the very smalladdition in group delay caused by thetransmultiplexer.The transmultiplexers described in thisarticle work in accordance with the conventionalmethod, i.e. analog/digitalconversion takes place at speech frequencies.An alternative method basedon digital filtering and fast Fouriertransformationhas been studied in detail.However, the conventional method offersmany technical and handling advantages,for example greater flexibilityand higher reliability. Moreover, thesubsystems that form part of the transmultiplexershave been developed andoptimized through several generationsof FDM and PCM systems, resulting in,for example, low power consumption.InstallationAnalog/digital conversion with individualequipment for each speech channelrequires extensive installation work, includingthe wiring up of a large numberof connections in the four-wire and signallinginterfaces. The installation oftransmultiplexers only necessitates theconnection of a few cables to easily accessibleconnectors at the front of theequipment Fig.8 shows the requiredconnections.The setting of various supergroup levelsis carried out by means of plug-in U-links. On the analog side the signallingtone levels can be set in a similar way.SynchronizationIn Ericsson's transmultiplexers the carriersand bit rates of the outgoing PCMstreams are derived from separateFig. 7, leftIndependent routes between digital exchanges ina mesh-shaped network containing both FDM andTDM linesFig. 8, rightExternal connections to ZAJ 2x30/601 Battery voltage -30 to -72 V2 Basic frequency. 12 or 124 kHz3 Supergroup input and output4 2x2048kbit/s input and output5 Alarm interface

188Fig. 9Synchronization of the transmultiplexerMO_•—,____Master oscillatorIncoming timingOutgoing timingTDM signalsFDM signalsFig. 10Synchronization of the transmultiplexer whenworking towards a synchronous digital networkSynchronization pathclocks. This means that problems areavoided that would otherwise arise, forexample because of the different frequencyaccuracy requirements that applyfor FDM and TDM signals respectively.This method also eliminates therisk of slip, and data signals can thereforebe transmitted over the speechchannels without excessive error rates.The timing signal for the outgoing PCMflow is generated by a built-in oscillatorin the PCM part concerned, fig.9. Thecarriers in the FDM part are derived froman external basic frequency. 12 or124kHz, which in accordance with normalFDM practice is available in the centralfrequency generating equipment ofthe terminal.The transmultiplexer can work towardsa synchronous digital network. In thiscase the recovered timing signal fromthe incoming PCM streams controls theinternal clocks so that the outgoing signalshave the same bit rate, fig. 10. Thecontrol signal is obtained by making asimple loop connection via two externalcables on the front of the equipment.SignallingSignalling information on carrier circuitsis usually transmitted by means ofchannel-associated outband signallingat 3825 Hz. In digital circuits one or moresignalling channels in the common signallingtimeslot, T16, of the PCM systemare used foreach speech channel. In thetransmultiplexer the signalling informationin the outband signal is converted,for each channel, to the correspondinginformation in timeslot 16 and vice versa.This method makes the transmultiplexerwholly transparent to differentsignalling diagrams as long as only onebit in T16 is used foreach speech channelOn the FDM side the equipment caneasily be matched to outband signallingwith either high or low level. Only lowlevel is suitable for continuous signallingdiagrams. The equipment can alsobe adapted to either of the two possiblesignalling diagrams where "tone" correspondsto "1" or "0".Signalling system R2, in its original ormodified form, is widely used for internationaland national traffic The systemis designed to transmit the line signallingin the signalling channel, whereasthe register signalling is carried out bymeans of MFC in the speech channel.The equipment meets the relevantCCITT recommendations for signallingsystem R2. For example, it is equippedwith pilot receivers for interruption control.In the analog version of system R2 a linkisestablished foreach channel betweenbit a and the signalling frequency,whereas bit b in the FDM-PCM directionis used to transmit alarm informationfrom the pilot receiver.In FDM systems there are only two signallingconditions in each direction perspeech channel, and the analog versionof system R2 therefore contains certaintiming conditions which supplementthe two signalling conditions. In the digitalversion of system R2. however, twobits (a and b) per speech channel areused to transmit the corresponding information,which amounts to four signallingconditions. This means that simplerterminal circuits can be used in theswitching equipment.Using the digital version of R2 meansthat extensive recoding of the signallinginformation has to be carried out in thetransmultiplexer However, this recodingcan be arranged for all 60 channelsby adding just two control units containingmicroprocessors The generalnature of processor control means thatadaptation can also be made to othersignal conversion requirements.The transmultiplexer is transparent tochannel-associated inband signalling. Avariant of the transmultiplexer withoutsignalling equipment is available fornetworks with inband signallingCommon channel signalling with a capacityof 64 kbit/s is normally not possibleon circuits containing transmultiplexers,since the transmission capacityof a transmultiplexer channel is limitedto what the band 300-3400 Hz permits.This applies for all transmultiplexers, regardlessof design approach However,common channel signalling at a reducedrate can be used on a speechchannel via modems.

189The transmultiplexer with no signallingequipment offers economical advantagesin networks where common channelsignalling is used and the signallingis transmitted via a separate circuitReliabilityTransmission equipments require ahigh degree of reliability. Breakdowns inthe long-distance network, with the consequentloss of traffic, can be very expensive.Poor reliability in the otherparts of the network can lead to unacceptablyhigh maintenance costs.Ericsson has developed several generationsof FDM and PCM systems. One ofthe objectives in this development workhas been to obtain high reliability. Thisaim has been met by adhering to strictdesign rules and by careful choice ofcomponents. At the same time the numberof components has been kept low.The power dissipation has also beenkept low, and since this gives a low operatingtemperature it is another factorcontributing to high reliability The reliabilityof the transmultiplexer is expectedto be very high since most componentsare well-proven types that havebeen used in the individual FDM andPCM systems.The structure of a system has an effecton its function. For example, it determinesthe consequences of differentfaults. The structure of Ericssons transmultiplexersis such that very few itemsare common to many channels. Theprobability that a fault will affect severalchannels is low, and the probability of atotal breakdown is extremely low. Sincethe transmultiplexer is thus built up ofsystem components with very high reliabilityand has been given the most adequatestructure, the prerequisites havebeen created for very good operationalreliability of the equipment. This is confirmedby the calculations of the MTBF(Mean Time Between Failures) that havebeen carried outFlexibilityMany administrations use the supergroupas the smallest extension unit,which gives many advantages as regardshandling and administration.However, the group may be a more suitablebasic unit in certain network configurationsand for special temporary requirementsIn Ericsson's transmultiplexerthe groups are therefore accessibleboth towards the TDM and the FDMside. A program channel, high-speeddata modem, through-connectionequipment etc. can be connected towardsthe FDM side.The equipment can be made transparentto a channel having a transmissionspeed of 64 kbit/s. This means thatcommon channel signalling can be usedover a transmultiplexer circuit. For example,remote subscriber stages forAXE 10 can be connected over a carriercircuit via transmultiplexers supplementedby external modems, fig. 11.This can solve certain network problems,for example in large-mesh ruralnetworks. The transmission capacity requiredfor the remote subscriber stage isoften no more than 30 channels, andthus the loss of the 12 channels that areneeded for the 64 kbit/s transmission isquite acceptable. The necessary synchronizationinformation is transmittedvia the data modem.Individual speech channels are also accessibleand can be used, for example,for connecting data modems in eitherdirection.Fig. 11Transmission of a synchronous 64kbit/s channelover a carrier system via transmultiplexersT16BG64 kbit/s interlaceBasic group interface

Fig. 13The M5/BYB bay equipped with three transmultiplexersfor 60 channelsBMBuilding moduleFor traffic towards the TDM side unitscan be provided which permit the connectionof eight 64 kbit's data channels,in place of eight of the speech channels.The data channels, which give an extremelylarge data transmission capacity,can for example be used to transmitdata bit streams from a digital data multiplexerSuch a unit can be connectedin during operation without disturbingother traffic. The available bits in timeslotTO are also accessible.The transmultiplexer can be partiallyequipped to suit lower capacity requirements,resulting in lower costs.For AXE 10 it is possible to connect avariant of the transmultiplexer direct tothe digital group selector In this casethe ETC (Exchange Terminal Circuit) isnot required in AXE 10, and the transmultiplexercan be integrated into thestructure of the system, which is advantageousfrom the point of view of handlingand maintenance.Operation and maintenanceEricssons transmultiplexers do not requireany preventive maintenance. Thesystem parts are designed so that veryhigh stability is obtained throughoutthelife of the equipment.The equipment contains alarm monitoringand alarm transmission in accordancewith CCITT recommendations.All alarm information is available in aspecial alarm unit. This unit also transferssome alarms between the FDM andTDM sides. All transmultiplexer alarmscan be allocated to the urgent or nonurgentcategory by means of straps inthe alarm unit.Reminder indication can be initiated bymeans of a push-button. When an alarmoccurs, a light emitting diode on thefront of the unit lights. Several units inthe subsystems are also equipped withlight emitting diodes, which simplifyfault tracing The alarm unit also containsservice alarm outputs, whichprovide alarm information for the wholeequipment as well as individual subsystemsat carefully chosen levels. Thealarm outputs can be connected toalarm equipment of the traditional typeor to Ericssons computer-controlledalarm system, ZAN101.Fig. 12ZAJ 2x30/60, transmultiplexer for 60 channelsDetailed measurements can be carriedout from various test points. The FDMpart contains short-circuit proof testpoints at important system points andalso access points for connection to thespeech frequency and signalling interfaces.The PCM part contains short-circuitproof test points for, for example,the transmitted and received bit streamand the transmitter and receiver timing.On the FDM receive side manual levelregulation can be carried out individuallyfor each channel as well as for eachbasic group. This means that the residuallevel deviations, such as thosecaused by a complex network configuration,can be compensated. Incominggroup or supergroup pilot frequenciescan be monitored for interruptions by apilot receiver, which can be included inthe equipment. The frequency of thesend clocks can be adjusted. IncomingTDM bit streams are supervised for lossof timing and bit error rate. All test andmeasuring points are readily accessibleat the front of the equipment.

191Fig. 15Control unit with microprocessorThe basic principle of the transmultiplexerasregards alarms is that it shouldbehave as a PCM multiplexer towardsthe TDM side and as a carrier systemtowards the FDM side. There are certainextra features, however, such as thetransmission of the AIS (Alarm IndicationSignal)Fault-clearing maintenance consists ofchanging faulty printed board assemblies,and a suitably dimensioned stockof spare parts must therefore be available.The conventional structure ofEricssons transmultiplexers meansthat many of the boards are also used inother Ericsson multiplex equipments,so the stock of spares can be shared,with a consequent reduction in maintenancecosts.System structureThe description above applies in the relevantparts for all Ericsson transmultiplexers.The mechanical constructionand functional design of three differentequipments are described separatelybelow.ZAJ 2x30/60The transmultiplexer for 60 channels.ZAJ2x30/60, fig. 12, converts two2048kbit/s PCM streams to a standardizedbasic supergroup in the frequencyband 312-552 kHz and vice versaThe equipment is built up using Ericsson'sBYB construction practice. Allequipment for the transmultiplexer ismounted in a pre-wired triple magazinehaving a width of 12 building modules(488 mm) and a height of 18 buildingmodules (732mm). The magazine is anindependent unit which also containstwo d.c./d.c. converters. The magazinecan be mounted in M5/BYB bays 2 , inBYB rows or in a BYB cabinets. All cablingis accessible from the front. An M5/BYB bay can be equipped with threetransmultiplexers, fig. 13.As far as possible the transmultiplexerhas been built up of the same parts asEricsson's other multiplexer systems.The functional structure is shown infig 14. The TDM part contains two identicalPCM units. These subsystems containwell-tried system componentswhich also form part of AXE 10 as well asseparate PCM equipments 3 . The FDMpart contains units from Ericsson's FDMsystems 4 . The continuous technical developmentalso affects the FDM field.For example, hybrid type thick film circuitshave been replaced by monolithiccircuits in the signalling circuits, whichmeans that it has been possible to reducethe number of integrated circuitsby a third on the send side and by half onthe receive side. The necessary carrierand signalling frequencies are generatedinternally with the aid of phaselocked oscillators, which are controlledby an incoming basic frequence of 12 or124kHz The group pilot frequencies,84.08 or 104.08 kHz, can either be generatedinternally or obtained from an externalsource. The supergroup pilot frequency,411.92 or 547.92 kHz, is obtainedfrom an external source. FDM terminalsnormally contain central fre-Fig. 14Block diagram of the transmultiplexerZAJ 2x30/60RflRacif nrnnn inlprf.irr

Fig. 17ZAJ 5x24/2x60, transmultiplexer for120 channelsquency generating equipment for basicand pilot frequencies.The conversion between the outbandsignals of the FDM side and the bits intime-slot 16 of the TDM side takes placepartly in the channel units in the FDMpart and partly in units from Ericsson'slatest equipment for E&M signalling,ZAK01-3 5 . The signalling part can besupplemented by a control unit, fig. 15,which carries out the necessary recodingbetween the analog and the digitalversion of the CCITT signalling systemR2.The transmultiplexer is powered by twod.c./d.c. converters which are fed from abattery voltage of between -30 and-72VThe equipment contains an alarm unitwhich collects all alarms and generatesservice alarms etc. at different systemlevels. The unit also handles the transferof alarms between the FDM and TDMsides. The alarm unit is provided withthe standard alarm interface for transmissionequipments in th BYB constructionpractice.ZAJ 24/2x12The transmultiplexer for 24 channels,ZAJ24/2x12, fig. 16, converts a 1544kbit/s PCM stream to two standardizedbasic groups in the frequency band 60-108 kHz and vice versa.The equipment is built up using Ericsson'sBYB construction practice but isintended for rack mounting in the standard19" construction practice. Allequipment for the transmultiplexer ismounted in a pre-wired double magazinehavingaheight of 12 bu ilding modules(488 mm) and a width of 19"(483 mm). The magazine occupies 11mounting spaces in a standard 19" rack.The magazine is an independent unitand contains two d.c./d.c. converters.Fig. 16ZAJ 24/2x12, transmultiplexer tor 24 channels

Fact panelZAJ 2x30/60CapacityDigital interfaceAnalog interface60 channels2048 kbit/s, HDB3, A-lawBasic supergroup,312-552 kHz193ZAJ 24/2x12CapacityDigital interfaceAnalog interfaceZAJ 5x24/2x60CapacityDigital interfaceAnalog interfaceReferences24 channels1544 kbit/s, Bipolar, (i-lawBasic group, 60-108 kHz120 channels1544 kbit/s, Bipolar, ^i-lawBasic supergroups,312-552 kHz1 CCITT Rec. G.791, 792, 793 and 794.Yellow Book, Vol. Ill-Fascicle III.3.(G.794 Proposed rec.)2. Hallberg, P.-A. and Viklund, B.:Construction Practice BYB for TransmissionEquipments. Ericsson Rev. 57(1980):4, pp. 124-128.3. Hamacher, H.-H. and Pettersson, G.:First-Order PCM Multiplex in the BYBConstruction Practice. Ericsson Rev.57 (1980):4, pp. 129-137.4. Asarnoj, R. et al.: Channel TranslatingEquipment in the M5 ConstructionPractice. Ericsson Rev. 52 (1975):3/4,pp. 106-115.5. Larsson, L.-E.: PCM Signalling Equipmentin the BYB Construction Practice.Ericsson Rev. 58 (1981):3, pp.132-141.The equipment is built up of the samesystem components as Ericsson's FDMand D4 systems These components arecharacterized by a high degree of reliabilityand low power consumptionThe ordinary transmultiplexer containsequipment for conversion betweenPCM signalling and FDM outband signallingat 3825 Hz, but a version withoutsignalling equipment is also available.All necessary pilot, signalling and carrierfrequencies are generated internally,with the aid of a built-in quartzoscillator. Alternatively it is possible tocontrol the equipment by means of anexternally generated basic frequency,12 or 124kHz, injected via an auxiliaryunit using the phase locking technique.The transmultiplexer is powered by twod.c./d.c. converters, which are fed with abattery voltage of between -30 and-72V. The equipment is provided withseveral alarm circuits which monitorboth internal and external functionsAlarms are indicated by light emittingdiodes on the front of the unit, whichsimpifies fault tracing.ZAJ 5X24/2X60The transmultiplexer for 120 channels,ZAJ5x24/2x60, fig. 17, converts five1544 kbit/s PCM streams to two standardizedbasic supergroups in the frequencyband 312-552 kHz and vice versa.The group interface is also accessible.The equipment is built up in Ericsson'sM5 construction practice and ismounted in a bay having a height of 51building modules (2134mm). The bay isconstructed for use also on data floors,and it is thus possible to run the cablingto the bay via the base plate.The equipment is built up of Ericsson'swell-proven system components. Allwiring between the various subsystemsin the bay is completed during the manufacture,and hence it is simply a questionof plugging in towards the FDM partwhen installing. The transmultiplexer isan independent unit, with d.c./d.c. convertersand carrier frequency generation.The basic and pilot frequencies areobtained from the central frequencygenerating equipment of the terminal.The subsystems contain equipment thathandles the signal conversion from outbandsignalling at 3825Hz to PCM signallingand vice versa. The transmultiplexeris equipped with the normalalarm and supervision functions.The transmultiplexer can be equippedwith pilot receivers for interruption controlworking on the group or supergrouppilots.The transmultiplexer for 120 channelsoffers economic and handling advantages,among other reasons becausethe MDF equipment at both the channeland group level can be dispensed with.This transmultiplexer has been developedat the request of customers whouse the supergroup as the smallest extensionunit.ConclusionThe transmultiplexer will be in use duringthe foreseeable future for varioustypes of through-connection betweenFDM and TDM networks. The efficienttransmission and well defined interfacesof the transmultiplexers makethem simple and flexible enough to beused in many different stages of the currentdigitalization process.Ericsson's transmultiplexers. which arebuilt up of system components that havebeen developed and tested through severalgenerations of equipments, are allflexible and have a high degree of reliability.These characteristics, togetherwith their low maintenance costs andgood performance, enable the transmultiplexersto become functional andeconomic components in telecommunicationnetworks.

ERITEX10 for Teletex and WordProcessingThomas Augustinsson and Bjorn SjostrandEricsson Information Systems AB has developed an electronic word processorwith facilities for teletex communication, ERITEX 10.The authors discuss the need for and purpose of this type of machine anddescribe the system structure and the design of the various units that make upthe system.UDC 654.145651.9:681 3Fig. 2ERITEX 10, electronic typewriter for teletex communicationand word-processingTeletex is a new international text communicationservice which has recentlybeen standardized by CCITT. Teletexmakes it possible to transfer documentsbetween data memories in terminalsthat are connected via telecommunicationor data networks.All teletex communication is carried outmemory to memory, which means thatthe transmission of documents takesplace very quickly. Local work at the terminals,such as input and printout, is notdisturbed when sending and receivingtakes place, fig. 1.The introduction of special interfaceequipment between teletex and telexnetworks makes it possible to offer communicationwith all national and internationaltelex terminals as a part of theteletex service. In several respects teletexoffers improved facilities for textcommunication compared with telex.Not only is the transmission speed approximately30 times higher than fortelex, but teletex also makes it possibleto transmit most national alphabets, andthis gives the received printout a highquality.ERITEX 10 is the first member of a newfamily of office equipment It can beused as a word processor and as a terminalfor office communication, fig. 2 and3. ERITEX 10 consists of an economicallydesigned keyboard, a line displaywith 40 characters, a daisy-wheel printerwith 105 characters and circuits for connectionto an external DCE (Data Circuitterminating Equipment). The electronictypewriter ERITEX 10 is- a teletex terminal for external communication- a terminal for internal communication- a word processor with an electronicmemory- an electronic archive.The teletex communication inERITEX 10 is in accordance with CCITTRecommendations F.200, S 60, S.61,S.62 and S 70. ERITEX 10 can be connectedto a teletex service which uses adata network for circuit switching(CSDN), a data network for packetswitching (PSDN) or the public telephonenetwork (PSTN). Teletex communicationcan take place either viapublic telecommunication networks orvia private networks. It makes no differenceto the user whether ERITEX 10 isconnected to a data network or a telephonenetwork.By internal communication is meanttraffic with other ERITEX units or withother terminals within an organization.The communication can take place viaaPBX or a local data network.ERITEX 10 used as a word processor offersa large number of editing functionsfor altering, adding, removing and transferringtext The word-processing functionsare supported by an extensive internaltext memory for documents andmessages, together with memories fordifferent formats, for frequently usedphrases and for the last line entered.Some of the usual word-processingfunctions in ERITEX 10 are:- indented left-hand margin- straight right-hand margin- automatic carriage return, wrap-

195THOMAS AUGUSTINSSONBJORN SJOSTRANDEricsson Information Systems ABFig. 1The basic principle for teletex communicationFig. 3The ERITEX 10 terminal with a floppy disc unitcontaining a memory for 1.2 million characters, adata modem DCE for teletex communication anda telephone set- page riffling- division into pages- joining two pages into one- tabulation- centering- underlining- bold type- stop codes- memory function- deletion- choice of document type.An electronic archive in the form of aseparate floppy disc unit can be connectedto ERITEX 10 when largeamounts of text have to be stored. Theunit holds two 51/4" floppy discs, eachwith a storage capacity of approximately300 pages. The floppy disc unit also includesa file management system, andthe operator can therefore easily open,define, store and read files. Documentscan also easily be copied from one floppydisc to another in the floppy disc unit.System descriptionMicroprocessors are used to performthe various functions in ERITEX 10 Thesystem is described with reference tothe block diagram in fig. 4.The four motors included in the printingand striking mechanism are controlledby servo circuits and power amplifiers.The necessary sensors are controlled bytheir own microprocessor (motor controlunit), which is connected to themain system bus via a parallel/seriesconverter (USART).The central processing unit (CPU) consistsof a microprocessor with the associatedmemories, fig. 5. It handles interruptsignals, keyboard functions andstore functions. This CPU controls theflow of information on the main systembus and the execution of certain pro-Fig. 4

Fig. 5The central processing unit in ERITEX10. Theprinted board contains a microprocessor andRAMs and PROMsgrams that are stored in the programmableread-only memory (PROM) The randomaccess memory (RAM), which hasstandby battery power, is divided into aline memory, aformat memory, a phrasememory and a text memory.When the terminal is switched on, theline memory is automatically activated.This memory is emptied of its contentswhen automatic or manual carriage returnis initiated. The information istransferred to the text memory. The linememory holds 350 characters.The format memory holds informationconcerning margins, tabulations, firstline of writing etc.The phrase memory is used to store frequentlyused expressions, which canthen be retrieved and inserted in thetext. The storage capacity, together withformat information is 1000 characters.The fourth and largest primary memory,the text memory, is used for temporarystorage of received and input documents.Its storage capacity is 42k byte,which corresponds to 20-25 normalpages.Communication unitThe communication unit is connected tothe main system bus. It comprises allnecessary circuits for connection to adata circuit terminating equipment,DCE, and the circuits required for connectionto a secondary memory It alsocontains a two-wire interface for communicationwith other ERITEX units.Line displayThe front of the machine includes a linedisplay unit, fig. 6, above the keyboard,where a limited part of the stored textcan be shown. The display makes it possibleto check the input text. It is also ahelp in correcting and adding to printedtext, as well as in making up pages.Five of the 40 characters in the line displayare used for state information, i.e.information regarding the operatingFig. 6The line display holds 40 characters, of which 34are used tor text and 5 for state information. Thekeyboard is of a low profile type andergonomically designed. Approximately 300characters can be obtained by means of differentkey combinations

197mode of the machine and which functionsare activated. Of the remainder, 34positions are used for text, and one forseparating the line of text from the stateinformation. It is possible to display thename of the document and its contents,as well as riffling the pages. All textsdefined in accordance with CCITTRecommendation S.61 can be shown onthe line displayThe line display has its own charactergenerator and is connected to the busvia an interface, UPI (Universal PeripheralInterface).KeyboardThe keyboard has been designed in accordancewith modern ergonomic principles,fig. 6. It is of a low profile type,with an average height of only 59 mmfrom the table to the keys. This enablesthe operator to work in the correct sittingposition.The keyboard comprises all keys andlight emitting diodes required to controland use ERITEX10. It also contains abuzzer with two different signals. Thesesignals are used for acknowledgementor fault indication. ERITEX10 has alphanumericalkeys and function keys, with alayout in accordance with the internationalstandard for an ordinary typewriter.The function keys are grouped outsidethe alphanumeric keys in such a waythat they are easy to find and combinewith other keys. The top row of alphanumerickeys is also used for functionsin combination with a command key.The alphanumeric keys give direct accessto 105 characters, but up to 308characters can be obtained by combiningkeys.At the ends of the keyboard there arefour controls with associated light emittingdiodes for setting- operating mode- line spacing- key pressure- character spacing.The keyboard is connected to the mainsystem bus via a special processor (keyboardCPU).Printing unitThe printing unit is of daisy-wheel typeand gives a printing speed of 16 charactersper second. The printer can beequipped with different wheels. Eachwheel has a set of 105 characters. Aneven greater range of characters can beobtained by combining these. A specialteletex wheel is used for internationalteletex traffic. Most characters in thelanguages originating from Latin can bewritten with the teletex wheel.Fig. 7The printing unit can be equipped with differentdaisy-wheels, including a teletex wheel for internationaltraffic. The printing speed is 16 Charactersnpr second

198Fig. 8The floppy disc unit with two 51/4" floppy discs. Ithas a total capacity of 1.2 million charactersSecondary memoryThe secondary memory is a separatefloppy disc unit for two 51/4" floppydiscs, fig. 8. Each disc holds some600000 characters, which correspondsto approximately 300 pages of size A4.The reading and writing in the secondarymemory is controlled by a file managementsystem and a processor in thefloppy disc unit.Paper feederTwo types of paper feeder are availablefor the ERITEX 10 terminals, a sprocketdrive feeder and a cut sheet feeder,figs.9 and 10. The maximum printingwidth is 165 characters, allowing the paperto be used for lengthwise or transverseformats. The appropriate paperfeeder is simply placed on top of theterminal without needing any specialfixing arrangements.SoftwareThe software in ERITEX 10 is dividedinto a number of process-orientatedprograms, which interwork with the aidof signals. The exchange of signals betweenthe various programs is controlledby an operating system. The mostimportant parts of the software areword-processing, file management andteletex communication. Fig. 11 shows ablock diagram of the software structure.OPERATING SYSTEMThe main program in the operating systemsupervises and controls the programhandling by means of signals fromdifferent buffers. Signals to these buffersare generated by an interrupt routinein the operating system, which scansdifferent units and circuits in the hardware.WORD PROCESSING SOFTWAREThe programs in the word-processingsoftware scan the keyboard, feed the informationto the line display and printer,interpret the commands given by the operator,fetch, edit, store and erase text inthe internal memories and check thecodes and characters. In addition to certainbasic programs there are programsfor- editing- interpretation of commands- code checking- store management- printout.Basic programsThe line display, keyboard and printerare all controlled by their own programsfor code conversion, interpretation ofinput characters and output of charactersvia the buffers of the units.EditingThe main editing program works onepage at a time in the indicated documentand is responsible, with the aid of thestore management program, for settingup, storing and erasing pages and documents.A page can also be divided intotwo, and two pages can be joined intoone. Text pages for editing are transferredfrom the main memory to the editingmemory, where the editing is carriedout.The editing program also ensures thatthe relevant command is shown on theline display.Fig. 9, leftCut sheet feederFig. 10, rightPaper feed with sprocket drive

199Command interpretationEach command received by the editingprogram from the basic program forkeyboard scanning must be interpreted.This is done by a special program, whichis initiated by the key COMMAND.Code checkingA program checks that every inputcharacter conforms to the format andcode rules that apply for the type of documentin question. This program canalso be used to change a document or apage to another type of document.In the case of teletex and telex, the programalso checks that the set of charactersand codes are correct for the specifiedformat. In addition the program carriesout the necessary checks when twodocument pages are joined into one.Store managementERITEX10 has an internal text memoryfor storing and editing. This is dividedinto an editing memory and a communicationmemory. The editing memory isused by the basic programs as well asthe editing program for line and pageediting, and also by the printing programfor the printing of pages. The communicationmemory is used by the storemanagement program for storing, retrievingand erasing documents and individualpages, and by the teletex programfor the storing and handling oftransmitted and received documents.These two memories share a commonstorage area. The memory sizes and theboundary between them can vary.The store management program containsfunctions for the administration ofthese memories, such asFig. 11The software structure in ERITEX10

200- finding space for a document or apage- releasing areas when a documentorapage is erased- searching for documents by theirname- checking that document names arenot duplicated- packing and reorganizing the informationin the memory.The phrase and format memories areused by the basic programs and the editingprogram for the storing and handlingof phrases and formats.PrintoutThe printing program prepares andchecks the printout of the documentpage or the document selected by theoperator. The document page to beprinted is transferred to the editingmemory if it is not already stored therefor editing.Printout of documents that are stored inthe floppy disc memory takes place onlyafter each page concerned has beentransferred to the editing memory viathe communication memory.If the communication memory is not sufficientfor the number of received documents,the information that cannot beaccommodated is automatically printed.Each printed document page is normallyretained in the memory concerned,with the exception of any receivedpages that are automatically outputwhen the whole storage area hasbeen used up.FILE MANAGEMENT SOFTWAREThe file management software, whichcarries out the storing, retrieving andother handling of documents, is dividedinto two parts. One part is placed in theterminal and is controlled by its operatingsystem, and the other part is placedin the microprocessor of the floppy discunit.Terminal partThe transmission of data and interruptsignals takes place over the bus betweenthe terminal and the floppy discunit. The interrupt routine of the operatingsystem checks when the informationcan be transmitted via the bus system tothe file management software, and givespriority to teletex communication.The file management software in the terminalpart compiles and forwards pagesand documents from received teletexmessages, and transfers them betweenthe communication memory and thesecondary memory where they are automaticallystored on floppy discs.Floppy disc partThe software in the floppy disc part consistsof a number of programs that carryout and check such functions as- creating, opening, reading, erasing,changing the name of and copyingdocuments- reading or printing blocks of data.A program controls the transmission ofinformation at the interface to the floppydisc hardware. Another program controlsand supervises the signal buffers.TELETEX SOFTWAREThe teletex communication softwarecarries out the procedures defined byCCITT for the teletex service.The teletex software is divided into levelsin the way defined in the OSI (OpenSystems Interconnection) model andconforms to CCITT recommendationsfor the international teletex service(F.200, Basic Teletex Service). It is dividedinto programs for the following levels:- application- presentation- session- transport (S.70)- link and network (X.75 and X.21).Application levelThe application program checks thegiven commands for sending or receiving,and initiates the actions to be takenby the store management program andother programs. When the document isstored on a floppy disc the file managementprogram is also initiated whennecessaryThe information to the operator concerningthe transmission and receptionin progress, any faults etc. is controlledby this program, which also handles logginginto an internal log file, where informationis stored regarding transmittedand received teletex messages. The logfile can only be erased when output hasbeen correctly effected.

202The ERITEX10 software Includes specialtest programs for testing the operationof both the mechanical and theelectronic units. If a fault occurs, theseprograms can be initiated by servicetechnicians in order to locate the faultyunit. The various parts of the terminalare easy to replace.SummaryThe introduction of teletex constitutesan important advance towards the officeof the future. In teletex an internationalmethod has been created for the transmissionof documents between terminalsof different manufacture. Teletex istherefore a basic service in Ericsson sintegrated systems for office automationand office communicationERITEX10 is the first terminal fromEricsson that is equipped for teletexcommunication. It meets stringent demandsas regards function, ergonomicdesign, quiet operation, integration withother products from Ericsson InformationSystems AB and convenient servicing.Fig. 13ERITEX 10 terminal with teletex communication

Local Area Radio System-LARSHans LindbladSRA Communications AB, a member of the Ericsson Group, has developed aradio system for certain military applications. The system is designated LARS(Local Area Radio System) and is intended for radio communication withingeographically limited areas, such as military bases, training premises and largestore areas.The author describes the functions offered by the system for stationary, portableand mobile stations and the technology used. The maintenance aspects,operational reliability and environmental factors are also discussed.HANS LINDBLADSRA Communications ABThe system is specially designed to meetthe requirements of military organizations.It offers full communication facilitiesbetween all users. A commandingofficer who wants to use a certain radiochannel can use his priority and enter acall between subordinates.UDC 621 396.7Fig. 1Simplified model of LARS^"^~ Direct traffic to and from the command centre (HQ)Other direct traffic"" ,^~ Relay trafficRadio system LARS (Local Area RadioSystem) is used for communication betweenthe commanding officers at theoperational centre of an establishmentand the staff, for internal communicationbetween members of the staff andfor connection to PBXs The operationalcentre has permanently installed equipment,the other equipments are portableor designed for installation in vehiclesor tents, fig.1. The system also allowsthe stationary radio equipment at thecommand centre, with its efficient aerialsystem, to be used as a relay station betweenunits placed at a distance fromeach other within the base area. The systemalso contains facilities for establishinglinks with adjacent base areas.LARS enables staff to establish communicationwith each other and withcommanding officers and thus use vehiclesand personnel efficiently. Transportcan quickly be directed to the correctplace. Messages can be delivered tostaff or units without delay. LARS thereforecontributes to the smooth and efficientoperation of an establishment.Speech privacy can be ensured bymeans of digital ciphering equipment.The radio equipment has been preparedfor the transmission of data (data rate9.6 or 12 kbit/s). Transvertex. a subsidiaryof SRA, has developed special encryptionequipment which can be connectedto the various radio units. Thisequipment is so light that it can be usedtogether with the portable radio version.LARS is flexible and easy to adapt to suitdifferent applications and the varyingrequirements and needs of customers.

.2Fig. 3A soldier equipped with the portable stationFig. 2Control set tor a command centre with parts ofthe stationary radio equipment mounted in a rackin the backgroundStationary radio equipmentThe stationary equipment of the commandcentre constitutes the central partof the system, fig. 2. It consists of controlunits, a central unit, filter unit, transmitterand receiver. All units are constructedfor mounting in a 19" rack,which gives a high degree of flexibilityand makes it easy to adapt the equipmentto suit the relevant communicationneeds.The control unit can be connected up tofive stationary radio stations, which canbe used simultaneously. Open or selectivecalling is chosen individually foreach channel Open calling (squelchcontrol) means that the receiver is connectedin for listening immediately thereis a carrier on the channel. Selectivecalling means that the channel is openedfor listening by means of the DTMFC(Dual Tone Multi Frequency Code) signal,and the operator need not listen toirrelevant traffic on the channel. Severalcontrol units can be connected to thestationary radio equipment. Each unit isserved by two operators, one of whichhas priority.The central unit in LARS is controlled bya microcomputer. It administers and supervisesselective calling, the setting upof relay traffic, connection to PBXs, prioritycalls and other traffic functions.The central unit also controls automaticroutines which test the system duringoperation and give an alarm if a fault isdetected.The stationary transmitter has an outputpower of approximately 35W. A specialpower amplifier can be connected,which raises the output power to approximately250WAll units in the stationary radio systemare powered by 24 V d.c. from a batteryfedrectifier system. This makes the radiosystem immune to variations andtemporary failure of the local mains supplyand improves the system reliability.Portable radio stationThe portable radio station is simple touse and light enough to be easily carried,fig. 2. The radio unit is small andflat, and the microphone, the soundsource and most of the operating devicesare placed in a separate unit whichis designed for one-hand operation.This makes the station ergonomicallywell suited to the special usages thatapply for portable military equipment.The station is powered by rechargeableNiCd cells which are mounted in a plugincassette.The aerial consists of a quarter-waveblade aerial mounted on a flexible joint,This type of aerial gives a good compromisebetween manageability and aerialperformance in the relevant frequencyband.Special carrying devices have been developedfor different combinations withother equipment carried by the user(combat harness, hand weapons etc.).This portable radio station is unique inits combination of small dimensions,robust structure, multitude of channelsand system functions and large batterycapacity.Mobile radio stationThe mobile radio staiton is a small, compactunit which is plugged into a cassettein the vehicle, fig. 4. The main advantageof this installation method is

Fig. 5One of the thick film hybrids that form part of theportable equipment205Fig. 6Block

206as transmitter noise, two-signal selectivityand microphony, i.e. unwantedmodulation due to mechanical effects,can be directly related to the VCO.The control computer in the central unit,which administers all traffic functions, isprogrammed to carry out advanced automaticfault testing. It performs plausibilityanalyses of, for example, the responsesgiven by the system to externalsignals and control data It also providesan outgoing combined alarm and a localindication of the type of fault.The two-way transmission of state informationbetween the control unit and thecentral unit takes place over a four-wirecircuit, with the data transmitted in serialform. In this way approximately 100wires are replaced by four. The controlunit is equipped with a microcomputerwhich converts the state information tothe appropriate formThe standard DTMFC signalling is usedfor selective calling. The radio operatorcan thereby call staff at the commandcentre selectively, or activate functionsin the system, such as setting up relayconnections or connections to PBXs.The reasons for choosing this signallingsystem are its high sensitivity and highF. (crystal-controlled) frequency accuratecassette station partly dismantledc\/. Moreover, the complex digital signalprocessing circuits required for the systemare available in the form of small,low-current LSI circuits.Maintenance aspectsSpecial attention has been paid to themaintenance aspects of the completesystem as well as the components.The radio equipment is designed for automatictesting Frequency selection,keying, squelch disconnection etc. canbe carried out electrically via a specialconnector on the unit, where the lowfrequency signals, internally generatedvoltages and squelch state can also bemeasured. The modulation in the stationarytransmitter can also be determinedby means of deviation measurementsAll the equipment has been given a modularstructure with a view to simplifyingservice and maintenance, fig. 7. Theelectronic equipment is logicallygrouped on printed boards, which areconnected via internal cables and connectors.Hybrid circuits constitute specialfunction blocks in the miniaturizedcircuits. For maintenance purposesthese blocks are considered as components.The possibility of efficient maintenanceof military radio equipment is extremelyimportant and the customer often requiresthat the contract includesguarantees regarding the mean time torepair (MTTR).Operational reliabilityThe operational reliability of the equipmenthas been assured by using onlywell-tried component types of goodquality.Certain stages in the production processinclude tests and measurements inaccordance with a special quality controlprogram. In addition, before theequipment undergoes its final factorytesting it is subjected to a bump test andburning-in in accordance with a specialprogram. The result of this is equipmentwith such high and even quality that thecustomers can be given guarantees ofoperational reliability in the form of

207Technical data for LARSSelective calling systemLF interface betweenstationary unitsNominal levelTransmission of states betweencontrol unit andcentral equipmentTemperature rangeStationary equipmentMobile equipmentVibrationsStationary equipmentMobile equipmentBumpsStationary equipmentMobile equipmentPower supplyStationary equipmentCassette stationPortable stationDTMFC signalling600 ohms,balanced-3.5 dBm600 bauds serialdata in a currentloop+5 to +55°C-40 to + 55°C395525 g, 1200 bumps40 g, 6000 bumps24 V d.c.12 V car battery10.8 V NiCd batteryRadio units Station- Portable Cassetteary radio radio radioBandwidth 2.5 MHz 2.5 MHz 2.5 MHzFrequency band 150 MHz 150 MHz 150 MHzBandwidth 0.5 MHzFrequency band 140 MHzNumber ofchannels 100+20 100 100Channel spacing 25 kHz 25 kHz 25 kHzTraffic mode one- or two-frequencysimplexModulation FM (6 dB/octave LF characmethodteristic)Output power 35 W >1 W 15 WWith a separatepower stage 250 WSensitivity (EMK)0.6ixV(12 dB SIN AD)Battery life with -normal operation0.8 i(V-10 h0.6 nV~8 h(in carryingcase)EnvironmentThe equipment is designed to the environmentalrequirements set by theSwedish Armed Forces for mobile andstationary radio equipment.The specification of environmental conditionscomprises temperature limits foroperation, data for bump tests, vibrationtests, static humidity tests and cyclic humiditytests.Thisapplies forall units, butthe requirements are different for stationaryand mobile equipment Moreover,the portable radio station and thecassette station installed in the carryingcase also have to be waterproof.In order to be able to meet the bump andvibration requirements the radio equipment,particulary the portable andmobile units, have been given a constructionthat makes them especiallysturdy, robust and durable. The user isassured of a working radio even duringthe most difficult operating conditionsSummaryLARS (Local Area Radio System) hasbeen developed for stationary and locallydelimited military applications. Forsuch applications the system providesan almost optimum solution for thecommunication requirements.However, the mobile units can also interworkregardless of the other units,and they can therefore also be used toadvantage for tactical communicationThe sturdy construction and high operationalreliability makes them suitable forthis, perhaps the most demanding of allmilitary applications.In addition to its ability to withstand adverseenvironmental conditions and itshigh operational reliability, LARS hasfeatures which enable it to meet thestringent demands made by the militaryuser, and thus put it in a different categoryfrom conventional, civil landmobilesystems Some such featuresare:- Channel allocation. All users musthave access to all radio channels. Initiallyeach local system is allocated aspecific group of channels, and eachstaff category within the system is allocateda specific channel.- Traffic mode. The two traffic modesavailable are single-frequency or twofrequencysimplex. The latter mode isused for setting up connections towardsrelay stations and PBXs. Thebasic principle is that all users mustbe able to follow the traffic on thechannel they have been allocatedConsequently the traffic is normallyopen, i.e. squelch controlled. Selectivecalling is used towards staff at thecommand centre. They themselvescan choose between selective andopen monitoring of the radio channelin question. Selective calling is alsoused for requesting services from thesystem, for example the setting up ofrelay circuits.- Priority. The basic principle is that allmembers of the staff at the operatingcentre shall have the same degree ofpriority. This means, for example, thatseveral operators can connect in toone and the same radio channel simultaneously,in which case thespeech is interleaved. This is essentialso that all operators are able to sendout important messages quickly.The new base radio system of the SwedishAir Force is a current LARS project. Itis intended to meet the communicationneeds inside air bases. The order wasplaced in the spring of 1980 and theequipment is now being delivered. Thedevelopment work was carried out incollaboration with the Swedish AirForce.

Load Study of theAXE 10 Control SystemBerth Eklundh and David RappThe Department of Telecommunication Systems at the Lund Institute ofTechnology in Sweden has, with the support of Ericsson and the SwedishTelecommunications Administration, carried out a study of the traffic capacity ofthe AXE 10 control system with the APZ210 central processor.The authors describe the various methods used and the result of the study.In the analog version of AXE 10 theswitching system consists of link connectedunits. The proper dimensioningof link connected systems has long beenknown, and in this respect systemAXE 10 presents no new problems. Inthe digital version the dimensioning isalso so ample that only routine checkingis necessary.In the AXE 10 system the equivalent ofthe registers and markers in crossbarsystems is a complex of computers, thesystem being stored program controlled.The function of the computers ismore intricate than that of the markers,and the division between different interworkingfunctional units is complex.The available mathematical methodshave not been sufficient. It has beennecessary to develop new methods inorder to be able to solve the problemThese methods will also be useful forother, similar problems.The structure of AXE 10The design and function of AXE 10 hasbeen described previously in several articlesin Ericsson Review 123 . A briefsummary of the function of the controlsystem is given here as a background tothe description of the study that followsAXE 10 is built up in a hierarchic structurewith systems, subsystems, functionalblocks and functional units, fig. 1.The interworking between differentfunctional blocks is by means of signals.When a subfunction is completed in onefunctional block, the block sends signalsto one or more other functionalblocks. This initiatesothersubfunctionsin these blocks, which in their turn sendout new signalsFig. 2 shows a diagram of the structureof system AXE 10 The parts that are ofimportance to the capacity analysis arethe processor system with the centralprocessor, CP, and the regional processors,RP. CP and RP are connectedvia a bus system with separate buses forgroups of RPs. The main work of CPconsists of complex, infrequent tasks,such as number analysis, while RPhandles routine work, e.g. scanning theline circuits.Other units of importance in this connectionare RPH-I and RPH-O. whichFig. 1The functional levels of system AXE 10HardwareRegional software• Central softwareFig. 2The structure of system AXE 10SSSGSSTSSMASIOSRPSRPCPSCPU-ACPU-BPSRSDSRPH-IRPH-0Subscriber switching subsystemGroup switching subsystemTrunk and signalling subsystemMaintenance subsystemInputoutput subsystemRegional processor subsystemRegional processorCentral processor subsystemCentral processing unit ACentral processing unit BProgram storeReference storeData storeUnit for sensing incoming calls from RPUnit for distributing outgoing signals to RP

209BERTH EKLUNDHDAVID RAPPDepartment of Telecommunication SystemsLund Institute of Technology. Swedenhandle the interworking with the regionalprocessors RPH-I scans all RP busescyclically using a polling process, andthe signals stored in RP are transmittedto CP. RPH-0 transmits signals to differentRPs in a corresponding wayThe processors in stored program controlledsystems have different prioritylevels for the jobs they have to perform.Thus certain |obs are allowed to passothers. This method of working complicatesthe calculations of processor capacityIn CP the job management is mainly carriedout by micro programs. An interruptsystem is used which has four programlevels and which includes job buffers forqueue administration, fig.3.The malfunction level. MFL. has top priorityAn interrupt signal isgenerated forMFL when a fault detecting circuit hasfound a hardware fault which must becleared quickly.The normal work of CP is carried out atthe traffic handling level, THL, and at thebasic level, BAL, with sublevelsin accordancewith fig.3. BAL has the lowestpriority. Job management at levels THLand BAL uses the job buffers JBA-JBDfor storing different signal messagesSuch a message contains the receivingblock number and the signal number togetherwith the data being transmitted inthe signal. An interrupt signal is automaticallygenerated when a signal messageis placed in a buffer. Such a signalsent to THL then interrupts work in progressat level BAL If other work is alreadybeing carried out at THL, it mustalways be completed before the next jobcan be started.All signals from RP, and most signalsbetween the CP program blocks aretemporarily stored in the job buffers. Inaddition level THL receives a clock interruptsignal every 10ms so that signalsinaccordance with a job table can be sentto program blocks that measure time.Fig. 3Job management in CPPCU Priority control unitMFL Malfunction levelTRL Tracing levelTHL Traffic handling level with sublevels 1-3BAL Basic level with sublevels 1-2MAU Maintenance unitTRU Tracing unitJBA Job buffer AJBB Job buffer BJBC Job buffer CJBD Job buffer DEES Execution stoppedE

210ModelsIn order to carry out a capacity analysisof the system it is necessary to create amodel that describes the internal"mechanism" of the system. A model isalways a simplification of reality. However,in order for a model to be of anyuse, the essential functional characteristicsmust be adequately represented.The practicability of the modelas a working tool is of course also dependenton how suitable it is for mathematicaltreatment or computer simulation.The following models have been used inthe project of analysing the capacity ofthe AXE 10 processor:- the Queue Flow Model- the simulation model- analytical models• queueing network models• models of the central processor.During the work on the models certainassumptions were made regarding thebehaviour of the subscribers. On thebasis of the results of many measurementsthe call arrival process has beenassumed to be a Poisson process. Theoperating times of different functionalblocks in CP and RP have been determinedby means of measurements, orsimply by counting the number of instructionsin each block.THE QUEUE FLOW MODELA Queue Flow Model shows how thevarious equipments (with their programs)interwork when carrying outtheirjobs. This model formsthebasisforfurther work on the project.Fig. 4 shows a basic Queue Flow Modelof the whole AXE 10 processor system.This model takes into consideration CP,RP, RPH-I and RPH-O, all of which aredepicted as individual Queue Flow Modelsin themselves. For example, CP isdivided into a server, the processor itself,and several queues, JBA-JBD.The connections between the differentunits in the model represent the flowpaths, i.e. the paths of the signals in thesystem. Physically they consist of thebus system. Most signal sequences atthe various priority levels use thesepaths, but to a varying extent. A completeQueue Flow Model would thereforebe extremely complicated. More-Fig. 4Basic Queue Flow Model for the AXE 10 processorsystemCPJBA-JBDMALTHLBALRPRP-LICRPH-IRPH-0Central processorJob queuesMalfunction levelTraffic handling levelsBasic levelsRegional processorsRegional processor tor line circuitsUnit for sensing incoming calls from RPUnit for distributing output signals to RP

211over, the type of traffic handled by anexchange differs depending on its sizeand application. In order to obtain amanageable model a configuration hastherefore been studied for a traffic casewhere the traffic handling signals mainlyuse a single priority level. This case isused in fig. 5 to give a somewhat simplifieddescription of the conversion to aQueue Flow Model. The process ofevents when detecting a call attempt isas follows: A signal for the detected callis created in RP, NEW CALL, and istransmitted to CP. This means that thesignal comes in over the RP buses andRPH-I to JBB. If there is a queue, thesignal has to wait there, after which afunctional block in CP is activated,which in its turn initiates one or moresignals, in this case two. One of theseimmediately activates a new functionalblock in CP, which in its turn generates anew signal, this time to an RP. The othersignal generates a shorter sequence forcertain secondary functions, and thissequence is eventually completed. Thisgeneration of several output signalsleads to activities being performed inparallel, a fact which is of importance forthe model.The setting-up sequence can thus be describedas a job, NEW CALL, arriving atRP, where it receives a certain service,and then continuing to CP. where it receivesservice several times. Sometimesthe job receives new service immediately,at other times it has to queue inJBB. Sometimes the job leaves CP directly,and other times it returns to anRP. The basic principle is the same regardlessof traffic case or level.The Queue Flow Model contains no informationregarding, for example, thearrival process and service time in differentsubsystems Such factors arespecified in connection with the furtherchoice of model. In this respect simulationmodels offer better possibilitiesthan the analytical models.SIMULATION MODELA simulation model can be made "asaccurate as one likes". It is possible tomodel the internal behaviour of a systemin the most minute detail, and the simulationprogram will then behave exactlylike the reality it depicts. However, themodel development should not betakenso far in practice. There must be a balancebetween the effort put into themodel and the results that can be expected.Fig. 5aSignal flow diagram for level THL3RP functional block'[ CP functional blockFig. 5bQueue Flow Model for level THL3

212Queue theory,queueing network theoryQueues occur because of a temporary shortageof resources. This applies in many every-dayoccurrences, in the community as well as intechnology. In the telephony field queueingproblems aroused great interest almost sincethe first telephone exchanges were built. Thedevelopment of telecommunication technologyhas given rise to a multitude of problems, whichhave often required sophisticated mathematicaltreatment. The article deals with a difficultqueueing problem in modern telephony.Some basic conceptsThe purpose of queueing theory is to provide ananswer to the question of how long the waitingtime will be under different conditions. The limitfor what is acceptable must be decided individuallyin each case, depending on the overallfunction of the telecommunication system andthe expected reactions of the subscribers.Configurations. The queueing systems are builtup of servers. A simple queueing system consistsof a number of servers (one or several)working in parallel and carrying out the sametype of service, for example the devices in telephoneexchanges that work in delay systems.This type of system is treated with the aid ofqueueing theory (in a restricted sense).More complicated systems contain differenttypes of servers that must interwork in order tobe able to carry out their tasks. Such systemswork in several stages, e.g. the processors insystem AXE 10. The crossbar systems of typesARM and ARE (transit exchanges) also containsuch service systems. Such systems are treatedwith the aid of queueing network theory.The distribution function for the interarrivaltimes to the queue. Most common (and mostrealistic) is to assume an exponential distribution.If there is only one server the mean interarrivaltime must be longer than the mean servicetime, otherwise the queue will eventually becomeinfinite. The service time is either constantor varies with different call categories.There are a great number of queue handlingmethods. The most common one is FIFO (FirstIn, First Out). The calls can also be fetched fromthe queue at random. Other alternatives canmean different priority categories for differenttypes of calls.The result of a theoretical or empirical solutionto a queue problem is often given as a series ofcurves showing the distribution of the waitingtime for the calls.There were several reasonsforstarting asimulation study of AXE 10. The simulationresult was needed for comparisonswith analytical results, and the constructionof a simulation programmeprovides further insight into the behaviourof the system. Simulation alsohas an intrinsic value as a study of methods.Rather extensive compromises had tobe made when preparing the program.The central and regional processors andthe communication between them aredescribed in a fairly detailed way, whereasthe hardware connected to the regionalprocessors has been left out ofthe model. The functional blocks for theprocessing of incoming calls are describedin detail for the sequences ofevents considered to be of the greatestinterest.Jobs that are generated internally withinthe exchange are usually handled at alower priority level than jobs which concernthe traffic handling. However, certaininternal jobs have a degree of prioritysuch that they compete with thetraffic handling, and hence these havebeen included in the model.The simulation program comprises approximately8000 SIMULA statements. Itwas developed using a UNIVAC 100/80,and both the compiler capacity and theavailable storage space were fully utilized.ANALYTICAL MODELSIt is not possible to create a mathematicalanalytical model of the whole AXE 10processor system containing all the elementsdescribed by the Queue FlowModel, the signal operating times in differentprocessors, the behaviour of thesubscribers etc. The reason for this isthat there are no general analyticalmethods for networks of queues. It istherefore necessary to create differentmodels, depending on what features areto be studied. For overall analyses themost suitable methods are those in thebranch of queueing theory known asqueueing network theory. For detailedstudies, on the other hand, many usefulmethods are to be found in the branch ofqueueing theory that deals with queueingsystems consisting of only onestage 56 .Queueing network modelsA so-called Markovian queueing networkmodel requires simplification ofthe central processor structure. Thus itis necessary to refrain from giving certaintypes of signals priority. Moreover,no consideration can be paid to the factthat certain functional blocks generatemore than one signal. Finally an approximationhas to be made of the distributionof operating time.The Markovian queueing networkmodel can provide:1 Information regarding the mean flowsin different parts of the system, andthe mean load on different parts of thesystem.2 Information regarding the length ofqueues, the waiting time in differentparts of the system and sometimesthe total time through the whole system.Much of the structure in the Queue FlowModel for traffic handling, THL, can beretained in a Markovian queueing networkmodel of system AXE 10, but parallelactivities cannot be described in thelatter model. This is one of the majordisadvantages of this type of model.Fig. 6Model of the central processor (CP)

213Models of CPA comparison between the results obtainedfrom the Markovian queueingnetwork model and the simulation resultsshowed large differences. Thismeant that certain system parts had tobe studied in more detail. It was foundsuitable to study CP on its own, removedfrom the system. If the quite reasonableassumption is also made that the callarrival process is a Poisson process,there are a number of methods available.This results in a so-called M/G/1queueing system in accordance with theusual queueing theory designations 56 .This means that previously omitted factorscan be included in the model, suchas priorities, feedback to CP and parallelsignals. In addition a larger number ofquantities can be calculated, includingthe distribution of the total time in CP.The final CP model is shown in fig. 6.All traffic handling at levels THL1 -3 hashigher priority than the maintenancefunctions at levels BAL1-2. The priorityis of the preemptive-resume category,i.e. no BAL job can be started while aTHL job is waiting to be serviced, and aBAL job being serviced is interrupted if aTHL job arrives (preemptive), and is resumedwhen all THL jobs are finished(resume). The waiting times for THL jobsare therefore not affected by the situationat the BAL levels. This means thatthe THL levels can be studied separately.Further simplification can bemade when studying JBB, since the regularinterrupts from levels MAL-THL2generate jobs of approximately constantlength, which normally take up nomore than 5-10% of a primary interval.It was found that good approximate resultscould be obtained by adding a constantload to the JBB load.Parallel activities are introduced into themodel in the followig way. As soon asany signal has activated a functionalblock in CP, i.e. when CP is busy, newsignals to CP are generated at a constantintensity. This gives an obvious improvementof the model, in spite of thefact that the intensity is low, since afunctional block in CP only generatesmore than one signal in less than 10% ofall cases. The resultant model of JBB isshown in fig, 7.This Queue Flow Model of the CP byitself, together with the previous assumptionsregarding the calling processand the distribution of operatingtimes, makes it possible to determinequeue lengths and waiting times.The final queue model is called a feedbackqueue model, and methods for thistype of model have been published 7 . Themethod can be modified to include theparallel activities discussed above.However, a final model of, for example,the waiting time to dialling tone meansthat the whole processor system mustbe considered, since the signalling sequenceup to this stage in the call handlingincludes processing in differentRPs, RPH and CP several times. Becauseof the internal relations in the system,the delays in different processorswill be dependent on each other. Thisdependence is difficult to include in ananalytical model. The waiting time to diallingtone must therefore be describedin the model as the sum of a number ofindependent delays. This is an approximation,the accuracy of which is revealedby a comparison with the simulationresults. Classic models for queueswith priority can be used for calculatingvarious quantities at levels BAL (JBCand JBD), on condition that the arrivalprocesses are Poisson processes. Iffeedback and parallel activities are alsointroduced into the model at these levels,very complicated problems will beobtained, particularly if it is the distributionsof the delays of certain signal sequencesthat are sought.Fig. 7Working model of JBB

214Load Number Waiting time (s) is less thano of for 90% of for 99% ofcalls/s the calls the calls0.750.900.954757610.3800.4370.5520.4090.4870.652Table 1Maximum waiting time to dialling tone for 90 %and 99% of the calls, at different loadsResults and comparisonsTo conclude it can be stated that theavailable analytical methods for the systemas a whole gives a summary pictureof its behaviour. Using appropriate assumptions,different parts of the queueingsystem can be treated separately, formore detailed structural studies.However, it is impossible to form anygeneral conclusions of the effects ofsuch approximations. Comparisonsmust therefore always be made withmeasurements on real systems and withsimulations.The numerical results given in this sectionare based on certain simplifications.Only one traffic case has beenconsidered, and the total load from levelsMAL-THL2 towards JBA has beenassumed to be 10%.The operating time distribution at differentpriority levels is an important systemparameter. The distribution for jobsat level THL3 towards JBB is shown inthe form of a histogram in fig.8, but itsexact shape varies with the compositionof the traffic.In order to simplify the analysis the operatingtime distribution was approximatedto continuous distributions. Theoriginal distribution was replaced by ahyperexponential distribution, well fittedto the original, the blue curve infig.8. There was also great similarity toan exponential distribution, and such adistribution was therefore tried, the redcurve in fig.8.The distribution of the RP delay was assumedto be uniform (t,, t ( + At). Theconstant part refers to the action time inthe hard ware equipment for subscribersand corresponds to, for example, thetime required to activate a relay. Theother part is given by the clock pulseinterval (At) in an RP.The work flows to various parts of theprocessor system can be determinedwith the aid of the Queue Flow andqueueing network models. The workflows and the mean operating times arethen used to calculate the offered loads.The curves on the left of fig. 9 show themeasured load on CP from THL3 jobsvia JBB, the curve designated Sim, andits confidence interval (97.5%). Analyticallythe load on CP has been calculatedin three ways. Curve no. 3 showsthe mean value of the total operatingtime per call, multiplied by the callingrate. Curves nos. 1 and 2 are calculatedwith the aid of the queue network model.Curve no. 2 shows the load when considerationis paid to parallel activities.The total time, i.e. the waiting time plusthe operating time, through CPforTHL3jobs determines the load properties ofthe system. Since the operating time isalready known, only the waiting time inJBB needs to be investigated. The righthandside of fig. 9 shows the mean waitingtime for an arbitrary signal, whicharrives from an RP and which passesonce through CP. The blue area marksthe confidence interval. Since both theload and the mean waiting time are measured,the uncertainty of the result fallsFig. 8The distribution of the operating time in CP forjobs at level THL3 towards JBBMeasured values, histogram (1)H2-E2. approximation (2)Approximation with an exponential distibution (3)

ETable 2The table shows the different combinations ofoperating time distributions for CP and RP, thedifferent models of RPH that have been simulatedand how different measured quantities have beenaffected by alterations. Distributions in accordancewith fig. 8Operating time dis- Operating timetribution for CP distribution forRPOriginal (1)H 2-E 2 (2)Exponential (3)RectangularExponentialExponential (3) with Exponentialall signals bufferedWithout any parallel signalsRPH Measured quantity Effect ofchangeArrival process to JBB SmallOriginal Waiting times, JBB SmallModified Queue lengths, JBB SmallWithout RPH Busy time, JBB SmallWaiting time to dialling tone Depend, on RPWithout RPHAcertain reductionofall quantitiesLarge reduction of all quantitiesFig. 9Left: The load on CP from THL 3 jobs via JBB as afunction of the number of calls/sRight: The mean waiting time W JBB for an arbitrarysignal to JBB^^ Confidence areaConfidence interval1 Queueing network model2 Modified queueing network model3 >. x mean operating timewithin a certain area. This is also markedin fig.9. The results from analytical calculationsare also included in fig.9. Thequeueing network model was used(curve no. 1, right-hand side) and the resultswere also adjusted to take the parallelactivities into consideration (curveno. 2, right-hand side). The latter curveclearly shows an improvement, but asystematic deviation is still obvious.This behaviour is characteristic of allstudied parameters and is probablycaused partly by dependency mechanismswhich have not yet been fully clarifiedand partly because the model of theparallel activities is fairly simple. For example,a number of parallel signals tovarious RPs have been omitted.Table 1 shows the most important systemparameter, waiting time to diallingtone. This time includes not only severalpassages through CP but also activitiesin different RPs.Detailed investigations were carried outwith the aid of the simulation programand various analytical calculations, inorder to find the causes of the systematicdeviations. The exact signal lengthsin the simulation were replaced by thedistributions in fig. 8. The polling strategyand the cycle time for RPH were alsochanged, and the RPH function wasomitted in one calculation. Finally theuniform distribution in RP was replacedby an exponential distribution. Table2shows the possible combinations, thequantities that were studied and thepossible effects of the changes. Themean value and variance were comparedfor all measured quantities.In the analytical studies related to themodel of CP by itself for level THL3,fig. 7, was studied how the distributionof the operating time affected queuelengths and waiting times in CP. Smalldifferences in the results were foundalso in this case. The deviations are sosmall because the operating time distributionfor CP differs very little fromthe exponential distribution. The distributionof the RP operating times wasalso changed, and the effects of thechanges were small.The most important result of these studiesis that priorities and parallel activitieshave greater effect than differencesin the operating time distribution.Conclusions and summaryMethodsThe experience gained from these studiesshows that there is no point in tryingto build up a very detailed model of thewhole system. It appears to be more usefulto construct more accurate modelsof different subsystems, and then to tryto combine the results from these partmodels to give an overall picture of thewhole system. This applies particularlyin those cases where one part of thesystem, in this case CP, carries thegreatest part of the load. It has also beenshown that intuitive ideas about what isessential and not essential are oftenfalse, and can easily lead to the wrongline being chosen in the analysis work.

A Probability D(t. y)Fig. 10The distribution of the waiting time to diallingtone (DTD) with a load o = 0.9SimulationsAnalytical calculations without any correction forparallel activitiesAnalytical calculations with correction for parallelactivitiesCurve Blue RedMean value for DTD (s) 0.375 0.360Standard deviation (s) 0.048 0.029D(t, o) = P(DTD>t, o)o = LHL + LJBBLHL = 0.1o = 0.957 Calls/sPractical resultsFig. 10 shows an example of the distributionof the waiting time to diallingtone with the load Q as parameter. A correspondingvalue for the total number ofcalls has also been given. Such curvescorrespond to the practical knowledgethat is required concerning the capacityof the control system. Table 1 showshow long the maximum waiting time willbe for 90% and 99% of the calls withdifferent loads, based on the results ofthe simulation.The table shows that with 47 calls/s thewaiting time is less than 0.380s for 90%and less than 0.409s for 99% of thecalls, i.e. an increase by only 0.029s.With 61 calls/s the corresponding valuesare 0.552s and 0.652s.It is a normal requirement that the waitingtime to dialling tone must not exceed1 second for 99% of the calls. The resultsof these studies show that this requirementis met with good marginseven with a very high load on the controlsystem.The other results of the simulations thathave been carried out also confirm theinformation that has previously beengiven regarding the traffic capacity ofthe AXE 10 control system 1 .CommentsThe work on improving the models andfinding the dependency mechanismshas continued after this article was preparedfor publication. Models have beendeveloped which show that the combinedfeedback (CP-CP.CP-JBB,CP-RP-JBB), together with signalbranching, are the predominant structuralelements of the system.The experience gained from the projecthas also led to a simplification of thesimulation model, and it has been possibleto make it smaller and faster, whileretaining its precision.References1. Meurling, J.: Presentation of AXE 10Switching System. Ericsson Rev. 53(1976):2, pp. 54-59,2. Eklund, M. et al.: AXE 10 - System-Description. Ericsson Rev. 53(1976):2,pp. 70-89.3. Ericson, B. and Roos, S.: Digital GroupSelector in the AXE 10 System.Ericsson Rev. 55 (1978):4, pp. 140-149.4. Persson, K. and Sundstrdm, S.: Digitallocal exchanges AXE 10. Ericsson Rev.58 (1981):3, pp. 102-110.5. Kleinroch, L: QueueingSystems. Voll:Theory. John Wiley & Sons. 1975.6. Kleinroch. L: Queueing Systems. VolII: Computer Applications. John Wiley& Sons, 1976.7. Takacs. L: A Feedback Queue. TheBell System Technical Journal, March1963.

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