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82Mbit/s line interface unit is replaced byan 8 Mbit/s line interface unit and thecorresponding 2 Mbit/s buffer unit isreplaced by the branching unit.The stop branching, see fig. 9c, is basedon leak branching. The connection betweenTerminal III and Terminal II ismade via Terminal I by looping of thetributary interfaces in Terminal I. Theexamples in figs. 9b and 9c show thatone fully-equipped digital multiplexercan be saved in certain branching configurations.Alarm functions andmaintenanceThe digital multiplexer complies withLM Ericsson's general alarm philosophy,i.e. alarms from equipment placedin shelves cause bay frame alarms andcentralized station alarms. In addition,visual alarm indications are provided foreach individual equipment. Primaryalarms are generated automatically ormanually in the equipment itself (nearend) or are received from the far end.The alarms result in secondary alarmswhich are processed and indicated visuallyat the near end, transferred to thebay frame and station alarm units andtransmitted to the far end. The relationshipbetween primary and resulting(secondary) alarms is determined mostlyby PROMs (programmable read onlymemories) and in some cases by wirestrappings.Fig. 10 shows the alarms provided in thedigital multiplexer, ZAK 30/120, that arecommon for all tributaries. In addition,an LED indication of failure of the 2Mbit/s tributary timing is provided foreach 2 Mbit/s buffer unit and detectionof branching timing and framing failurefor each branching unit. The providedalarm functions and LED indications(situated on the transmit control unitCUT 8/34) permit easy location ofequipment failures and simplify equipmentmaintenance. In fig. 10 the correspondingalarm function for the thirdorder digital multiplexer is indicated inbrackets.The transmit control unit of the multiplexercontains a pseudo random bitsequence generator RTG. The signalsfrom this generator can be used eitherinstead of incoming tributary signals orfor measuring purposes. For partlyequipped multiplexers the empty tributaryinputs can be connected to thepseudo random signal source in orderto shape the spectrum of the higher ordersignal.Fig. 12Shelf for two digital multiplexersThe same shelf houses either two multiplexers ZAK30/120-2 or two multiplexers ZAK 120/480
83Technical dataZAK 30/120-2 ZAK 120/480Nominal bit rate 8448 kbit/s 34368 kbit/sTolerance ±30x10~ 6 ±20x10" 6Frame structure See fig. 2Frame alignment routinessee CCITT Recommendations G 742 G751JitterTributary input mask See fig. 6Jitter gain See fig. 7Digital link interfaces D1, D2 and D375 L> unbalSee CCITT Recommendation G 703 G 703Permitted attenuation of the input signal 0-6 dB 0-12 dBExternal and reference clock frequency 2112 kHz 8592 kHz(1/4 of gross digit clock rate.)Alarm functions See fig 10Criteria See CCITT Rec. G742 G 751Alarm indication signals (AIS):Transmitted on tributary outputs when themultiplexer is in the alarm state256 onesThe alarm indication state is activated atIthe reception of256 onesat the higher order signal inputSignalling channel bit 12E- and M-type interface, gross digit rate of 9.96 kbit/s 22.375 kbit/sPower supplyBattery 24, 36, 48, 60 V d.c. ( + 20%-15%)Mains 110, 127, 220 Va.c. ( + 10% —10%)Power consumption 22 W 48 WDimensionsBay frameMultiplex shelfHeight 2743 mmWidth 600 mmDepth 225 mmHeight 244 mmWidth 473 mmDepth 225 mmFig. 11 shows the spectrum of a multiplexerworking with only one tributarywith and without transmission ofrandomized signals in the empty tributarybit position. The smooth spectraldensity of the randomized line signal reducesthe risk of crosstalk.Mechanical designThe digital multiplexers are built in theM5 design. A standard M5 shelf accomodatestwo second order digitalmultiplexers, ZAK 30/120-2 (see fig. 12)or two third order digital multiplexersZAK 120/480. The same multiplexershelf is used for different branchingalternatives.The second order digital multiplexer isequipped with a special interface betweenthe 2 Mbit/s line interface unitand the 2 Mbit/s buffer unit (Ta interface).The same interlace exists in the30-channel p.cm. terminal as an option,i.e. when combining both 30-channelp.cm. terminals and a second orderdigital multiplex equipment within thesame bay frame. This makes possible adirect interconnection based on the Tainterface which saves two 2 Mbit/s lineinterface units per tributary. The digitallink interface resp. external clock interfaceconsist of easily accessiblecoaxial contacts situated on the left sideof the shelf.References1. Fremstad, P., Karl, H. and Karlsson,S.: Multiplex and Radio-RelayEquipment for a 120-channel PCMsystem Ericsson Rev. 52 (1975):2,pp. 76-89.2. Axelson, K., Harris, P.-O. and Storesund,E.: M5 Construction Practicefor Transmission Equipment.Ericsson Rev. 52 (1975):3/4, pp.94-105.
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