1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G0!30%#43G0G0/ amended at Geneva, 1980 and at Melbourne, 1988)1 GeneralThe second order digital multiplex equipment using positive/zero/negative justification, considered below, isintended for use on di
2、gital paths between countries using 2048 kbit/s primary multiplex equipments, such as the PCMmultiplex equipment described in Recommendation G.732 or any identical equipment.2 Bit rateThe nominal bit rate should be 8448 kbit/s. The tolerance on that rate should be 30 parts per million (ppm).2 Fascic
3、le III.4 - Rec. G.7453 Frame structureTable 1/G.745 gives:- the tributary bit rate and the number of tributaries;- the number of bits per frame;- the bit numbering scheme;- the bit assignment;- the bunched frame alignment signal.4 Loss and recovery of frame alignment and consequent actionLoss of fra
4、me alignment should be assumed to have taken place when five consecutive frame alignment signalshave been incorrectly received in their predicted positions.Recovery of frame alignment should take place in the case of receiving without errors at least two consecutiveframe signals in their predicted p
5、ositions.As soon as frame alignment has been lost and until it has been recovered, a definite pattern should be sent to alltributaries from the output of the demultiplexer. The equivalent binary content of this pattern, called the AlarmIndication Signal (AIS), at 2048 kbit/s is a continuous stream o
6、f binary 1s.5 Multiplexing methodCyclic bit interleaving in the tributary numbering order and positive/zero/negative justification with two-command control are recommended.The justification control signal should be distributed and use the Cjn-bits (n = 1, 2, 3, see Table 1/G.745).Correction of one e
7、rror in command is possible.Positive justification should be indicated by the signal 111, transmitted in each of two consecutive frames;negative justification should be indicated by the signal 000 transmitted in each of two consecutive frames, and nojustification by the signal 111 in one frame follo
8、wed by 000 in the next frame. Bits 5, 6, 7 and 8 in Set IV (see Table1/G.745) are used for negative justification of tributaries 1, 2, 3 and 4 respectively, and bits 9 to 12 for positivejustification of the same tributaries.Besides, when information from tributaries 1, 2, 3 and 4 is not transmitted,
9、 bits 5, 6, 7 and 8 in Set IV areavailable for transmitting information concerning the type of justification (positive or negative) in frames containingcommands of positive justification control and intermediate amount of jitter in frames containing commands of negativejustification.Fascicle III.4 -
10、 Rec. G.745 3Table 1/G.745 gives the maximum justification rate per tributary.6 JitterThe amount of jitter that should be tolerated at the input of the multiplexer and the demultiplexer should beaccording to Rec. G.823, 3.1.1. The amount of jitter at the output of the multiplexer and the demultiplex
11、er should bestudied and specified.7 Digital interfaceThe digital interfaces at 2048 kbit/s and 8448 kbit/s should be in accordance with Recommendation G.703.8 Timing signalIt might be desirable to be able to derive the multiplexer timing signal from an external source as well as froman internal one.
12、9 Service digitsSome spare bits per frame are available for service functions (bits from 5 to 8 in Set II and bit 8 in Set III) fornational and international use. Bits 5, 6, 7 and 8 in Set II are available for a digital service channel between twoterminals (using 32 kbit/s Adaptive Delta Modulation)
13、 and bit 8 in Set III is available for ringing up a digital servicechannel. Utilization of other spare bits is under study.TABLE 1/G.7458448-kbit/s digital multiplexing frame structure using positive/zero/negative justificationTributary bit rate (kbit/s) 2048Number of tributaries 4Frame structure Bi
14、t numberFrame alignment signal (11100110)Bits from tributariesJustification control bits (Cj1) (see Note)Bits for service functionsBits from tributariesJustification control bits (Cj2) (see Note)Spare bitsBits from tributariesJustification control bits (Cj3) (see Note)Bits from tributaries available
15、 for negative justificationBits from tributaries available for positive justificationBits from tributariesSet I1 to 89 to 264Set II1 to 45 to 89 to 264Set III1 to 45 to 89 to 264Set IV1 to 45 to 89 to 1212 to 264Frame lengthFrame durationBits per tributaryMaximum justification rate per tributary1056
16、 bits125 s256 bits8 kbit/sNote - Cjnindicates nth justification control bit of the jth tributary.4 Fascicle III.4 - Rec. G.74510 Fault conditions and consequent actions10.1 The digital multiplex equipment should detect the following fault conditions:10.1.1 Failure of power supply.10.1.2 Loss of inco
17、ming signal at 2048 kbit/s at the input of the multiplexer.Note - When using separate circuits for the digital signal and the timing signal, loss of either or both shouldconstitute loss of the incoming signal.10.1.3 Loss of the incoming signal at 8448 kbit/s at the input of the demultiplexer.Note 1
18、- The detection of this fault condition is required only when it does not result in an indication of loss offrame alignment.Note 2 - When using separate circuits for the digital signal and the timing signal, loss of either or both shouldconstitute loss of the incoming signal.10.1.4 Loss of frame ali
19、gnment.10.1.5 Alarm indication received from the remote multiplex equipment at the 8448 kbit/s input of the demultiplexer(see 10.2.2).10.2 Consequent actionsAfter detection of a fault condition appropriate actions should be taken as specified in Table 2/G.745. Theconsequent actions are as follows:10
20、.2.1 Prompt maintenance alarm indication generated to designate that the performance is below acceptablestandards and maintenance attention is required locally. When detecting the AIS at the 8448 kbit/s input of thedemultiplexer, the prompt maintenance alarm indication associated with loss of frame
21、alignment should be prohibited(see Note 1 below).Note - The location and provision of any visual and/or audible alarm activated by this prompt maintenancealarm indication is left to the discretion of each Administration.10.2.2 Alarm indication to the remote multiplex equipment generated by changing
22、from the state 0 to the state 1 bit 7of set III at the 8448 kbit/s output of the multiplexer.10.2.3 AIS (see Note 2 below) applied to all the four 2048 kbit/s tributary outputs from the demultiplexer.10.2.4 AIS (see Note 2 below) applied to the 8448 kbit/s output of the multiplexer.10.2.5 AIS (see N
23、ote 2 below) applied to the time slots of the 8448 kbit/s signal at the multiplexer outputcorresponding to the relevant 2048 kbit/s tributary.Note 1 - The bit rate of the AIS at the output of the corresponding demultiplexer should be as specified for thetributaries. The method of achieving this is u
24、nder study.Note 2 - The equivalent binary content of the AIS at 2048 kbit/s and 8448 kbit/s is a continuous stream ofbinary 1s.Fascicle III.4 - Rec. G.745 5TABLE 2/G.745Fault conditions and consequent actionsConsequent actions (see 10.2)AIS appliedEquipmentpartFault condition(see 10.1)Promptmaintena
25、ncealarmindicationgeneratedAlarmindication tothe remotemultiplexergeneratedTo all thetributariesTo thecompositesignalTo the relevanttime slotsof thecompositesignalMultiplexeranddemultiplexer_Failureof power supplyYes Yes, ifpracticableYes, ifpracticableYes, ifpracticableMultiplexeronlyLoss of incomi
26、ng signalon a tributaryYes YesLoss of incomingsignal at 8448 kbit/sYes Yes YesDemultiplexerLoss of frame alignment Yes Yes YesonlyAIS received fromthe remote multiplexerNote - A Yes in the table signifies that a certain action should be taken as a consequence of the relevant fault condition. Anopen
27、space in the table signifies that the relevant action should not be taken as a consequence of the relevant fault condition,if this condition is the only one present. If more than one fault condition is simultaneously present the relevant action shouldbe taken if, for at least one of the conditions, a Yes is defined in relation to this action.
