1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G0!30%#43G0G0/ further amended)1 GeneralThe second order digital multiplex equipment using positive justification, described below, is intended for useon digital paths between countries using 2048 kbit/
2、s primary multiplex equipments.2 Bit rateThe nominal bit rate should be 8448 kbit/s.The tolerance on that rate should be 30 parts per million (ppm).3 Frame structureTable 1/G.742 gives:- the tributary bit rate and the number of tributaries;- the number of bits per frame;- the bit numbering scheme;-
3、the bit assignment;- the bunched frame alignment signal.2 Fascicle III.4 - Rec. G.742TABLE 1/G.7428448-kbit/s multiplexing frame structureNote - Cjiindicates the ith justification control bit of the jth tributary.4 Loss and recovery of frame alignment and consequent actionLoss of frame alignment sho
4、uld be assumed to have taken place when four consecutive frame alignment signalshave been incorrectly received in their predicted positions.When frame alignment is assumed to be lost, the frame alignment device should decide that such alignment haseffectively been recovered when it detects the prese
5、nce of three consecutive frame alignment signals.The frame alignment device having detected the appearance of a single correct frame alignment signal, shouldbegin a new search for the frame alignment signal when it detects the absence of the frame alignment signal in one of thetwo following frames.N
6、ote - As it is not strictly necessary to specify the detailed frame alignment strategy, any suitable framealignment strategy may be used provided the performance achieved is at least as efficient in all respects as that obtainedby the above frame alignment strategy.5 Multiplexing methodCyclic bit in
7、terleaving in the tributary numbering order and positive justification is recommended.Tributary bit rate (kbit/s) 2048Number of tributaries 4Frame structure Bit numberFrame alignment signal (1111010000)Alarm indication to the remote digital multiplex equipmentBit reserved for national useBits from t
8、ributariesJustification control bits Cj1(see Note)Bits from tributariesJustification control bits Cj2(see Note)Bits from tributariesJustification control bits Cj2(see Note)Bits from tributaries available for justificationBits from tributariesSet I1 to 10111213 to 212Set II1 to 45 to 212Set III1 to 4
9、5 to 212Set IV1 to 45 to 89 to 212Frame lengthBits per tributaryMaximum justification rate per tributaryNominal justification ratio848 bits206 bits10 kbit/s0.424Fascicle III.4 - Rec. G.742 3The justification control signal should be distributed and use the Cjn-bits (n = 1, 2, 3, see Table 1/G.742).P
10、ositive justification should be indicated by the signal 111, no justification by the signal 000. Majority decisionis recommended.Table 1/G.742 gives the maximum justification rate per tributary and the nominal justification ratio.6 Jitter6.1 Jitter transfer characteristicA 2048 kbit/s signal, modula
11、ted by sinusoidal jitter, should be subject to a muldex jitter transfer characteristicwithin the gain/frequency limits given in Figure 1/G.742. The equivalent binary content of the test signal should be1000.Note 1 - The frequency f0should be less than 20 Hz and as low as possible (e.g. 10 Hz),taking
12、 into account the limitations of measuring equipment.Note 2 - To achieve accurate measurements, the use of a selective method is recommendedwith a bandwidth sufficiently small referred to the relevant measurement frequency, but notwider than 40 Hz.Note 3 - The need to tolerate spurious responses gre
13、ater than -19.5 dB in the frequencyrange f6 to f7is for further study.FIGURE 1/G.7426.2 Tributary output jitterThe peak-to-peak jitter at a tributary output in the absence of input jitter should not exceed 0.25 UI whenmeasured in the frequency range up to 100 kHz.When measured with an instrument inc
14、orporating a bandpass filter having a lower cutoff frequency of 18 kHz,a roll-off of 20 dB/decade and an upper limit of 100 kHz, the peak-to-peak output jitter should not exceed 0.05 UI with aprobability of 99.9% during a measurement period of 10 s.Note - For interfaces meeting the national high Q o
15、ption, detailed in Recommendation G.703, the lower cutofffrequency for the above measurement should be 700 Hz.6.3 Multiplex signal output jitterIn the case where the transmitting timing signal is derived from an internal oscillator, the peak-to-peak jitter at4 Fascicle III.4 - Rec. G.742the 8448 kbi
16、t/s output should not exceed 0.05 UI when it is measured within the frequency range fromf1 = 20 Hz to f4 = 400 kHz.7 Digital interfacesThe digital interfaces at 2048 kbit/s and 8448 kbit/s should be in accordance with Recommendation G.703.8 Timing signalIf it is economically feasible, it may be desi
17、rable to be able to derive the multiplexer timing signal from anexternal source as well as from an internal source.9 Service digitsTwo bits per frame are available for service functions. Bit 11 of Set I is used to transmit an alarm indication tothe remote multiplex equipment when specific fault cond
18、itions are detected in the multiplex equipment (see 10 below).Bit 12 of Set I is reserved for national use. On the digital path crossing the border, this bit is fixed at 1.10 Fault conditions and consequent conditions10.1 Fault conditionsThe digital multiplex equipment should detect the following fa
19、ult conditions:10.1.1 Failure of power supply.10.1.2 Loss of an incoming signal at 2048 kbit/s at the input of the multiplexer.Note - Where separate circuits are used for the digital signal and the timing signal then loss of either or bothshould constitute loss of the incoming signal.10.1.3 Loss of
20、the incoming signal at 8448 kbit/s at the input of the demultiplexer.Note 1 - The detection of this fault condition is required only when it does not result in an indication of loss offrame alignment.Note 2 - Where separate circuits are used for the digital signal and the timing signal, then loss of
21、 either or bothshould constitute loss of the incoming signal.10.1.4 Loss of frame alignment.10.1.5 Alarm indication received from the remote multiplex equipment at the 8448 kbit/s input of the demultiplexer(see 10.2.2 below).10.2 Consequent actionsFurther to the detection of a fault condition, appro
22、priate actions should be taken as specified by Table 2/G.742.The consequent actions are as follows:10.2.1 Prompt maintenance alarm indication generated to signify that performance is below acceptable standards andmaintenance attention is required locally. When the Alarm Indication Signal (AIS) (see
23、Note 2 under 10.2.5 below) at8448 kbit/s is detected at the input of the demultiplexer, the prompt maintenance alarm indication associated with loss offrame alignment should be inhibited, while the rest of the consequent actions are in accordance with those associated inTable 2/G.742 with the fault
24、condition.Note - The location and provision of any visual and/or audible alarm activated by this maintenance alarmindication is left to the discretion of each Administration.Fascicle III.4 - Rec. G.742 510.2.2 Alarm indication to the remote multiplex equipment generated by changing from the state 0
25、to thestate 1 bit 11 of Set I at the 8448 kbit/s output of the multiplexer.10.2.3 AIS (see Notes 1 and 2 below) applied to all four 2048 kbit/s tributary outputs from the demultiplexer.10.2.4 AIS (see Notes 1 and 2 below) applied to the 8448 kbit/s output of the multiplexer.10.2.5 AIS (see Note 2 be
26、low) applied to the time slots of the 8448 kbit/s signal at the output of the multiplexer,corresponding to the relevant 2048 kbit/s tributary.The method of transmitting the AIS at the output port of the multiplexer in time slots corresponding to a faultyinput tributary, should be such that the statu
27、s of the justification control digits is controlled so as to ensure that the AIS iswithin the tolerance specified for that tributary.TABLE 2/G.742Fault conditions and consequent actionsConsequent actions (see 10.2)AIS appliedEquipmentpartFault condition(see 10.1)Promptmaintenancealarmindicationgener
28、atedAlarmindication tothe remotemultiplexequipmentgeneratedTo all thetributariesTo thecompositesignalTo therelevant timeslots of thecompositesignalMultiplexeranddemultiplexerFailure of power supply Yes Yes, ifpracticableYes, ifpracticableMultiplexeronlyLoss of incoming signalon a tributaryYes YesLos
29、s of incoming signalat 8448 kbit/sYes Yes YesDemultiplexerLoss of frame alignment Yes Yes YesonlyAlarm indicationreceived from theremote multiplexequipmentNote - A Yes in the table signifies that a certain action should be taken as a consequence of the relevant fault condition. An open spacein the t
30、able signifies that the relevant action should not be taken as a consequence of the relevant fault condition, if this condition isthe only one present. If more than one fault condition is simultaneously present the relevant action should be taken if, for at least oneof the conditions, a Yes is defin
31、ed in relation to this action.6 Fascicle III.4 - Rec. G.742Note 1 - The bit rate of the AIS at the output of the multiplexer equipment or at the output of the demultiplexerequipment should be in accordance with the interface specifications.Note 2 - The equivalent binary content of the AIS at 2048 kb
32、it/s and 8448 kbit/s is nominally a continuousstream of 1s. The strategy for detecting the presence of the AIS should be such that the AIS is detectable even in thepresence of an error ratio 1 10-3. However, a signal, with all bits except the frame alignment signal in the 1s state,should not be mistaken for an AIS.10.3 Time requirementsThe fault detection and the application of the consequent actions listed in 10.2.2 to 10.2.5, including thedetection of AIS, should be completed within a time limit of 1 ms.
