1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G0!30%#43G0G0/- unidirectional synchronous 384 kbit/s channels (see Figure 1b/G.735).The 384 kbit/s channel is based on the allocation of 6 x 64 kbit/s time slots, e.g. for setting up sound-programme ci
2、rcuits according to Recommendations J.41 and J.42.Because these circuits are specified as unidirectional, the equipment for insertion/extraction has to be separatedas shown in Figure 1b/G.735.1 General characteristics1.1 Fundamental characteristics for voice channel encodingThe encoding law used is
3、the A-law as specified in Recommendation G.711. The sampling rate, load capacityand the code are also specified in that Recommendation.The number of quantized values is 256.Note - The inversion of bits 2, 4, 6 and 8 is covered by the encoding law and is applicable only to voicechannel time slots.1.2
4、 Bit rateThe nominal bit rate is 2048 kbit/s. The tolerance on this rate is 50 parts per million (ppm).1.3 Timing signalIt should be possible to derive the transmit timing signal from any of the following:a) from the received 2048 kbit/s signal,b) from an external source at 2048 kHz (see 5),c) from
5、an internal oscillator.Note - The provision of a timing signal output, available for the purpose of synchronizing other equipments, isan option that might be required depending upon national synchronization arrangements.1.4 Types of access:a) access for bidirectional synchronous 64 kbit/s channels (
6、see Figure l a/G.735);b) access for unidirectional synchronous 384 kbit/s channels (see Figure 1b/G.735).Note - The synchronous insertion of a digital sound programme signal into a 384 kbit/s channel requires theinternal regeneration of a timing signal T synchronized by the 2048 kbit/s signal I1. Th
7、e timing signal is used for2 Fascicle III.4 - Rec. G.735synchronizing the sampling frequency of the analogue/digital converters producing the digital sound programme signal.FIGURE 1/G.735PCM multiplex equipment operating it 2048 kbit/sand offering access to digital sound-programme signalsand/or to s
8、ynchronous 64 kbit/s data channels2 Frame structure and use of derived channel time slots2.1 Frame structure of 2048 kbit/s signalRefer to 2.3 of Recommendation G.704. Bit 1 of the frame should be used in accordance with 2.3.3 ofRecommendation G.704, i.e. for a CRC check bit procedure.2.2 Use of der
9、ived channel time slots2.2.1 Telephone channelsIt should be possible to assign channel time slots 1 to 15 and 17 to 31 to thirty telephone channels numberedfrom 1 to 30.2.2.2 64 kbit/s accessThe number of accessible channel time slots should be at least four and the equipment shall allow access to a
10、nyFascicle III.4 - Rec. G.735 3of channel time slots 1 to 15 and 17 to 31.Note - Equipment exists which provides access to at least four channel time slots in the following order ofpriority: 6 - 22 - 14 - 30 - 2 - 18 - 10 - 26 - 4 - 20 - 12 - 28 - 8 - 24 - 5 - 21 - 13 - 29 - 1 - 17 - 9 - 25 - 3 - 19
11、 - 11 - 27 - 7 -23 - 15 - 31.2.2.3 384 kbit/s accessThe time slot allocation for digital channels with a bit rate at 384 kbit/s is given in Table 1/G.735.TABLE 1/G.735384 kbit/s channels (Note 1)Digital sound-programme accesspointsABCDE1-2-317-18-194-5-620-21-227-8-923-24-2510-11-1226-27-2813-14-152
12、9-30-31I3, T, E3Figure 1b/G.735Note 1 - The five possible 384 kbit/s channels in a 2048 kbit/s stream are numbered A to E. Preferably the channel pairs A-B and C-Dshould be used for stereophonic transmission.Note 2 - If the channel time slot 16 which is assigned to signalling as covered in 5 is not
13、needed for signalling, it may be used forpurposes other than a voice channel encoded within the PCM multiplex equipment.3 Frame alignment and CRC proceduresAn illustration of the procedure is given in Figure 2/G.706.3.1 Loss of frame alignmentRefer to 4.1.1 of Recommendation G.706.3.2 Recovery of fr
14、ame alignmentRefer to 4.1.2 of Recommendation G.706.3.3 CRC multiframe alignment in TSORefer to 4.2 of Recommendation G.706.3.4 CRC bit monitoringRefer to 4.3 of Recommendation G.706.4 Fault conditions and consequent actions4.1 Fault conditionsThe PCM multiplex equipment should detect the following
15、fault conditions:4.1.1 Failure of power supply.4.1.2 Failure of codec (except when using single channel codecs).As a minimum requirement, this fault condition should be recognized when for at least one signal level in therange -21 to -6 dBm0, the signal-to-quantizing noise ratio performance of the l
16、ocal codec is 18 dB or more below thelevel recommended in Recommendation G.712.4 Fascicle III.4 - Rec. G.7354.1.3 Loss of incoming signals at the 64 kbit/s and 384 kbit/s tributary input ports.Note 1 - This detection is not mandatory when contradirectional interfaces are used.Note 2 - The detection
17、of this fault condition is not mandatory for channel time slot 16 when channel associatedsignalling is used and the signalling multiplex equipment is situated within a few metres of the PCM multiplexequipment.4.1.4 Loss of the incoming signal at 2048 kbit/s.Note 1 - The detection of this fault condi
18、tion 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, the loss of either or bothshould constitute loss of the incoming signal.4.1.5 Loss of frame alignment.4.1.6 Excessive bit e
19、rror ratio detected by monitoring the frame alignment signal.4.1.6.1 With a random bit error ratio of 10-4, the probability of activating the indication of fault conditionwithin a few seconds should be less than 10-6.With a random bit error ratio of 10-3, the probability of activating the indication
20、 of fault conditionwithin a few seconds should be higher than 0.95.4.1.6.2 With a random bit error ratio of 10-3, the probability of deactivating the indication of fault condition within afew seconds should be almost 0.With a random bit error ratio of 10-4, the probability of deactivating the indica
21、tion of fault condition within afew seconds should be higher than 0.95.Note - The activating and the deactivating period specified as “a few seconds“ is intended to be in the order of4 to 5 seconds.4.1.7 Alarm indication received from the remote PCM multiplex equipment (see 4.2.3).4.2 Consequent act
22、ionsFurther to the detection of a fault condition, appropriate actions should be taken as specified in Table 2/G.735.The consequent actions are as follows:4.2.1 Service alarm indication generated to signify that the service provided by the PCM multiplex is no longeravailable. This indication should
23、be forwarded at least to the switching and/or signalling multiplex equipment dependingupon the arrangements provided. The indication should be given as soon as possible and not later than 2 ms afterdetection of the relevant fault conditionThis specification, taking into account the specification giv
24、en in 4.2.5, is equivalent to recommending that theaverage time to detect a loss of frame alignment or a loss of the incoming 2048 kbit/s signal and to give the relevantindication should not be greater than 3 ms.When using common channel signalling the indication should be forwarded to the switching
25、 equipment bymeans of a separate interface on the PCM multiplex equipment.4.2.2 Prompt maintenance alarm indication generated to signify that performance is below acceptable standards andmaintenance attention is required locally. When the AIS (see General Notes below to 4.2) at 2048 kbit/s input isd
26、etected, the prompt maintenance alarm indication associated with loss of frame alignment (see 4.1.5) and excessiveerror ratio (see 4.1.6) should be inhibited, while the rest of the consequent actions are in accordance with thoseassociated in Table 2/G.735 with the two fault conditions.Note - The loc
27、ation and provision of any visual and/or audible alarm activated by the alarm indications given in 4.2.1 and 4.2.2, is left to the discretion of each Administration.4.2.3 Alarm indication to the remote end transmitted by changing bit 3 of channel time slot 0 from the state 0 to thestate 1 in those f
28、rames not containing the frame alignment signal. This should be effected as soon as possible.Fascicle III.4 - Rec. G.735 54.2.4 Transmission suppressed at the analogue voice-frequency outputs.4.2.5 AIS applied to all 64 kbit/s and 384 kbit/s outputs (see General Notes below 4.2). For 64 kbit/s outpu
29、ts, thisaction should be taken as soon as possible and not later than two ms after the detection of the fault condition.4.2.6 AIS applied to relevant time slots in the composite 2048 kbit/s output signal (if suspension of incoming 64kbit/s and/or 384 kbit/s signals is provided).General Notes to 4.2N
30、ote 1 - The equivalent binary content of the alarm indication signal (AIS) is a continuous stream of binary 1s.The strategy for detecting the presence of the AIS should be such that with a high probability the AIS is detectable evenin the presence of random errors having a mean error rate of 1 in 10
31、3. Nevertheless, a signal in which all the binaryelements, with the exception of the frame alignment signal, are in the state 1, should not be taken as an AIS.Note 2 - All timing requirements quoted apply equally to restoration, subsequent to the fault condition clearing.5 SignallingText as in Recom
32、mendation G.732.6 Interfaces6.1 Audio frequency interfaceThe analogue audio frequency interfaces should be in accordance with Recommendations G.712, G.713, G.714and G.715.6.2 Digital interfacesThe digital interfaces at 2048 kbit/s should be in accordance with Recommendation G.703.The digital interfa
33、ces at 64 kbit/s should be of either the codirectional or the contradirectional type specified inRecommendation G.703. The specifications for 64 kbit/s interfaces are not mandatory for channel associated signalling.The interface for external synchronization of the transmitting timing signal should b
34、e in accordance withRecommendation G.703.The need to define a digital interface operating at 384 kbit/s is under study.Note 1 - It should be noted that, according to the principle of minimizing the number of different types ofinterfaces, the information rate of 384 kbit/s will be offered to customer
35、s at the user/network interface level using the2048 kbit/s interface as defined in Recommendations I.431 and G.703.Note 2 - In the case of the 64 kbit/s codirectional interface, the design of the input ports should take account ofthe need to provide octet alignment, to allow controlled slips when th
36、e tributary timing and that of the multiplexer timingsource are plesiochronous, and to absorb jitter and wander up to the limits given in Recommendation G.823.6 Fascicle III.4 - Rec. G.735TABLE 2/G.735Fault conditions and consequent actions for the PCM multiplex equipmentConsequent actions (see 4.2)
37、Equipment part Fault conditions(see 4.1)ServicealarmindicationgeneratedPromptmaintenancealarmindicationgeneratedAlarmindicationremote endtransmittedTransmissionsuppressed atthe analoguevoice-frequencyoutputsAIS appliedto all64 kbit/s and384 kbit/soutputsAIS to therelevant timeslot of the2048 kbit/sc
38、ompositesignalMultiplexerand demulti-Failure of powersupplyYes Yes Yes(if practicable)Yes(if practicable)Yes(if practicable)Yes(if practicable)plexerFailure of codec Yes Yes Yes YesMultiplexeronlyLoss of incomingsignal at64 kbit/s and/or384 kbit/s inputs(see Note under 4.1.3)Yes YesLoss of incomings
39、ignal at2048 kbit/sYes Yes Yes Yes YesDemultiplixerLoss of framealignment (seeNote 2 ofRec. G.706, 4.2)Yes Yes(see 4.2.2)Yes Yes YesonlyError ratio1 10-3on theframe alignmentsignalYes Yes(see 4.2.2)Yes Yes YesAlarm indicationreceived from theremote endYesNote - A Yes in the table signifies that an a
40、ction should be taken as a consequence of the relevant fault condition. An open space in thetable signifies that the relevant action should not be taken as a consequence of the relevant fault condition, if this condition is the onlyone present. If more than one fault condition is simultaneously pres
41、ent, the relevant action should be taken if, for at least one of theconditions, a Yes is defined in relation to this action.7 JitterFascicle III.4 - Rec. G.735 77.1 Jitter at 2048 kbit/s output7.1.1 In the case where the transmitting timing signal is derived from an internal oscillator, the peak-to-
42、peak jitter atthe 2048 kbit/s output should not exceed 0.05 UI when it is measured within the frequency range from fl= 20 Hz tof4 = 100 kHz. See Figure 2/G.823.7.1.2 In the case where the transmitting timing signal is derived from an external source having no jitter, the peak-to-peak jitter at the 2
43、048 kbit/s output should not exceed 0.05 UI when it is measured within the frequency range fromfl = 20 Hz to f4= 100 kHz.7.1.3 In the case where the transmitting timing signal is derived from the incoming 2048 kbit/s signal having nojitter, the peak-to-peak jitter at the 2048 kbit/s output should no
44、t exceed 0.10 UI when it is measured within thefrequency range from fl= 20 Hz to f4= 100 kHz. The equivalent binary content of the test signal applied at the2048 kbit/s input shall be a pseudo-random bit sequence of length 215- 1 as specified in Recommendation O.151.Note - It may be necessary to inc
45、lude a frame alignment signal in the test signal to enable the measurement to becarried out.7.2 Jitter at tributary outputs7.2.1 Jitter at 64 kbit/s outputIn the case where the incoming 2048 kbit/s signal has no jitter, the peak-to-peak jitter at the 64 kbit/s outputshould not exceed 0.025 UI when i
46、t is measured within the frequency range from fl= 20 Hz to f4= 10 kHz. Theequivalent binary content of the test signal applied to the 2048 kbit/s input shall be a pseudo-random bit sequence oflength 215- 1 as specified in Recommendation O.151Note - In order to carry out this measurement without invo
47、king AIS at the 64 kbit/s output, it will normally benecessary to include a frame alignment signal in the test signal.7.2.2 Jitter at 384 kbit/s outputSince the physical and electrical characteristics of a 384 kbit/s interface are identical to those of the 2048 kbit/sinterface, the specification of
48、this parameter is the same as that given in 7.1.3 above.7.3 Jitter transfer functions7.3.1 The jitter transfer function between the 2048 kHz external synchronisation signal and the 2048 kbit/s outputsignal should not exceed the gain/frequency limits given in Figure 2/G.735. The 2048 kHz signal shall
49、 be modulatedwith sinusoidal jitter.Some Administrations require that equipment be fitted with jitter reducers. In this case, the jitter transferjunction should not exceed the gain/frequency limits given in Figure 3/G.735.7.3.2 In the case where the transmitting timing is derived from the incoming signal, the jitter transfer junctionbetween the 2048 kbit/s input and 2048 kbit/s output shall be as specified in 7.3.1.Note 1 - The 2048 kbit/s test signal shall be modulated by sinusoidal jitter. The equivalent binary content of thetest
