1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G0!30%#43G0G0/- unidirectional synchronous 320 kbit/s channels (see Figure 1b/G.738).The 320 kbit/s channel is based on the allocation of 5 x 64 kbit/s time slots, e.g. for setting up sound-programme ci
2、rcuits according to Recommendations J.43 and J.44. Because these circuits are specified as unidirectional,the equipment for insertion/extraction has to be separated as shown in Figure 1b/G.738.1 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 voice-channeltime slots.1.
4、2 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 accessa) access for bidirectional synchronous 64 kbit/s channels (
6、see Figure l a/G.738);b) access for unidirectional synchronous 320 kbit/s channels (see Figure 1b/G.738).Note - The synchronous insertion of a digital sound-programme signal into a 320 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 forsynchronizing the sampling frequency of the analogue/digital converters producing the digital sound programme signal.2 Fascicle III.4 - Rec. G.7382 Frame structure and use of derived channel time slots2.1 Frame structure of 2048 kbit/s signalRefer to 2.3 of Recommendation
8、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.FIGURE 1/G.738PCM multiplex equipment operating at 2048 kbit/s and offering access to digitalsound-programme signals and/or to synchronous 64 kbit/s data channels2.2 Use of de
9、rived channel time slots2.2.1 Telephone channelsIt should be possible to assign channel time slots 1 to 15 and 17 to 31, to 30 telephone channels numbered from1 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 any
10、of channel time slots 1 to 15 and 17 to 31.Fascicle III.4 - Rec. G.738 3Note - 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 - 31.2.2.3 320 kbit/s accessThe time slot allocation for digital channels with bit rate at 320 kbit/s is given in Table 1/G.738.TABLE 1/G.738320 kbit/s channels (Note 1) Digitalsound-programmeABCDE Faccess points1 - 2 -3 - 4 -56 - 7 -8 - 9 -1011 - 12- 13 -14 - 1517 - 18- 19 -20
12、 - 2122 - 23- 24 -25 - 2627 - 28- 29 -30 - 31I3, T, E3Figure 1b/G.738Note 1 - The six possible 320 kbit/s channels in a 2048 kbit/s stream are numbered A to F. Preferably the channel pairs A-B, C-Dand E-F should be used for stereophonic transmission.Note 2 - If the channel time slot 16 which is assi
13、gned to signalling as covered in 5 is not needed for signalling, it may be usedfor purposes 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
14、of Recommendation G.706.3.2 Recovery of frame 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 multip
15、lex equipment should detect the following conditions:4.1.1 Failure of power supply.4.1.2 Failure of codec (except when using single channel codecs).4 Fascicle III.4 - Rec. G.738As a minimum requirement, this fault condition should be recognized when for at least one signal level in therange -21 to -
16、6 dBm0, the signal-to-quantizing noise ratio performance of the local codec is 18 dB or more below thelevel recommended in Recommendation G.712.4.1.3 Loss of incoming signals at the 64 kbit/s and 320 kbit/s tributary input ports.Note 1 - This detection is not mandatory when contradirectional interfa
17、ces are used.Note 2 - The detection 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
18、 - 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, the loss of either or bothshould constitute loss of the incoming signal.4.1.5 Loss of f
19、rame alignment.4.1.6 Excessive bit error 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 condition within afew seconds should be less than 10-6.With a random bit error ratio of 10-3, the prob
20、ability of activating the indication of fault condition within afew 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 pr
21、obability of deactivating the indication of fault condition within afew seconds should be higher than 0.95.Note - The activating and 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 equipmen
22、t (see 4.2.3).4.2 Consequent actionsFurther to the detection of a fault condition, appropriate actions should be taken as specified in Table 2/G.738.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 longerav
23、ailable. This indication should 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 condition.This specification, taking i
24、nto account the specification given 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 sho
25、uld be forwarded to the switching equipment bymeans of 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 at 2048 kbit/s input is de
26、tected (see General Notes below to 4.2), the prompt maintenance alarm indication associated with loss of frame alignment (see 4.1.5) and excessiveFascicle III.4 - Rec. G.738 5error ratio (see 4.1.6) should be inhibited, while the rest of the consequent actions are in accordance with thoseassociated
27、in Table 2/G.738 with the two fault conditions.Note - The location 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
28、 channel time slot 0 from the state 0 to thestate 1 in those frames not containing the frame alignment signal. This should be effected as soon as possible.TABLE 2/G.738Fault conditions and consequent actions for the PCM multiplex equipmentConsequent actions (see 4.2)EquipmentpartFault conditions(see
29、 4.1)Service alarmindicationgeneratedPromptmaintenancealarmindicationAlarmindicationremote endtransmittedTransmissionsuppressed atthe analoguevoice-frequencyoutputsAIS appliedto 64 kbit/sand320 kbit/soutputsAIS appliedto therelevant timeslot of the2048 kbit/scompositesignalMultiplexerandFailure of p
30、owersupplyYes Yes Yes(if practicable)Yes(if practicable)Yes(if practicable)Yes(if practicable)demulti-plexerFailure of codec Yes Yes Yes YesMultiplexeronlyLoss of incomingsignal at 64 kbit/sand/or 320 kbit/sinputs (see Notesunder 4.1.3)Yes YesLoss of incomingsignal at2048 kbit/sYes Yes Yes Yes YesDe
31、multi-Loss of framealignment (seeNote 2 Rec. G.706, 4.2)Yes Yes(see 4.2.2)Yes Yes Yesplexer onlyError ratio 1 10-3on the framealignment signalYes Yes(see 4.2.2)Yes Yes YesAlarm indicationreceived from theremote endYesNote - A Yes in the table signifies that an action should be taken as a consequence
32、 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 present, the relevant action should be tak
33、en if, for at least one of theconditions, a Yes is defined in relation to this action.4.2.4 Transmission suppressed at the analogue voice-frequency outputs.6 Fascicle III.4 - Rec. G.7384.2.5 AIS applied to all 64 kbit/s and 320 kbit/s outputs (see General Notes below to 4.2). For 64 kbit/s outputsth
34、is actions should be taken as soon as possible and not later than 2 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 incoming64 kbit/s and 320 kbit/s signals is provided).General Notes to 4.2Note 1 - T
35、he equivalent binary content of the alarm indication signal (AIS) is a continuous stream of binary 1s.The strategy for detecting the presence of AIS should be such that with a high probability the AIS is detectable even inthe presence of random errors having a mean error rate of 1 in 103. Neverthele
36、ss, 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 Recommendation G.7
37、32.6 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 interfaces at 64 kbi
38、t/s should be of either the codirectional or the contradirectional type specified inRecommendation G.703. The specification for 64 kbit/s interfaces are not mandatory for channel-associated signalling.The interface for external synchronization of the transmitting timing signal should be in accordanc
39、e withRecommendation G.703.The need to define a digital interface operating at 320 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 320 kbit/s will be offered to customers at the user/
40、network interface level using the2048 kbit/s interface as defined in Recommendation I.431 and Recommendation G.703.Note 2 - In the case of the 64 kbit/s codirectional interface, the design of the input ports should take into accountthe need to provide octet alignment, to allow controlled slips when
41、the tributary timing and that of the multiplexer timingsource are plesiochronous, and to absorb jitter and wander up to the limits given in Recommendation G.823.7 Jitter7.1 Jitter at 2048 kbit/s output7.1.1 In the case where the transmitting timing signal is derived from an internal oscillator, the
42、peak-to-peak jitter atthe 2048 kbit/s output should not exceed 0.05 UI when it is measured within the frequency range from f1= 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 a
43、t the 2048 kbit/s output should not exceed 0.05 UI when it is measured within the frequency range from f1 = 20 Hz to f4= 100 kHz.Fascicle III.4 - Rec. G.738 77.1.3 In the case where the transmitting timing signal is derived from the incoming 2048 kbit/s signal having nojitter, the peak-to-peak jitte
44、r at the 2048 kbit/s output should not exceed 0.10 UI when it is measured within thefrequency range from f1= 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.
45、151.Note - It may be necessary to include 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 ou
46、tputshould not exceed 0.025 UI when it is measured within the frequency range from f1= 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.151.Note - In order to c
47、arry out this measurement without invoking 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 320 kbit/s outputSince the physical and electrical characteristics of a 320 kbit/s interface are identical to those of the 2048 k
48、bit/sinterface, the specification of 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 kbit/s external synchronization signal and the 2048 kbit/s outputsignal should not exceed the gain/frequency limits given in F
49、igure 2/G.738. The 2048 kHz signal shall be modulatedwith sinusoidal jitter.Some Administrations require that equipment be fitted with jitter reducers. In this case, the jitter transferfunctions should not exceed the gain/frequency limits given in Figure 3/G.738.7.3.2 In the case where the transmitting timing is derived from the incoming signal, the jitter transfer functionbetween the 2048 kbit/s input and the 2048 kbit/s output shall be as specified in 7.3.1.Note 1 - The 2048 kbit/s test signal shall he modulated by sinusoidal jit
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