1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G0!30%#43G0G0/ further amended)1 General characteristics1.1 Fundamental characteristicsThe encoding law used is the -law as specified in Recommendation G.711. The sampling rate, load capacityand the cod
2、e are also specified in that Recommendation.The number of quantized values is 255. Two character signals are reserved for zero value (11111111 and01111111).In some networks the all-0 character signal (00000000) is eliminated to avoid loss of timing information to thedigital line, resulting in 254 qu
3、antized values.1.2 Bit rateThe nominal bit rate is 1544 kbit/s. The tolerance on this rate is 50 parts per million (ppm).1.3 Timing signalIt should be possible to derive the transmitting timing signal of a PCM multiplex equipment from an internalsource, from the incoming digital signal and also from
4、 an external source.2 Frame structureRefer to 3.1.1 and 3.1.2 of Recommendation G.704 for frame structure and use of derived channel time slots.3 Loss and recovery of frame alignmentThe strategy for the loss and recovery of frame alignment should be according to Recommendation G.706, 2.1.4 Fault con
5、ditions and consequent actions4.1 Fault conditionsThe PCM multiplex equipment should detect the following conditions:4.1.1 Failure of power supply.4.1.2 Loss of incoming signals at 1544 kbit/s.4.1.3 Loss of frame alignment.4.1.4 Alarm indication received from the remote PCM multiplex equipment.2 Fas
6、cicle III.4 - Rec. G.7334.2 Consequent actionsFurther to the detection of a fault condition, appropriate actions should be taken as specified in Table 1/G.733.The consequent actions are as follows:4.2.1 A service alarm indication should be generated to signify that the service provided by the PCM mu
7、ltiplex is nolonger available. This indication should be forwarded to the switching and/or signalling equipment depending upon thearrangement provided.4.2.2 The service alarm described in 4.2.1 above should be used to automatically remove the associated circuitsfrom service and to restore them to se
8、rvice when frame alignment has been recovered.Note - The removal of the associated circuits described in 4.2.2 above should be done in such a way that thecircuits are not needlessly removed in the case of a brief isolated loss of frame alignment but are removed in the case ofa permanent or intermitt
9、ent loss of frame alignment.It is also important to minimize the impact of signalling errors which may occur during periods of loss of framealignment. These functions should be provided in the PCM multiplex equipment or in the switching/signallingequipment.4.2.3 A prompt maintenance alarm indication
10、 should be generated to signify that performance is below acceptablestandards and maintenance attention is required locally.4.2.4 An alarm indication to the remote end should be generated by either forcing bit 2 in every channel time slot tothe value 0 or by modifying the S-bit as described in 3.1.3
11、.2.2 of Recommendation G.704 for the 12 frame multiframeor by sending a frame alignment alarm sequence (1111111100000000) as described in 3.1.1.3 (A)-(3) ofRecommendation G.704 for the 24-frame multiframe.4.2.5 Transmission should be suppressed at the analogue outputs.4.2.6 Rapid indication of loss
12、of frame alignmentAn indication should be given to the Signalling System No. 6 equipment (digital version) when the PCMmultiplex equipment (local end only) detects a loss of frame alignment. The average time to detect and give an indicationof random bits in the frame alignment signal bit positions s
13、hould not be greater than 3 ms. This indication will serve thesame function as that provided by the data carrier failure alarm in the analogue version (see Recommendation Q.2751).5 Signalling5.1 Signalling arrangementRefer to 3.1.3 of Recommendation G.704.5.2 Loss of multiframe alignment in case of
14、channel associated signalling on 12 frame multiframeLoss of multiframe alignment is assumed to have taken place when loss of frame alignment occurs.5.3 Minimization of quantizing distortion in case of channel associated signallingIn the signalling frame only seven bits are available for encoding of
15、voice frequencies. In order to minimize thequantizing distortion, the decoder output values are shifted slightly. All even numbered decoder output values yn, arechanged to be equal to the next higher decision value, i.e. xn+1,. All odd numbered decoder output values yn+1arechanged to be equal to the
16、 same numbered decision value, i.e. xn+1, as shown on Figure 1/G.733.When suppression of the all 0 character signal is required, the value of the seventh bit is forced to be 1 when allthe other bits of the character signal have the value 0.Fascicle III.4 - Rec. G.733 3TABLE 1/G.733Fault conditions a
17、nd consequent actions for the PCM multiplex equipmentConsequent actionsEquipment part Fault conditionsService alarmindicationgeneratedPromptmaintenancealarmindicationAlarmindication tothe remote endgeneratedTransmissionsuppressed atthe analogueoutputsMultiplexer anddemultiplexerFailure of powersuppl
18、yYes Yes Yes(if practicable)OptionalLoss of incomingsignals at 1544 kbit/sYes Yes Yes YesDemultiplexer only Loss of framealignmentYes Yes Yes YesAlarme indicationreceived from theremote endOptional Yes OptionalNote 1 - A Yes in the table signifies that an action should be taken as a consequence of t
19、he relevant fault condition. An openspace in the table signifies that the relevant action should not be taken as a consequence of the relevant fault condition, ifthis condition is the only one present. If more than one fault condition is simultaneously present the relevant action should betaken if,
20、for at least one of the conditions, a Yes is defined in relation to this action.Note 2 - Indications of additional fault conditions, such as codec failure and excessive bit errors, are left to the discretion ofindividual Administrations.FIGURE 1/G.733Seven-bit codec transfer characteristic4 Fascicle III.4 - Rec. G.7336 InterfacesAnalogue: Refer to Recommendations G.712, G.713 and G.714.Digital: Refer to Recommendation G.703.Reference1 CCITT Recommendation Data channel failure detection, Vol. VI, Rec. Q.275.
