1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G0!30%#43G0G0/ amended at Melbourne, 1988)1 GeneralThe 60-channel transcoder implements a conversion between two 30-channel 2048 kbit/s PCM streams and one60-channel 2048 kbit/s ADPCM stream. In the 30-
2、channel 2048 kbit/s streams, the telephone signals are coded using 64 kbit/sA-law PCM as specified in Recommendation G.711. In the 60-channel 2048 kbit/s stream, the telephone signals are codedusing 32 kbit/s ADPCM as specified in Recommendation G.721. Figure 1/G.761 indicates the nomenclature used
3、for the threedifferent signal ports A, B and C.Note 1 - Administrations should take into account the guidance given in Recommendation G.721 concerning the useand transmission performance of 32 kbit/s ADPCM .Note 2 - It should be noted that the transcoder equipment described in this Recommendation ha
4、s a limited capabilityof transparently transmitting 64 kbit/s data channels and this should be taken into account in the planning of networks whichare likely to evolve into an ISDN (see 3.8).This Recommendation is divided into two parts:- 2 contains the interface requirements associated with the por
5、t C. These requirements are not only applicable tothe 60-channel transcoder equipment, but could be applied, in the future, to other equipment such as a 60-channel multiplex terminal, a 60-channel terminating unit at a TDM switch, or a TDMA terminal. In these lattercases, the A and B interfaces woul
6、d be virtual. As well as point-to-point operation, account has been taken ofmulti-destination operation in TDMA applications.- 3 contains the requirements which are specific to a 60-channel transcoder equipment realization.FIGURE 1/G.76160-channel transcoder ports2 Characteristics of a 2048 kbit/s s
7、ignal organized in 64 kbit/s and/or 32 kbit/s time slots (port C)2.1 Interface CThe electrical characteristics of the 2048 kbit/s interface are in accordance with Recommendation G.703, 6.2.2 Frame structureThe frame structure is in accordance with Recommendation G.704, 2.3, with bit 1 of time slot 0
8、 used for the cyclic2 Fascicle III.4 - Rec. G.761redundancy check (CRC) procedure.Time slots 1 to 15 and 17 to 31 each corresponds to:- either two 4-bit samples of telephone signals coded using 32 kbit/s ADPCM originating from the same incomingPCM stream (A or B); the bit ordering of the 32 kbit/s s
9、ignals is such that the 4-bit words are transmitted in bitorder starting with bit 1 (see 4.2.2 and 4.2.3 of Recommendation G.721). Bits 1 to 4 correspond to the first32 kbit/s signal and bits 5 to 8 correspond to the second 32 kbit/s signal;- or a digital signal at 64 kbit/s.Where stream C is transm
10、itting 60 telephone signals, the numbering of the channels and the correspondence betweenthe 64 kbit/s PCM channels in streams A and B and the 32 kbit/s ADPCM channels in stream C are given in Table 1/G.761.TABLE 1/G.761Organization of 2048 kbit/s frame for 60-channels at 32 kbit/s (stream C)Note 1
11、- The organization of the frame has been chosen to facilitate the direct time slot transfer described in 3.8.Note 2 - TS16 is foreseen for signalling but speech channels can be transmitted when necessary. These are numbered 31A and 31Brespectively for Bits 1-4 and Bits 5-8 (see 2.5).Note 3 - The num
12、bering of the channels from 1 to 30 and the correspondence with the time slots in PCM stream A (respectively B)are given in Recommendation G.735, 2.2.3 Allocation of bits in time slot 0The allocation of the bits in time slot 0 is given in Recommendation G.704, 2.3, with bit 1 of time slot 0 used for
13、 thecyclic redundancy check procedure.Bits 3 to 8 in TS0 of those frames not containing the frame alignment signal are used to transmit:- alarm indications associated with PCM streams A or B (see 2.6.2 and 2.6.3 dealing with alarm indications);- spare bits associated with PCM streams A or B (see 3.3
14、).8-bit time slotnumber0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Bits 1-4 ofchannel- 1A 1B 3A 3B 5A 5B 7A 7B 9A 9B 11A 11B 13A 13B 15ABits 5-8 ofchannel- 2A 2B 4A 4B 6A 6B 8A 8B 10A 10B 12A 12B 14A 14B 16A8-bits time slotnumber16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31Bits 1-4 ofchannelNote 2 15B 17
15、A 17B 19A 19B 21A 21B 23A 23B 25A 25B 27A 27B 29A 29BBits 5-8 ofchannelNote 2 16B 18A 18B 20A 20B 22A 22B 24A 24B 26A 26B 28A 28B 30A 30BFascicle III.4 - Rec. G.761 32.4 Frame alignment and CRC proceduresThe strategy for loss and recovery of frame alignment and CRC multiframe alignment is given inRe
16、commendation G.706, 4.2.5 Allocation of bits in TS16TS16 can be used:- either for signalling purposes; namely channel associated signalling (see 2.5.1) and common channelsignalling (see 2.5.2);- or, as envisaged in 5.1 of Recommendation G.704, for the transmission of telephone signals; in this case
17、twosamples of telephone signals each coded with 4 bits. Used in this way, stream C can transmit up to 62 telephonesignals. Bits 1 to 4 and 5 to 8 of stream C will correspond to 64 kbit/s PCM signals transmitted in TS16 of PCMstreams A and B respectively.2.5.1 Channel associated signallingThe allocat
18、ion of bits in TS16 will depend on the number of signalling bits per channel.2.5.1.1 Two or less signalling bits per channelThis applies to the digital version of Signalling System R2 (see Recommendation Q.421) specified for internationalapplications. This also applies to a number of national signal
19、ling systems.TS16 is organized in multiframes. Each multiframe contains 16 consecutive frames, numbered from 0 to 15. Themultiframe repetition frequency is 500 Hz.2.5.1.1.1 Allocation of bits in TS16 frame 0Table 2/G.761 indicates the content of TS16 frame 0.Bits 1 to 4 are fixed at 0 and constitute
20、 the multiframe alignment signal.Bits 5 and 8 are used to indicate “AIS in TS16“ of PCM streams A and B (see 2.6.5).Bits 6 and 7 are used to transmit the remote alarm indications associated with the multiframe of PCM streams A andB (see 2.6.6).TABLE 2/G.761Content of TS16 frame 02.5.1.1.2 Allocation
21、 of bits in TS16 frames 1 to 15Table 3/G.761 indicates the content of TS16 in frames 1 to 15.Bit number123456780000X5X6X7X84 Fascicle III.4 - Rec. G.761This allocation of bits provides each 32 kbit/s channel with two signalling channels at 500 bit/s nominated “a“ and “b“as defined in Recommendation
22、G.704, 5.1.3.2.2.To minimize the risks of simulation of the multiframe alignment signal, special processing of certain signalling bits iscarried out as described in 2.5.1.1.3.In the case of direct transfer of some 64 kbit/s time slots of PCM streams A or B, the four bits of TS16 associatedwith the t
23、ransferred time slots will be transparently transmitted and allocated in accordance with Table 7/G.704. They will notbe subject to the special processing described in 2.5.1.1.3. The four bits of time slot 16 associated with each of the time slotsnot used in PCM streams A and B because of the direct
24、transfer will be restituted by the transcoder with the following values:a = 0; b = 1; and d = 1, in conformity with Table 9/G.704.The signalling distortion of any signalling channel will not be greater than Error! Bookmark not defined. 2 ms.TABLE 3/G.761Content of TS16 frames 1 to 15Note - The organ
25、ization of the multiframe ensures consistency with the frame and multiframeorganization of Recommendation G.704, 5.1.3 and allows the possibility of the mixed use instream C of 32 kbit/s and 64 kbit/s channels with their associated signalling.Time slot 16 bit number 1 2 3 4 5 6 7 8Signalling Channel
26、a bChannela bChannela bChannela bFrame 1 1A 2A 15B 16BFrame 2 1B 2B 17A 18AFrame 3 3A 4A 17B 18BFrame 4 3B 4B 19A 20AFrame 5 5A 6A 19B 20BFrame 6 5B 6B 21A 22AFrame 7 7A 8A 21B 22BFrame 8 7B 8B 23A 24AFrame 9 9A 10A 23B 24BFrame 10 9B 10B 25A 26AFrame 11 11A 12A 25B 26BFrame 12 11B 12B 27A 28AFrame
27、13 13A 14A 27B 28BFrame 14 13B 14B 29A 30AFrame 15 15A 16A 29B 30BFascicle III.4 - Rec. G.761 52.5.1.1.3 Special processing of signalling bitsThe signalling bits to be transmitted in bit 2 (respectively 4, 6 and 8) of TS16 (frames 1 to 15) are calculated fromBn-1, Bn-2, Bn-3, bn-1and bnin accordance
28、 with Table 4/G.761 where:i) bnis the signalling bit before processing;ii) Bnis the signalling bit after processing, andiii) the subscripts n - 3, n - 2, and n - 1 relate to previous signalling bits pertaining to the same telephone channel;more specifically, if bnis a bit with a given number (2, 4,
29、6 or 8) in any time slot 16 of frames 1 to 15, then bn-1is the bit with the same number, one multiframe earlier.Note 1 - It follows from the above that there are 60 individual and independent processing operations at the sametime.The reverse processing (after transmission) is in accordance with Tabl
30、e 5/G.761. The reverse processed value bn, isdeduced from the successive received bits Bn-3, Bn-2, Bn-1, Bnand from the previous value bn-1. In the absence of transmissionerrors in stream C, bn= bnand there is no increase in signalling distortion. When this is not so, the error multiplication factor
31、lies between 2 and 4.When, in the case of fault conditions on multiframe A or B (see Table 9/G.761), the signalling bits need to be forcedto state 1, this should be implemented on the unprocessed signalling channels (i.e. before the special processing at the send endor after the reverse processing a
32、t the receive end). This does not apply to the cases of “partial AIS stream A (B)“ considered in 2.6.2 where the AIS is all 1s, unprocessed.2.5.1.1.4 Loss and recovery of multiframe alignmentThe multiframe alignment should be assumed to have been lost when two consecutive multiframe alignment signal
33、shave been received in error.The multiframe alignment should be assumed to have been recovered following detection of an all-zero 4-bit wordformed by the first four bits of a time slot 16 and an all-zero 4-bit word one multiframe period later.2.5.1.2 More than two signalling bits per channelSee 3.8.
34、2.5.2 Common channel signallingTS16 of stream C can be used for common channel signalling. In this case, its content corresponds, without anymodification, to that of TS16 of either PCM stream A or PCM stream B. It should be noted that the simultaneous transfer ofTS16 from both streams A and B is not
35、 envisaged in this case.Time slot 16 of PCM stream B (or A) not used because of the direct transfer of time slot 16 of PCM stream A (or B)in time slot 16 of the PCM stream C will be restituted by the transcoder in the form of an all-0s signal or an all-1s signal.2.6 Alarm indicationsThe following al
36、arm indications can be transmitted in stream C.2.6.1 AIS stream CThis means that a fault, common to the 60 channel has been detected in the send side “AIS stream C“ is transmitted asan all ones configuration in stream C.2.6.2 AIS stream A (respectively B)This means that a fault, common to the 30 cha
37、nnels of stream A (respectively B), has been detected in the send side.6 Fascicle III.4 - Rec. G.761For the send side, the following applies:When “AIS stream A“ and “AIS stream B“ are present simultaneously, then “AIS stream C“ should be transmitted.When “AIS stream A“ (respectively B) is present, b
38、ut not “AIS stream B“ (respectively A), then the information bitsand the signalling bits associated with stream B (respectively A) should be transmitted normally and an all ones configurationshould be transmitted in the time slots associated with stream A (respectively B) in stream C and the corresp
39、onding bits inTS16. In addition, bit 7 (respectively 8) of TS0 not containing the frame alignment signal in stream C should be set to 1 toindicate the “AIS stream A“ (respectively B) (see Table 6/G.761). This configuration in stream C is nominated “partial AISstream A“ (respectively “partial AIS str
40、eam B“).For the receive side, the following applies:Partial AIS stream A (respectively B) will be considered as being present if bit 7 (respectively bit 8) is detected atstate 1 on three consecutive occasions.Partial AIS stream A (respectively B) will be considered as having ceased if bit 7 (respect
41、ively 8) is detected at state 0on three consecutive occasions.TABLE 4/G.761Processing of signalling bits (send side)Note - Other states may be possible, just after powering-on, which may be avoided by careful design.Input State Outputbnbn-1Bn-3Bn-2Bn-1Bn000000000111111111010 or 10 or 10 or 11001100
42、or 10 or 10 or 11001000001111000001111000110011000110011001010101001010101100100110101011001Fascicle III.4 - Rec. G.761 7TABLE 5/G.761Processing of signalling bits (receive side)Note - Other states may be possible, just after powering-on, which may be avoided by careful design.TABLE 6/G.761Use of bi
43、ts 7 and 8 of TS0 not containing the frame alignment signal in stream C2.6.3 Alarm indication to the remote end for stream A (respectively B)In the send side, bit 3 of TS0, not containing the frame alignment signal of stream A (respectively B) should betransferred to bit 3 (respectively 4) of the co
44、rresponding TS0 of stream C.In the receive side, bit 3 (respectively 4) of TS0 not containing the frame alignment signal of stream C should betransferred to bit 3 of the corresponding TS0 of stream A (respectively B).2.6.4 AIS in TS16 of stream CFor channel associated signalling, this means that a f
45、ault condition, common to the signalling information associatedInput State OutputBnBn-3Bn-2Bn-1bn-1bn00110101010101010100000000001111111100000011110000111100001100110011001101010 or 10 or 10 or 10 or 10 or 10 or 111000011100101100101101001Bit number 7 8 Meaning1 0 AIS Stream A0 1 AIS Stream BStates
46、0 0 Normal1 1 Indicates that the safeguarding option(see 3.10) is being used8 Fascicle III.4 - Rec. G.761with all 60 channels of stream C has been detected in the send side. “AIS in TS16 of stream C“ is transmitted as an all onesconfiguration in TS16.2.6.5 AIS in TS16 stream A (respectively B)For ch
47、annel associated signalling, this means that a fault, common to the 30 channels of stream A (respectively B),has been detected in the send side.For the send side, the following applies:When “AIS in TS16 stream A“ and “AIS in TS16 in stream B“ are present simultaneously, then “AIS in TS16 ofstream C“
48、 should be transmitted. When “AIS in TS16 stream A“ (respectively B) is present, but not “AIS in TS16 stream B“(respectively A), then the signalling information of stream B (respectively A) should be transmitted normally, and thesignalling bits of TS16 of stream C associated with stream A (respectiv
49、ely B) should be transmitted as an all onesconfiguration. In addition, bit 5 (respectively 8) of TS16 frame 0 should be set to 1 to indicate “AIS in TS16 stream A“(respectively B).For the receive side, the following applies:AIS in TS16 stream A (respectively B) will be considered as being present if bit 5 (respectively 8) of TS16 frame 0 isdetected at state 1 on two consecutive occasions.AIS in TS16 stream A (respectively B) will be considered as having ceased if bit 5 (respectively 8) is detected atst
