1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 8 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU05“,)#G0G0$!4!G0G0.%47/2+342!.3-)33)/. G0G03).!,).G0G0!.$G0G037)4#().).4%2(b) that 64 kbit/s satellite channels on TDMA systems are not yet operational;(c) that 64 kbit/s channels on SCPC systems are under in
2、vestigation;(d) that for an interim period, only 48, 50 or 56 kbit/s channels via a satellite will be available in manycases;(e) that there is a requirement of a multiplexing scheme for the interworking between two networkswhere both use 10-bit envelope structure but transmission is at a gross bit r
3、ate of 56 kbit/s, typically via an SCPCsatellite system with forward error correction.recommendsthat the fundamental parameters for a multiplexing scheme using a 10-bit envelope structure for transmissionvia a 56-kbit/s SCPC satellite channel should be as described in this Recommendation.1 Gross bit
4、 rateFor transmission on the international digital satellite link, the multiplexed bit stream shall have a gross bit rateof 56 kbit/s. The fundamental multiplex structure shall have a gross bit rate of 54 kbit/s and shall utilize paddingtechniques for transmission on the 56 kbit/s bearer channel. On
5、 the tributary channel interface, each transmitted andreceived tributary channel data stream has the 10-bit envelope structure as recommended in Recommendation X.51.The adaptation to the SCPC 56-kbit/s channel is achieved by suppressing the A bit of each envelope within themultiplex system.2 Fundame
6、ntal multiplexFor the fundamental multiplexing of information bearer channels, the following applies:2.1 The signal elements of each individual channel shall be assembled in 9-bit envelopes, in which bit 1 is astatus bit (S bit) (see Note), and bits 2-9 are information bits, as in Figure 1/X.56.2 Fa
7、scicle VIII.3 - Rec. X.56The addition of the status bit results in a 1212% increase in bit rate, so that the bearer channel rates are:10.8 kbit/s for the 9.6 kbit/s data signalling rate;5.4 kbit/s for the 4.8 kbit/s data signalling rate;2.7 kbit/s for the 2.4 kbit/s data signalling rate;675 bit/s fo
8、r the 600 bit/s data signalling rate.Note - A status bit (S bit) is associated with each envelope and in conjunction with the associated 8-bit databyte conveys call control information (cf. Recommendations X.21, X.21 bis, X.60, X.71 and X.50).2.2 A 9-bit envelope interleaved structure shall be used.
9、2.3 These interleaved envelopes will appear on the 54 kbit/s fundamental multiplex as follows:- 10.8 kbit/s channels will repeat every 5th envelope,- 5.4 kbit/s channels will repeat every 10th envelope;- 2.7 kbit/s channels will repeat every 20th envelope;- 675 bit/s channels will repeat every 80th
10、envelope.2.4 Both structures suitable for handling homogeneous (with respect to bearer rates) mixes of bearer channelsand structures suitable for handling heterogeneous mixes of bearer channels are required, with the constraint that thedivision of any 10.8 kbit/s bearer channel of the multiplex shal
11、l be homogeneous providing either two 5.4 kbit/s, four2.7 kbit/s or sixteen 675 bit/s bearer channels.3 Method of framing3.1 Overall structureThe residual 2 kbit/s capacity obtained by carrying the fundamental 54 kbit/s multiplex on the 56 kbit/s bearershall be distributed so that a padding bit is i
12、nserted after each group of 27 bits from the fundamental multiplex (see alsoFigure 2/X.56).Fascicle VIII.3 - Rec. X.56 3The frame length shall be 2240 bits in the case of a synchronized bearer, i.e. 2160 bits or 240 envelopes fromthe fundamental multiplex interleaved with 80 padding units.When justi
13、fication is used (for national purposes) in the case of a non-synchronized bearer, the last padding bitin the frame can be deleted or an extra padding bit added when needed, resulting in a variable frame length of2240 1 bit. (This can allow a maximum speed tolerance of approximately 4.5 parts in 104
14、.)The padding bits shall contain the framing pattern, justification service digits and housekeeping signalling(alarms, etc.).3.2 Framing3.2.1 Frame alignment patternsThe frame alignment method is based on the use of 4 equidistantly distributed frame alignment patternswritten into the padding bits, d
15、ividing the frame into 4 sub-frames. Each sub-frame alignment pattern starts with the 14-bit pattern:11111001101010followed by a 2-bit sub-frame identifier unique to the sub-frame, i.e.:SF1 = 00,SF2 = 01, SF3 = 10, SF4 = 11.3.2.2 Framing strategy3.2.2.1 Loss of frame alignmentThe criterion for loss
16、of frame alignment shall be three consecutive frame alignment patterns including thesub-frame identifier in error.The frame alignment shall also be considered lost if the first received frame alignment pattern including sub-frame identifier after reframing is in error.3.2.2.2 ReframingThe criterion
17、for reframing shall be the detection of one valid frame alignment pattern.3.2.2.3 Reframing procedureAfter loss of frame alignment:- the outgoing envelopes shall be set to all ones;- the state shall be signalled to the distant end; and- a parallel hunt for a valid frame alignment pattern shall be st
18、arted.After a valid frame alignment pattern is found:- the two following padding bits shall be accepted as sub-frame identifiers and be used to set the frame andsub-frame counter(s) as applicable;- the blocking of the outgoing data channels shall be removed; and- the signalling of out-of-frame alarm
19、 to the distant end shall be terminated.4 JustificationThe 56 kbit/s bearer carrying the 9-bit envelope multiplex normally shall be locked to the data stream, andtherefore justification on international links is not required. However, justification could be required for national4 Fascicle VIII.3 - R
20、ec. X.56purposes. To achieve this, plus/minus justification shall be used in which four repeated justification service signalsoccupy the 3 bits immediately following each sub-frame identifier. The last padding bit of the frame is used as ajustification digit.The repeated justification service signal
21、s are:010 no justification (i.e. one padding bit at end of frame);100 one justification bit has been added (i.e. two padding bits at end of frame);001 the justification bit has been deleted (i.e. no padding bit at end of frame).In evaluating the signals in one frame, a majority decision of the four
22、received signals is used. In case of nomajority, no justification shall be assumed.If framing is lost, no justification shall be assumed before reframing has occurred.5 Housekeeping signals and functionsThe padding bits not used for framing and justification shall be available for housekeeping infor
23、mationsignals, for both international and national use. The definition and allocation of the available housekeeping bits is leftfor further study.6 Allocation and use of padding bits (20 bits) in one sub-frame (560 bits) for framing, justification andhousekeepingThe allocation of padding bits in one
24、 sub-frame numbered P1 to P20 is described below and shown inFigure 3/X.56.P1-P14 Framing pattern: 14 bitsCode 11111001101010P15-P16 Sub-frame identifier: 2 bitsCode 00, 01, 10 or 11For P17-P20, two alternatives exist:a) Synchronous transmission bearerP17-P20 International housekeeping bits A, B, C
25、and D (cf. Recommendation X.50)b) Asynchronous transmission bearerP17-P19 Justification service signals: 3 bitsCode 001, 010, 100P20 In the first three sub-frames (SF1, SF2, SF3), may remain as housekeeping bits as above.Their use is for further study.P20(P21) In the last sub-frame (SF4), is used for justification:Justification bit(s) . 0, 1, 2 bit(s)Code , 0, 00Fascicle VIII.3 - Rec. X.56 5
