1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 8 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU05“,)#G0G0$!4!G0G0.%47/2+342!.3-)33)/. G0G03).!,).G0G0!.$G0G037)4#().amended at Malaga-Torremolinos, 1984)The CCITT,considering(a) that Recommendation X.50 sets out the fundamental parameters for a multiplexi
2、ng scheme for theinterworking of networks where at least one makes use of the 8-bit envelope structure or of the four 8-bit envelopesgrouping,(b) that there is a requirement for a multiplexing scheme for the interworking between two networkswhere both use 10-bit envelope structure,unanimously declar
3、es the viewthat the following fundamental parameters shall be used between networks using the 10-bit envelope structure.1 Gross bit rateFor transmission on the international link the multiplexed bit stream shall have a gross bit rate of 64 kbit/s.The fundamental multiplex structure shall have a gros
4、s bit rate of 60 kbit/s and shall utilize padding techniques fortransmission on the 64 kbit/s international bearer.2 Fundamental multiplexFor the fundamental multiplexing of information bearer channels, the following applies:2.1 The signal elements of each individual channel shall be assembled in 10
5、-bit envelopes, in which bit 1 is astatus bit (see Note), bit 2 is an envelope alignment bit, and bits 3-10 are information bits, as in Figure 1/X.51.The addition of the status and the envelope alignment bits results in a 25% increase in bit rate, so that thebearer channel rates are:12.0 kbit/s for
6、the 9.6-kbit/s data signalling rate;6.0 kbit/s for the 4.8-kbit/s data signalling rate;3.0 kbit/s for the 2.4-kbit/s data signalling rate;750 bit/s for 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 data byte
7、conveys call control information (cf. Recommendations X.21, X.21 bis, X.60, X.71 and X.50).2 Fascicle VIII.3 - Rec. X.512.2 A 10-bit envelope interleaved structure shall be used.2.3 These interleaved envelopes will appear on the 60 kbit/s fundamental multiplex as follows:- 12.0-kbit/s channels will
8、repeat every 5th envelope;- 6.0-kbit/s channels will repeat every 10th envelope;- 3.0-kbit/s channels will repeat every 20th envelope;- 750-bit/s channels will repeat every 80th envelope.2.4 Both structures suitable for handling homogeneous (with respect to bearer rates) mixes of bearer channelsand
9、structures suitable for handling heterogeneous mixes of bearer channels are required, with the constraint that thedivision of any 12-kbit/s bearer channels of the multiplex shall be homogenous providing either two 6-kbit/s, four3-kbit/s or sixteen 750-bit/s bearer channels.3 Method of framing3.1 Ove
10、rall structureThe residual 4-kbit/s capacity obtained by carrying the fundamental 60-kbit/s multiplex on the 64-kbit/s bearershall be distributed so that a padding bit is inserted after each group of 15 bits from the fundamental multiplex (see alsoFigure 2/X.51).The frame length shall be 2560 bits i
11、n the case of a synchronized bearer, i.e. 2400 bits or 240 envelopes fromthe fundamental multiplex interleaved with 160 padding bits.When justification 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 add
12、ed when needed, resulting in a variable frame length of2560 1 bit. (This can allow a maximum speed tolerance of approximately 4 parts in 104.)The padding bits shall contain the framing pattern, justification service digits and housekeeping signalling(alarms, etc.).3.2 Framing3.2.1 Frame alignment pa
13、tternsThe frame alignment method is based on the use of 4 equidistantly distributed frame alignment patternsFascicle VIII.3 - Rec. X.51 3written into the padding bits, dividing the frame into 4 subframes. Each subframe alignment pattern starts with the 14-bit pattern:11111001101010followed by a 2-bi
14、t subframe identifier unique to the subframe, 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 of frame alignment shall be three consecutive frame alignment patterns includingsubframe identifier in error.The frame alignment shall
15、also be considered lost if the first received frame alignment pattern includingsubframe identifier after reframing is in error.3.2.2.2 ReframingThe criterion for reframing shall be the detection of one valid frame alignment pattern.3.2.2.3 Reframing procedureAfter loss of frame alignment:- the outgo
16、ing 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 started.After a valid frame alignment pattern is found:- the two following padding bits shall be accepted as subframe identifiers and be used to
17、 set the frame andsubframe counter(s) as applicable,- the blocking of the outgoing data channels shall be removed, and- the signalling of out of frame alarm to the distant end shall be terminated.4 JustificationThe 64-kbit/s bearer carrying the 10-bit envelope multiplex normally shall be locked to t
18、he data stream andtherefore justification on international links is not required. However, justification could be required for nationalpurposes. To achieve this, plus minus justification shall be used in which four repeated justification service signalsoccupy the 3 bits immediately following each su
19、bframe identifier. The last padding bit of the frame is used as ajustification digit.The repeated justification service signals 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 h
20、as been deleted (i.e. no padding bit at end of frame).In evaluating the signals in one frame a majority decision of the four 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.4 Fascic
21、le VIII.3 - Rec. X.515 Housekeeping signals and functionsThe padding bits not used for framing and justification shall be available for housekeeping informationsignals, for both international and national use. The definition and allocation of some of the available housekeeping bitsis left for furthe
22、r study. The following allocation is recommended.5.1 International housekeeping bitsEight bits A, B, C, D, E, F, G, and H (cf. Recommendation X.50) are allocated for international housekeepingsignals.The bit A is used to convey to the distant end alarm indications detected at the local end correspon
23、ding to:- absence of incoming pulses,- loss of frame alignment,and the bit A shall be assigned such that:- A equals 1 means no alarm,- A equals 0 means alarm.The other bits B, C, D, E, F, G and H are reserved to convey further international housekeeping signals. Theexact use is under study. Pending
24、the result of the study these bits shall be set to binary 1.5.2 Cyclic error-controlA cyclic error-control (cf. Recommendation V.41) to be used end-to-end on the international 64-kbit/s link isrecommended but not mandatory. The multiplex frame (2560 bits) is divided modulo 2 by the polynomialx16 + x
25、12 + x5 + 1 and the resulting reminder (16 bits), the check bits, are sent in the next frame, 4 bits in eachsubframe. An error is detected at the receiving end by comparing the check bits generated locally, by dividing thereceived multiplex frame with the same polynomial, and the check bits received
26、 in the following frame. The errordetection shall be blocked in the out-of-frame state.5.3 National housekeeping signalsA total of 48 housekeeping bits, 12 in each subframe, remains for national housekeeping signals, of which thefollowing are foreseen:Network status 1-4 bitsMultiplex channel allocat
27、ion (depending on number of speed classes and coding) 5-10 bitsInternal and external alarms 1-4 bitsThese signals could possibly be extended for international use. Housekeeping bits not used in one networkshall be set to binary 1.6 Allocation and use of padding bits (40 bits) in one subframe (640 bi
28、ts) for framing, justification andhousekeepingThe allocation of padding bits in one subframe numbered P1 to P40 is described below and shown inFigure 3/X.51.P1-P4 International housekeeping bits A, B, C, and D(cf. Recommendation X.50)P5-P8 Error check bits 4 bitsP9-P20 National housekeeping bits 12
29、bitsP21-P34 Framing pattern. 14 bitsCode 11111001101010Fascicle VIII.3 - Rec. X.51 5P35-P36 Subframe identifier 2 bitsCode 00, 01, 10 or 11For P37-P40 two alternatives exist:I - Synchronous transmission bearerP37-P40 International housekeeping bits E, F, G and H(cf. Recommendation X.50)II - Asynchro
30、nous transmission bearerP37-P39 Justification service signals 3 bitsCode 001, 010, 100P40(P41) Justification bit(s) 0, 1, 2 bit(s)Code , 0, 00Only the justification bit(s) in the last subframe (SF4) is used for justification.7 Transmission of the 48 kbit/s user data signalling rateGenerally, Recomme
31、ndation X.51 bis applies.Optionally, on bilateral agreement, the scheme described in this Recommendation may be applied also totransmit the 48 kbit/s user data signalling rate. Using this option, the bearer channel rate of the fundamental multiplexdescribed in 2 becomes 60 kbit/s, permitting only one single channel to be conveyed.
