1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 8 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU05“,)#G0G0$!4!G0G0.%47/2+342!.3-)33)/. G0G03).!,).G0G0!.$G0G037)4#().amended at Geneva, 1976 and 1980)The establishment in various countries of public synchronous data networks creates a need to standardize a
2、preferred multiplexing scheme to be used on international links between these countries.The CCITT,consideringthat the resolution of the fundamental parameters of a multiplexing scheme is urgently needed for theinterworking of data networks using different envelope structures;unanimously declares the
3、 following view:1 Division 11.1 This Recommendation sets out the fundamental parameters of a multiplexing scheme for interworking ofnetworks that make use of the following structures:a) 8-bit envelope (see Explanatory Note 1 below);b) four 8-bit envelopes grouping (see Explanatory Note 2 below);c) 1
4、0-bit envelope (see Explanatory Note 3 below), in the case where at least one of the networks isstructured according to a) or b).1.2 For interworking between two networks both of which utilize the 10-bit envelope structure as identified in 1.1 c) above, Recommendation X.51 will apply.1.3 Paragraph 2
5、 of this Recommendation deals with the basic multiplexing parameters which shall be used in anyapplication of this Recommendation.1.4 Paragraph 3 of this Recommendation, in addition to 2, applies to the interworking between two networksboth of which utilize the 8-bit envelope structure, as identifie
6、d in 1.1 a) above.1.5 Paragraph 4 of this Recommendation, in addition to 2, applies to the interworking of networks as identifiedin 1.1 above in cases other than those described in 1.2 and 1.4 above with due regard to the transit situations.1.6 The use of the status bit, besides that indicated in th
7、is Recommendation, should comply withRecommendations X.21 and X.21 bis, together with Recommendation X.71 for connections using decentralizedsignalling and with Recommendation X.60 for connections using common channel signalling.2 Division 22.1 The multiplex gross bit rate of 64 kbit/s should be sta
8、ndardized for international links and framinginformation for the channels should be contained within the 64 kbit/s capability.2.2 For the basic multiplexing of information bearer channels, the following applies:i) structures suitable both for handling homogeneous (with respect to bearer rates) mixes
9、 of bearer channelsand structures suitable for handling heterogeneous mixes of bearer channels are required;2 Fascicle VIII.3 - Rec. X.50ii) the signal elements of each individual channel should be assembled in 8-bit envelopes;iii) an 8-bit envelope interleaved structure should be used;iv) for the m
10、ultiplex signal framing a distributed framing pattern should be used, employing the framing bitsof consecutive 8-bit envelopes but taking into account the requirements for service digits (housekeepingdigits);v) these interleaved 8-bit envelopes will appear on the 64 kbit/s bearer as follows:- 12.8 k
11、bit/s channels will repeat every 5th 8-bit envelope;- 6.4 kbit/s channels will repeat every 10th 8-bit envelope;- 3.2 kbit/s channels will repeat every 20th 8-bit envelope;- 800 bit/s channels will repeat every 80th 8-bit envelope.2.3 The following multiplexing structure is recommended:i) the multip
12、lexing structure will comprise 80 8-bit envelopes;ii) this structure will allow the multiplexing of channels at the bearer rates indicated in 2.2 v) above;iii) within each 12.8 kbit/s channel, only a homogeneous mixture of subrate channels will be allowed;iv) a 72-bit long framing pattern is recomme
13、nded. This pattern is part of the 80-bit pattern which isgenerated according to the primitive polynomial:1 + x4+ x7of the 27Galois field with the forcing configuration1001101and which is reproduced in Table 1/X.50, showing 8 bits (“A“ to “H“) reserved for housekeeping;v) the first F bit, indicated a
14、s “A“ in Table 1/X.50 is used to convey to the distant end alarm indicationsdetected at the local end corresponding to:- absence of incoming pulses;- loss of frame alignment;vi) the “A“ bit shall be assigned such that:“A“ equals 1 means no alarm;“A“ equals 0 means alarm;vii) the other F bits indicat
15、ed as “B“, “C“, “D“, “E“, “F“, “G“, and “H“ in Table 1/X.50 are reserved to conveyfurther international housekeeping information. The exact use of the remaining housekeeping bits isunder study. Pending the resolution of the housekeeping requirements, these bits are provisionally fixedto:“B“ equals 1
16、, “C“ equals 1, “D“ equals 0,“E“ equals 0, “F“ equals 1, “G“ equals 1, “H“ equals 0.Fascicle VIII.3 - Rec. X.50 32.4 For frame synchronization, the following general requirements are recommended:i) the frame synchronization method should be insensitive as far as possible to bit errors, error bursts
17、andshort bursts of the alarm indication signal (AIS) generated by transmission equipment;ii) when a slip occurs in the transmission equipment, a fast frame alignment should be possible.2.5 In addition to 2.4 above, the frame synchronization method should offer the following performances:i) the frame
18、 alignment recovery time, after a slip and in absence of bit errors should be less than 120envelopes, with 95% probability;ii) the time from the start of a disturbance as defined in 2.4 i) to any action affecting the data channelsshall be including the transmission of alarm to the distant end define
19、d in 2.3 v) and vi) greater than x(x in the range 1-20 ms);iii) a random error rate of 1 in 104shall not cause any frame alignment recovery action.3 Division 33.1 For interworking between two networks, both of which utilize the 8-bit envelope structure, as identified in 1.1 a) above, each individual
20、 channel should be assembled into single 8-bit envelopes. As an alternative to themultiplexing structure recommended in 2.3 above, other structures may be used by bilateral agreement. One of thepreferred structures is described below:i) the multiplexing structure will comprise 20 8-bit envelopes;ii)
21、 the structure will allow the multiplexing of channels at the bearer rates 12.8 kbit/s, 6.4 kbit/s and3.2 kbit/s indicated in 2.2 v) above;iii) within each 12.8 kbit/s channel only a homogeneous mixture of subrate channels will be allowed;iv) a 19-bit long framing pattern is recommended. The pattern
22、 is part of the 20-bit pattern which is generatedto the primitive polynomial:1 + x2+ x5of the Galois 25field with the forcing configuration01110and is reproduced in Table 2/X.50;v) the first F bit indicated as “A“ in Table 2/X.50, is used as stated in 2.3 v) above,vi) the sense of “A“ will be in acc
23、ordance with 2.3 vi) above.3.2 For frame synchronization, the general requirements and the performances should be as recommended in 2.4 and 2.5 above.4 Division 4For the interworking of networks as identified in 1.1 above, in cases other than those described in l.2 and1.4, the following shall apply.
24、4 Fascicle VIII.3 - Rec. X.504.1 A network using the 10-bit envelope structure shall interwork with other networks, as identified in 1.1 a)and b) above, by offering the same characteristics as a network using the four 8-bit envelopes grouping. Therefore inthe following, the term “network providing t
25、he four 8-bit envelopes grouping“ will cover the case of a network usingeither four 8-bit envelopes grouping as identified in 1.1 b) or the 10-bit envelope structure, as identified in 1.1 c).4.2 When either end of an international connection, carrying point-to-point or switched service, terminates i
26、n anetwork providing four 8-bit envelopes grouping, the use of the four 8-bit envelopes grouping may be required on theinternational connection carrying point-to-point or switched services. This is subject to further study.4.3 The alignment of the four 8-bit envelopes grouping shall be subject to th
27、e following conditions:i) the method of alignment shall allow switched and non-switched point-to-point data circuits to beconveyed;ii) alignment shall be monitored and maintained at both ends of an international connection in the twonetworks providing the four 8-bit envelopes grouping by means of a
28、pattern(s) on bit SDof the four 8-bitenvelopes grouping throughout the data phase of the call;iii) transit exchanges shall not overwrite the SDbit once they have through connected;iv) alignment shall be established at both ends of an international connection in the two networks providingthe four 8-b
29、it envelopes grouping prior to the through connection of the originating exchange.Note 1 - The interexchange signalling procedures are expected to allow iv) above without unacceptable delayin call set-up.Note 2 - Problems caused by imitation of the SDpattern(s) should be studied further.4.4 The spec
30、ific strategy of the four S-bit envelopes grouping alignment would be the subject of further study.4.5 In the case where one of the networks is not providing the four 8-bit envelopes grouping, there is norelationship between any four 8-bit envelopes grouping and any character structure outgoing from
31、 that network.Moreover this network will not originate any alignment information for the outgoing four 8-bit envelopes grouping.4.6 When links containing a four 8-bit envelopes grouping are connected in tandem to provide a trunk, the four8-bit envelopes grouping alignment information shall be mainta
32、ined across the connection.4.7 In transit switching, the four 8-bit envelopes grouping alignment information shall be maintained once thetransit exchange has through connected.4.8 When links containing single 8-bit envelopes are connected in tandem to provide a trunk, the information andstatus bits
33、of the 8-bit envelopes shall be transparently conveyed across the connection.5 Division 5To regulate transmission of 64-kbit/s streams, the following will apply:i) In networks where the 64-kbit/s transmission paths have an octet structure throughout (i.e. 8-bit timeslots are generally available), th
34、e 8-bit envelopes of the data multiplex signal should be aligned with theoctet structure. This relationship will be established across 64-kbit/s interfaces by means of the 8-kHztiming signal.ii) In networks where an octet structure is not utilized throughout (i.e. 8-bit time slots are not generallya
35、vailable on 64-kbit/s transmission paths), the 8-bit envelopes need not necessarily be aligned with theoctet structure. On 64-kbit/s interfaces the 8-kHz timing signal may not be used; in that case the framinginformation is conveyed across this interface fully within the multiplex.iii) Whether on an
36、 international link the 8-bit envelopes have to be aligned with the octet structure isprovisionally subject to bilateral agreement but further study is required.Fascicle VIII.3 - Rec. X.50 5EXPLANATORY NOTESNote 1 - 8-bit envelopeIn an 8-bit envelope, bit 1 is reserved for framing purposes, bits 2-7
37、 are information bits of the channel, and bit8 is a status bit (see Figure 1/X.50).The addition of the framing and the status bits results in a 33% increase in bit rate, so that bearer channel ratesare:12.8 kbit/s for the 9.6 kbit/s data signalling rate;6.4 kbit/s for the 4.8 kbit/s data signalling
38、rate;3.2 kbit/s for the 2.4 kbit/s data signalling rate;800 bit/s for the 600 bit/s data signalling rate.The status bit is associated with each envelope and, in conjunction with the information bits, conveys callcontrol information.Note 2 - Four 8-bit envelopes groupingA group of four 8-bit envelope
39、s is assembled on a single channel as a 32-bit group providing 24 informationbits. This gives the possibility of accommodating three 8-bit characters, e.g. P, Q, R, as in Table 3/X.50.TABLE 3/X.50F P1P2P3P4P5P6SA8-bit envelope AF P7 P8Q1Q2Q3Q4SB8-bit envelope BF Q5Q6Q7Q8R1R2SC8-bit envelope CF R3R4R
40、5R6R7R8SD8-bit envelope DStatus bit SDis used to provide the alignment information of the four 8-bit envelopes grouping.Status bits SA, SBand SCin conjunction with the 24 information bits convey call control information.When the three 8-bit characters P, Q and R are accommodated as above described,
41、status bits SA, SBand SCare respectively associated with those characters.The four 8-bit envelopes grouping is applied on a per channel basis. For example, for the 12.8-kbit/s bearerrate, the four 8-bit envelopes group recurs after twenty 8-bit envelopes of the multiplexed stream, as in Figure 2/X.5
42、0.6 Fascicle VIII.3 - Rec. X.50Note 3 - 10-bit envelopeIn a 10-bit envelope, bit 1 is a status bit, bit 2 is reserved for envelope alignment purposes and bits 3-10 areinformation bits of the channel (see Figure 3/X.50).The addition of the envelope alignment and the status bits results in a 25% incre
43、ase in bit rate, so that bearerchannel rates are:12.0 kbit/s for 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.The status bit is associated with each envelope and, in conjunction with the associated 8-bit byte informationbits, conveys call control information.
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