1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$)4!,G0G0.%47/2+3).4%27/2+).G0G0“%47%.%47/2+3G0G0“!3%$G0G0/.G0G0$)- interworking hierarchy between networks which incorporate the different digital hierarchies based onRecommendation G.702;- interworking arrang
2、ements between networks incorporating the different hierarchies and encoding laws;and,- interconnection by plesiochronous operation between networks which each has an independentsynchronization.This Recommendation is applicable also to ISDNs for transport of B channels specified in RecommendationI.4
3、12.Note - The future specifications on channels and their bit rates to support ISDN broadband services forcustomer-to-customer applications may require additional interworking arrangement specifications other than thosespecified below.2 Terms and definitionsThe terms used in this Recommendation and
4、not defined below are described in Recommendations G.701 orI.112.2.1 z-operationConversion of the -law character signal “00000000“ (all-zero octet) into the -law character signal“00000010“, where “l “ is the bit numbered seven in the octet (see Recommendation G.711).Note - Bit number indicates the c
5、hronological order of transmission of bits in serial processing.2.2 1.5/2 Mbit/s multiplex system conversion (1.5/2 Mbit/s MSC)A function which embodies the following properties:1) termination of a digital link operating at a digital hierarchical level of 1544 kbit/s;2) termination of a digital link
6、 operating at a digital hierarchical level of 2048 kbit/s; and,3) rearrangement of 64 kbit/s channels between 1544 kbit/s and 2048 kbit/s digital terminations.Note - The hierarchical levels and the frame structures are specified in Recommendations G.702 and G.704,respectively.2.3 pulse density requi
7、rement (PDR) at 1544 kbit/sThe minimum requirement for an entire 1544 kbit/s digital signal is that there should be no more than 15 binary2 Fascicle III.5 - Rec. G.802“0“s between successive binary “1“s and that there should be an average binary “1“s density of at least one in every eightbits. This
8、requirement is due to the design of a number of existing systems (see Recommendation G.703.)Moreover, the requirement for an octet-structured source in a 1544 kbit/s digital link is that at least one binary“1“ should be contained in any octet.3 Unrestricted 64 kbit/s transfer capability of a digital
9、 linkNewly introduced digital transmission systems should have the capability to provide bit sequence independencefor 64 kbit/s digital links. This capability should be activated as soon as unrestricted 64 kbit/s transfer capability can bepractically realized.During a transition period, however, 56
10、kbit/s bit sequence independent transfer capability may be provided bybilateral agreement. (Important constraints on the data formats transmitted by source data terminal equipment are givenin Annex 1 to this Recommendation.)4 Encoding law conversion between A-law and -law4.1 Encoding law on an inter
11、national digital linkInternational digital links between countries which have adopted different PCM encoding laws (A-law or-law) should carry signals encoded in accordance with the A-law specified in Recommendation G.711.Where both countries have adopted the same law, that law should be used on digi
12、tal links between them.4.2 Conversion ruleA-law/-law conversion necessary between countries which have adopted different PCM encoding laws will beperformed according to Recommendation G.711 by the -law country. The conversion includes the even-bit inversion ofthe A-law character signal.Note - Locati
13、on of the conversion function in a -law country is a national matter depending upon the structureof domestic digital networks, and is left to the discretion of the Administrations in the -law country.4.3 Control of conversion functionIn switched public network applications enabling/disabling of the
14、conversion function should be under controlof the international switching system, and will be carried out on a call-by-call or during-a-call basis depending upon theservice category requested by the signalling protocol.It should also be possible to enable/disable this conversion function manually an
15、d/or via an operator terminal ona per-channel or semi-permanent basis. This capability would be necessary for configuring leased line circuits notpassing through the international switching system, or if the international switching system were not capable ofcontrolling this function.Note - Control o
16、f conversion function in ISDN environment is specified in I.300-series and I.500-seriesRecommendations.5 Interworking hierarchyFor international interworking between networks using different digital hierarchies specified inFascicle III.5 - Rec. G.802 3Recommendation G.702, the following interworking
17、 hierarchy should be employed:2048 - 6312 - 44 736 - 139 264 kbit/s.For interworking between networks with different digital hierarchies but with 1544 kbit/s primary level,however, levels other than those specified for the above interworking hierarchy may be employed (e.g. 1544 kbit/s).Note 1 - Nati
18、onal networks with a 1544 kbit/s primary level may offer transit of international traffic of6312 kbit/s composed of three 2048 kbit/s signals or of 44 736 kbit/s containing twenty-one 2048 kbit/s signals. Thesenetworks will provide the property of bit sequence independence at 6312 and 44 736 kbit/s
19、and hence at 2048 kbit/s.Note 2 - The frame structures for 2048-6312 kbit/s, 6312-44 736 kbit/s and 44 736-139 264 kbit/s multiplexingstages are specified in Recommendations G.747, G.752 and G.755, respectively.6 Interworking arrangementsBased on the general specifications described in the previous
20、Sections, establishment of an international digitalinterconnection between networks using the different digital hierarchies and speech encoding laws should conform to theinterworking arrangements specified in Table 1/G.802.7 Transport of a 1544 kbit/s signal within a G.704-structured 2048 kbit/s sig
21、nalFor international leased line applications, the transmission of 1544 kbit/s signals may be considered using aspecial mapping into point-to-point 2048 kbit/s signals. Annex B to this Recommendation specifies the method for thismapping.Note - The possible development of specific mappings of 8448 or
22、 34 368 kbit/s signals into 44 736 kbit/ssignals is not precluded.8 Synchronization of an international digital link8.1 Links not synchronized to the national networksWhere independently synchronized national networks are interconnected via an international digital link, thetiming of which is indepe
23、ndent of the national networks, the link should be operated in a plesiochronous mode with theaccuracy specified in Recommendation G.811.8.2 Links synchronized to the network in the transmitting countryWhere independently synchronized national networks are interconnected via an international digital
24、link, thetiming of which is synchronized to the national network in the transmitting country, the plesiochronous operation will beperformed in the receiving country.4 Fascicle III.5 - Rec. G.802TABLE 1/G.802Interworking arrangementsType of information Voice or voiceband data Non-voiceinformationSign
25、allinginformation(Note 1)Encodinglaw at IRP(Note 2)FunctionPCMG.711ADPCMG.721SB-ADPCMG.722. .Network (Note 3) A B A B A B A B A B1.5/2 Mbit/sMSC X X X X XA/ and /Aconversion X Z-operation X(Note 4) X(Notes 4and 5) X(Notes 4and 6) X(Note 4)Transcoding XXNot allowedX May be appliedNote 1 - Signalling
26、information is transferred on unrestricted channels between International Switching Centers (ISCs).Note 2 - IRP = Interworking reference point between Network A and Network B.Note 3 - “A“ is a network within the country incorporating the A-law and 2048 kbit/s-based digital hierarchy. “B“ is a networ
27、k withinthe country incorporating the -law and 1544 kbit/s-based digital hierarchy.Note 4 - Z-operation in the -law country will be applied when the link in that country contains transmission systems that have to meetPDR; in this case unrestricted 64 kbit/s transfer capability cannot be provided due
28、 to PDR and the bit sequence independent transfercapability is restricted to 56 kbit/s.Note 5 - 32 kbit/s digital signals, which are voice or voiceband data signals encoded in accordance with the ADPCM algorithmspecified in Recommendation G.721, do not contain a “0000“ code word. (See Recommendation
29、 G.721.) This implies that even whenPDR exists in the -law country, these signals will not be affected by the z-operation and will be transferred transparently.Note 6 - 64 kbit/s audio signals, where the audio signals having the bandwidth of 50 to 7000 Hz are encoded at 64, 56 or 48 kbit/s inaccorda
30、nce with the coding algorithm specified in Recommendation G.722, do not contain an all-zero octet. (See RecommendationG.722.) This implies that even when PDR exists in the -law country, these signals will not be affected by the z-operation and will betransferred transparently.Fascicle III.5 - Rec. G
31、.802 5ANNEX A(to Recommendation G.802)Impact on terminal equipment designed to work with 56 kbit/sbit sequence independent transfer capabilityDuring a transition period 56 kbit/s bit sequence independent transfer capability may be provided by bilateralagreement. In this case a 56 kbit/s bit sequence
32、 independent transfer capability requires that the source data terminalequipment (DTE) fix the eighth bit of each octet to binary “1“. This must be done on both ends of the digital connectioneven if one portion of the connection has unrestricted 64 kbit/s transfer capability. Failure to keep the eig
33、hth bit fixed tobinary “1“ will cause any all-zero octet to be converted to “00000010“ by z-operation in the -law country.ANNEX B(to Recommendation G.802)Mapping method of a 1544 kbit/s signalinto a G.704-structured 2048 kbit/s signalThe following is a means of accommodating a bit synchronous 1544 k
34、bit/s signal, which may be unstructured orstructured, within a G.704-structured 2048 kbit/s frame, for the purpose of providing leased line applications at1544 kbit/s only. The 1544 kbit/s signal is transmitted transparently without regard to its frame structure within the 2048kbit/s signal.The 193
35、bits of an arbitrary 125 s period of the 1544 kbit/s signal should be accommodated within aG.704-structured 2048 kbit/s frame as follows:TS 0: Frame alignment signal according to Recommendation G.704TS 1-15 TS 17-25 193 contiguous bits of the 1544 kbit/s signalBit 1 in TS 26 TS 16, 27-31: Reserved f
36、or possible accommodation of additional information at up to 384 kbit/s (Note 2)Note 1 - In cases where only the 1544 kbit/s signal is to be transported, the timing of the 1544 kbit/s (or2048 kbit/s) outgoing signal should be derived from the 2048 kbit/s (or 1544 kbit/s) incoming signal for each dir
37、ectionof transmission.Note 2 - In some cases, e.g. where information is transported by the reserved time-slots, the timing of theoutgoing signal should be traceable to the national reference clock conforming to Recommendation G.811. This willrequire the use of 125 s slip buffers.Note 3 - The maximum
38、 capacity available to end-users for transparent transport of their information is1536 kbit/s and not 1544 kbit/s. Depending on the national regulations some network operators may offer the user ofpart of the 8 kbit/s overhead associated with a 1544 kbit/s signal for performance monitoring and its reporting.
