G704同步帧结构.docx

1、G704同步帧结构INTERNATIONAL TELECOMMUNICATION UNIONCCITT G.704THE INTERNATIONALTELEGRAPH AND TELEPHONECONSULTATIVE COMMITTEEGENERAL ASPECTS OF DIGITALTRANSMISSION SYSTEMS;TERMINAL EQUIPMENTSSYNCHRONOUS FRAME STRUCTURES USED AT PRIMARY AND SECONDARY HIERARCHICAL LEVELSRecommendation G.704Geneva, 1991FOREW

2、ORD The CCITT (the International Telegraph and Telephone Consultative Committee) is a permanent organ of the International Telecommunication Union (ITU). CCITT is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telec

3、ommunications on a worldwide basis. The Plenary Assembly of CCITT which meets every four years, establishes the topics for study and approves Recommendations prepared by its Study Groups. The approval of Recommendations by the members of CCITT between Plenary Assemblies is covered by the procedure l

4、aid down in CCITT Resolution No. 2 (Melbourne, 1988). Recommendation G.704 was prepared by Study Group XVIII and was approved under the Resolution No. 2 procedure on the 5th of April 1991._CCITT NOTES1) In this Recommendation, the expression “Administration” is used for brevity to indicate both a te

5、lecommunication Administration and a recognized private operating agency.2) A list of abbreviations used in this Recommendation can be found in Annex B.ITU1991All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, includi

6、ng photocopying and microfilm, without permission in writing from the ITU.Recommendation G.704SYNCHRONOUS FRAME STRUCTURES USED ATPRIMARY AND SECONDARY HIERARCHICAL LEVELS(Malaga-Torremolinos, 1984; amended at Melbourne, 1988, revised 1990)1 General This Recommendation gives functional characteristi

7、cs of interfaces associated with: network nodes, in particular, synchronous digital multiplex equipment and digital exchanges in IDNs for telephony and ISDNs, and PCM multiplexing equipment. Section 2 deals with basic frame structures, including details of frame length, frame alignment signals, cycl

8、ic redundancy check (CRC) procedures and other basic information. Sections 3 to 6 contain more specific information about how certain channels at 64 kbit/s and at other bit rates are accomodated within the basic frame structures described in 2. Electrical characteristics for these interfaces are def

9、ined in Recommendation G.703. Note 1 This Recommendation does not necessarily apply to those cases where the signals that cross the interfaces are devoted to non-switched connections, such as those for the transport of encoded wideband signals (e.g. broadcast TV signals or multiplexed sound-programm

10、e signals which need not be individually routed via the ISDN), see also Annex A to Recommendation G.702. Note 2 The frame structures recommended in this Recommendation do not apply to certain maintenance signals, such as the all 1s signals transmitted during fault conditions or other signals transmi

11、tted during out-of-service conditions. Note 3 Frame structures associated with digital multiplexing equipments using justification are covered in each corresponding equipment Recommendation. Note 4 Inclusion of channel structures at other bit rates than 64 kbit/s is a matter for further study. Recom

12、mendations G.761 and G.763 dealing with the characteristics of PCM/ADPCM transcoding equipment contain information about channel structures at 32 kbit/s. The more general use of those particular structures is a subject of further study.2 Basic frame structures2.1 Basic frame structure at 1544 kbit/s

13、2.1.1 Frame length: 193 bits, numbered 1 to 193. The frame repetition rate is 8000 Hz.2.1.2 F-bit The first bit of a frame is designated an F-bit, and is used for such purposes as frame alignment, performance monitoring and providing a data link.2.1.3 Allocation of F-bit Two alternative methods as g

14、iven in Tables 1/G.704 and 2/G.704 for allocation of F-bits are recommended.TABLE 1/G.704Multiframe structure for the 24 frame multiframeF-bitBit number(s) in each channel time slotSignalling channelFrame numberwithin multiframeBit number withinAssignmentsdesignation a)multiframeFASDLCRCFor characte

15、r signal a)For signalling a) 01 1 m 1-8 02 194 e11-8 03 387 m 1-8 04 580 0 1-8 05 773 m 1-8 06 966 e21-7 8 A 07 1159 m 1-8 08 13520 1-8 09 1545 m 1-8 10 1738 e31-8 11 1931 m 1-8 12 21241 1-7 8 B 13 2317 m 1-8 14 2510 e41-8 15 2703 m 1-8 16 28960 1-8 17 3089 m 1-8 18 3282 e51-7 8 C 19 3475 m 1-8 20 3

16、6681 1-8 21 3861 m 1-8 22 4054 e61-8 23 4247 m 1-8 24 44401 1-7 8 DFAS Frame alignment signal (. . . 001011 . . .).DL 4 kbit/s data link (message bits m).CRC CRC-6 block check field (check bits e1 a e6).a) Only applicable in the case of channel associated signalling (see 3.1.3.2).TABLE 2/G.704Alloca

17、tion of F-bits for the 12-frame multiframeFrame numberFrame alignment signalMultiframe alignment signalor signalling112S30 4S Note For multiframe structure, see 3.1.3.2.2.2.1.3.1 Method 1: Twenty-four-frame multiframe Allocation of the F-bit to the multiframe alignment signal, the CRC check bits and

18、 the data link is given in Table 1/G.704.2.1.3.1.1 Multiframe alignment signal The F-bit of every fourth frame forms the pattern 001011 . . . 001011. This multiframe alignment signal is used to identify where each particular frame is located within the multiframe in order to extract the cyclic redun

19、dancy check code, CRC-6, and the data link information, as well as to identify those frames that contain signalling (frames 6, 12, 18 and 24), if channel associated signalling is used.2.1.3.1.2 Cyclic redundancy check (CRC) The CRC-6 is a method of performance monitoring that is contained within the

20、 F-bit position of frames 2, 6, 10, 14 18 and 22 of every multiframe (see Table 1/G.704). The CRC-6 message block check bits e1, e2, e3, e4, e5, and e6 are contained within multiframe bits 194, 966, 1738, 2510, 3282 and 4054 respectively, as shown in Table 1/G.704. The CRC-6 message block (CMB) is a

21、 sequence of 4632 serial bits that is coincident with a multiframe. By definition, CMB N begins at bit position 1 of multiframe N and ends at bit position 4632 of multiframe N. The first transmitted CRC bit of a multiframe is the most significant bit of the CMB polynomial. In calculating the CRC-6 b

22、its, the F-bits are replaced by binary 1s. All information in the other bit positions will be identical to the information in the corresponding multiframe bit positions. The check-bit sequence e1 through e6 transmitted in multiframe N+1, is the remainder after multiplication by x6 and then division

23、(modulo 2) by the generator polynomial x6+x+1 of the polynomial corresponding to CMB N. The first check bit (e1) is the most significant bit of the remainder; the last check bit (e6) is the least significant bit of the remainder. Each multiframe contains the CRC-6 check bits generated for the preced

24、ing CMB. At the receiver, the received CMB, with each F-bit having first been replaced by a binary 1, is acted upon by the multiplication/division process described above. The resulting remainder is compared on a bit-by-bit basis, with the CRC-6 check bits contained in the subsequently received mult

25、iframe. The compared check bits will be identical in the absence of transmission errors.2.1.3.1.3 4 kbit/s data link Beginning with frame 1 of the multiframe (see Table 1/G.704) the first bit of every other frame is part of the 4 kbit/s data link. This data link provides a communication path between

26、 primary hierarchical level terminals and will contain data, an idle data link sequence or a loss of frame alignment alarm sequence. The format to be used for the transmission of data over the m-bits of the data link is still under study. The idle data link pattern is also under study. A loss of fra

27、me alignment alarm sequence is used when a loss of frame alignment (LFA) condition has been detected. After a loss of frame alignment condition is detectd at local end A, a 16-bit LFA sequence of eight 1s eight 0s (1111111100000000) will be transmitted in the m-bits of the 4 kbit/s data link continu

28、ously to remote end B.2.1.3.2 Method 2: Twelve-frame multiframe Allocation of the F-bit to the frame alignment signal, multiframe alignment signal and signalling is given in Table 2/G.704.2.2 Basic frame structure at 6312 kbit/s2.2.1 Frame length The number of bits per frame is 789. The frame repeti

29、tion rate is 8000 Hz.2.2.2 F-bits The last five bits of a frame are designated as F-bits, and are used for such purposes as frame alignment, performance monitoring and providing a data link.2.2.3 Allocation of F-bits Allocation of the F-bits is given in Table 3/G.704.TABLE 3/G.704Allocation of F-bit

30、sFrame numberBit number78578678778878911100m2101003xxxam4e1e2e3e4e5 m Data link bit.a Remote end alarm bit (1 state = alarm, 0 state = no alarm).ei CRC-5 check bit (i = 1 to 5).x Spare bits, to be set at state 1 if not used.2.2.3.1 Frame alignment signal The frame and multiframe alignment signal is 110010100, and is carried on the F-bits in frames 1 and 2, excluding bit