1、INTERNATIONAL TELECOMMUNICATION UNIONITU-T G.975TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU(10/2000)SERIES G: TRANSMISSION SYSTEMS AND MEDIA,DIGITAL SYSTEMS AND NETWORKSDigital sections and digital line system Optical fibresubmarine cable systemsForward error correction for submarine systemsITU-T
2、Recommendation G.975(Formerly CCITT Recommendation)ITU-T G-SERIES RECOMMENDATIONSTRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKSINTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS G.100G.199GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER-TRANSMISSION SYSTEMSG.200G.299INDIVIDUAL CHAR
3、ACTERISTICS OF INTERNATIONAL CARRIER TELEPHONESYSTEMS ON METALLIC LINESG.300G.399GENERAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONESYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION WITHMETALLIC LINESG.400G.449COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY G.450G.499TESTING EQU
4、IPMENTS G.500G.599TRANSMISSION MEDIA CHARACTERISTICS G.600G.699DIGITAL TERMINAL EQUIPMENTS G.700G.799DIGITAL NETWORKS G.800G.899DIGITAL SECTIONS AND DIGITAL LINE SYSTEM G.900G.999General G.900G.909Parameters for optical fibre cable systems G.910G.919Digital sections at hierarchical bit rates based o
5、n a bit rate of 2048 kbit/s G.920G.929Digital line transmission systems on cable at non-hierarchical bit rates G.930G.939Digital line systems provided by FDM transmission bearers G.940G.949Digital line systems G.950G.959Digital section and digital transmission systems for customer access to ISDN G.9
6、60G.969Optical fibre submarine cable systems G.970G.979Optical line systems for local and access networks G.980G.989Access networks G.990G.999For further details, please refer to the list of ITU-T Recommendations.ITU-T G.975 (10/2000) iITU-T Recommendation G.975Forward error correction for submarine
7、 systemsSummaryThe present Recommendation is primarily concerned with the implementation of a Forward ErrorCorrection (FEC) function in the multigigabit-per-second optical fibre submarine cable systems. Theapplications being addressed in this Recommendation are both optically amplified repeateredsys
8、tems and repeaterless optical systems (described in ITU-T G.973 3). The use of this FECfunction in submarine terminal transmission equipment is not mandatory.SourceITU-T Recommendation G.975 was revised by ITU-T Study Group 15 (1997-2000) and approvedunder the World Telecommunication Standardization
9、 Assembly (Montreal, 27 September 6 October 2000).ii ITU-T G.975 (10/2000)FOREWORDThe International Telecommunication Union (ITU) is the United Nations specialized agency in the field oftelecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ ofITU. ITU-T is
10、responsible for studying technical, operating and tariff questions and issuing Recommendationson them with a view to standardizing telecommunications on a worldwide basis.The World Telecommunication Standardization Assembly (WTSA), which meets every four years,establishes the topics for study by the
11、 ITU-T study groups which, in turn, produce Recommendations on thesetopics.The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1.In some areas of information technology which fall within ITU-Ts purview, the necessary standards areprepared on a collaborative
12、 basis with ISO and IEC.NOTEIn this Recommendation, the expression “Administration“ is used for conciseness to indicate both atelecommunication administration and a recognized operating agency.INTELLECTUAL PROPERTY RIGHTSITU draws attention to the possibility that the practice or implementation of t
13、his Recommendation mayinvolve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence,validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or othersoutside of the Recommendation development process.As of the date o
14、f approval of this Recommendation, ITU had received notice of intellectual property,protected by patents, which may be required to implement this Recommendation. However, implementors arecautioned that this may not represent the latest information and are therefore strongly urged to consult theTSB p
15、atent database. ITU 2001All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means,electronic or mechanical, including photocopying and microfilm, without permission in writing from ITU.ITU-T G.975 (10/2000) iiiCONTENTSPage1 Scope 12 References 13 Term
16、s and definitions. 14 Abbreviations . 25 FEC features . 25.1 General principles of the FEC function 25.2 Error monitoring capability 35.3 Interest of the FEC function for submarine systems 45.4 Inter-terminal channels. 46 Definition of the FEC function. 46.1 Definitions 46.2 Forward Error Correction
17、 algorithm 46.3 Properties of the RS(255,239) code 56.4 FEC frame structure 56.4.1 FEC encoder and FEC decoder architectures 56.4.2 Framing structure 86.4.3 Scrambling 86.4.4 Redundancy ratio. 87 FEC function performance . 97.1 Theoretical FEC function performance 97.2 Coding gain 10Appendix I FEC f
18、rame structure (optional) 11I.1 FEC encoder and FEC decoder architecture. 11I.2 Framing structure 14I.3 Scrambling 14I.4 Redundancy ratio 15ITU-T G.975 (10/2000) 1ITU-T Recommendation G.975Forward error correction for submarine systems1ScopeThe present Recommendation is primarily concerned with the
19、implementation of a Forward ErrorCorrection (FEC) function in the multigigabit-per-second optical fibre submarine cable systems. Theapplications being addressed in this Recommendation are both optically amplified repeateredsystems and repeaterless optical systems (ITU-T G.973 3). The use of this FEC
20、 function insubmarine terminal transmission equipments (TTEs) should not be considered as mandatory.It is not the intention of this Recommendation to pursue the transverse compatibility of the system.Therefore the selection of the FEC frame structures described in this Recommendation is a matter ofj
21、oint engineering.The transmission data rates under consideration in this Recommendation are 2.5 Gbit/s STM-16(ITU-T G.707 1) and integer multiples of 2.5 Gbit/s (interleaved STM-16 tributaries).Clause 5 presents the main features of the FEC function implemented in the submarine systems, andin partic
22、ular the error monitoring facility.Clause 6 provides the definition of the forward error correction algorithm to be used, which is aReed-Solomon code, and gives guidelines for the implementation of this algorithm in the submarineTerminal Transmission Equipments (TTEs).Clause 7 is dedicated to the me
23、asurement of the performance of this Reed-Solomon code and theexpected gain on the optical transmission power budget.2 ReferencesThe following ITU-T Recommendations and other references contain provisions which, throughreference in this text, constitute provisions of this Recommendation. At the time
24、 of publication, theeditions indicated were valid. All Recommendations and other references are subject to revision;users of this Recommendation are therefore encouraged to investigate the possibility of applying themost recent edition of the Recommendations and other references listed below. A list
25、 of the currentlyvalid ITU-T Recommendations is regularly published.1 ITU-T G.707 (1996), Network node interface for the synchronous digital hierarchy (SDH).2 ITU-T G.972 (1997), Definition of terms relevant to optical fibre submarine cable systems.3 ITU-T G.973 (1996), Characteristics of repeaterle
26、ss optical fibre submarine cable systems.3 Terms and definitionsThis Recommendation uses the following terms defined in other Recommendations: Synchronous Digital Hierarchy (SDH): See ITU-T G.707 1. Synchronous Transport Module (STM): See ITU-T G.707 1. Optical fibre submarine cable system: See ITU-
27、T G.972 2. Terminal Transmission Equipment (TTE): See ITU-T G.972 2. Optical power budget: See ITU-T G.972 2. Service channel: See ITU-T G.972 2.2 ITU-T G.975 (10/2000) Order wire channel: See ITU-T G.972 2. Line error ratio: See ITU-T G.972 2. Forward Error Correction (FEC): See ITU-T G.972 2. FEC
28、frame: See ITU-T G.972 2. FEC encoder: See ITU-T G.972 2. FEC decoder: See ITU-T G.972 2.4 AbbreviationsThis Recommendation uses the following abbreviations:BER Bit Error RatioEDFA Erbium-Doped Fibre AmplifierFEC Forward Error CorrectionGF Galois FieldRS Reed-Solomon5 FEC features5.1 General princip
29、les of the FEC functionThe FEC function defined in this Recommendation works on the STM-16 basis. When M (M integerdifferent from 0) STM-16 signals are interleaved to achieve M 2.5 Gbit/s transmission data rates,the encoding is performed before the interleaving of the M STM-16 tributaries and the de
30、coding isperformed after the de-interleaving of the optical line signal.The FEC function essentially comprises: a FEC encoder in the transmit Terminal Transmission Equipment (TTE) that acceptsinformation bits and adds computed redundant symbols, producing encoded data at a higherbit rate; a FEC deco
31、der in the receive Terminal Transmission Equipment (TTE) that performs theerror correction while extracting the redundancy to regenerate the data that was encoded bythe FEC encoder.Figure 1 outlines the fact that the encoding and decoding procedures are performed at the TerminalTransmission Equipmen
32、t (TTE) level only, on electrical signals, and benefit the overall optical fibresubmarine cable system, which comprises the optical fibre and possibly optical modules such asoptical amplifiers using EDFA technology.ITU-T G.975 (10/2000) 3T1520650-96OpticalReceiverFECencoderOpticalReceiverFECencoderS
33、TM-16STM-16(x M)STM-16STM-16(x M)OpticalTransmitterFECdecoderOpticalTransmitterFECdecoderOpticalsectionNoiseSITRANSMIT TTERECEIVE TTEDe-interleaver a low complexity of both the FEC encoder and the FEC decoder; a coding structure compatible with binary transmissions, providing that a demultiplexingop
34、eration is performed; an important correcting capacity of burst errors. This intrinsic property of the Reed-Solomoncodes is even enhanced by the interleaving of elementary RS(255,239) codecs. Thistechnique, implemented on the 2.5 Gbit/s optical fibre submarine cable systems, puts theerror correcting
35、 capacity to bursts of 1024 bits maximal length, for 16 interleaved codecs.In addition, the Reed-Solomon codes remain among the most efficient codes which can beimplemented using the state-of-the-art hardware and software technology.6.4 FEC frame structureAnother FEC frame structure (Optional) is gi
36、ven in Appendix I for information.6.4.1 FEC encoder and FEC decoder architecturesIn order to enhance the immunity of the optical fibre submarine cable system to burst errors, severalRS(255,239) codes can be interleaved. In Figures 2 and 3, (n) denotes the interleaving order (n is anon-zero integer).
37、Given the interleaving to depth “n“ of RS(255,239) codes, the architectures of both the FEC encoderand the FEC decoder are detailed in Figures 2 and 3 respectively.6 ITU-T G.975 (10/2000)1/n1/81/81/8n/18/18/18/1888888T1520660-96STM-16at 2.5 Gbit/sFEC frameat 2.66 Gbit/sMultiplexersubframe (8 n)subfr
38、ame 1RS(255,239)encoder #1RS(255,239)encoder #2RS(255,239)encoder #nDemultiplexerFraming structure(insertion)n Number of interleaved codecsFigure 2/G.975 FEC encoder architectureT1520670-961/n1/81/81/8n/18/18/18/1888888subframe 1DemultiplexerMultiplexerSTM-16at 2.5 Gbit/sFEC frameat 2.66 Gbit/ssubfr
39、ame (8 n)RS(255,239)decoder #nRS(255,239)decoder #2RS(255,239)decoder #1Framing structure(extraction)n Number of interleaved codecsFigure 3/G.975 FEC decoder architectureITU-T G.975 (10/2000) 7For data integrity, the digital multiplexer and the digital demultiplexer represented in Figures 2 and 3are
40、 strictly symmetrical. In addition, the same digital multiplexers and the same digitaldemultiplexers are used for both the FEC encoder and the FEC decoder.Due to the fact that each elementary Reed-Solomon algorithm process byte information andtherefore works on 8 parallel data stream, the demultiple
41、xers deliver (8 n) data stream to the (n)interleaved codecs while the multiplexers do the reverse operation.Provided the FEC encoder and the FEC decoder architectures, the FEC frame construction isdescribed in Figure 4.T1520680-961 8 n8 n + 1 16 n1912 n1920 n 2040 nFRAMINGSTRUCTURESTM-16 DATA REDUND
42、ANT DATAFECframeData bytefrom FEC encoder #nMSBLSBMSBData bytefrom FEC encoder #1subframe 1subframe 2subframe 3subframe 4subframe 5subframe 6subframe 7subframe 8subframe 9subframe 8 nsubframe length = 255 bitscolumn 1 column 2 column 239 column 240 column 255n Number of interleaved codecsLSBcolumn1c
43、olumn240column255Figure 4/G.975 FEC frame constructionDue to the interleaving of RS(255,239) codes to depth n, the FEC frame is (2040 n) bits long andis made of (8 n) bit interleaved subframes.And, as a consequence of the symmetry of the digital demultiplexers and the digital multiplexers oneach sid
44、e of the Reed-Solomon algorithms, the sequence of STM-16 data bits within the FEC frameis identical to that within the STM-16 input signal.8 ITU-T G.975 (10/2000)6.4.2 Framing structureA framing structure is added in the FEC frame to possibly insert a FEC frame alignment word,which is required to pe
45、rform the synchronization of the FEC frame with the FEC decoder structure atthe receive Terminal Transmission Equipment (TTE).The remaining spare bits can be used for carrying tributary markers, order wire channels or servicechannels.The FEC frame presented in 6.4.1 can be divided into (8 n) 255 bit
46、s long subframe. Each subframe(see Figure 5) contains the following information: bit 1 of each subframe carries the framing structure either the FEC Frame Alignment Word,the tributary markers for STM-16 data stream identification in systems carrying multiples ofSTM-16 (if required) or order wire cha
47、nnels or service channels for inter-terminalcommunication; bits 2 to 239 of each subframe carry the STM-16 information; bits 240 to 255 of each subframe carry the redundant bits, computed by the RS(255,239)algorithm.T1520690-96STM-16 dataredundantcodes238 bitssubframe length = 255 bits16 bits1 bitfr
48、amingstructureFigure 5/G.975 Content of the subframes of the FEC frame6.4.3 ScramblingSTM-16 data in the FEC frame are already scrambled as shown in ITU-T G.707 1. Hence,rescrambling of the FEC frame is not generally required.However, when implemented in the submarine Terminal Transmission Equipment
49、 (TTE), thescrambling facility should possibly be inhibited.The scrambling of the FEC frame may be carried out in the following procedure: the FEC frame isscrambled, with the exception of the framing structure bits of the FEC frame, by a x7+ x + 1polynomial initiated at each frame on the first bit which follows the framing structure in the FECframe.The first bits of the scrambler sequence are 1111111. Thereafter, the scrambler runs continuouslythroughout the complete FEC
