1、ITU-T RECHN*G-S8L 94 = 48b259L 0592298 730 INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU DIGITAL SECTIONS AND DIGITAL LINE SYSTEMS G.981 (O1 /94) PDH OPTICAL LINE SYSTEMS FOR THE LOCAL NETWORK ITU-T Recommendation G.981 (Previously “CCITT Recommendation”
2、) FOREWORD The -T (Telecommunication Standardization Sector) is a permanent organ of the International Telecommunication Union (ITU). The -T is responsible for studying technical, operating and tariff questions and issuing Recommen- dations on them with a view to standardizing telecommunications on
3、a worldwide basis. The World Telecommunication standardization Conference (WTSC), which meets every four years, establishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. The approval of Recommendations by the Members of the ITU-T is cov
4、ered by the procedure laid down in WTSC Resolution No. 1 (Helsinki, March 1-12, 1993). IT-T Recommendation G.981 was prepared by IT-T Study Group 15 (1993-1996) and was approved under the WTSC Resolution No. 1 procedure on the 20th of January 1994. NOTE In this Recommendation, the expression “Admini
5、stration” is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. O ITU 1994 All 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 microfi
6、lm, without permission in writing from the ITU. ITU-T RECMN*G*SBL 94 = 4862571 0592300 ILS 1 CONTENTS Page General . 1 1.1 Scope 1 1.2 Objectives . 1 1.3 Definitions 1 1.4 Applications and configurations . 1 2 Functions 4 3 Optical transmission . 3.1 Duplex method 3.2 Transmission medium . 3.3 Defin
7、ition of margins 3.4 Transmitter 3.5 Receiver 3.6 Optical characteristics of one-fibre line terminations . 3.7 Optical path . 4 System performance . 4.1 Error performance . 4 4 4 5 5 6 6 6 7 7 4.2 Jitter 7 5 Operation and maintenance 7 6 Power feeding . 8 7 Environmental conditions 8 8 Safety consid
8、erations . 8 Recommendation 6.981 (O/94) i ITU-T RECMN*G.S8L 94 W 4862591 0592303 055 Recommendation 6.981 PDH OPTICAL LINE SYSTEMS FOR THE LOCAL NETWORK (Geneva, 1994) 1 General 1.1 Scope This Recommendation covers digital line systems for the transmission of signals based on the plesiochronous hie
9、rarchy on optical fibre cables in the local network and includes systems conveying the following bit rates: I - 1544 kbit/s; - 2048 kbit/s; - 34 368 kbit/s. This Recommendation is applicable together with the relevant Recommendations on digital sections for the respective applications. Those include
10、: - - G.962 for ISDN primary rate access at 2048 kbivs; G.963 for ISDN primary rate access at 1544 kbids. 1.2 Objectives The aim of this Recommendation is to achieve longitudinal compatibility of different digital line systems on elementary cable sections, Le. the possibility of installing digital l
11、ine systems, produced by different manufacturers, on the same optical fibre cable. The systems should: - be in line with the error performance specified in Recommendations G.821 (local grade) and G.826 and should harmonize with the relevant Recommendations; - - - NOTE - The possibility of transverse
12、 compatibility is for further study. be able to provide access to the majority of customers without the use of regenerators; be able to support the relevant maintenance functions; comply with various national regulations concerning EMI. 1.3 Definitions For the purpose of this Recommendation, the dig
13、ital optical line systems can be represented as in Figure 1. The systems have no intermediate regenerators. 1.4 Applications and configurations The transmission system may be applied for ISDN-primary rate access, leased lines or access to other network capabilities using the bit rates defined in 1.1
14、. Possible configurations in the access network are shown in Figure 2. Recommendation 6.981 (OW4) 1 ITU-T RECMN*G*SB1 94 4862593 0592302 T91 m a) Two-fibre system T1512170-92/401 b) One-flbre system NTi Line termination function, customer side (ndwork termination) for ISDN applications: network term
15、ination 1 as defined in Recommendation 1.41 1 Line termination function, network side for ISDN applications: line termination as defined in Recommendation 1.112 LT C Optical connector the equipment T Equipment interface according to Recommendation G.73, aistomer side for ISDN applications: user nshn
16、rork interface as defined in Recommend the use of one fibre for both directions, applying: a) b) c) time division to separate the directions (TDM); or wavelength division to separate the directions (WDM); or subcarrier division to separate the directions (SCDM). 3.2 Transmission medium Single mode o
17、ptical fibres conforming to Recommendation G.652 should be used. Performance of such systems is beyond the scope of this Recommendation. NOTE - Some Administrations may use already installed multimode fibres conforming to Recommendation G.651. 4 Recommendation G.981 (O94) ITU-T RECMN*G.SBL 94 = 4862
18、593 0592305 ?TO W 3.3 Definition of margins For the purposes of this Recommendation, the total system margin is subdivided into two main contributions. The allocation of these margins is shown in Figure 4. FIGURE 4/G.981 Ailocation of margins 3.3.1 Cable margin (Mc) The cable margin (Mc) covers allo
19、wances for: i) future modifications to the cable configuration (additional splices, increased cable lengths, etc.); i) fibre cable performance variations due to environmental factors; iii) degradation of any connector and optical passive devices between points S and R when provided. 3.3.2 Equipment
20、margin (Me) The equipment margin (Me) covers allowances for the effect of time and environmental factors on equipment performance (e.g. launched power, receiver sensitivity, equipment connector degradations). NOTES 1 The design margin, which covers the allowance for the tolerances on the characteris
21、tics of the various components of the system, is not considered because worst-case values for such characteristics are reflected in the specifications of clause 4. 2 The system margin is in relation to a BER threshold of 1 x 10-10 even though for practical reasons the measurements of receiver sensit
22、ivity may be carried out at other thresholds. 3 The worst-case approach adopted in this Recommendation leaves some additional margin in operating systems which can be considered as an unallocated margin. 3.4 Transmitter 3.4.1 Optical source type The systems may employ either MLM-lasers (multi-longit
23、udinal mode lasers) or LEDs (light-emitting diodes) as sources. Lasers should not require cooling. 3.4.2 Spectral characteristics The spectrum and wavelength ranges of the optical source are determined by the characteristics of the optical path and the minimum performance required. NOTE - For single
24、-mode systems the lower wavelength limit is determined from consideration of cut-off wavelength and possibly dispersion effects, while the upper wavelength limit is due to consideration of attenuation, in particular OH peak related excess losses, and possibly dispersion. The possibility of specifyin
25、g spectral characteristics of the line interface is for further study. Recommendation G.981 (OV94) 5 ITU-T RECNN*G-SBL 74 4b257L 059230b b37 = Maximum attenuation at 1310 nm (dB) 24 12 12 12 3.5 Receiver Maximum Minimum optical Maximum dispersion relevant return loss at S propagation delay length (k
26、m) (PS) n.a. n.a. n.a. n.a. 20 85 17 n.a. n.a. 17 20 85 The optical receiver dynamic range should be sufficient to automatically compensate for equipment production tolerances and the effects of temperatures and ageing, as well as for the total range of optical path attenuation. The maximum optical
27、path penalty should be I 1 dB. The optical path penalty takes into account the transmitter spectral characteristics and the fibre dispersion. 3.6 Optical characteristics of one-fibre line terminations For one-fibre systems the optical reflectance of the line termination as seen from reference point
28、SR should be less than -20 dB. 3.7 Optical path The optical path specifications serve two purposes: - to indicate to the network planner the worst-case characteristics the optical path may exhibit to assure satisfactory operation with any equipment; to indicate to the equipment designer the worst-ca
29、se path characteristics the equipment must be able to accommodate. - The overall characteristics of the optical path between reference points S and R are determined by: - the characteristics of: - the cabled optical fibres; - splices; - connectors; - other passive optical devices (if any); - the cab
30、le margin. Optical fibre cable specifications are given in Recommendation G.652. Overall optical path specifications between reference points S and R are given in Table 1 together with the following subclauses. TABLE UG.981 Opticai path specifications Nominal bit rate 1544 kbit/s or 2048 kbis I I 34
31、 368 kbits Two fibres One fibre Two fibres One fibre Allowance between S and R at 1 x 10-10 BER n.a. Not applicable 6 Recommendation G.981 (OU94) 3.7.1 Attenuation Attenuation specifications are given in terms of worst-case values, including losses to splices, connectors, any passive optical devices
32、 (if used) and the cable margin. The values are given for the reference wavelength of 1310 nm (value for further study). The values of fibre attenuation at other wavelengths may be calculated with the procedure specified in Appendix VG.652. 3.7.2 Dispersion The maximum dispersion is determined by th
33、e worst-case chromatic dispersion coefficient and the maximum dispersion relevant length. The worst-case chromatic dispersion coefficient is specified in Recommendation G.652. Table 1 therefore only specifies the maximum dispersion relevant length. NOTE-The length specification in Table 1 refers to
34、chromatic dispersion only. The network operator may choose to deploy fibres with better than worst-case chromatic dispersion, thus allowing longer spans in terms of real length, if the original system were dispersion limited. 3.7.3 Optical return loss At the considered bit rates and using LED or MLM
35、 lasers this parameter is not expected to be relevant for systems employing one fibre per direction. However it may be relevant for systems employing one fibre for both directions with time or subcarrier division to separate directions. The optical return loss includes reflections in the optical pat
36、h between S and R but not at the receiving connector and at the electro-optical receiving device. 4 System performance 4.1 Error performance The digital line systems described in this Recommendation are required to provide error performance in accordance with the relevant section Recommendations und
37、er all specified working conditions. The transmitter and receiver should be designed so that a BER I 1 x 10-10 is obtained when operating over an optical path between points S and R as specified in 3.7 with no external electromagnetic or electrostatic interference. 4.2 Jitter Jitter is specified in
38、the relevant Recommendations on digital sections. 5 Operation and maintenance The required minimum operation and maintenance functions are specified in the relevant Recommendations on digital sections. System specific operation and maintenance functions are for further study. The necessity and speci
39、fication of an F interface at the LT (for local supervision, workstation) and/or a Q-interface at the LT (for connection to the Telecommunications Management Network according to Recommendation G.77 1) is for further study. Recommendation 6.981 (OV94) 7 ITU-T RECMN*G.981 94 = 4862591 0592308 40T = 6
40、 Power feeding Both NTl and LT are fed locally. The details of power feeding should comply with national standards. Power feeding of NTl in the application of ISDN primary rate access is defined in Recommendation 1.431. Other power requirements are for further study. 7 Environmental conditions Clima
41、tic conditions and electromagnetic compatibility requirements are under study in IEC. Appropriate references include EC 721-3-1 for climatic conditions and IEC 801-2 and 801-3 for electromagnetic compatibility. Equipment shall meet all specifications when normal operating conditions are fulfilled. 8
42、 Safety considerations The Recommendations for guidance for the safe use, maintenance and service of line systems on optical fibre cables with operating wavelengths between 400 nm and 3000 nm are to be found in IEC 825-1 and IEC 825-2. 8 Recommendation 6.981 (OY94) ITU-T RECMN*G-983 94 W 4862593 0592309 34b m Printed in Switzerland Geneva, 1994
