1、STDmITU-T RECMN R*ZL-ENGL L77b qBb257L Ob24LZL1 127 INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU SERIES R: TELEGRAPH TRANSMISSION Vo ice-f req u e n cy te I eg rap h y R.21 (08196) 9600 bit/s modem standardized for use in the telegraph TDM system ITU-T
2、Recommendation R.21 (Previously “CCITT Recommendation”) - STD-ITU-T RECMN RDZL-ENGL 199b m 48b2591 Ob24125 Ob3 m ITU-T R-SERIES RECOMMENDATIONS TELEGRAPH TRANSMISSION Telegraph distortion R. 1-R. 19 Special cases of alternating current telegraphy Transmission quality Correction of signals Telegraph
3、maintenance Time-division multiplexing Transmission quality above 50 bauds Definitions Availability and reliability of international telegraph circuits R.40-R.49 R.50-R.59 R.60-R.69 R.70-R.99 R. 100-R. 119 R. 120-R. 139 R. 140-R.149 R. 150-R. 159 For further details, please refr to ITU-TList oftieco
4、mmendations. STDvITU-T RECMN R-21-ENGL L99b 4862591 Db24L2b TTT FOREWORD The IT-T (Telecommunication Standardization Sector) is a permanent organ of the International Telecommunication Union (ITU). The IT-T is responsible for studying technical, operating and tariff questions and issuing Recommen- d
5、ations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years, establishes the topics for study by the IT-T Study Groups which, in their turn, produce Recommendations on these topics. T
6、he approval of Recommendations by the Members of the IT-T is covered by the procedure laid down in WTSC Resolution No. 1 (Helsinki, March 1-12, 1993). ITU-T Recommendation R.21 was prepared by ITU-T Study Group 14 (1993-1996) and was approved under the WTSC Resolution No. 1 procedure on the 16th of
7、August 1996. NOTES 1. telecommunication administration and a recognized operating agency. 2. follows: In this Recommendation, the expression “Administration” is used for conciseness to indicate both a The status of annexes and appendices attached to the Series R Recommendations should be interpreted
8、 as - - an annex to a Recommendation forms an integral part of the Recommendation; an appendix to a Recommendation does not form part of the Recommendation and only provides some complementary explanation or information specific to that Recommendation. O ITU 1997 All rights reserved. No part of this
9、 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 the ITU. 1 Recommendation R.21 (08/96) 1 2 3 4 5 6 7 8 9 10 11 12 11 CONTENTS Scope Normative references . Line signal structure
10、. Requirements to received signals frequency tolerance Transmitted power levels . Modulation method and signai space diagram . Data signalling and modulation rates . Received signal conditions . Interchange circuits Synchronization Special measures for raising modem immunity . Testing Facilities Rec
11、ommendation R.21 (08/96) Page 1 1 2 2 2 2 2 4 4 4 5 5 STD-ITU-T RECMN R.2L-ENGL L79b 9 4b259L b24128 872 Recommendation R.21 9600 BITIS MODEM STANDARDIZED FOR USE IN THE TELEGRAPH TDM SYSTEM (Geneva, I996) The ITU-T, considering (4 that V-Series modems are intended to be used for data transmission;
12、(b) that characteristics of these modems may preclude their use in telegraph TDM systems; (c) requirements for reliability and quality of the aggregate signai transmission over a bearer circuit; that telegraph TDM systems serve a great number of subscribers ( 46) simultaneously and claim higher (d)
13、other one is damaged; that a telegraph TDM system should have the possibility to transfer information in one direction when the (e) working signal without the use of sped synchronization pattern and handshaking procedure; that it is important for TDM systems that resynchronization of modems should b
14、e carried out by means of the (f) sequences “zeros” and “ones”, that in telegraph TDM systems special measures have already been taken to exclude the occurrence of long recommends the following 1 Scope This Recommendation denes a modem for use in telegraph TDM systems. The principal characteristics
15、of this Recommendation are for use on 4-wire leased telephone-type circuits. Other applications, such on the GSTN are for further study. 2 Normative references The following Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this R
16、ecommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision: all users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and othe
17、r references listed below. A list of the currently valid ITU-T Recommendations is regularly published. CCITT Recommendation H.22 (1 984), Transmission requirements of international voice-frequency telegraph links (at 50, IO0 and200 bauds). ITU-T Recommendation M. 1020 (1993), Characteristics of spec
18、ial quality international leased circuits with special bandwidth conditioning. ITU-T Recommendation M. 1025 (1993), Characteristics of special qualis, international leased circuits with basic bandwidth conditioning. CCITT Recommendation V.2 (1980), Power levels for data transmission over telephone l
19、ines. ITU-T Recommendation V. 10 (1993), Electrical characteristics for unbalanced double-current interchange circuits operating at data signalling raies nominally up to I O0 kbit/s, - - - - - Recommendation R.21 (08/96) 1 - STD-ITU-T RECMN R-ZL-ENGL L77b m 4Bb2591 Ob24127 709 - ITU-T Recommendation
20、 V.24 (1993), List of definitions for interchange circuits between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE). - ITU-T Recommendation V.28 (1 993), Electrical characteristics for unbalanced double-current interchange circuits. CCITT Recommendation V.54 (1988), Loop te
21、st devices for modems. - 3 Line signal structure 3.1 Carrier frequencies The total number of carrier frequencies is to be 71. 3.1.1 orthogonal within the: Subchannel signals should be presented as sections of harmonic oscillation of multiple frequencies that are T = l/Af interval where Afis the dist
22、ance between carrier frequencies equal to 43.636 Hz. 3.1.2 By minimum dispersion of total additive and interference noises best 67 frequencies from 71 carrier frequencies fi = Af(i + 6), i = 1,2, ., 71 from 305.455 Hz to 3360 Hz (i.e. operation bandwidth is 3054.545 Hz) are chosen to be working freq
23、uencies. 3.1.3 used as auxiliary frequencies for handling the signal at the receiver side. 3.1.4 signal interference immunity. Two pilot frequencies 436.364 and 3229.094 are added to the working frequencies. These pilot frequencies are Six other frequencies (after selection of better ones) are used
24、to provide structural redundancy in order to raise 4 The carrier frequency tolerance allowance at the transmitter is 4 O. 1 Hz. The receiver must be able to operate normally at a frequency shift of 2 7 Hz. Requirements to received signals frequency tolerance 5 Transmitted power levels The transmitte
25、d power level of the aggregate signal must conform to Recommendation V.2. 6 Modulation method and signal space diagram 6.1 The data stream to be transmitted is divided into groups of four consecutive data bits (quadbits). These groups are formed into modulo 1 of 252 bits and then distributed between
26、 63 working subchannels on the quadbit basis. The first bit in time (Ql) of each quadbit is used to determine the signal element amplitude to be transmitted. The second (Q2), third (Q3) and fourth (44) bits are encoded as a phase change relative to the phase of the immediately preceding signal eleme
27、nt (see Table 1). 6.2 The absolute amplitude of the transmitted signal element is detennined by the first bit (Ql) of the quadbit irrespective of the value of the absolute phase of the signal element (see Table 2). 6.3 The initial establishing of the absolute phase is not required. 6.4 The absolute
28、phase diagram of transmitted signal elements at 9600 bit/s must conform to Figure 1. 7 7.1 The length of the working bit is 25.417 mc, the guard interval is 2.5 ms. 7.2 signalling rate (9600 bit/s), the single error correction should be applied. Data signalling and modulation rates The data signalli
29、ng rate shall be 9600 bids 4 0.01%. The modulation rate is 39.34 bauds in each subchannel. The line signalling in the mode is 9914.753 bids. Due to the difference between the line signalling and the data 2 Recommendation R.21 (08/96) STD-ITU-T RECMN R-21-ENGL 199b LiBb259L Ob24130 Li20 D 7.3 informa
30、tion part of the pattern = 244 bits; the number of redundancy bits = 8. The Hamming code with the following parameter is used: the total length of the coded word = 252 bits; the TABLE 1R.21 Q2 43 I Q4 Phase change (Note) O“ 45“ 90“ 135“ i 80“ 225“ 270 315“ NOTE - The phase change is the actual phase
31、 shift in the transition region from one signalling element to the following signalling element. TABLE 2R.21 I Phase change I Absolute amplitude 1 QI I O“, 45“, 90, 135“, 180“, 225O, 270“, 315 0.567 or 1 O or 1 I 90“ *. -. -. - -.- 1 /; 0.567 *. . *. 45“ t -9. . 8 . . 8 . 1 l,;r * /* /* I-.- .=3 1 5
32、“ *.- *.- I,- 270“ T1402450961d01 FIGURE 1R.21 Signal space diagram at 9600 bis Recommendation R21 (08/96) 3 - STD-ITU-T RECMN R.2L-ENGL 177b 48b257L Ob24131 3b7 8 Received signal conditions 8.1 M.1025 and over circuits of lower quaiity at the discretion of the Administration concerned. The modem mu
33、st operate on special quality leased circuits conforming to Recommendations M.1020 and 8.2 In most cases the telephone type bearer circuits conforming to Recommendation H.22 could be used. 8.3 When the modem is operating on a composite channel where the number of retransmit sections is more than two
34、, the automatic “rough” non-adaptive correction of the amplitude/fiequency (MC) and signal transfer delay/fiequency (STD) characteristics of the bearer circuits should be used. The tolerances on STD and AFC within the working fiequency range of the modem should be f 0.5 ms and * 1 dB respectively. 8
35、.4 subject for further study. The selection of necessary testing signals for automatic line distortion correction in the bearer circuit is the 9 Interchange circuits 9.1 Table 3 lists the V.24 circuits which apply. List of interchange circuits (Table 3) TABLE 3R.21 Interchange circuits 1- No. 102 10
36、3 104 109 113 115 141 142 Description Signal ground or common return Transmitted data Received data Data channel received line signal detector Transmitter signal element timing Receiver signal element timing (DTE source) Local loopback (optional) Test indicator (optional) 9.2 Threshold of circuit 10
37、9 - - greater than -26 dBm: circuit 109 ON; less than -31 dBm: circuit 109 OFF. 9.3 dation V. 1 O). The electrical characteristics of the interchange c.-cuits comply with Recommendation V.28 (Recommen- 10 Synchronization 10.1 used for this purpose. 10.2 The use of the special synchronizing signals i
38、n the modem is not provided. Only working signals should be The detection accuracy of the significant modulation transition shall be 1-2% of the unit element length. 4 Recommendation R.21 (08/96) STD-ITU-T RECMN RaZL-ENGL 197b m 48b2593 Ob24332 273 W 10.3 10.4 interruptions in the bearer circuit. Th
39、ese values are subject to further study. The retiming period shall not exceed 0.6-1.0 s with the probability of 0.9. The synchronization period should be not less than 10 s in the presence of 0.1-0.3 ms impulse noise and short 11 The following information is provided to assist equipment manufacturer
40、s. Special measures for raising modem immunity 11.1 To improve immunity in the presence of interference and fiequency shift distortions in the bearer circuits, it is necessary to multiply the receiving signal by the auxiliary reference signal of special form. The characteristics of this reference si
41、gnal are for further study. Improvement of frequency selectivity in the receiver subchannels 11.2 To improve the modem immunity in the presence of AFC irregularity, it is necessary to redistribute the power levels between subchannels. In this way the proper predistortion coefficients should be intro
42、duced for all subchannels on the sending side. Power level redistribution between subchannels 11.3 To improve the modem immunity, the structure redundancy of subchannels should be predetermined. These subchannels could be used as an alternative for replacing the “low” quality subchannels. The criter
43、ia of subchannel selection is for further study. The use of signal structure redundancy 12 Testing Facilities Optionally, test loop 3, as specified in Recommendation V.54, could be provided. Recommendation R21 (08/96) 5 - STDOITU-T RECMN R-21-ENGL Lb qBbt591 ObZLi133 13T m Series A Series B Series C
44、 Series D Series E Series F Series G Series H Series I Series J Series K Series L Series M Series N Series O Series P Series Q Series R Series S Series T Series U Series V Series X Series Z ITU-T RECOMMENDATIONS SERIES Organization of the work of the ITU-T Means of expression General telecommunicati
45、on statistics General tariff principles Telephone network and ISDN Non-telephone telecommunication services Transmission systems and media Transmission of non-telephone signals Integrated services digital network Transmission of sound-programme and television signals Protection against interference
46、Construction, installation and protection of cables and other elements of outside plant Maintenance: international transmission systems, telephone circuits, telegraphy, facsimile and leased circuits Maintenance: international sound-programme and television transmission circuits Specifications of mea
47、suring equipment Telephone transmission quality Switching and signalling Telegraph transmission Telegraph services terminal equipment Terminal equipments and protocols for telematic services Telegraph switching Data communication over the telephone network Data networks and open system communication Programming languages Printed in Switzerland Geneva, 1997
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