1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 - TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU-!).4%.!.#% ).4%2.!4)/.!,G0G04%,%2!0(G0G03934%-3!.$G0G00(/4/4%,%2!0(G0G042!.3-)33)/.3%44).G0G050G0G0!.$G0G0,).).G0G050G0G0!.).4%2.!4)/.!,G0G06/)#%G13 Type II are those links which contain one or more 3-kHz se
2、ctions, or contain a mixture of 3-kHz and4 -kHz sections; Type III are those links which are routed over audio-frequency line plant.3.2 The method to be used and the procedure to be followed in setting up and lining up a voice-frequencytelegraph link are the same as those given in Recommendation M.5
3、80 4 for public telephone circuits as far as it applies.The test signals to be used for these three types of link and the limits of the loss/frequency characteristics atintermediate sub-control stations are the same as those given in Recommendation M.580 4 for public telephonecircuits.3.3 The overal
4、l loss/frequency characteristics of types I, II and III voice-frequency telegraph links are given inTables 1/M.810, 2/M.810 and 3/M.810 respectively.3.4 The nominal relative power level of the test signals at the input and output of the link will be those normallyused by the Administration concerned
5、.If the voice-frequency telegraph terminal stations are remote from the terminal international centres, theAdministration should arrange the nominal transmission loss of the national section so that the levels at the input andoutput of the voice-frequency telegraph link are respected, and to permit
6、the conventional national levels to be used atterminal international centres.3.5 For voice-frequency telegraphy the use of the edge-channels of a group should be avoided if at all possiblesince these may introduce greater distortion than other channels of the group.4 Limits for the overall loss of a
7、 voice-frequency telegraph link4.1 Nominal overall loss at 1020 HzThe nominal relative power levels at the extremities of the voice-frequency telegraph link are those levelsnormally used in the national network of the countries concerned so that it is not possible to recommend a particularnominal va
8、lue for the overall loss.2 Fascicle IV.2 - Rec. M.810The nominal relative power level at the input to the link and the absolute power level of the telegraph signals atthis point must be such that the limits concerning the power level per telegraph channel at a zero relative point on carriersystems a
9、re respected (see Annex A).Some Administrations have bilateral agreements to reduce the total mean power level of frequency-shiftvoice-frequency telegraph systems to 13 dBm0 (50 W0). The CCITT encourages such reduction where feasible.These Administrations have made their own determination of the fea
10、sibility of operating at the reduced level. As a guide,other Administrations may wish to use the line parameters suggested in Annex B.4.2 Overall loss/frequency distortionThe variation with frequency of the overall loss of the link with respect to the loss at 1020 Hz must not exceedthe following lim
11、its:4.2.1 Type I Links with 4-kHz sections throughoutTABLE 1/M.8104.2.2 Type II Links with one or more 3-kHz sections or with a mixture of 3-kHz and 4-kHz sectionsTABLE 2/M.810Frequency range (Hz) Overall loss relative to that at 1020 HzBelow 300 Not less than 2.2 dB; otherwise unspecified300- 400 2
12、.2 to +4.0 dB400- 600 2.2 to +3.0 dB600-3000 2.2 to +2.2 dB3000-3200 2.2 to +3.0 dB3200-3400 2.2 to +7.0 dBAbove 3400 Not less than 2.2 dB; otherwise unspecifiedFrequency range (Hz) Overall loss relative to that at 1020 HzBelow 300 Not less than 2.2 dB; otherwise unspecified300- 400 2.2 to + 4.0 dB4
13、00- 600 2.2 to +3.0 dB600-2700 2.2 to +2.2 dB2700-2900 2.2 to +3.0 dB2900-3050 2.2 to + 6.5 dBAbove 3050 Not less than 2.2 dB; otherwise unspecifiedFascicle IV.2 - Rec. M.810 34.2.3 Type III Links on audio-frequency line plantTABLE 3/M.8104.2.4 Application of RecommendationsFigure 1/M.810 shows, in
14、respect of loss/frequency distortion, the relationship of the Recommendationsrelating to international voice-frequency telegraph links. In practice, in the majority of cases, the international linebetween terminal international centres will be well within the limits of Recommendation M.580 4 and no
15、additionalequalization will be needed to meet the overall requirement of this Recommendation.d01-scFIGURE 1/M.810.D01Frequency range (Hz) Overall loss relative to that at 1020 HzBelow 300 Not less than 1,7 dB; otherwise unspecified300- 400 1.7 to + 4.3 dB400- 600 1.7 to +2.6 dB600-1600 1.7 to +1.7 d
16、B1600-2400 1.7 to + 4.3 dB2400-2450 1.7 to +5.2 dB2450-2520 1.7 to +7.0 dBAbove 2520 Not less than 1.7 dB; otherwise unspecified4 Fascicle IV.2 - Rec. M.8104.3 Change of overall loss due to a changeover to the reserve line or section4.3.1 The nominal relative power level at 1020 Hz of the normal and
17、 reserve lines or sections at the changeoverpoints for a particular direction of transmission should be the same. This level will be that normally used in the nationalnetwork of the country concerned.4.3.2 Change in overall loss at 1020 HzBearing in mind that the overall loss of the normal line (or
18、section) and the reserve line (or section) are bothsubject to variations with time, these variations being, in general, uncorrelated, it is not possible to assign a limit to thechange of insertion loss at 1020 Hz introduced by the changeover procedure.4.3.3 Values of overall loss over the frequency
19、band, relative to the value at 1020 HzThe overall-loss/frequency distortion characteristic of the link when established over the normal route shouldbe within 2 dB of that of the link when established over the reserve route. This limit applies over the frequency bands300-3400 Hz, 300-3050 Hz or 300-2
20、520 Hz as appropriate.There should ordinarily be no difficulty in achieving the limit when only one portion of the link, for example,the international telegraph line, or one section, has a reserve section. However, when two or more portions of the link areseparately associated with reserve portions
21、it becomes administratively difficult to ensure that all combinations ofnormal and reserve portions comply with the limit. In these circumstances the best that can be done is to ensure that theoverall-loss/frequency characteristics of corresponding normal and reserve portions are as much alike as po
22、ssible.Careful attention should be paid to the impedance of normal and reserve sections at the point where they are connectedto the changeover apparatus so that errors due to changing mismatch losses are minimized. A suitable target would befor all impedances concerned to have a return loss against
23、600 ohms, non-reactive, of not less than 20 dB over theappropriate band of frequencies.5 Measurement of noise voltage on a voice-frequency telegraph link5.1 Uniform-spectrum random noiseThe psophometrically weighted noise voltage should be measured at the ends of the voice-frequency telegraphlink in
24、 both directions of transmission. The unweighted noise voltage should also be measured using aCCITT psophometer without the weighting network.The mean psophometric noise power referred to a point of zero relative level should not exceed 80 000 pW (41 dBm0p).Note 1 If recourse is made to synchronous
25、operation, a higher noise level might be tolerated (such as 30 dBm0p for a particular telegraph system).Note 2 In principle it would be desirable to specify a value of unweighted noise power level. However, sucha value cannot be specified in unqualified terms. If the noise power is uniformly distrib
26、uted over the band 300-3400 Hzand if there is no significant noise power outside this band then the level of the unweighted noise power will beapproximately 2.5 dB higher than the value of the weighted noise power, using the weighting filters specified inRecommendation O.41 5. However, on a practica
27、l telegraph link neither of these conditions is likely to be met. Theoverall-loss/frequency distortion will affect the within-band noise distribution and, in a telegraph installation, there islikely to be significant noise power outside the band, particularly at low frequencies.As a consequence, it
28、is not possible to recommend a limit for the unweighted noise power level and theCCITT psophometer with the telephone weighting networks should continue to be the instrument used for specifyingand measuring random noise power levels on international voice-frequency telegraph links.Fascicle IV.2 - Re
29、c. M.810 55.2 Impulsive noiseImpulsive noise should be measured with an instrument complying with Recommendation O.71 6 andH.13 7. (See also Recommendation V.55 8.)The number of counts of impulsive noise which exceeds 18 dBm0 should not exceed 18 in 15 minutes.6 Crosstalk6.1 The near-end crosstalk r
30、atio (between the go and return telephone channels) of the link should be at least43 dB.6.2 The crosstalk ratio between the link and other carrier circuits is restricted by the Recommendation cited in 9to not worse than 58 dB.Crosstalk in any audio cables forming part of the terminal national sectio
31、ns should not normally significantlyworsen the crosstalk ratio.7 Group-delay/frequency distortionPractical experience obtained up to the present shows that it is not necessary to recommend limits forgroup-delay/frequency distortion for 50-baud voice-frequency telegraph links even when they are compo
32、sed of severalsections each provided on telephone channels of carrier systems. There is little practical experience with higher-speedtelegraph systems.It may happen that under adverse conditions some telephone channels of the link are of insufficient quality toprovide 24 telegraph channels. In such
33、a case a better combination of telephone channels must be chosen for thetelegraph service.8 Frequency errorThe frequency error introduced by the link must not be greater than 2 Hz.9 Interference caused by power supply sourcesWhen a sinusoidal test signal is transmitted over the link at a level of 0
34、dBm0 the level of the strongestunwanted side component should not exceed 45 dBm0.Note The limit of 45 dBm0 is based on a test-signal level of 0 dBm0, and this level should be used whenmaking this test.10 Variation of overall loss with time10.1 Before a voice-frequency telegraph link is placed into s
35、ervice it is desirable that a test signal in each directionof transmission should be monitored at the distant end with a level-recording instrument for a minimum of 24 hours.Where possible the instrument should be capable of detecting level variations of duration as short as 5 ms.10.1.1 The differen
36、ce between the mean value and the nominal value of the overall transmission loss should notexceed 0.5 dB.10.1.2 The standard deviation about the mean value should not exceed 1.0 dB.However in the case of circuits set up wholly or partly on older-type equipment, and where the internationalline consis
37、ts of two or more circuit sections, a standard deviation not exceeding 1.5 dB may be accepted.11 Amplitude hits, short interruptions in transmission and phase hitsSuch impairments to voice-frequency telegraph links reduce the quality of telegraph transmission. Forexample, phase hits in excess of 110
38、 will cause errors in telegraph transmission. Amplitude hits, short interruptions intransmission and phase hits should be reduced to the minimum possible, bearing in mind the need to meet the error rateobjectives given in Recommendations F.10 10 and R.54 11.6 Fascicle IV.2 - Rec. M.81012 Record of r
39、esultsAll measurements made during the lining-up of the link are reference measurements and should be carefullyrecorded and a copy sent by the sub-control stations to the control station in accordance with Recommenda-tion M.570 12.13 Information concerning voice-frequency telegraph terminal equipmen
40、tInformation concerning international voice-frequency telegraphy is given in Annex A and Annex B.14 Marking of circuits used for voice-frequency telegraphyAny interruption of a voice-frequency telegraph link, even of very short duration, spoils the quality of thetelegraph transmission. It is therefo
41、re desirable to take great care when making measurements on circuits used forvoice-frequency telegraphy. To draw the attention of staff to this, all equipments used for voice telegraphy links shouldbear a special identification mark in the terminal exchanges and, where necessary, in repeater station
42、s where the circuitsare accessible.ANNEX A(to Recommendation M.810)Basic characteristics of telegraph equipments usedin international voice-frequency telegraph systemsA.1 Limiting power per channelA.1.1 Amplitude-modulated voice-frequency telegraph systems at 50 baudsAdministrations will be able to
43、provide the telegraph services with carrier telephone channels permitting theuse of 24 voice-frequency telegraph channels (each capable of 50 bauds) on condition that the power of the telegraphchannel signal on each channel, when a continuous marking signal is transmitted, does not exceed 9 microwat
44、ts at zerorelative level points.For 18 telegraph channels only, the power so defined may be increased to 15 microwatts per telegraph channel,so that even telephone channels with a relatively high noise level can then be used.The power per telegraph channel should never exceed 35 microwatts, however
45、few channels there may be.These limits are summarized in Table A-1/M.810.TABLE A-1/M.810Limiting power per telegraph channel when sending a continuous marking signalin amplitude-modulated voice-frequency telegraph systems at 50 baudsSystemLimiting power per telegraph channel whensending a continuous
46、 marking signalW0 dBm012 telegraph channels or less 35 14.518 telegraph channels 15 18.324 (or 22) telegraph channels 9 20.5Fascicle IV.2 - Rec. M.810 7A.1.2 Frequency-shift voice-frequency telegraph systems at 50 baudsThe total average power transmitted to the telephone-type circuit is normally dep
47、endent on the transmissioncharacteristics and length of the circuit as follows:a) For circuits with characteristics not exceeding the limits given in Annex B, the total average powertransmitted by all channels of a system should preferably be limited to 50 microwatts at a point of zerorelative level
48、. This sets, for the average power of a telegraph channel (at a point of zero relative level), thelimits given in Table A-2/M.810.b) For other circuits, the total average power transmitted by all channels of a system is limited to135 microwatts at a point of zero relative level. This sets, for the a
49、verage power of a telegraph channel(at a point of zero relative level), the limits given in Table A-3/M.810.Note The values in Tables A-2/M.810 and A-3/M.810 assume the provision of a pilot channel on thetelegraph bearer.TABLE A-2/M.810Normal limits for the power for telegraph channel in FMVFT systemsfor bearer circuits with caracteristics not exceeding the limits given in Annex BTABLE A-3/M.810Normal limits for the power per telegraph channelin FMVFT systems for other bearer circuitsNumber oftelegraph channels
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