1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU).4%2.!4)/.!,G0G0!.!,/5%G0G0#!22)%2G0G03934%-3%.%2!,G0G0#(!2!#4%2)34)#3G0G0#/-/.G0G04/G0G0!,!.!,/5%G0G0#!22)%2G0G13G042!.3-)33)/.G0G03934%-30),/43G0G0/.G0G02/503 G0G0350%22/503 G0G0%4# )45G134G0G02ECOMMENDATION
2、G0G0 (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation G.241 was published in Fascicle III.2 of the Blue Book. This file is an extract fromthe Blue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, thecontents of the file are identical
3、 to the Blue Book version and copyright conditions remain unchanged (see below).2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1988, 1993All rights reserved. No part of this p
4、ublication may be reproduced or utilized in any form or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle III.2 - Rec. G.241 1Recommendation G.241Fascicle III.2 - Rec. G.241PILOTS ON GROUPS, SUPERGROUPS, ETC.(amended at G
5、eneva, 1964; further amended)1 Use of pilotsExperience has shown that, without the use of a group pilot transmitted throughout a group link, adequatestability of the channels of individual group links cannot be guaranteed in spite of the care given to the maintenance ofthe carrier systems on which t
6、hey are routed.It may be necessary, in the first place, to place an automatic regulator, controlled by the pilot, at the end of someof the group sections forming the group link to compensate for inevitable variations in attenuation on each of thesections. This regulator is not, of course, designed t
7、o correct automatically for faults.It is desirable for the regulator to have a range of at least 4 dB. While no maximum range is specified, noteshould be taken that too great a range can prove unsatisfactory, e.g. due to noise or the masking of faults. A maximumrange of approximately 7 dB has been f
8、ound satisfactory by some Administrations.An alarm should be given when the amplitude of the pilot at the input of the regulator departs from its nominalvalue by more than 4 dB. The conditions governing the use of these regulators are given in Recommendation M.1601.It is also necessary to provide fo
9、r measuring the level of the group pilot at the ends of group sections where it isnot planned to use a regulator. In these cases, too, an alarm should be given when the level of the pilot departs from itsnominal value by more than 4 dB.Precisely similar considerations apply to the use of supergroup,
10、 mastergroup and supermastergroup pilots, andalso to the use of basic 15-supergroup assembly pilots.Note - When a group is through-connected from a cable section (on coaxial or symmetric pairs) to an open-wireline, transmission of the group pilot over the open-wire line, which is an advantage as reg
11、ards maintenance of thecomplete group can, to a certain extent, facilitate “tapping“ of conversations by means of radio receivers of a particulartype in the territory traversed by the open-wire line. However, this risk of “tapping“ is less than the similar risk arisingfrom inadequate suppression of
12、the carrier, because the frequency of the group pilot is more remote from the nearbycarrier frequency, so that the quality of the overheard conversation would be necessarily degraded.2 Nominal characteristics of pilots (group, supergroup, etc.)When group, supergroup, etc., pilots are considered nece
13、ssary, they should be permanently transmitted.The frequency and the level of these pilots are shown in Table 1/G.241.2 Fascicle III.2 - Rec. G.241TABLE 1/G.241Frequency and level of pilotsa)The group pilots 84.080 and 84.140 kHz and the supergroup pilots 411.860 and 411.920 kHz are used over groups
14、andsupergroups transmitting telephone channels and, in some cases, wide spectrum signals (data, facsimile, etc.). For each group(or supergroup) the two pilots at 84.080 and 84.140 kHz (or 411.860 and 411.920 kHz) should be transmittedsimultaneously. However, only one of these two pilots need be used
15、 if there is agreement between the Administrationsconcerned (including the Administrations of transit countries).It is now apparent that transmission of wide spectrum signals (data, facsimile, etc.) may demand use of the pilots104.080 kHz and 547.920 kHz instead of those previously used. These latte
16、r pilots may also be used on groups andsupergroups carrying only telephone channels. The choice of pilots to be used is a matter of agreement between theAdministrations concerned (including the Administrations of transit countries).b)However, the use of the pilots at 104.080 and 547.920 kHz might le
17、ad to the following difficulties:1) The group pilot at 104.080 kHz is incompatible with the line pilots situated at 4 kHz from one end of a group, which areto be found in the following systems: open-wire systems using frequency allocation 1 as shown in Figure 1/G.311; symmetric-pair systems using va
18、riant B as shown in Figure 5/G.322, especially the transistorized system described inRecommendation G.323.2) If the frequency allocation in the supergroup comprises groups A-E in accordance with Figure 2c/G.322 and 3/G.322, asupergroup pilot at 547.92 kHz will appear at frequency 103.92 kHz in group
19、 A. This frequency is liable to causedifficulties when group A is used for telephony. To avoid any disturbance, it might be necessary to introduce new routingrestrictions.3) Difficulties would arise if these pilots were used on groups having terminal equipment with carrier frequency spacing of6 kHz
20、in accordance with Recommendation G.234, unless one further channel is abandoned in some groups.Note - These difficulties have already arisen in some cases with the pilots recommended at present.4) The choice of these frequencies would make it very difficult to use signalling at the virtual carrier
21、frequency of atelephone channel in conformity with Recommendation Q.21 2. However, this point (and the preceding one) can beconsidered to be of purely national interest.c)The supergroup pilot at 411.920 kHz may also be used when the supergroup contains one or more groups transmittingwideband signals
22、. It is impossible to route a group equipped with a pilot at 104.080 kHz in the position of group 3 in asupergroup with a pilot at 411.920 kHz.d)This pilot, after modulation of the 15-supergroup assembly to position No. 3 (see procedure 2 of Recommendation G.211, 1), appears at the frequency of 11 0
23、96 kHz; this is identical with the frequency of the basic supermastergroup pilot.Pilot forFrequency(kHz)Absolute power levelat a zero relative level point(dBm0)Basic groupBasic supergroupBasic mastergroupBasic supermastergroupBasic 15-supergroup assembly (No. 1)84.080a)84.140b)104.080a), b), c)411.8
24、60a)411.920a), c)547.920a), b)1 55211 0961 552d)202520252020202020Fascicle III.2 - Rec. G.241 33 Tolerances on the sent level of pilotsThe following values are recommended for the frequency accuracy of the various pilots:Pilot frequency 84.080 kHz and 411.920 kHz . 1 HzPilot frequency 84.140 kHz and
25、 411.860 kHz . 3 HzPilot frequency 104.080 kHz and 547.920 kHz . 1 HzPilot frequency 1552 kHz1). 2 HzPilot frequency 11 096 kHz 10 HzNote - These tolerances can be taken as a basis for the specifications of the associated pilot receiving filters andstop filters, allowance also being made for recomme
26、ndations concerning the accuracy of master oscillators.The following recommendations are made concerning the tolerances for the sent pilot level:1) The design of equipment should be such as to allow the sum of errors in the level of any group, etc., pilotas transmitted, due to finite level adjustmen
27、t steps, change in number of groups supplied, and lack ofadjustment facilities in individual groups, to be kept within 0.1 dB.2) The change in output level of the pilot generator with time (which is a factor included in equipmentspecifications) must not exceed 0.3 dB during the interval between two
28、maintenance adjustments, e.g. inone month.3) To reduce pilot level variations with time, it is advisable to have a device to give an alarm when thevariation at the generator output exceeds 0.5 dB, the zero of the warning device being aligned asaccurately as possible with the lining-up level of the t
29、ransmitted pilot.The attention of Administrations is drawn to the difficulty which could result from an appreciable reduction inthe absolute power level of the pilot sent to line; such a reduction is liable to cause “near singing“, resulting from theoperation of the automatic gain-control amplifiers
30、. It would be desirable to make arrangements for overcoming thisdifficulty if it should arise.4 Harmonics of pilots4.1 It is recommended that the levels of harmonics of group and supergroup pilots should not exceed the valuesgiven in Table 2/G.241. The point where these limits should be met is the d
31、istribution frame (or equivalent point) at theoutput of the next higher stage of modulation, e.g. the supergroup distribution frame in the case of the group pilot.Account should be taken of the change of frequency.TABLE 2/G.241Maximum level of harmonics of pilotsNote - If the system includes 3-kHz s
32、paced channels a maximum level of -73 dBm0 is recommended._1)This pilot, after modulation of the 15-supergroup assembly to position No. 3 (see procedure 2 of Recommendation G.211, 1) appears at the frequency 11 096 kHz; this is identical with the frequency of the basic supermastergroup pilot.PilotNo
33、minalfrequencyMaximum level of harmonicsof pilot(kHz)Secondharmonics(dBm0)Thirdharmonics(dBm0)Each higherharmonics(dBm0)GroupGroupSupergroupSupergroup84.080 or84.140104.080411.920 or411.860547.9207367(see note)756767677367(see note)757575754 Fascicle III.2 - Rec. G.2414.2 In the case of the pilot (1
34、552 kHz) for a mastergroup, it is recommended that the level of the second harmonicof the pilot should not exceed - 50 dBm0 and the level of each other higher harmonic should not exceed 75 dBm0measured at the output of the next higher stage of modulation.4.3 In the case of the pilot (11 096 kHz) for
35、 a supermastergroup, it is recommended that the level of any harmonicshould not exceed -75 dBm0 measured at the output of the next higher stage of modulation.4.4 In the case of the pilot (1552 kHz) for 15-supergroup assemblies, it is recommended that the level of thesecond harmonic should not exceed
36、 -50 dBm0, measured at the output of the supergroup translating equipment.Where the 15-supergroup assembly is not combined with other assemblies, there is no particular requirement onthe level of the third and higher harmonics.Where the 15-supergroup assembly is combined with other assemblies, the l
37、evel of the third and higher orderharmonies should not exceed -75 dBm0, measured at the combined output.5 Protection of group, supergroup, etc., pilots against interference by noiseAutomatic regulators operated by group, supergroup, etc., reference pilots should be so designed that theinterfering ef
38、fect of noise does not exceed 0.02 dB for any significant period. If, for example, the regulator operates onthe mean signal voltage, this corresponds to a long-term interfering signal of -20 dB relative to the pilot level. When theinterference is of short duration compared with the time constant of
39、the regulator, high levels of interference may beexperienced without causing an error in regulation exceeding 0.02 dB.5.1 Group and supergroup pilotsIf the pilot pick-off filter has a bandwidth of 50 Hz (25 Hz on each side of the nominal pilot frequency) the ratiobetween pilot and noise will always
40、be considerably greater than 20 dB in the case of carrier systems over land-lines.This ratio is still respected if the unweighted power of the noise in a telephone channel reaches 106pW at zero relativelevel (level of -30 dBm0), which very rarely occurs on radio-relay links conforming to the conditi
41、ons ofRecommendation G.441.In the case of very long group or supergroup links on such radio-relay links, the pilot-to-noise ratio will besmaller than 20 dB only for a period of less than some ten-thousandths of any month. In that case the resultant error inregulation will be negligible, as the durat
42、ion of the very high-level noise will be short compared with the necessarilylong time-constant of the regulator. In any case, such high-level bursts are not expected to occur with any significantfrequency and the chief factor limiting the interference caused to a pilot by noise is therefore the effe
43、ctive bandwidth ofthe pick-off filter.5.2 Other pilotsSimilar consideration applies also to mastergroup, supermastergroup and basic 15-supergroup assembly pilots.However, the bandwidth of the pick-off filter will certainly be greater than 50 Hz and more reliance will have to beplaced on the relative
44、ly long time-constant of the regulator to minimize the effect of short-duration high-level noise.Note 1 - Recommendations concerning the protection and suppression of pilots at certain points appear inRecommendation G.243.Note 2 - When use is made of procedure 1, described in Recommendation G.211, t
45、he spacing between the11 096 kHz supermastergroup pilot and the audio-frequencies transposed in the adjacent channels is 28 kHz and 60kHz.This same spacing is only 4 kHz with procedure 2, described in Recommendation G.211.In view of this, a supermastergroup regulator is not necessarily suitable for
46、the transmission of a 15-supergroupassembly over a supermastergroup link.Fascicle III.2 - Rec. G.241 56 Protection of group or supergroup pilots against signals transmitted in telephone channelsThis protection is ensured in the channel and group translating equipment, in accordance with Recommendati
47、onG.232, 12 and the Recommendation cited in 4.7 Protection of group or supergroup link pilots transmitting wide-spectrum signals7.1 To protect the group or supergroup link pilots (used to establish wideband circuits) against other wide-spectrum signals (data, facsimile, etc.), it is recommended that
48、 the power spectrum emitted about the pilot frequency belimited in the equipment which transmits these signals. This limitation is so calculated that the group or supergroupregulators installed on the link will not receive interference of more than 0.1 dB, and the values to be specified thereforedep
49、end on the characteristics of the regulators (passband of the pilot filters, regulation operating time constant).With regard to continuous spectrum signals, the spectrum density in the band f0 25 Hz must not exceed70 dBm0/Hz.The limits to be set for discrete components are fixed by the Figure 1/G.241 which allows for the existingcharacteristics of regulators activated by pilots at frequencies (f0) of 84.08 or 104.08 kHz in group links and of 411.92 or547.92 kHz in supergroup links.Such a limitation of the t
