1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU).4%2.!4)/.!,G0G0!.!,/5%G0G0#!22)%2G0G03934%-3).$)6)$5!,G0G0#(!2!#4%2)34)#3G0G0/ the CCITT has therefore defined an 18 MHz systemwhich offers a transmitting capacity of 3600 telephone channels in the case of pu
2、re telephone application. Alternatively, thesystem may be used for the transmission of up to two TV channels or one TV signal plus 1800 telephone channels. Anotherpossibility is that the bandwidth above 12 435 kHz could be used for the provision of an 8448 kbit/s digital path.1 Arrangement of line f
3、requencies for telephonyThe arrangement of line frequencies most suitable for the network of a particular Administration depends to a highdegree on the organization of this network with respect to the interconnection with and through connection to the othersystems existing in this network. On the ot
4、her hand, it is very desirable to limit the number of different frequency plans for the18 MHz system.The CCITT therefore recommends that in any case one of the following three plans should be applied. However, ininternational connections between countries which use different modulation procedures (s
5、ee Recommendation G.211) and inthe absence of any special arrangements between the interested Administrations including, if necessary, the Administrations oftransit countries, Plan 1 is to be preferred.1.1 Frequency arrangement of Plan 1Plan 1 uses the first modulation procedure described in Recomme
6、ndation G.211.The telephone channels should first be assembled into basic supermastergroups. The four supermastergroups aretransmitted to line in accordance with the frequency arrangement of Figure 1/G.334.Note - The arrangement of the supermastergroups No. 1, 2 and 3 is the same as in Plan 1A of th
7、e 12-MHz system(Recommendation G.332) and supermastergroup No. 4 corresponds to its arrangement in Plan 1 of the 60-MHz system(Recommendation G.333).1.2 Frequency arrangement of Plan 2This Plan uses the second modulation procedure described in Recommendation G.211.The telephone channels should first
8、 be assembled into basic (No. 1) 15-supergroup assemblies. Four 15-supergroupassemblies are transmitted to line in accordance with the frequency arrangement shown in Figure 2/G.334.Note - The arrangement of the 15-supergroup assemblies Nos. 1, 2 and 3 is the same as in Plan 2 of the 12 -MHzsystem (R
9、ecommendation G.332).2 Fascicle III.2 - Rec. G.334FIGURE 1/G.334Plan 1 frequency arrangement for 18 MHz systemsFIGURE 2/G.334Plan 2 frequency arrangement for 18 MHz systems1.3 Frequency arrangement of Plan 3This Plan uses the first modulation procedure described in Recommendation G.211, but adds a f
10、urther intermediatefrequency position.The telephone channels should first be assembled into basic supermastergroups. The four supermastergroups are thentranslated into the position of the supermastergroups Nos. 6-9 as in Plan 1 of the 60-MHz system (Recommendation G.333).Fascicle III.2 - Rec. G.334
11、3By translating with an additional 40 480 kHz carrier frequency, these supermastergroups are transmitted to line inaccordance with the frequency arrangement of Figure 3/G.334.Note 1 - This arrangement is best suited to those networks which need frequent direct through-connections betweenthe 18-MHz a
12、nd 60-MHz systems. It therefore makes use of a wider frequency band for through-connection than the basicsupermastergroup. The arrangement is also suitable for the interconnection of 18-MHz systems and for the interconnectionbetween 18-MHz systems and 60-MHz systems via the basic supermastergroup 85
13、16-12 388 kHz, because the relatively largefrequency space between the supermastergroups permits the use of simpler through supermastergroup filters.Note 2 - This arrangement can handle also 15-supergroup assemblies by bringing them first into the frequency bandof the basic supermastergroup (15-supe
14、rgroup assembly No. 3).FIGURE 3/G.334Plan 3 frequency arrangement for 18 MHz systems2 Pilots and additional measuring frequencies2.1 Line-regulating pilotsIt is recommended that 18 480 kHz be used for the main line regulating pilot1).In any regulated-line section crossing a frontier, it is recommend
15、ed that in both directions of transmission theAdministration on the sending side should, if requested, permanently transmit an auxiliary line-regulating pilot at 308 kHz toprovide facilities for additional regulation, for example.For Frequency Plans 1 and 2 as defined under 1 above, 4287 kHz and/or
16、12 435 kHz may be used as additionalauxiliary line-regulating pilots on request of the Administration on the receiving side.The frequency accuracy recommended for the pilots is 1 10-5.The power level of the main and auxiliary line-regulating pilots should be adjusted at the point of injection to hav
17、e avalue of -10 dBm0. The harmonics of the 308 kHz and 4287 kHz pilots should each have a level not greater than -70 dBm0._1)18 480 kHz is a multiple of 308 kHz (60 308) and of 440 kHz (42 440).4 Fascicle III.2 - Rec. G.334Equipment should be designed in such a way that these pilots may be blocked a
18、t the end of a regulated-line section, sothat their level shall be at least 40 dB below that of the pilots used on other sections.The following tolerances for the level of these pilots are recommended:2.1.1 The design of equipment should be such as to allow the error in the level of any pilot as tra
19、nsmitted, due to finitelevel adjustment steps, to be kept within 0.1 dB.2.1.2 The change in output level of the pilot generator with time (which is a factor included in equipment specifications)must not exceed 0.3 dB during the interval between two maintenance adjustments, e.g. in one month.2.1.3 To
20、 reduce pilot level variations with time, it is advisable to have a device to give an alarm when the variation at thegenerator output exceeds 0.5 dB, the zero of the warning device being aligned as accurately as possible with the lining-uplevel of the transmitted pilot.2.2 Frequency comparison pilot
21、sAdministrations wishing to make an international frequency comparison shall choose the frequency 300, 308 or (forPlans 1 and 2 only) 4200 kHz for this purpose. International comparison of national standards is relatively rare. During aspecified period of time, it will always be possible to use for
22、such comparisons one of the frequencies mentioned above, eventhough it may normally be used for other purposes.It is recommended that the frequency comparison pilot be transmitted at a power level of -10 dBm0. The harmonicsof the frequency comparison pilots should each have a level not higher than -
23、70 dBm0.2.3 Additional measuring frequenciesFrequencies that may be used as additional measuring frequencies are given in Table 1/G.334.The absolute frequency variation of additional measuring frequencies below 4 MHz should never be outside limits of 40 Hz from their nominal value. For frequencies a
24、bove 4 MHz, the relative frequency variation referred to the nominal valueshould never exceed 1 10-5.The power level of the additional measuring frequencies should be adjusted at the point of injection to have a valueof -10 dBm0. The harmonics of the additional measure frequencies below 9 MHz should
25、 each have a level not higher than-70 dBm0 as transmitted to the line. The additional measuring frequencies should not be permanently transmitted. They willonly be transmitted for as long as is necessary for actual measurement purposes.Arrangements should be made in equipment for the 12-MHz system,
26、so that the 308 kHz line-regulating pilot isprotected from disturbances from a pilot or additional measuring frequency of the same frequency coming from a 4-MHzsystem when this protection is not already provided by the equipment of the 4-MHz system.Note - Some Administrations use new manual or autom
27、atic methods of equalizing attenuation distortion, e.g.equalizers based on the Cosine function, using frequencies which do not appear in the list of additional measuring frequenciesrecommended by the CCITT.Obviously no additional measuring frequency which might leave the national network should be s
28、ent at the samefrequency as one of the pilots recommended by the CCITT.Fascicle III.2 - Rec. G.334 5TABLE 1/G.334Frequency plan1Frequency plan2 Frequency plan3(kHz)(see Note 1)(kHz)(see Note 2)(kHz)(kHz)1 5922 9125 6086 9288 248 (see Note 3)8 4729 79211 1125608081 3041 8002 2962 7923 5365 3927 1288
29、2488 4728 8649 60811 34410561 5522 0482 5443 0403 2883 7845521 8723 1924 7586 2727 5929 15810 67212 67814 40815 72812 77613 45214 94016 67611 99213 55815 07216 392Note 1 - Additional measuring frequencies to be sent or measured on request.Note 2 - Other additional measuring frequencies which can be
30、sent.Note 3 - A frequency of 8248 kHz can be used as a radio-relay link line-regulating pilot. In such a case, theprecautions shown in Recommendation G.423 should be applied.3 Hypothetical reference circuit3.1 General considerationsThe hypothetical reference circuit is 2500 km long and is divided in
31、to nine homogeneous sections of 280 km each.3.2 ModulationThe three line-frequency allocations recommended in 1 above need different numbers of modulating stages to bringan audio signal into the line-frequency position. This has to be reflected in the constitution of the hypothetical referencecircui
32、t.On the above basis, the hypothetical reference circuits, as shown in Figure 4/G.334 and Figure 5/G.334, arerecommended by the CCITT.6 Fascicle III.2 - Rec. G.334FIGURE 4/G.334Diagram of a hypothetical reference circuitfor 18 MHz systems (Plan 1 and Plan 3)FIGURE 5/G.334Diagram of a hypothetical re
33、ference circuitfor 18 MHz systems (Plan 2)3.2.1 Hypothetical reference circuit for the Plan 1 frequency allocationThis is shown in Figure 4/G.334. It has, for each direction of transmission, a total of:- two pairs of channel modulators, each pair including translation from the audio-frequency band t
34、o the basicgroup and vice versa;- three pairs of group modulators, each pair including translation from the basic group to the basic supergroup andvice versa;- five pairs of supergroup modulators, each pair including translation from the basic supergroup to the basicmastergroup and vice versa;- seve
35、n pairs of mastergroup modulators, each pair including translation from basic mastergroup to the basicsupermastergroup and vice versa;- nine pairs of supermastergroup modulators, each pair including translation from basic supermastergroup to thefrequency band transmitted on the coaxial cable and vic
36、e versa.3.2.2 Hypothetical reference circuit for the Plan 2 frequency allocationThis is shown in Figure 5/G.334. It has, for each direction of transmission, a total of:- two pairs of channel modulators, each pair including translation from the audio-frequency band to the basicgroup and vice versa;-
37、three pairs of group modulators, each pair including translation from the basic group to the basic supergroup andvice versa;- six pairs of supergroup modulators, each pair including translation from the basic supergroup to the basic15-supergroup assembly and vice versa;- nine pairs of 15-supergroup
38、assembly modulators, each pair including translation from the basic 15-supergroupassembly to the frequency band transmitted on the coaxial cable and vice versa.Fascicle III.2 - Rec. G.334 73.2.3 Hypothetical reference circuit for the Plan 3 frequency allocationThis is shown in Figure 4/G.334. It dif
39、fers from that for Plan 1 only by the fact that the supermastergroup modulatorsconsist of two translating stages.4 Circuit noiseIn accordance with Recommendation G.222 the system is to be designed in such a way as to obtain a meanpsophometric noise power of 3 pW0p per km of line or less as a design
40、objective for the worst telephone channel in the2500-km hypothetical reference circuit as defined under 3 above.5 Matching of repeater and line impedancesThe present Recommendation refers only to 18-MHz systems on 2.6/9.5-mm coaxial pairs in which the nominalspacing between repeaters is approximatel
41、y 4.5 km.The sum N of the three terms defined as in G.332, 5 must in this case be equal to at least 48 dB at 300 kHz and to atleast 55 dB at all frequencies above 800 kHz. Between 300 and 800 kHz the permissible limit in decibels varies linearly withthe frequency.6 Relative levelsLevels in the main
42、station (see Recommendation G.213).When one part of the frequency band is transmitted without demodulation, the same value of -33 dBr is recommendedat the output of the direct through-connection filter.7 Power feedingRecommendation G.341, 7.1 and 7.2, applies.8 Monitoring and fault tracing bandsFreq
43、uency bands for monitoring and fault tracing signals should be situated below 300 kHz and/or above 18 480 kHz,that is, leaving a clear band for traffic signals.9 Use of 18-MHz systems for television transmission9.1 General remarksIn 9 all additional requirements are summarized which are recommended
44、in the case of television transmission onthe 18-MHz system. The characteristics of the television signal in the first intermediate frequency allocation (transmit sideconditions) are dealt with in Recommendation J.77 1.9.2 Circuit noiseIf the 18-MHz system is used for television transmission on the b
45、asis of a hypothetical reference circuit of a length of2500 km, the mean value of the thermal noise of the line should not exceed 1 pW0p/km. Experience has shown that a meanvalue of 1.5 pW0p/km total noise of the line is sufficient when measured according to normal telephone conditions.9.3 Matching
46、of repeater impedances and line impedanceFor television programme transmission a value of at least 70 dB for the magnitude N, defined inRecommendation G.332 5, is recommended in the band occupied by television signals.9.4 Line-frequency allocation of the television channels8 Fascicle III.2 - Rec. G.
47、3349.4.1 TV transmission onlyThe 18-MHz system can provide two television channels. The line-frequency allocation is shown in Figure 6/G.334.The television channels are capable of transmitting the signals of all television systems defined by the CCIR having a videobandwidth not exceeding 6 MHz.Note
48、1 - Two recommended modulating methods are shown in Annex A.Note 2 - A television channel-pair pilot can be provided at the mean of the two carrier frequencies, i.e. 9570 kHz(3 3190 kHz). It is recommended that this pilot be transmitted at a power level of -10 dBm0. The harmonics should have alevel
49、of not higher than -50 dBm0.FIGURE 6/G.334Line-frequency allocation of two television channelson the 18 MHz system9.4.2 Mixed telephone-television transmissionOne television channel and a maximum of two 900-channel groups can be provided. Two line-frequency allocationsare possible:a) the upper television channel 2* of Figure 6/G.334;b) the lowest television channel (TV channel No. 1) of the 60-MHz television line-frequency allocation ofFigure 4/G.333.Note 1 - The modulation methods for a) and b) conform to the first modulation steps of F
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