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/ further amended)Preliminary noteThe present Recommendation describes a 6 MHz system which may be used for transmitting a maximum
2、of1,260 telephone channels.A system of this kind can be produced by halving the length of the elementary cable section of a 1.3 MHzsystem (as described in Recommendation G.341) if this length is 6 km, corresponding to a nominal repeater spacing of3 km for the 6 MHz system.1 Line frequenciesThe CCITT
3、 recommends the three frequency allocation plans in Figure 1/G.344, each plan forming a wholewithin the line-frequency band.Plans 1 and 2 show the supergroup allocations and Plan 3 the mastergroup allocations.In Plan 1, the supergroups are assembled by means of carriers produced from a single freque
4、ncy at 124 kHz.There are two possible methods of assembling the supergroups in the band 4404 to 5636 kHz. The first is to usecarriers at 4092, 4340, 4588, 4836 and 5084 kHz and to keep the upper modulation band (the first two frequenciesbeing the carriers corresponding to supergroups 15 and 16). The
5、 second method is to translate the assembly ofsupergroups 4 to 8, which are those of the basic mastergroup, using a carrier frequency of 6448 kHz obtained bymultiplying by 4 the carrier frequency of 1612 kHz corresponding to supergroup 5.In Plan 2, the five supergroups reversed in band 4332 to 5564
6、kHz correspond to mastergroup 4 of the12 MHz line allocation, but they also represent a plan conveniently obtained with supergroup and group carrierfrequencies.Plan 3 consists of mastergroups 1-4 of the 12 MHz system (see 1 of Recommendation G.332).2 Fascicle III.2 - Rec. G.344FIGURE 1/G.344Allocati
7、on of line frequencies in international 6 MHz carrier system on 1.2/4.4 mm coaxial cable pairs2 Pilots and additional measuring frequencies2.1 Line-regulating pilotsThe frequencies recommended are 308 kHz on the one hand, and 4287 kHz or 6200 kHz on the other.Note - The pilot at 4287 kHz cannot be u
8、sed with television transmissions.In every instance, the recommended stability is 1 10-5, the power level recommended is -10 dBm0, whilethe tolerances at this level are the same as in Recommendation G.332, 2.1. The harmonics of the 308 kHz pilotshould each have a level not higher than -70 dBm0.2.2 F
9、requency comparison pilotsPlans 1 and 2 - The same recommendation as for the 4 MHz system (Recommendation G.343, 2.2).Plan 3 - The same recommendations as for the 12 MHz system (Recommendation G.332, 2.2).Fascicle III.2 - Rec. G.344 32.3 Additional measuring frequenciesPlans 1 and 2 - All the additi
10、onal measuring frequencies given in Recommendation G.343 (supergroups)should be used. In addition, in the frequency band above 4287 kHz, the following additional measuring frequencies arerecommended:- Plan 1: 5680 kHz,- Plan 2: 5608 kHz.However, the harmonics of the additional measuring frequencies
11、below 2.8 MHz should comply with therelevant conditions indicated in Recommendation G.343.Plan 3 - The additional measuring frequencies recommended for the 12 MHz system in the same frequencyband (Recommendation G.332) should be used.3 Hypothetical reference circuitsSame Recommendations as for the 4
12、 MHz system (see 3 of Recommendation G.343).4 NoiseRecommendation G.341, 4 applies.5 Matching of the coaxial-pair impedance and repeater impedancesFor an elementary cable section about 3 km in length the sum N of the three terms defined inRecommendation G.332, 5) must be at least equal to 60 dB at a
13、ll frequencies above 300 kHz.A figure of 50 dB is recommended at 60 kHz. Between 60 and 300 kHz the acceptable limit variesprogressively.6 Relative levels and interconnection6.1 Relative levels at repeater output at 4287 MHz:Approximately -17 dBr.6.2 Pre-emphasis characteristicsIt has not been possi
14、ble to reach agreement for recommending a pre-emphasis characteristic applicable to allcases. The pre-emphasis used in practice varies between 7 and 14 dB. Some Administrations use a pre-emphasischaracteristic corresponding to the formula:For the constants, a, b and fr , the following figures may be indicated:1) a = 10 b = 2.20 fr= 5.75 MHz2) a = 24 b = 8.50 fr= 6.40 MHz7 InterconnectionInterconnection should be in conformity with Recommendation G.352.8 Power-feeding and alarm systemsRecommendation G.341 also applies to systems conforming to the present Recommendation.
