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 two types of systems on coaxial cable pairs providing 300 te
2、lephonechannels in the approximate frequency band 0.06 to 1.3 MHz. The length of the elementary cable section is about 6 kmfor the first type of system and about 8 km for the second. The first is to be preferred when it is planned to equip thecable with 6 MHz repeaters later on, the second when it i
3、s planned to install systems belonging to the other family onthe cable later on, i.e. the 4 MHz system, 12 MHz system or 18 MHz system.1 Line frequenciesThe system will carry 300 telephony channels, transmitted to line:- either between 60 kHz and 1300 kHz as supergroups Nos. 1-5 of the 4 MHz system
4、(Figure 1 a)/G.341);- or between 64 kHz and 1296 kHz as a mastergroup with erect channel sidebands (Figure 1 b)/G.341).FIGURE 1/G.341Line-frequency arrangements for international carrier 1.3-MHz systemson 1.2/4.4-mm coaxial pair2 Pilots and additional measuring frequencies2.1 Line-regulating pilotTh
5、e CCITT recommends that 1364 kHz be used for the main line-regulating pilot on all regulated-line sections2 Fascicle III.2 - Rec. G.341crossing a frontier. The main line-regulating pilot is used for automatic correction of cable attenuation with thetemperature.In any regulated-line section crossing
6、a frontier, it is recommended that in both directions of transmission theAdministration on the transmitting side permanently transmit an auxiliary line-regulating pilot at 60 or 308 kHz, as theAdministration on the receiving side may choose, so as to provide for additional regulation, for example.Th
7、e frequency accuracy recommended for the pilots is 1 10-5.The power level of these pilots should be adjusted at the output of the transmit amplifier to have a nominalvalue of - 10 dBm0. The harmonics of the 60 and 308 kHz pilots should each have a level not higher than -70 dBm0.The tolerances for th
8、is level are the same as in Recommendation G.332, 2.1.Note - Some systems in use employ a pilot at -1.2 Nm0.2.2 Frequency-comparison pilotsFor national frequency comparison, it is recommended that a 60 or 308 kHz pilot be used. Should internationalfrequency comparison appear desirable, the Administr
9、ations concerned will reach agreement on which of these twofrequencies they will use.The power level of a frequency-comparison pilot should be adjusted at the output of the transmit amplifier, toa nominal value of -10 dBm0. The harmonics of the frequency-comparison pilots should each have a level no
10、t higherthan -70 dBm0.2.3 Additional measuring frequenciesFrequencies that can be used as additional measuring frequencies are as follows:- supergroups Nos. 1 to 5 frequency allocation: (60), (308), 556, 808, 1056, 1304 kHz;- mastergroup frequency allocation: (60), (308), 804, 1052, 1304 kHz.Note -
11、One of the two frequencies in brackets will be used for the auxiliary line-regulating pilot.The power level of these additional measuring frequencies should be adjusted, at the output of the transmitamplifier, to have a nominal value of -10 dBm0. The harmonics of the additional measuring frequencies
12、 below 650kHz should each have a level at this point not higher than -70 dBm0.Note - Some systems in use employ additional pilots at -1.2 Nm0.The additional measuring frequencies should not be permanently transmitted. They will be transmitted onlyfor as long as is necessary for actual measurement pu
13、rposes.3 Hypothetical reference circuitThe CCITT has defined two hypothetical reference circuits, one for supergroup arrangement and the other formastergroup arrangement. Both are 2500 km long and are divided into nine homogeneous sections of 280 km each.3.1 Hypothetical reference circuit used in su
14、pergroup arrangement1)This hypothetical reference circuit (see Figure 2/G.341) has, for each direction of transmission, a total of:- three pairs of channel modulators, each pair including translation from the audio-frequency band to thebasic group and vice versa;- six pairs of group modulators, each
15、 pair including translation from the basic group to the basicsupergroup_1)This hypothetical reference circuit is also used for 4 MHz and 6 MHz systems transmitting supergroups on1.2/4.4 mm coaxial pairs and for systems providing two supergroups on symmetric pairs.Fascicle III.2 - Rec. G.341 3and vic
16、e versa;- nine pairs of supergroup modulators, each pair including translation from the basic supergroup to thefrequency band transmitted on the coaxial cable and vice versa.It will be seen that there is a total of 18 modulations and 18 demodulations for each direction of transmission,assuming that
17、each modulation or demodulation is carried out in a single stage.FIGURE 2/G.341Diagram of the hypothetical reference circuit for 1.3-MHz systemson coaxial cable using supergroup arrangement3.2 Hypothetical reference circuit used in mastergroup arrangementThis hypothetical reference circuit (see Figu
18、re 3/G.341) has, for each direction of transmission, a total of:- three pairs of channel modulators, each pair including translation from the audio-frequency band to thebasic group and vice versa;- three pairs of group modulators, each pair including translation from the basic group to the basicsupe
19、rgroup and vice versa;- six pairs of supergroup modulators, each pair including translation from the basic supergroup to thefrequency band of the basic mastergroup and vice versa;- nine pairs of mastergroup modulators, each pair including translation from the basic mastergroup to thefrequency band t
20、ransmitted on the coaxial cable and vice versa.FIGURE 3/G.341Diagram of a hypothetical reference circuit for 1.3-MHz coaxial pair systemsusing mastergroup arrangement4 Circuit noiseThe general target noise values for cable systems (see Recommendation G.222) apply also to systems on1.2/4.4 mm coaxial
21、 pairs, with the conditions given in Recommendation G.223.In practice, it is sufficient to check by calculation that, for every telephone channel as defined by the relevant4 Fascicle III.2 - Rec. G.341hypothetical reference circuit, the mean psophometric power at the end of the channel, referred to
22、a zero relative levelpoint, does not exceed 10 000 pW0p during any period of one hour.5 Matching of the coaxial pair impedance and the repeater impedancesThe sum N of three terms defined in Recommendation G.332, 5 must be at least equal to:- 54 dB for a 6 km elementary cable section;- 52 dB for an 8
23、 km elementary cable section.These figures have been calculated so as to get a ripple in the attenuation/frequency characteristic notexceeding 0.8 dB at the end of a homogeneous section 280 km long. It has been assumed that the reflected currents addin phase in all the elementary cable sections of t
24、his homogeneous section (the spacing of the buried repeaters, on asmall coaxial pair, generally being very regular). In addition, it has been assumed that it is highly improbable that atelephone channel will be on more than one homogeneous section of the hypothetical reference circuit in the lower p
25、artof the band of line frequencies. At higher frequencies, N should be well above the limit.6 Relative levels and interconnection6.1 Relative levels and cabling loss for any repeater section6.1.1 The loss on any 6 km elementary cable section should be 35 dB at 1300 kHz. The relative power level at t
26、heinput of the cable section (output of the repeater equipment) should be -13 dBr at 1300 kHz. Each Administration mayso select the pre-emphasis characteristic that the level at this point and at frequency 60 kHz lies in the range -18 to -28dBr.6.1.2 The nominal loss on any 8 km elementary cable sec
27、tion should be 49 dB at 1300 kHz. The relative levels atthe input of any cable section are not strictly standardized, values of -3.5 dBr and -4.3 dBr at the top channel are beingused in connection with pre-emphasis values of 9 dB and 10 dB respectively.6.2 Frontier sectionFor interconnection between
28、 two systems using different pre-emphasis characteristics, unless there are specialarrangements between the Administrations concerned, the following recommendation will be applied:6.2.1 In a 6 km elementary cable section crossing a frontier, the level at the end of the cable section (input of therep
29、eater equipment) should be equal to -48 dBr at 1300 kHz.As it may be necessary to insert equipment at the frontier crossing to eliminate the monitoring or fault-locatingfrequencies used in each country or to terminate the remote power supply section, it is possible that the sending relativepower lev
30、el at 1300 kHz may be less than -13 dBr. It is then necessary that the frontier section should be less than 6 kmlong. If the difference between the pre-emphasis characteristics used in both countries is too great to be compensatedfor in this way, one of the Administrations concerned, chosen by mutua
31、l agreement, will have to make up for thisdifference at the attended receiving station on its territory which lies closest to the frontier.6.2.2 For interconnection between two different systems of this type with 8-km elementary cable sections, therelative level at the frequency 1300 kHz should be -
32、4.0 dBr at the input of the frontier cable section. According toRecommendation G.352 one of the Administrations concerned, chosen by mutual agreement, will have to make up forthe slight differences in relative level and pre-emphasis at the attended repeater station which lies closest to the frontier
33、.6.3 Relative levels in a terminal station; interconnection with other systemsRecommendation G.213 explains the general principles to be adopted to facilitate interconnection of differentsystems in terminal stations.7 Power-feeding and alarm systemsFascicle III.2 - Rec. G.341 57.1 Power-feeding acro
34、ss a frontierIn the absence of a special agreement between the Administrations concerned with a power-feeding sectioncrossing a frontier, it is recommended that each Administration power-feed only those repeater stations in its owncountry. Many Administrations use looped power-feeding on the two sid
35、es of a power-feeding station, half of each ofthe sections between this station and the adjacent power stations being so fed; they can close the loop at their frontierstations. Agreements will be necessary if, for example, the frontier is very far from the mid-point between the twonearest feeding st
36、ations, or if the Administrations concerned use looped power-feeding on the entire section betweentwo feeding stations.If the repeater stations in a country are fed from another country, special precautions will be required toprotect the staff working on the cables.7.2 Remote power-feeding systemsTh
37、e CCITT is studying these systems from the following viewpoints:- precautions to be taken to protect staff against normal voltages and remote power-feed currents, or theuse of voltages and currents which are innocuous to persons working in repeater stations or on lines;- protection of staff and equi
38、pment against induced voltages and currents;- trouble in remote power-feeding operation caused by induced voltages and currents.7.3 Supervision and alarms in a frontier sectionThis should be governed by agreement between the Administrations concerned. In particular, it is necessary atthe points of i
39、nterconnection between two systems that if frequencies are used for monitoring or for locating faults theybe attenuated to a level of -50 dBm0 on the receiving sides to prevent any disturbance to similar frequencies used in thesystem farther down the line.Note - Frequencies sent only over a system already withdrawn from service because of a fault may be selectedby each Administration on the national level.
