1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU42!.3-)33)/.G0G0-%$)!G0G0#(!2!#4%2)34)#3#(!2!#4%2)34)#3G0G0/ amended at Geneva, 1980)1 PreambleThis Recommendation relates to symmetric pair cables which have been developed for the transmission ofsignals with
2、bit rates of the order of 6 to 34 Mbit/s, but they are not ruled out for the transmission of lower or higher bitrates, subject to the use of an appropriate regeneration section; in most cases they can also be used for basebandtransmission of videophone or television signals.These cables fall into tw
3、o categories, according to whether or not the cable is intended for use in both directionsof transmission in the same cable.2 Parameters to be measuredThose parameters which, for digital system transmission, have to be measured by a particular method or atfrequencies different from those defined in
4、Recommendation G.611, are: characteristic impedance, attenuationcoefficient, and far-end crosstalk between pairs on the same direction of transmission. If the cable is intended for usewith both directions of transmission within the same cable, it is also necessary to measure the near-end crosstalk b
5、etweenpairs intended for different directions of transmission.2.1 Characteristic impedanceThe characteristic impedance may be measured:- either in the sinusoidal mode, when the measured pair will be terminated by an impedance constantlyequal to that measured by the bridge, except when the length is
6、sufficient for the measurement result to beindependent of the termination impedance;- or by a pulse echo meter1), when the impedance of the pair being measured is compensated by anadjustable balancing network graduated to show the impedance value. The pair being measured isterminated by an identical
7、 network.2.2 Attenuation coefficientThe attenuation per km of the pairs is derived from that value to be obtained on an elementary cable section,allowance being made for the tolerance accepted on the length of these sections.Note - In the case of looped measurement, a check should be carried out to
8、ensure that the near-end crosstalkattenuation between the ends of the circuit being measured is sufficient.2.3 CrosstalkCrosstalk may be specified either in sinusoidal mode, at a frequency near the timing half-frequency of thesystem concerned, or in digital mode2)._1)This method is similar to the on
9、e used for coaxial pairs, but with a symmetrical measuring head andnetworks. The pulse duration is equal to 100 ns; the echo is not corrected.2)An example of a digital technique is given in Supplement No. 19.2 Fascicle III.3 - Rec. G.6122.3.1 Far-end crosstalk measurementThe far-end crosstalk measur
10、ements are carried out on pairs used in the same direction of transmission at afrequency above about 100 kHz; if this frequency is not the timing half-frequency of the system, the value to bespecified will be corrected to the factor 20 log10f 3).2.3.2 Near-end crosstalk measurementsIf it is intended
11、 to transmit in both directions on the same cable, these measurements are conducted on aprototype length, either in sinusoidal mode or digital mode, between pairs used for opposite directions of transmission.3 Description of pairs and cablesAdministrations which decide to use symmetrical pairs to tr
12、ansmit digital signals with a bit rate of the order of 6to 34 Mbit/s should, wherever possible, choose one of the types of cable described in 3.1 and 3.2 below.3.1 Cable designed for use with one cable for each direction of transmission3.1.1 The basic characteristics of the pairs are given in Table
13、1/G.612.3.1.2 The characteristics of cables constructed with these pairs are given in Table 2/G.612.3.2 Cables designed for transmission in both directions in the same cableTables 3/G.612 and 4/G.612 indicate the characteristics of the pairs which make up cable pairs and quad cablesrespectively.All
14、these cables consist of bundles protected by one or more copper or aluminium screens, the pairs in eachbundle being used for the same direction of transmission. For this reason, near-end crosstalk values relate only to pairs indifferent bundles.Note 1 - To make the presentation of Tables 3/G.612 and
15、 4/G.612 uniform, the values of characteristic impedance aregiven at 1 MHz (real part of Z1). The ratio between impedance Z1= X1- jY1at 1 MHz and impedance Zf = Xf- jYfatf MHz isXfX Y Y f and YfYf=+ =111 1/.The difference between the value of the real part of the impedance at 1 MHz and its value at
16、4 MHz is between2 and 3 . At 1 MHz, the imaginary part of the impedance is between 4 and 6 ; for frequencies above about 0.3 MHz,it varies in the inverse ratio to the square root of the frequency.Note 2 - For the same reason as in Note 1 above, the attenuation value is given at 1 MHz. At a frequency
17、f MHz (f 1), attenuation fis related to attenuation 1at 1 MHz by the ratio ff=1.Note 3 - The value of far-end crosstalk is reduced to a length of 1000 m by a correction of 10 logl0L if the cablelength L being measured is different from 1000 m. The crosstalk values indicated are the minimum limit val
18、ues for thespecification of systems. Where either of the above conditions is not fulfilled, the values are shown between brackets._3)For symmetrical pair star-quad cables the correction law 20 logl0f is used for pairs of the same quad only upto a certain characteristic frequency, above this frequenc
19、y the law 40 log10f must be used.Fascicle III.3 - Rec. G.612 3TABLE 1/G.612Pair characteristics Type I cableDiameter of conductors (mm)Average mutual capacitance of pairs (nF/km)Characteristic impedance ()a)Attenuation coefficient at 24 C (dB/km)a)0.6424.217813.5a)The attenuation and impedance measu
20、rement frequency is 3150 kHz.TABLE 2/G.612Set 1a)Set 2a)Nominal characteristic impedance Z0()(desired average at 3150 kHz)178Attenuation and crosstalkAttenuation at 3150 kHz to 24 C (dB/km)pair minimumpair maximumFar-end crosstalk (FEXT) loss at 3150 kHz dB for a 300 m (1000 feetlength)pair minimum
21、power summinimum pair-to-pair (0.1% point)11.814.3537.540.511.814.639.040.5DC resistance at 24C ( /km)maximum conductordesired average56.854.5Cable average mutual capacitance (nF/km)maximumminimumdesired averager.m.s standard deviation () of pairs within a cable (%)25.423.024.2 7Capacitance unbalanc
22、e to ground (pF/km)maximum paircable average 443 164DC dielectric strengthbetween conductors for ARPAP b) sheathcore and inner aluminium to shieldcore to inner aluminium and shield 1 500 V (applied for 1 s) 20 000 V (applied for 3 s) 5 000 V (applied for 3 s)a)Two sets of values for attenuation and
23、far-end crosstalk are given. The cable may meet either one of these sets,thus allowing a cable with lower loss to meet a less stringent crosstalk requirement.b)Aluminium-resin-polythene-aluminium-polythene.4 Fascicle III.3 - Rec. G.612TABLE 3/G.612Cable pairsCharacteristics Cable typeI II III IV VNo
24、minal characteristicimpedance Z0at 1 MHz () 160 160 140 120 145far-end crosstalk(minimum values referred to 1000m)(dB)1 MHz4 MHz17 MHz43a)43a)40564431645240Near-end crosstalk from 1 to 17 MHz(minimum values, dB)119 119 98 116 125Nominal attenuation coefficientat 1 MHzb) (dB/km at 10 C)7.0 9.3 10.5 9
25、.5 5.2Nominal capacity (nF/km) 28.5 28.5 31.5 38 30Diameter of conductors (mm) 0.8 0.6 0.65 0.9 1.2a)Far-end crosstalk measurements on elementary cable sections for pairs of this type are made in the digitalmode only (see Supplement No. 19). The maximum value specified is 30 mV.b)The real values sho
26、uld make it possible to meet the conditions required for an elementary cable section (TypeI: 56 2 dB at 4.2 MHz and 10C for 4 km; Type II: 56 2 dB at 4.2 MHz and 10 C for 3 km; Type III:below 55 dB at 3.15 MHz for 2.8 km).Fascicle III.3 - Rec. G.612 5TABLE 4/G.612Quad cablesCharacteristicsCable Type
27、IIINominal characteristic impedance Z0at 1 MHz () 165 120Far-end crosstalkDifferentquads1 MHz4 MHz13 MHz17 MHz463431564431(minimum values referred to 1000 m)(dB)Samequad1 MHz4 MHz13 MHz17 MHz(45)(25)a)(21)4634c)Near-end crosstalk, from 1 to 17 MHz (minimum values, dB) 125b)116Nominal attenuation coe
28、fficient at 1 MHz (dB/km at 10C) 8.8 9.5Nominal capacity (nF/km) 28 38Diameter of conductors (mm) 0.65 0.9a)For 34 Mbit/s transmission over each pair of a star quad, a balancing method is applied to the elementary cable section of 2km by means of systematic crossings every 500 m, which improves the far-end crosstalk values by at least 15 dB. Hence thevalues given in this box correspond to 500 m of cable.b)The value must be above 130 dB in 99% of cases.c)The transmission of 34 Mbit/s over each pair of a star quad is studied.
