1、 CCITT FASCICLE 111.1 * 48b2591 0502652 3 References Calculations of the stability of international connections established in accordance with the transmission and switchingplan, CCITT Green Book, Vol. 111-2, Supplement No. 1, ITU, Geneva, 1973. CCITT Recommendation 12-channel terminal equipments, V
2、ol. III, Rec. G.232, 0 2. CCITT manual Transmission planning of switched telephone networks, ITU, Geneva, 1976. CCITT Recommendation Reduction of the risk of instability by switching means, Vol. VI, Rec. Q.32. CCITT Recommendation Conventional telephone signal, Vol. III, Rec. G.227. CCITT Question 8
3、/XII, Annex 2, Contribution COM XII-No. 1, Study Period 1981-1984, Geneva, 1981. I 2 3 4 5 6 Recommendation G.123 CIRCUIT NOISE IN NATIONAL NETWORKS (Geneva, 1964; amended at Mar del Plata, 1968, Geneva, 1972, 1976 and 1980 and Melbourne 1988) 1 Noise induced by power lines The network performance o
4、bjective for the psophometric e.m.f. of the noise produced by magnetic and/or electrostatic induction from all the power lines affecting one or more parts of a chain of telephone lines) joining a subscribers set to its international centre should not exceed 1 millivolt, this being the value at the l
5、inel) terminais of the subscribers set (when receiving), it being assumed that the telecommunication installations inserted in that chain are balanced to earth as perfectly as possible, in conformity with the most modern equipment construction. It should be noted that, even in the case of perfectly
6、balanced lines), the insertion of equipment having too great a degree of unbalance to earth may cause unacceptable noise at the terminals of a subscribers receiver. In every national network, it is usually possible, in practice, to find switching centres such that some of the lines) that terminate a
7、t those centres (lines) in cable, conforming to CCITT specifications) are free from noise arising from neighbouring power lines. It is then sufficient to determine the psophometric e.m.f.s arising from all the power lines” affecting one or more parts of the chain of lines) joining such a centre to t
8、he subscribers set. 2 Noise contributed by transmission systems 2.1 Analogue sJJstems 2.1.1 Verj4ong-distance circuits (about 2500-25 O00 km) If an extension circuit more than 2500 km long is used in a large country, it will have to meet all the recommendations applicable to an international circuit
9、 of the same length (Recommendation G.153). This implies that the equipment design objective for the line noise in channels used to provide these circuits should not exceed 2 pWOp/km. I “Line” as used in this 5 1 should be understood as meaning subscribers line, trunk junction or trunk circuit. 138
10、Fascicle 111.1 - Rec. G.123 COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesCCITT FASCICLE 111.1 * 4862593 0502653 5 1.1.2 Circuit ranging in length from very short distances up to 2500 km These circuits should meet the requirements of
11、Recommendation G.152. This implies that according to the noise objectives of Recommendation (3.222 i the accumulated line noise should correspond to an average of not more than 3 pWOp/km and the noise power produced by the various modulating equipments should meet the provisions of the Recommendatio
12、n cited in 2. Taking account of the particular structure of a real circuit the pertinent Recommendations CCITT/G.326 3 (for cable systems) or CCIR/395 4 (for radio-relay systems) must be applied when assessing its noise performance. Note I - The permissible noise contributions from equipment do not
13、depend on whether the circuits form part of the international 4-wire chain or are connected to it by 2-wire switching. However, the circuit noise powers assume that the hypothetical reference connections of Recommendation G.103 are, or will be in future, reasonably typical of connections. They also
14、assume that the total length of circuits connecting the local exchange to the primary centre is not excessive. The attention of Administrations is drawn to a conclusion of studies carried out by the CCITT during the 1964-1968 Study Period, that if the additional percentage of “poor or bad” opinions
15、on the quality of connections due to noise introduced by the circuits connecting the local exchange to the primary centre is not to exceed one half of that caused by the presence in the connection of all other sources of circuit noise, then the noise contributed by each one of these circuits should
16、be limited to about 500 pWOp (mean for all the channels of the system during any hour). Note 2 - Under the above conditions and assuming the maximum noise values permitted for pairs of channel modulators (200 pWOp), group modulators (80 pWOp) and supergroup modulators (60 pWOp), a total noise power
17、of 500 pWOp will not be exceeded by a circuit connecting the local exchange to the primary centre (Figure 1/(3.103) when its length is less than about 50 to 100 km. Note 3 - In the case that those circuits are operated with compandors conforming to Recommenda- tion G.162, the permitted noise powers
18、are to be understood inclusive of the effect of the compandor gain. 2.2 Digital srstem Circuits provided by PCM systems which accord with the G.700 Series of Recommendations, in particular Recommendation G.7 12 5, will have an acceptable noise performance which is substantially independent of their
19、2.3 Mixed circuits The noise value in a circuit provided by both analogue and digital transmission systems depends on the whole length of analogue sections and of the number of codecs in a circuit. Noise limits and measurement methods for a mixed circuit are studied under Questions 261x11, 16IV and
20、18/IV. Noise in a national 4-wire automatic exchange2) 3.1 Definition of a connection through an exchange Noise conditions in a national 4-wire automatic exchange are defined by reference to a “connection” through this exchange. By “connection through an exchange” is to be understood the pair of wir
21、es corresponding to a direction of transmission and connecting the input point of a circuit incoming in the exchange to the output point of a different circuit outgoing from the exchange. These input or output points are those defined in Recommendation Q.45 (points A and D of Figure 1lQ.45 8) and ar
22、e not necessarily the same as the text access points defined in Recommendation M.640 9. In accordance with Recommendation Q.31 6), the limits are the same as in Recommendation Q.45 7. Fascicle 111.1 - Rec. G.123 139 - COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by
23、 Information Handling Services _ - CCITT FASCICLE 111.1 * II 4862593 0502654 7 m 3.2 Equipment design objective for the mean noise power during the busy-hour The mean of the noise over a long period during the busy-hour should not exceed the following values: 1) Psophometrically weighted noise: -67
24、dBmOp (200 pWOp), 2) Unweighted noise: -40 dBmO (100000 pW0) measured with a device with a uniform response curve throughout the band 30-20 O00 Hz. Note - A sufficient variety of connections should be chosen to ensure that the measurements are representative of the various possible routes through th
25、e exchange. 3.3 Equipment design objective for the impulsive noise during the busy-hour Noise counts should not exceed 5 counts in 5 minutes at the threshold level of -35 dBmO (see the Recommendation cited in lo for measurement procedure). Note - Figure 3/Q.45 li shows the maximum number of impulsiv
26、e noise counts acceptable in a 5-minute period. 4 Noise allocation for a national system (guide for planning purposes) The noise powers indicated in the following text are nominal values. Network planning should be such that the noise power entering the international network and attributable to nati
27、onal sending systems meets the limits of the following rule: The psophometric noise power introduced by the national sending system at a point of zero relative level on the first international circuit must not exceed either (4000 + 4L) or (7000 + 2L) pWp, whichever is less, and where L is the total
28、length in kilometres of the long-distance FDM carrier systems in the national chain. The corresponding quantities referred to the send virtual switching point are (1800 + 1.8L) and (3100 + 0.9L) pWp. The derivation of this rule is explained in Annex A. Note - A problem, which has already arisen in s
29、ome national networks, as regards the receiving direction, is that when losses are reduced the circuit noise becomes more noticeable, particularly during periods of no conversation. This is particularly relevant in the case of large countries in which the noise contribution from line systems is high
30、. Hence if an Administration complies with a recommendation concerning national noise power levels and then subsequently improves transmission, perhaps by introducing 4-wire switching in lower-order exchanges, it may find itself in a worse situation as regards noise. It follows that it is important
31、to preserve a proper balance between noise and loss. ANNEX A (to Recommendation G.123) Noise allocation for a national system A. 1 is desirable that the noise power arising in national networks be limited in terms o. the level appearing at the virtual switching points - the agreed interface between
32、the national and the international network. In order to do this, some particular distribution of losses within the national network must be assumed. The solution is to adopt an agreed reference connection in order to specify maximum noise power levels from national sources referred to the virtual sw
33、itching point of the international circuit. A.2 Having regard to the way in which national networks are constructed, it is appropriate to express the noise allowance in the form A + BL where A is a fixed allowance resulting from noise in exchanges and from short-haul multiplex systems, i? is an allo
34、wance for a noise rate per unit length from long-haul multiplex systems and L is the total length of these latter systems in the national portion of the international connection. Two such expressions are necessary, one for countries of average size and another for large countries (in the sense of Re
35、commendation (3.121). 140 Fascicle 111.1 - Rec. G.123 COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesCCITT FASCICLE 111.1 * 4862593 0502655 9 A.3 amount of noise injected into the international connection. This approach is comparativel
36、y straightforward in the national sending system and serves to limit the A.4 Average-sized countries (.e. not greater than 1500 km from the CT3 to the most remote local exchange) The relevant hypothetical reference chain for the national sending system is given in Figure A-1lG.123 3). The circuit be
37、tween the local exchange and the primary centre is assumed to be routed on an FDM carrier system of length not exceeding 250 km and operated at a nominal loss of 3 dB. The noise power on this circuit is taken to be the maximum value of 2000 pW0. The circuit between the primary centre and the seconda
38、ry centre is also assumed to be routed on an FDM carrier system of the same type. The line noise power rate of the two long-distance trunk circuits is assumed to be 4 pW/km and the total line length of these two circuits (LI + L2 in Figure A-l/G.123) approaches the limit of 1500 km arbitrarily defin
39、ing “a country of average size” in Recommendation G.121. It is thus assumed that the distance covered by the two short-haul systems is a very small proportion of the total length of the complete national sending system. Each exchange is assumed to contribute 200 pWp in accordance with 0 3 of the tex
40、t, or 4.31 6. . A line with a slope of 2 pW/km is constructed to pass through the same point and its intercept is seen to be 7000 pW. Hence the rule for large countries is 7000 + 2L pW0. (The 0.5-dB nominal loss of the last national circuit has been ignored for simplicitys sake.) 4- PWOP : 20000 ; 1
41、8000 ._ 3 o L .- - O m ._ 4- c f 16000 C m ._ c ._ E .- - L LOO0 2 a O 1000 a C cn o UJ .- a O References 500 1000 1500 2000 2500 3000 3500 km CCI-rr-43840 Total length of national long-distance FDM carrier system FIGURE A-2/G. 123 CCITT Recommendation Noise objectives for design of carrier-transmis
42、sion systems, Vol. III, Rec. G.222. Ibid., 5 4. CCITT Recommendation Noise on a real link, Vol. III, Rec. G.226. CCIR Recommendation Noise in the radio portion of circuits to be established over real radio-relay links for FDM telephony, Vol. IX, Rec. 395, ITU, Geneva, 1986. Fascicle 111.1 - Rec. G.1
43、23 - . COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling Services CCITT FASCICLE 111.1 * 4862571 0502b57 2 CCITT Recommendation Performance characteristics of PCM channels between 4-wire interfaces at voice frequencies, Vol. III, Rec. G.712. CCIT
44、T Recommendation Noise in a national 4-wire automatic exchange, Vol. VI, Rec. 4.31. KITT Recommendation Transmission characteristics of an international exchange, Vol. VI, Rec. 4.45. Ibid., Figure 1Q.45. CCITT Recommendation Four-wire switched connections and four-wire measurements on circuits, Yell
45、ow Book, Vol. IV, Rec. M.640, ITU, Geneva, 1981. CCITT Recommendation Transmission characteristics of an international exchange, Vol. VI, Rec. 4.45, Annex A. Ibid., Figure 3Q.45. Recommendation G.125 CHARACTERISTICS OF NATIONAL CIRCUITS ON CARRIER SYSTEMS (Geneva, 1964; amended at Mar del Plata, 196
46、8 and Geneva, 1972) Carrier circuits which are likely to form part of international connections should meet the requirements of Recommendation G.132 as far as attenuation distortion is concerned. The circuits should transmit all types of signai (e.g. speech, data, facsimile) which might normally be
47、expected, according to Recommendations over this part of the connection. Recommendations relating to the noise performance of national circuits are now to be found in Recommendation G.123 (circuit noise in national networks). 1.3 General Characteristics of the 4-wire chain formed by the internationa
48、l circuits and national extension circuits This subsection gives the overall characteristics recommended for the 4-wire chain defined in Recommen- dation G.lO1, 0 2. Recommendation G.131 STABILITY AND ECHO Geneva, 1964; amended at Mar del Plata, 1968, and Geneva, 1972, 1976, and 1980; Malaga-Torremo
49、linos, i984 and Melbourne, 1988) 1 Stabiliy of telephone transmission The nominal transmission loss of international circuits having been fixed, the principal remaining factors which affect the stability of telephone transmission on switched connections are: - - - the variation of transmission loss with time and among circuits (Recommendation G.151, 0 3); the attenuation distortion of the circuits (Recommendation G.151, 4 1); the distribution of stability balance return losses (Recommendation G.122, $0 2 and 3). Fascicl
