1、CCITT FASCICLE IV.1 * m 48b2571 0505413 O m SECTION 3 INTERNATIONAL TELEPHONE CIRCUITS 3.1 Bringing an international telephone circuit into senice Recommendation M.560 INTERNATIONAL TELEPHONE CIRCUITS - PRINCIPLES, DEFINITIONS AND RELATIVE TRANSMISSION LEVELS 1 General The purpose of this Recommenda
2、tion is to provide the necessary background information for other Recommendations in the M Series. The CCITT transmission plan and international telephone connections are explained. The Recommenda- tion also introduces the concepts of “virtual analogue switching points”, and their conventional relat
3、ive transmission levels. Appropriate definitions are given where necessary. Extracts from the relevant Recommendations in Volume III and from Recommendation 4.45 i are included in this Recommendation. 2 The CCITT Transmission Plan 2.1 Principles The CCITT transmission plan has been drawn up with the
4、 object of making use, in the international service, of the advantages offered by 4-wire switching. However, the recommendations in the plan are considered to be met if the use of technical media other than those described give an equivalent performance at the international centre. Note - Short tran
5、sfrontier circuits are not covered by the transmission plan; they should be the subject of agreement between the Administrations concerned. 2.2 International telephone connections A complete international telephone connection has three parts, as shown in Figure 1lM.560, namely: - an international ch
6、ain an international chain is made up of one or more 4-wire international circuits. These are connected on a 4-wire basis to other international circuits in transit international centres or to national systems in terminal international centres; two national systems, one at each end These may compris
7、e one or more 4-wire amplified national circuits with 4-wire interconnection, and circuits with 2-wire connection to terminal exchanges and subscribers. - - Fascicle IV.l - Rec. M.560 3 35 CCITT FASCICLE IV.1 * W 4862591 0505414 2 -7 Four-wire chain I ! 1- - I l -, I I l ./ I A A “ I ,. IC x : v I I
8、. I l I l l l I National system I International chain i National system I 1- CCllT - 36850 0 Transit international exchange a Terminal international exchange X National exchange e Subscribers set FIGURE 1/M.560 Constituent parts of an international telephone connection 2.3 International telephone ci
9、rcuits, virtual analogue switching points and relative transmission levels 2.3.1 analogue switching points” in the international centre. From a transmission planning point of view, an international telephone circuit is defined by its “virtual 2.3.2 Virtual analogue switching points Virtual analogue
10、switching points are theoretical points with specified relative levels. For circuits terminating at a digital international centre, the concept of virtual analogue switching points postulates the existence of ideal analogue-to-digital coders and digital-to-analogue decoders, via which the desired an
11、alogue points could be derived. The virtual analogue switching points may not- be the same as the points at which the circuit terminates physically in a switching equipment. These latter points are known as the circuit terminals; the exact position of the terminals is decided in each case by the Adm
12、inistration concerned (see Figure 2/M.560). For illustrative purposes, Figure 2/M.560 depicts the virtual analogue switching points for wholly digital and wholly analogue international telephone circuits. Recommendation M.562 deals in detail with circuits provided by a mixture of analogue and digita
13、l systems. 2.3.3 Relative transmission levels at virtual analogue switching points The virtual analogue switching points of an international 4-wire telephone circuit are fixed by convention - sending: -3.5 dBr; - receiving: at points of the circuit where the nominal relative levels at the reference
14、frequency are: -4.0 dBr, for analogue circuits and the analogue end of mixed analogue/digital circuits; -3.5 dBr for digital circuits. . The nominal transmission loss of circuits at the reference frequency between virtual analogue switching points is therefore 0.5 dB for both analogue and mixed anal
15、ogue/digital circuits and O dB for digital circuits. Two international circuits interconnected in an international centre are considered to be connected together directly at their virtual analogue switching points without any pad or amplifier between those virtual analogue switching points (see Figu
16、re 3/M.560). The relationship between the actual switching points and the virtual analogue switching points in a practical international exchange is illustrated in Figure 3/M.560. 336 Fascicle IV.l - Rec. M.560 CCITT FASCICLE IV-L * 4862571 0505415 4 I I International centre B International centre A
17、 -33 the other values are given as examples only. (Relative level at virtual analogue switching point of adjacent centre) 61 Hypothetical arrangement indicating possible position of the virtual analogue switching points of the two circuits CCITT 36870 Note - Underlined values of relative level refer
18、 to the circuit on the right of the point concerned. Values of relative level not underlined refer to the circuit on the left of the point concerned. In an actual switching centre the virtual analogue switching points would not physically exist. FIGURE 3/M.560 Example showing a simplified representation of a transit connection in an international exchange with actual arrangement and possible location of virtual analogue switching points Reference 11 CCITT Recommendation Transmission Characteristics of an International Exchange, Vol. VI, Rec. 4.45. 7 i r 338 Fascicle IV.l - Rec. M.560 !
