1、CCITT FASCICLE V1.L * = 4862593 0506230 2 M 3 4 5 CCIT Recommendation The transmission plan, Vol. III, Rec. G. 101. CCIT Recommendations G.120, G.121, G.122, G.123 and G.124; Subsection 1.2: General characteristics of national systems forming part of international connections of Volume III. CCIT Rec
2、ommendation General performance objectives applicable to all modern international circuits and national extension circuits, Vol. III, Rec. G. 15 1. CCIIT Recommendation Characteristics appropriate to long-distance circuits of a length not exceeding 2500 km, Vol. III, Rec. G.152. CCITT Recommendation
3、 Characteristics appropriate to international circuits more than 2500 km in length, Vol. III, Rec. G.153. CCIT Recommendation Pulse code modulation (PCM) of voice frequencies, Vol. III, Rec. G.711. CCIT Recommendation Hypothetical reference connections, Vol. III, Rec. G.103. 6 7 8 9 6.1 General reco
4、mmendations on the transmission quality for an entire international telephone Recommendation 4.41 connection MEAN ONE-WAY PROPAGATION TIME 1 The times in I- Recommendation are the means of the propagation times i the two directions of transmission in a connection. When opposite directions of transmi
5、ssion are provided by different media (e.g. a satellite channel in one direction and a terrestrial channel in the other) the two times contributing to the mean may differ considerably. 1 Limits for a connection It is necessary in an international telephone connection to limit the propagation time be
6、tween two subscribers. As the propagation time is increased, subscriber difficulties increase, and the rate of increase of difficulty rises, see b) below. Relevant evidence is given in the bibliography of Recommendation G.114 i. As a network performance objective, CCIT therefore recommends the follo
7、wing limitations on mean one-way propagation times when echo sources exist and appropriate echo control devices, such as echo suppressors and echo cancellers, are used: a) O to 150 ms, acceptable. Note - Echo suppressors specified in Reference 2 may be used for delays not exceeding 50 ms (see Refere
8、nce 3). 150 to 400 ms, acceptable, provided that increasing care is exercised on connections as the mean one-way propagation time exceeds about 300 ms, and provided that echo control devices, such as echo suppressors and echo cancellers, designed for long delay circuits are used. Above 400 ms, unacc
9、eptable. Connectins with these delays should not be used except under the most exceptional circumstances. b) c) . I) This Recommendation is an extract of Recommendation G.114 i. The suspensive points show where a passage in Recommendation G.114 has not been reproduced under 4.41. Fascicle W.1 - Rec.
10、 4.41 199 =CCITT FASCICLE VI-L * 4862593 0506233 4 -, 2 Values for circuits In the establishment of the general interconnection plan within the limits in 0 1 the one-way propagation time of both the national extension circuits and the international circuits must be taken into account. The propagatio
11、n time of circuits and connections is the aggregate of several components; e.g. group delay in cables and in filters encountered in FDM modems of different types. Digital transmission and switching also contribute delays. The conventional planning values given in Q 2.1 may be used to estimate the to
12、tal propagation time of specified assemblies which may form circuits or connections. 2.1 Conventional planning values of propagation time Provisionally, the conventional planning values of propagation time in Table VQ.41 may be used. 2.2 National extension circuits The main arteries of the national
13、network should consist of high-velocity propagation lines. In these conditions, the propagation time between the international centre and the subscriber farthest away from it in the national network will probably not exceed: a) In purely analogue networks 12 + (0.004 x distance in kilometres) ms. He
14、re the factor 0.004 is based on the assumption that national trunk circuits will he routed over high-velocity plant (250 km/ms). The 12 ms constant term makes allowance for terminal equipment and for the probable presence in the national network of a certain quantity of loaded cables (e.g. three pai
15、rs of channel translating equipments plus about 160 km of H 88/36 loaded cables). For an average-sized country (see Figure 2/G.103) the one-way propagation time will be less than 18 ms. In mixed analogue/digital networks the propagation time can generally be estimated by the equation given for purel
16、y analogue networks. However, under certain unfavourable conditions increased delay may occur compared with the purely analogue case. This occurs in particular when digital exchanges are connected with analogue transmission systems through PCM/FDM equipments in tandem, or transmultiplexers. With the
17、 growing degree of digitisation the propagation time will gradually approach the condition of purely digital networks. In purely digital networks between exchanges (e.g. an IDN) the propagation time as defined above will probably not exceed: b) c) 3 + (0.004 x distance in kilometres) ms. The 3 ms co
18、nstant term makes allowance for one PCM coder or decoder and five digitally switched exchanges. Note - The value 0.004 is a mean-value for coaxial cable systems and radio-relay systems; for optical fibre systems 0.005 is to be used. In purely digital networks between subscribers (e.g. an ISDN) the d
19、elay of c) above has to be increased by up to 3.6 ms if burst-mode (time compression multiplexing) transmission .is used on 2-W local subscriber lines. d) 2.3 International circuits International circuits2) will use high-velocity transmission systems, e.g. terrestrial cable or radio-relay systems, s
20、ubmarine systems or satellite systems. The planning values of 0 2.1 may be used. The magnitude of the mean one-way propagation time for circuits on high altitude communication satellite systems makes it desirable to impose some routing restrictions on their use. Details of these restrictions are giv
21、en in Recommendation 4.13. (See also Annex A to Recommendation G.114.) * 2, For short nearby links, telecommunications cables operated at voice frequencies may also be used in the conditions set out in the introduction to Sub-section 5.4 of Fascicle 111.2. 200 Fascicle YI.1 - Rec. Q.41 TABLE 1/Q.41
22、Transmission medium Contribution to one-way propagation time Remarks Terrestrial coaxial cable or radio relay system; FDM and digital transmission 4 p/km Allows for delay in repeaters and regenerators Optical fibre cable system; digital transmission 5 p/km Allows for delay in repeaters and regenerat
23、ors Submarine coaxial cable system 6 pskm I I Satellite system - 14 O00 km altitude - 36 O00 km altitude 110 ms 260 ms Between earth stations only FDM channel modulator or demodulator 0.75 ms a) 0.5 ms b, FDM compandored channel modulator or demodulator PCM coder or decoder 0.3 ms PCM/ADPCM/PCM tran
24、scoding 0.5 ms Transmultiplexer 1.5 msc) Digital transit exchange, digital-digital I 0.45 ms d, I I 1.5 ms d, I Digital local exchange, analogue-analogue 0.975 ms d, Digital local exchange, analogue subscriber line-digital junction 0.825 ms dl Digital local exchange, digital subscriber line-digital
25、junction Half the sum of propagation times in both directions of transmission Echo cancellers i mse) a) These values allow for group-delay distortion around frequencies of peak speech energy and for delay of intermediate higher order multiplex and through-connecting equipment. b, This value refers t
26、o FDM equipments designed to be used with a compandor and special filters. c, For satellite digital communications where the transmultiplexer is located at the earth station, this value may be increased to 3.3 ms. d, These are mean values; depending on traffic loading, higher values can be encounter
27、ed, e.g. 0.75 ms (1.950 ms, 1.350 ms or 1.250 ms respectively with 0.95 probability of not exceeding). (For details see Recommendation Q.551). e) Echo cancellers, when placed in service, will add a one-way propagation time of up to 1 ms in the send path of each echo canceller. This delay excludes th
28、e delay through any codec in the echo canceller. No significant delay should be incurred in the receive path of the echo canceller. Fascicle VI.1 - Rec. Q.41 20 1 CCITT FASCICLE VI.1 * 48b2591 0506213 8 -, References CCIT Recommendation Mean one-way propagation time, Vol. III, Rec. G. 1 14. CC1- Rec
29、ommendation Definitions relating to echo suppressors and characteristics of a far-end operated, differential, half-echo suppressor, Blue Book, Vol. III, Rec. G.161, ITU, Geneva, 1965. CCIT Recommendation Stability and echo, Vol. III, Rec. G.131, 0 2.2. i 2 3 6.2 (See Recommendations G.120 to G.125,
30、Fascicle 111.1.) General characteristics of national systems forming part of international connections 6.3 (Overall characteristics for the 4-wire chain are defined in Recommendation 4.40, 4 2.) General characteristics of the “4-wire chain“ formed by the international circuits and national extension
31、 circuits Recommendation 4.42 STABILITY AND ECHO (ECHO SUPPRESSORS) (See Recommendation G.131 in Fascicle 111.1 and Recommendation Q.115) 6.4 General characteristics of the 4-wire chain of international circuits ; international transit Recommendation 4.43 . . . . . . . TRANSMISSION LOSSES, RELATIVE
32、LEVELS 1) . 5.3 . Definitions 5.3.1 transmission reference point F: point de rfrence pour la transmission S: punto de referencia para la transmisin A hypothetical point used as the zero relative level point in the computation of nominal relative levels. At those points in a telephone circuit the nom
33、inal mean power level (- 15 dBm) defined in the Recommenda- tion G.223 2 shall be applied when checking whether the transmission system conforms to the noise objectives defined in Recommendation G.222 3. Note - For certain systems, e.g. submarine cable systems (Recommendation G.371 4), other values
34、Such a point exists at the sending end of each channel of a 4-wire switched circuit preceding the virtual switching point; on an international circuit it is defined as having a signal level of +3.5 dB above that of the virtual switching point. In frequency division multiplex equipment, a hypothetica
35、l point of flat zero relative level (i.e. where all channels have the same relative level) is defined as a point where the multiplex signal, as far as the effect of intermodulation is concerned, can be represented by a uniform spectrum random noise signal with a mean power apply. This Recommendation is an extract of Recommendation G.101 i. The suspensive points show where a passage in Recommendation (3.101 has not been reproduced under 4.43. t 202 Fascicle VI.1 - Rec. 4.43
