1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 6 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$!4!G0G0#/-5.)#!4)/.G0G0/6%2G0G04(%G0G04%,%0(/.%.%47/2+0/7%2G0G0,%6%,3G0G0 amended at Geneva, 1964 and 1980)The objectives in specifying data signal levels are as follows:a) To ensure satisfactory transmissio
2、n and to permit coordination with devices such as signalling receiversor echo suppressors, the data signal levels on international circuits should be controlled as closely aspossible,b) To ensure correct performance of multichannel carrier systems from the point of view of loading andnoise, the mean
3、 power of data circuits should not differ much from the conventional value of channelloading (-15 dBm0 for each direction of transmission: see Note below). This conventional value makesallowance for a reasonable proportion P (dependent on the transmission systems and probably less than50%; the value
4、 will have to be specified in subsequent studies) of the channels in a multichannel systembeing used for nonspeech applications at fixed power levels at about -13 dBm0 for each direction oftransmission.If the proportion of nonspeech applications (including data) does not exceed the above value P, th
5、e meanpower of -13 dBm0 for each direction of transmission would be allowable for data transmission also.However, assuming that the proportion of nonspeech circuits is appreciably higher than P (due to thedevelopment of data transmission) on international carrier systems, a reduction of this power b
6、y 2 dBmight be reasonable (these values require further study).Note - The distribution of long-term mean power among the channels in a multichannel carrier telephonesystem (conventional mean value of -15 dBm0), probably has a standard deviation in the neighbourhoodof 4 dB (see 2).c) It is probable t
7、hat Administrations will wish to fix specific values for the signal power level of datamodulators either at the subscribers line terminals or at the local exchanges. The relation between thesevalues and the power levels on international circuits depends on the particular national transmission plan;i
8、n any case, a wide range of losses among the possible connections between the subscriber and the inputto international circuits must be expected.d) Considerations a) to c) suggest that specification of the maximum data signal level only is not the mostuseful form. One alternative proposal would be t
9、o specify the nominal power at the input to theinternational circuit. The nominal power would be the statistically estimated mean power obtained frommeasurement on many data transmission circuits.For these reasons, the CCITTunanimously declares the following view:1 Data transmission over leased tele
10、phone circuits (private wires) set up on carrier systems1.1 The maximum power output of the subscribers equipment into the line shall not exceed 1 mW at anyfrequency.1.2 For systems transmitting tones continuously, e.g., frequency-modulation systems, the maximum power levelat the zero relative level
11、 point shall be -13 dBm0. When transmission of data is discontinued for any appreciable time,the power level should preferably be reduced to -20 dBm0 or lower._1)Recommendation V.2 corresponds to Recommendation H.15 1.2 Fascicle VIII.1 - Rec. V.21.3 For systems not transmitting tones continuously, e
12、.g., amplitude-modulation systems, the signalcharacteristics should meet all of the following requirements:i) The maximum value of the 1-minute mean power shall not exceed -13 dBm0.ii) Provisionally, the maximum value of the instantaneous power shall not exceed a level corresponding tothat of a 0 dB
13、m0 sine wave signal. This limit should be confirmed or amended after further study.iii) Provisionally, the maximum signal power determined for a 10-Hz bandwidth centred at any frequencyshall not exceed -10 dBm0. This limit should be confirmed or amended after further study.Note 1 - It is estimated t
14、hat the proportion of international circuits which are carrying data transmissions isapproximately 20%. If the proportion should reach a high level (approximately 50% or even less in the case of high-usage systems), the limits now proposed would need to be reconsidered.Note 2 - Supplement No. 16 3 o
15、f the Yellow Book, Volume III, gives information on the out-of-band powerof signals applied to leased telephone-type circuits.2 Data transmission over the switched telephone system2.1 The maximum power output of the subscribers equipment into the line shall not exceed 1 mW at anyfrequency.2.2 For sy
16、stems transmitting tones continuously, such as frequency- or phase-modulation systems, the powerlevel of the subscribers equipment should be fixed at the time of installation to allow for loss between his equipmentand the point of entry to an international circuit, so that the corresponding nominal
17、level of the signal at theinternational circuit input shall not exceed -13 dBm0.2.3 For systems not transmitting tones continuously, e.g. amplitude-modulation systems, the signalcharacteristics should meet all of the following requirements (see also Note 1 to 1.3):i) The maximum value of the 1-minut
18、e mean power shall not exceed -13 dBm0.ii) Provisionally, the maximum value of the instantaneous power shall not exceed a level corresponding tothat of a 0 dBm0 sine wave signal. This limit should be confirmed or amended after further study.iii) Provisionally, the maximum signal power determined for
19、 a 10 Hz bandwidth centred at any frequencyshall not exceed -10 dBm0. This limit should be confirmed or amended after further study.Note 1 - In practice, it is no easy matter to assess the loss between a subscribers equipment and theinternational circuit, so that 2 of the present Recommendation shou
20、ld be taken as providing general planningguidance.Note 2 - In switched connections, the loss between subscribers telephones may be high: 30 to 40 dB. The levelof the signals received will then be very low, and these signals may suffer disturbance from the dialling pulses sentover other circuits.If t
21、here is likely to be a heavy demand for international connections for data transmission over the switchednetwork, some Administrations might want to provide special 4-wire subscriber lines. If so, the levels to be used mightbe those proposed for leased circuits.References1 CCITT Recommendation Power levels for data transmission over telephone lines, Vol. III, Rec. H.51.2 Measurement of the load of telephone circuits, Green Book, Vol. III-2, Supplement No. 5, ITU, Geneva, 1973.3 Out-of-band characteristics of signals applied to leased telephone-type circuits, Vol. III, Supplement No. 16.
