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(Extract from the "LUE "OOK)
 NOTES

1 ITU-T Recommendation V.2 was published in Fascicle VIII.1 of the Blue Book. This file is an extract from the
Blue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, the
contents of the file are identical to the Blue Book version and copyright conditions remain unchanged (see below).

2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both a
telecommunication administration and a recognized operating agency.

 ITU 1988, 1993

All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or
mechanical, including photocopying and microfilm, without permission in writing from the ITU.
Recommendation V.21)
Fascicle VIII.1 - Rec. V.2

POWER LEVELS FOR DATA TRANSMISSION OVER TELEPHONE LINES

(New Delhi, 1960; amended at Geneva, 1964 and 1980)

The objectives in specifying data signal levels are as follows:

a) To ensure satisfactory transmission and to permit coordination with devices such as signalling receivers
or echo suppressors, the data signal levels on international circuits should be controlled as closely as
possible,
b) To ensure correct performance of multichannel carrier systems from the point of view of loading and
noise, the mean power of data circuits should not differ much from the conventional value of channel
loading (-15 dBm0 for each direction of transmission: see Note below). This conventional value makes
allowance for a reasonable proportion P (dependent on the transmission systems and probably less than
50%; the value will have to be specified in subsequent studies) of the channels in a multichannel system
being used for nonspeech applications at fixed power levels at about -13 dBm0 for each direction of
transmission.

If the proportion of nonspeech applications (including data) does not exceed the above value P, the mean
power 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 the
development of data transmission) on international carrier systems, a reduction of this power by 2 dB
might be reasonable (these values require further study).

Note - The distribution of long-term mean power among the channels in a multichannel carrier telephone
system (conventional mean value of -15 dBm0), probably has a standard deviation in the neighbourhood
of 4 dB (see [2]).
c) It is probable that Administrations will wish to fix specific values for the signal power level of data
modulators either at the subscriber’s line terminals or at the local exchanges. The relation between these
values and the power levels on international circuits depends on the particular national transmission plan;
in any case, a wide range of losses among the possible connections between the subscriber and the input
to international circuits must be expected.
d) Considerations a) to c) suggest that specification of the maximum data signal level only is not the most
useful form. One alternative proposal would be to specify the nominal power at the input to the
international circuit. The nominal power would be the statistically estimated mean power obtained from
measurement on many data transmission circuits.

For these reasons, the CCITT

unanimously declares the following view:

1 Data transmission over leased telephone circuits (private wires) set up on carrier systems

1.1 The maximum power output of the subscriber’s equipment into the line shall not exceed 1 mW at any
frequency.

1.2 For systems transmitting tones continuously, e.g., frequency-modulation systems, the maximum power level
at the zero relative level 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].

Fascicle VIII.1 - Rec. V.2 1

1.3 For systems not transmitting tones continuously, e.g., amplitude-modulation systems, the signal
characteristics 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 to
that 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 a 10-Hz bandwidth centred at any frequency
shall not exceed -10 dBm0. This limit should be confirmed or amended after further study.

Note 1 - It is estimated that the proportion of international circuits which are carrying data transmissions is
approximately 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] of the Yellow Book, Volume III, gives information on the out-of-band power
of signals applied to leased telephone-type circuits.

2 Data transmission over the switched telephone system

2.1 The maximum power output of the subscriber’s equipment into the line shall not exceed 1 mW at any
frequency.

2.2 For systems transmitting tones continuously, such as frequency- or phase-modulation systems, the power
level of the subscriber’s equipment should be fixed at the time of installation to allow for loss between his equipment
and the point of entry to an international circuit, so that the corresponding nominal level of the signal at the
international circuit input shall not exceed -13 dBm0.

2.3 For systems not transmitting tones continuously, e.g. amplitude-modulation systems, the signal
characteristics should meet all of the following requirements (see also Note 1 to § 1.3):
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 to
that 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 a 10 Hz bandwidth centred at any frequency
shall 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 subscriber’s equipment and the
international circuit, so that § 2 of the present Recommendation should be taken as providing general planning
guidance.

Note 2 - In switched connections, the loss between subscribers' telephones may be high: 30 to 40 dB. The level
of the signals received will then be very low, and these signals may suffer disturbance from the dialling pulses sent
over other circuits.

If there is likely to be a heavy demand for international connections for data transmission over the switched
network, some Administrations might want to provide special 4-wire subscriber lines. If so, the levels to be used might
be those proposed for leased circuits.

References

[1] 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.

2 Fascicle VIII.1 - Rec. V.2