INTERNATIONAL TELECOMMUNICATION UNION

ITU-T E.180/Q.35
TELECOMMUNICATION (03/98)
STANDARDIZATION SECTOR
OF ITU

SERIES E: OVERALL NETWORK OPERATION,

TELEPHONE SERVICE, SERVICE OPERATION AND
HUMAN FACTORS
Operation, numbering, routing and mobile services –
International operation – Tones in national signalling
systems

SERIES Q: SWITCHING AND SIGNALLING

International automatic and semi-automatic working –
Tones for use in national signalling systems

Technical characteristics of tones for the

telephone service

ITU-T Recommendation E.180/Q.35

(Previously CCITT Recommendation)
 ITU-T E-SERIES RECOMMENDATIONS
OVERALL NETWORK OPERATION, TELEPHONE SERVICE, SERVICE OPERATION AND HUMAN
FACTORS

OPERATION, NUMBERING, ROUTING AND MOBILE SERVICES

INTERNATIONAL OPERATION
Definitions E.100–E.103
General provisions concerning Administrations E.104–E.119
General provisions concerning users E.120–E.139
Operation of international telephone services E.140–E.159
Numbering plan of the international telephone service E.160–E.169
International routing plan E.170–E.179
Tones in national signalling systems E.180–E.199
Maritime mobile service and public land mobile service E.200–E.229
OPERATIONAL PROVISIONS RELATING TO CHARGING AND ACCOUNTING IN THE
INTERNATIONAL TELEPHONE SERVICE
UTILIZATION OF THE INTERNATIONAL TELEPHONE NETWORK FOR NON-
TELEPHONY APPLICATIONS
ISDN PROVISIONS CONCERNING USERS E.330–E.399
QUALITY OF SERVICE, NETWORK MANAGEMENT AND TRAFFIC ENGINEERING
NETWORK MANAGEMENT
TRAFFIC ENGINEERING
QUALITY OF TELECOMMUNICATION SERVICES: CONCEPTS, MODELS,
OBJECTIVES AND DEPENDABILITY PLANNING

For further details, please refer to ITU-T List of Recommendations.

 ITU-T Q-SERIES RECOMMENDATIONS
SWITCHING AND SIGNALLING

SIGNALLING IN THE INTERNATIONAL MANUAL SERVICE Q.1–Q.3

INTERNATIONAL AUTOMATIC AND SEMI-AUTOMATIC WORKING Q.4–Q.59
Basic Recommendations Q.4–Q.9
Numbering plan and dialling procedures in the international service Q.10–Q.11
Routing plan for international service Q.12–Q.19
General Recommendations relative to signalling and switching systems (national or Q.20–Q.34
international)
Tones for use in national signalling systems Q.35–Q.39
General characteristics for international telephone connections and circuits Q.40–Q.47
Signalling for satellite systems Q.48–Q.49
Signalling for circuit multiplication equipment Q.50–Q.59
FUNCTIONS AND INFORMATION FLOWS FOR SERVICES IN THE ISDN Q.60–Q.99
CLAUSES APPLICABLE TO ITU-T STANDARD SYSTEMS Q.100–Q.119
SPECIFICATIONS OF SIGNALLING SYSTEMS No. 4 AND No. 5 Q.120–Q.249
SPECIFICATIONS OF SIGNALLING SYSTEM No. 6 Q.250–Q.309
SPECIFICATIONS OF SIGNALLING SYSTEM R1 Q.310–Q.399
SPECIFICATIONS OF SIGNALLING SYSTEM R2 Q.400–Q.499
DIGITAL EXCHANGES Q.500–Q.599
INTERWORKING OF SIGNALLING SYSTEMS Q.600–Q.699
SPECIFICATIONS OF SIGNALLING SYSTEM No. 7 Q.700–Q.849
DIGITAL SUBSCRIBER SIGNALLING SYSTEM No. 1 Q.850–Q.999
PUBLIC LAND MOBILE NETWORK Q.1000–Q.1099
INTERWORKING WITH SATELLITE MOBILE SYSTEMS Q.1100–Q.1199
INTELLIGENT NETWORK Q.1200–Q.1999
BROADBAND ISDN Q.2000–Q.2999

For further details, please refer to ITU-T List of Recommendations.

ITU-T RECOMMENDATION E.180/Q.35

TECHNICAL CHARACTERISTICS OF TONES

FOR THE TELEPHONE SERVICE

Summary

This Recommendation sets the limits and recommended values of cadences, frequencies and levels for tones used in the
telephone service. Those technical characteristics are relevant either for audible tones applied within the network or for
those generated at the digital terminal equipment.

Source

ITU-T Recommendation E.180/Q.35 was revised by ITU-T Study Group 2 (1997-2000) and was approved under the
WTSC Resolution No. 1 procedure on the 9th of March 1998.

Recommendation E.180/Q.35 (03/98) i

 FOREWORD

ITU (International Telecommunication Union) is the United Nations Specialized Agency in the field of telecommuni-
cations. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of the ITU. The ITU-T is
responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to
standardizing telecommunications on a worldwide basis.

The World Telecommunication Standardization Conference (WTSC), which meets every four years, establishes the
topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics.

The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in WTSC
Resolution No. 1.

In some areas of information technology which fall within ITU-T’s purview, the necessary standards are prepared on a
collaborative basis with ISO and IEC.

NOTE

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

INTELLECTUAL PROPERTY RIGHTS

The ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the
use of a claimed Intellectual Property Right. The ITU takes no position concerning the evidence, validity or applicability
of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation
development process.

As of the date of approval of this Recommendation, the ITU had not received notice of intellectual property, protected by
patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may
not represent the latest information and are therefore strongly urged to consult the TSB patent database.

 ITU 1998

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.

ii Recommendation E.180/Q.35 (03/98)

 CONTENTS

Page

1 General............................................................................................................................................................ 1

2 Electrical levels for tones ................................................................................................................................ 1

3 Acoustical levels for tones .............................................................................................................................. 2

4 Dial tone.......................................................................................................................................................... 3

5 Ringing tone.................................................................................................................................................... 3

6 Busy tone and congestion tone........................................................................................................................ 4

7 Special information tone ................................................................................................................................. 6

8 Warning tone to indicate that a conversation is being recorded ...................................................................... 6

9 Payphone recognition tone .............................................................................................................................. 6

9.3 Duration and level ............................................................................................................................. 7

10 Call waiting tone ............................................................................................................................................. 7

11 Caller waiting tone .......................................................................................................................................... 7

12 Machine recognition of tones .......................................................................................................................... 8

Annex A Digital generation of tones ......................................................................................................................... 8

Annex B Examples for limitation of spurious components of the dial tone with respect to interference with the
frequencies recommended for pushbutton telephone sets in Recommendation Q.23...................................... 9
B.1 Method A (used by ATT) .................................................................................................................. 9
B.2 Method B (used by the Federal Republic of Germany) ..................................................................... 9

Recommendation E.180/Q.35 (03/98) iii

Recommendation E.180/Q.35
Recommendation E.180/Q.35 (03/98)

TECHNICAL CHARACTERISTICS OF TONES1

FOR THE TELEPHONE SERVICE
(revised in 1998)

1 General
Administrations are reminded of the advantages of standardizing audible tones as far as possible so that subscribers and
operators may quickly recognize any tone transmitted of whatever origin2.
Guidance on the application of tones and recorded announcements in various situations is given in Recommen-
dation E.182.
In considering the degree of standardization, the ITU-T took account of the nature of the various tones already in use. It
was also considered that Administrations introducing new tones would find it helpful to know the preferred limits of
cadence, frequency and level.
Limits for tone cadences and frequencies are set forth below, all working tolerances being included in the limits.
Besides the limits applying to specifications, limits have been laid down for application to existing exchanges.
These latter limits are herein called accepted limits, while those for new equipment are called recommended limits.
The present Recommendation covers the case where audible tones are applied within the network. However, the same
frequencies and cadences are to be applied if, in the ISDN, the audible tones are generated at the terminal equipment.
All Administrations and operating agencies are requested to submit a concise technical description of any new or changed
network tones to the ITU-T whenever such tones are introduced. The technical detail should include: level, frequencies,
cadence, duty cycle, modulation and any other parameters that are required to fully describe the new or changed network
tone.

2 Electrical levels for tones

For international purposes, the levels of the ringing tonethe busy tone, the congestion tonethe special information tone
and the warning tonehave to be defined at a zero relative level point at the incoming (in the traffic direction) end of the
international circuit.
The level of tones so defined must have a nominal value of –10 dBm0. The recommended limits should be not more than
–5 dBm0 nor less than –15 dBm0 measured with continuous tone.
For the special information tone, a difference in level of 3 dB is tolerable between any two of the three frequencies which
make up the tone.
For the power level of the dial tone, the point of reference is the local exchange, where the subscriber line is connected.
In the existing networks the absolute power at the 2-wire access in the direction towards the subscriber station is normally
in the range of –10 dBm ± 5 dB. However, with respect to interference with Multifrequency Pushbutton (MFPB)
receivers dial tone levels higher than –10 dBm should be avoided.

_______________
1 See Supplement No. 2 at the end of the Blue Book, Fascicle II.2, for particular values of tone cadences and frequencies in actual
use.
2 Recommendation E.181 specifies the information which could be given to users to facilitate recognition of foreign tones.

Recommendation E.180/Q.35 (03/98) 1

NOTE – The relative level of local exchanges in an analogue network is not fixed. For digital local exchanges the relative levels are
given in Recommendation Q.517. A preferred level range of digital tone generators is –8 dBm0 to –3 dBm0 corresponding with the
above level range at the output of local exchanges.

3 Acoustical levels for tones

When tones are generated by a source within a network, e.g. by a telephone exchange, the power level as perceived by the
user will be influenced by the characteristics of the subscriber’s line and the equipment between the source and the user’s
ear.

Furthermore, tones can be generated within the user’s equipment, triggered by signals from the exchange. In these
circumstances it is necessary to define the tone level in terms of the preferred range of sound pressure levels as heard by
the listener.

Research has shown that the preferred listening level for information tones is substantially independent of room noise,
circuit noise and tone cadence, but does vary over a range of tone frequencies. Figure 1 shows the recommended sound
pressure levels, with upper and lower limits of the recommended range, over a range of tone frequencies, based on these
experiments.

dBPa dBrap
10 104
Upper limit of recommended
range
8 102

6 100
Recommended
level
4 98
Sound pressure levels

2 96
Lower limit of recommended
range

Tone levels
0 94

–2 92

–4 90

–6 88

–8 86

–10 84

–12
200 300 400 500 600 700 800 1000 2000 Hz
T0207440-98/d01
Frequency

Figure 1/E.180 – Recommended listening level limits for tones

FIGURE 1/E.180...[D01] = 11.4 CM

It is emphasized that there is no one-to-one relationship between electrical and acoustical power levels. What acoustic
level will result from a given electrical level is dependent on various parameters such as the characteristics of the user’s
equipment.

It should be noted that the recommended sound pressure levels apply only to the most common situation of a user
listening via a telephone handset, held reasonably close to the ear so that normal "ear coupling loss" values apply.

When using a loudspeaking telephone or a headset, the preferred sound pressure level is generally lower than the
recommended levels.

2 Recommendation E.180/Q.35 (03/98)

4 Dial tone

4.1 It is recommended that dial tone should be a continuous tone.

4.2 It is recommended that dial tone should be:

– either a single frequency tone in the range 400-450 Hz;

– or a combined tone composed of up to three frequencies, with at least one frequency in each of the ranges
340-425 Hz and 400-450 Hz. The difference between any two frequencies should be at least 25 Hz.

4.3 Recognizing the local nature of "normal" use of dial tone, as well as the technical and economic consequences
and consequences on customer habits of changes in dial tone, the full range of existing dial tones, including
non-continuous tones as in Supplement No. 2 at the end of Blue Book, Fascicle II.2, are considered acceptable. However,
when adopting a new single frequency dial tone, Administrations are recommended to use 425 Hz.

4.4 Where digital tone generation is applied, the frequencies for dial tone should be the same as those
recommended for analogue generated tones (see Annex A).

4.5 In order to prevent interference of harmonics or spurious components of the dial tone with the frequencies
recommended for pushbutton telephone sets in Recommendation Q.23 and the MFPB signal reception specified in
Recommendation Q.24, the maximum permissible power level of harmonics or quantizing noise of the dial tone has to be
limited in a suitable way, depending on the specific characteristics of the implementations of the dial tone generator and
the MFPB receivers within the same exchange. Examples of such limitations for the dial tone generator are given in
Annex B.

NOTE – In cases of digital generation of the dial tone, the quantizing noise is composed of a number of spectral lines which depend
on the number of samples in the generating pattern. In order to reduce the amplitude of the quantizing components, the number of
samples should be chosen sufficiently high, thus spreading the quantizing distortion power more evenly over the whole spectrum.

5 Ringing tone

5.1 Ringing tone is a slow period tone, in which the tone period is shorter than the silent period.

The recommended limits for the tone period (including tolerances) are from 0.67 to 1.5 seconds. For existing exchanges,
the accepted upper limit for the tone period is 2.5 seconds.

The recommended limits for the silent period separating two tone periods are 3 to 5 seconds. For existing exchanges, the
accepted upper limit is 6 seconds.

The first tone period should start as soon as possible after the called subscriber’s line has been found.

Figure 2 shows the recommended and accepted limits for the ringing tone periods.

5.2 The ringing tone cadence should be similar to the cadence used for applying ringing current to the called
subscriber’s telephone set, but these two cadences need not be synchronized. The electrical parameters of the ringing
current must be evaluated by the Administration concerned to prevent shock hazard.

5.3 The recommended frequency for the ringing tone is between 400 and 450 Hz. The accepted frequency should
be not less than 340 Hz, nor more than 500 Hz. Frequencies between 450 and 500 Hz in the accepted frequency range
should, however, be avoided. Administrations adopting a new single frequency ringing tone are recommended to use 425
Hz.

The ringing tone frequency may be modulated by a frequency between 16 and 100 Hz, but such modulation is not
recommended for new equipment. If the accepted frequency is more than 475 Hz, no modulation by a lower frequency is
allowed.

Recommendation E.180/Q.35 (03/98) 3

 s
6

Accepted
values

5
Recom-
mended

Silent period (S)

values

2
0 0.67 1 1.5 2 2.5 3 4 s
T0207450-98/d02
Tone period (E)

Frequency:

Recommended interval: 400-450 Hz

Accepted interval: 340-500 Hz

Figure 2/E.180 – Ringing tone

FIGURE 2/E.180...[D02] = 11 CM

5.4 Where digital tone generation is applied, the frequency for ringing tone should be the same as that
recommended for analogue generated tones (see Annex A).

5.5 Where a special ringing tone (see A.2.6/E.182) is implemented, it must be a tone similar in most respects to the
national ringing tone that it represents. It may be constructed by making any of a wide variety of possible variations on
the ringing tone. As one example, if the ringing tone is modulated, the special ringing tone might stop the modulation for
the last 100 milliseconds of each ring burst to form the special ringing tone. However, the tone is constructed, it must be
shown by appropriate empirical testing with callers to cause callers to wait for an answer if they do not know about the
tone and its meaning. This test must be conducted with a sample of at least 100 persons who do not know the tone, and
must show that on at least 95% of occasions when they hear the special ringing tone, they interpret the tone to mean that
they should wait for an answer. The purpose of this requirement is to prevent confusion on the part of callers, especially
international callers, who are not knowledgeable about the tone.

6 Busy tone and congestion tone

6.1 The (subscriber) busy tone and the (equipment or circuit group) congestion tone are quick period tones in which
the tone period is theoretically equal to the silent period. The total duration of a complete cycle (tone period E + silent
period S) should be between 300 and 1100 milliseconds.

The ratio E/S of the tone period to the silent period should be between 0.67 and 1.5 ( recommended values).

For existing exchanges, or for tones to be used in a special way, it is accepted that the tone period may be up to 500
milliseconds shorter than the silent period (E ≥ S – 500 milliseconds). In no circumstances should the tone period be
shorter than 100 milliseconds.

Figure 3 shows the recommended and the accepted areas for the busy tone and the congestion tone periods.

4 Recommendation E.180/Q.35 (03/98)

 ms

1000

800

0
25
–
2 S
+
= E

E
+
S
E

=
11
00
600 Accepted
Silent period (S)

values

S
Recommen-
.67
E = 100

ded values
=0

400
E

S
1.5
=
E
200 E
+
S
=
30
0

0 200 400 600 800 1000 ms

T0207460-98/d03
Tone period (E)

Frequency:

Recommended interval: 400-450 Hz

Accepted interval: 340-500 Hz

Figure 3/E.180 – (Subscriber) busy tone and (equipment or circuit group)

congestion tone

FIGURE 3/E.180...[D03] = 15.2 CM

6.2 The busy tone (of the called subscriber) and the congestion tone (of switching equipment or circuit groups) can
be identical or almost identical, providing that this does not create any serious problems for the network and does not
cause the subscriber to become confused. However, a distinction between these two tones is desirable:

– to allow Administrations to assess the quality of service;

– for the convenience of experienced subscribers.

6.3 Where a distinct congestion tone is used, it is recommended that:

a) the same frequency should be used for the busy tone and the congestion tone;

b) the busy tone should have a slower cadence than the congestion tone, but both cadences should be within the limits
mentioned in 5.1 above.

6.4 The recommended frequency for the busy tone and for the congestion tone must be between 400 and 450 Hz.
The accepted frequency must not be less than 340 nor more than 500 Hz. Frequencies between 450 and 500 Hz in the
accepted frequency range should, however, be avoided. Administrations adopting a new single frequency for busy and
congestion tones are recommended to use 425 Hz.

6.5 Where digital tone generation is applied, the frequency for busy and congestion tones should be the same as
that recommended for analogue generated tones (see Annex A).

Recommendation E.180/Q.35 (03/98) 5

7 Special information tone
7.1 The special information tone is provided for all cases in which neither the busy nor the congestion tone can give
the required information to the calling subscriber in the case of call failure. There are three ways in which it may be used:
a) when in special cases no provision is made for recourse either to a recorded announcement or to an operator, the
equipment at the point which the calls have reached must:
1) either connect the special information tone to the call,
2) or preferably, if technically available, send an appropriate backward signal such that connection to the special
information tone will be made by equipment which is nearer to the caller;
b) when the call is connected to a recorded voice machine: the tone is then given during the silent intervals between
transmissions of the announcement;
c) under arrangements made at manual positions serving lines which have been abnormally routed so that by operating
a key the operators may send the special information signal when, for example, the calling subscriber fails to
understand the operator.

When the special information tone is applied with or without a recorded announcement, it should be recognized that
customers may refer to an operator if they fail to understand the meaning of the recorded announcement and/or the
special information tone.

7.2 The special information tone has a tone period theoretically equal in length to the silent period.

Tone period – The tone period consists of three successive tone signals, each lasting for 330 ± 70 milliseconds. Between
these tone signals there may be a gap of up to 30 milliseconds.

Silent period – This lasts for 1000 ± 250 milliseconds.

7.3 The frequencies used for the three tone signals are: 950 ± 50 Hz; 1400 ± 50 Hz; 1800 ± 50 Hz, sent in that
order.

8 Warning tone to indicate that a conversation is being recorded

Where a conversation is being recorded at a subscriber’s station, it is recommended that the Administration require the
use of a warning tone to indicate that the conversation is being recorded. When such a tone is applied, it is recommended
that:
a) it consists of a 350-500 ms pulse every 15 ± 3 seconds of recording time; and
b) the frequency of the tone should be 1400 Hz ± 1.5%.

9 Payphone recognition tone

9.1 Where Administrations see the necessity of application of a payphone recognition tone in order to allow
operators to recognize that a call originates at a payphone station or that the called number belongs to a payphone station,
it is recommended to use a payphone recognition tone.

The application of the tone will depend on the operational requirements of individual Administrations, e.g. in some cases
the tone will only be required on an incoming call to the payphone, whilst in others there may be a requirement for the
tone to be present on originating calls and throughout the period of the call.

9.2 The tone is a combination of two frequencies f1 and f2 in the range:

f1: 1100-1750 Hz
f2: 750-1450 Hz

with the ratio: f1/f2 = 1.2 to 1.5

and with a cadence (frequency sequence) as follows:

f1 on 200 ms, silence 200 ms, f2 on 200 ms, silence 2 s (one cycle is therefore 2.6 s).

6 Recommendation E.180/Q.35 (03/98)

9.3 Duration and level

9.3.1 A principal purpose of the payphone recognition tone in international telephony is to identify a called station as
a payphone where the possibility exists of attempted fraud on a collect call. For this purpose the tone must be produced as
soon as a payphone answers a call, it must be clearly audible to an operator, and it must cease before it can seriously
interfere with conversation.

When the tone is used on an incoming call to payphone, it should have, in addition to those characteristics defined in 9.2,
a duration of 5 complete cycles (13 s).

9.3.2 If the tone is used to identify payphones which are originating calls, its duration is not specified.

9.3.3 The specification in 9.3.1 applies only to the first five cycles of the tone when the payphone is the receiving
station.

For use throughout a call or during conversation, the level and duration of the tone have to meet two contradictory
requirements:

– the public exchange operator should be able to detect and recognize the tone in the presence of the highest expected
levels of speech;

– the tone should not interfere unduly with normal conversation.

Experience of customer reaction to the tone requires that the time during which the tone is applied should be as short as
possible, subject to operational requirements. Similarly, the level of the tone should be as low as possible and
significantly lower than the recommended levels for other tones (e.g. –20 dBm at the payphone output). The duration of
the tone and the level at which it is applied are interdependent factors: the shorter the duration the higher the level and
vice versa. (Further studies on the recommended levels and duration will be carried out.)

10 Call waiting tone

10.1 The call waiting tone is used to advise a subscriber who is engaged on a call that another subscriber is
attempting to call.

10.2 The tone is intended to be sufficiently alerting to succeed in its purpose without interfering with existing
conversation.

10.3 The recommend specification of the tone is one or more cycles defined by a frequency f in the range:

f : 400 to 450 Hz

and with a cadence (frequency sequence) as follows:

a) f on 300 to 500 ms, silence 8 to 10 s (f = 300 ms is preferable to the longer tone since the ongoing conversation
would be interrupted for a briefer interval); or

b) f on 100 to 200 ms, silence 100 to 200 ms, f on 100 to 200 ms (the total to be no more than 500 ms); 8 to 10 s
silence.

Other tones may be acceptable.

10.4 The second and subsequent cycles may be at a lower level than the initial one.

10.5 Where the tone continues for more than one cycle, it should preferably cease when it is no longer possible to
accept the waiting call.

11 Caller waiting tone

11.1 This tone advises a caller that a called station, though busy, has a call waiting service active.

11.2 It is intended that, if this tone is not correctly interpreted by subscribers, it be misinterpreted as the ringing tone.

Recommendation E.180/Q.35 (03/98) 7

11.3 To dissuade a caller from waiting indefinitely, the tone may cease 30 seconds3 after it starts and may be
replaced by busy tone, or an Administration may decide to disconnect the calling station.
11.4 The caller waiting tone consists of a ringing tone followed, after a silent interval of 0 to 200 ms, by one of the
following:
a) the tone defined in 10.3 a);
b) the pair of tones defined in 10.3 b);
c) another call waiting tone in use by an Administration, provided that it can be appended to each sounded part of the
ringing tone.
11.5 The caller waiting tone, as defined in 11.4, should be distinguishably different from the ringing tone when
directly compared with it.

12 Machine recognition of tones

The UIT-T appreciates the value of machine recognition of tones for the purpose of service observations, maintenance,
testing or for the collection of statistics where equivalent electrical signals do not exist. However, the UIT-T considered,
at Mar del Plata in 1968, that such machine recognition should not be a substitute for electrical signals. Where machine
recognition of audible tones is to be introduced, the tone frequencies and cadences must be within close limits of
precision.
For dial tone, ringing tone, busy and congestion tones a working frequency tolerance of ± 1% should be met.
NOTE – The figure of 1% is taken as a compromise out of several national specifications which vary between ± 0.5% and ± 1.5%.
(See also Supplement No. 3 of the Blue Book, Fascicle II.2.)

Annex A

Digital generation of tones

The practice of several Administrations and equipment designers for digital generation of tones is known to deviate
largely:
– in the frequency chosen within the recommended range;
– in the power level which varies with the national application;
– in the mechanism of generation of tones and signal frequencies where, in part, the same equipment is used.
Therefore, it was found difficult to standardize on a fixed number of samples with a coded bit-stream, which represents
one frequency with one distinct power level.
On the other hand, there is no necessity for standardizing digital generated tones in a more stringent way than analogue
generated tones for the following reasons:
– It is to the interest of Administrations that subscribers should not be confused by hearing different tones for the same
purpose within their national networks. Consequently, the practice already in use for analogue generated tones
should be maintained for reasons associated with the human factor.
– The advantages that can be achieved by standardizing the code words for the tones in order to allow automatic
recognition of tones by monitoring the bit stream seem to be so small that they do not justify a stringent restriction
on all possible methods for digital generation of any frequency allocated with any level.

_______________
3 The specification of this time needs further study.

8 Recommendation E.180/Q.35 (03/98)

– For a long period of time, a mixture of analogue and digital networks will exist. Thus, machine recognition of tones
will have to be performed also with analogue receivers.

However, when Administrations have full freedom to make new decisions about tones in future networks, especially with
respect to an all-digital network, they may consider a preferred solution for the digital generation of dial tone, busy tone,
congestion tone and ringing tone having a uniform frequency of 425 Hz, as recommended by UIT-T.

Annex B

Examples for limitation of spurious components of the dial tone

with respect to interference with the frequencies recommended
for pushbutton telephone sets in Recommendation Q.23

B.1 Method A (used by ATT)

The total distortion power should be at least 33 dB less than the dial tone power, and the distortion power in any 100 Hz
band above 500 Hz should be at least 40 dB less than the dial tone power.

B.2 Method B (used by the Federal Republic of Germany)

In the frequency range from 500 to 2000 Hz [i.e. the range of Multifrequency Pushbutton (MFPB) frequencies] the
distortion power in any 100 Hz band should be at least 40 dB below the dial tone power. In addition, in the frequency
range above 2000 Hz up to 4000 Hz, the total distortion power should be at least 25 dB below the dial tone power.

Recommendation E.180/Q.35 (03/98) 9

 ITU-T RECOMMENDATIONS SERIES

Series A Organization of the work of the ITU-T

Series B Means of expression: definitions, symbols, classification

Series C General telecommunication statistics

Series D General tariff principles

Series E Overall network operation, telephone service, service operation and human factors
Series F Non-telephone telecommunication services

Series G Transmission systems and media, digital systems and networks

Series H Audiovisual and multimedia systems

Series I Integrated services digital network

Series J Transmission of television, sound programme and other multimedia signals

Series K Protection against interference

Series L Construction, installation and protection of cables and other elements of outside plant

Series M TMN and network maintenance: international transmission systems, telephone circuits,
telegraphy, facsimile and leased circuits
Series N Maintenance: international sound programme and television transmission circuits
Series O Specifications of measuring equipment
Series P Telephone transmission quality, telephone installations, local line networks
Series Q Switching and signalling
Series R Telegraph transmission

Series S Telegraph services terminal equipment

Series T Terminals for telematic services

Series U Telegraph switching

Series V Data communication over the telephone network

Series X Data networks and open system communications

Series Y Global information infrastructure

Series Z Programming languages