International Telecommunication


International Standards


Administrations and operating companies throughout the world carry on studies

of technical and other problems related to the inter-working of their respective

national telecommunication systems to provide a worldwide telecommunications

network. Two international committees exist for this purpose :

1) The International Telegraph and Telephone Consultative Committee (CCITT),


2) The International Radio Consultative Committee (CCIR).

They operate under the auspices of the International Telecommunication Union (ITU).

They promulgate their decisions in the form of Recommendations, which are published

by ITU. Generally, these Recommendations cover features of international circuits, but

where essential, they deal with relevant characteristics of the national systems which

may form part of international connections. This compendium collects, in condensed form ,

major Recommendations dealing with telephone, telegraph , and data-transmission circuits

and equipment.

November 21, 2020 7:17 PM


Recommendations of the CCITT


The CCITT develops new Recommendations, and updates existing ones, through

the activities of Study Groups, whose reports are acted on at Plenary Assemblies,

which meet at intervals f 3 or 4 years. The resulting Recommendations of the

Second Plenary Assembly, New Delhi, 1960, were published by the ITU in a number

of volumes, called collectively the Red Book. The subsequent study periods

culminated in the Third Plenary Assembly, Geneva, 1964 (Blue Book);

the Fourth Plenary Assembly, Mar del Plata, 1968 (White Book); and the

Fifth Plenary Assembly, Geneva, 1972 (Green Book). This compendium refers

to Green Book Recommendations, designated thus L (G.101), (H.31), (V.2), etc.

November 21, 2020 7:18 PM


Recommendations of the CCIR


The CCIR also functions with Study Groups and Plenary Assemblies.

The Eleventh Plenary Assembly was held at Oslo in 1966, the

Twelfth Plenary Assembly at New Delhi in 1970, and the

Thirteenth Plenary Assembly at Geneva in 1974.

After each Plenary Assembly , the ITU publishes volumes which contain

the currently accepted Recommendations, including such

Recommendations of the Plenary Assemblies at London (1953),

Warsaw (1956), Los Angeles (1959), and Geneva (1963) which are

still effect. No color coding is used. This compendium deals with those

Recommendations which treat point-to-point radio relay systems.

A purpose of those Recommendations is to make the performance of

such systems compatible with metallic line systems which follow the

CCITT Recommendations. References to the CCIR Recommendations

are made this : (CCIR,391).

November 21, 2020 7:18 PM


Zero-Relative-Level Points and Relative Levels


Many CCIT and CCIR Recommendations specify signal or noise levels at "a point

of zero relative level", or in dBm0 or pWp0, etc, where "0" (zero) stands for

"measured art or referred to a point of zero relative level".

1) 2-wire switching systems :


In 2-wire switching systems, the sending end terminals of a long-distance circuit

have long been considered to be at a point of zero relative level. The relative levels

of all other points are calculated from this reference point, as the algebraic sum of

all transmission losses and gains from it to the point in question. Any point in a circuit

with the same relative level as the sending terminals is a point of zero relative level,

which may be written 0 dBr (dB relative level). The American term for relative level is

transmission level. Thus : "Zero-transmission-level point" (0TLP).

For convenience in comparing circuit noise performance, it is customary to convert

absolute noise measurements made at the receiving ends of circuits having various

relative levels, to absolute power levels, at a zero-relative-level point. For example,

50 dBmp of noise at a 7-dBr point would be reported as 43 dBm0p. Signaling-tone

levels are similarly expressed. For example, a tone introduced at a 3.5-dBr point

with an absolute power level of 18.5 dBm may be referred to as a 15-dBm0 signal.

The latter designation would apply to such a tone no matter where it appeared; the

"0" denotes that its level is referred to a point of zero relative level . (Refer to table 1).

Statistics of speed power, requirements for linearity and limiting, system loading

factors, cross-talk, and noise have become well known in terms of their values at

points of zero relative level. The proper performance of voice repeaters, carrier

terminal and line equipment, radio relay systems etc., depends on adherence to

the relative levels for which they were designed. Many relative levels associated

with such equipment have been standardized.

November 21, 2020 7:23 PM


2) 4-Wire Switching Systems


In 4-wire switching systems, it is often considered desirable to handle speech and signaling at

lower values of absolute power through the switching equipment than is customary in 2-wire

systems. In 1964, the CCITT adopted a relative level of 3.5 dBr for the sending end of a 4-wire

circuit, at the "virtual" switching points. These are theoretical points; their exact location depends

on national practice , and the CCITT considers it unnecessary to define them. (In American

commercial system, 2 dBr is widely used).

Therefore, to ensure that carrier and other transmission equipment will be subjected to the

same absolute speech and signaling power levels as in 2-wire systems, determination of

relative levels in 4-wire circuits must take into account the relative level of the virtual

switching points. In a 4-wire circuit, there may be no actual point of zero relative level.

Nevertheless, standards will continue to refer many requirements to a zero relative point.

(Refer to Table 2).

Currently, many transmission measurements are made with a standard 800 or 1000-hertz

test tone, with an absolute power of 1 milliwatt at a zero-relative-level point : a power of

0 dBm0. The actual level applied is adjusted to the relative level of the sending point. The

test-tone level in dBm will be numerically equal to the relative level in dBr at any point in the

circuit, but it is not proper to express relative levels in dBm, since dBm represents absolute

power levels. If the standard-test-tone power is ever changed to another value, such as

10 dBm0, as has been tentatively proposed, the distinction between relative levels and

test-tone levels will be more apparent.

November 21, 2020 7:46 PM


Psophometric Noise and Power


The CCITT calls a noise measuring set a "psophometer". A psophometer includes

a device for measuring power through a weighting network. For measurements

on commercial telephone circuits, a weighting characteristic is usedwhich results

in the objective instrument measurements approximately paralleling the results

of subjective tests with human observes using modern telephone sets. The CCITT

weighting characteristic for commercial circuits is nominally identical with the

American F1A line weighting. Psophometric noise power may be expressed in

dBm0 "psophometrically weighted", or dBm0p. The conventional conversion

equation used between dBm0p and dBa0 (F1A) is

     dBm0p = dBa0-84

Psophometric Weighting for Commercial Telephone Circuits :


Frequency (hertz)    Level (dB)

100               -41.0

150               -29.0

200               -21.0

250               -15.0

300               -10.6

400               - 6.3

500               - 3.6

600               - 2.0

700               - 0.9

800                0.0

900               + 0.6

1000              + 1.0

1100              + 0.6

1200               0.0

1350              - 0.65

1500              - 1.30

1750              - 2.22  

2000              - 3.00

2250              - 3.60

2500              - 4.20 

2750              - 4.87

3000              - 5.60

3500              - 8.5

4000             - 15.0

4200             - 18.7

4500             - 25.0

4700             - 29.4

5000             - 36.0

November 21, 2020 7:49 PM


Psophometric Weighting Factor


If uniform-spectrum random noise is measured in a 3.1-kilohertz band with

a flat attenuation/frequency characteristic, the noise level must be reduced

by 2.5 decibels to obtain the psophometric power level. For another

bandwidth B, the weighting factor will be equal to :

      2.5+10 log10 (B/3.1) decibels.

When B=4 kilohertz, for example, this gives a weighting factor of 3.6 decibels.

November 21, 2020 7:50 PM


Psophometric Power


Where power addition of noise can be assured, it has been found convenient for

calculations and design of international circuits to use the concept of

"psophometric power".

psophometric power = (psophometric voltage)2/600

                 = (psophometric emf)2/(4x600)

A convenient unit is the picowatt (pW) = 10-12 watt, so that

psophometric power in pW = (psophometric voltage in mV)2/0.0024

November 21, 2020 7:51 PM


Conventional Telephone Signal


For the calculation or measurement of cross-talk noise between adjacent

channels, of the balance return loss for echo, and generally speaking ,

when it is desired to simulate the speech currents transmitted by a

telephone channel, the CCITT recommends the use of a conventional

telephone signal. This signal may be produced by passing the output of

a generator of a uniform-spectrum random noise signal ("white noise")

through a weighting network with a characteristic as shown in Fig.1.

The amount of this signal that appears in another circuit because of

cross-talk , etc., is measured with a psophometer or weighted-noise

measuring set, with standard psophometric weighting for commercial

telephone circuits.

November 21, 2020 7:53 PM


Telephone Circuit Loading


To simplify calculations when designing carrier systems on cables or radio links,

the CCITT has adopted a conventional value to represent the mean absolute

power level , at a point of zero relative level, of the speech-plus-signaling

currents, etc., transmitted over a telephone channel in one direction of

transmission during the busy hour, which is -15 dBm ( -1.73 nepers)

(mean power = 31.6 microwatts); this is the mean with time and the mean for

a large batch of circuits. This total mean power of about 32 microwatts is

conventionally distributed as follows (nominal mean power) : 10 microwatts,

all signaling and tones; 22 microwatts, to include speech currents

(including echoes), carrier leak, and telegraph signals, based on a speech

activity factor of 0.25 for one direction of a telephone channel. No account is

taken of pilot signals, which are assumed to be an integral part of the carrier

system, not affecting telephone channel power.

November 21, 2020 7:53 PM


Conventional Load


It will be assumed for the calculation of inter-modulation noise below

the overload point that the multiplex signal during the busy hour can

be represented by a uniform-spectrum random noise signal, the mean

absolute power level of which, at a zero-relative point, n(P), is given by

      n(P) = -15+10 log10N dB

      for N = 240 or more


      n(P) = -1+4 log10N dB

      for values of N between 12 and 240

when N is the total number of telephone channels in the system.

Typical values so calculated are as follows :

      N             n(P), dB

   ----------       -------------------

     12             3.3

     24             4.5

     36             5.2

     48             5.7

     60             6.1

    120             7.3

      .               .

      .               .

      .               .

    240            8.8

    300            9.8

    600           12.8

    960           14.8

   1800           17.5

   2700           19.3

Assumed : No pre-emphasis, and use of independent amplifiers for each direction.

November 21, 2020 7:54 PM


Power Levels


For cross-talk reasons , each component of a short-duration signal should

not exceed the following absolute power levels, at a zero-relative-level point.

Signaling Frequency      Absolute Power Level at Zero-Relative-Level Point

  (hertz)                       (dBm0)

--------------------------    ------------------------------------------------------------------

  800                           -1

 1200                          -3

 1600                          -4

 2000                          -5

 2400                          -6

2800, 3200                -8

November 21, 2020 7:55 PM


Private Telegraph Transmission on a Rented International Circuit,

with Alternative Private Telephone Service


The frequency of 1500 hertz is recommended for private telegraph

transmission between subscribers permanently connected via a

rented international circuit. The permissible power for a continuously

transmitted telegraph marking signal is 0.3 milliwatt at a

zero-relative-level point (-5 dBm0).

November 21, 2020 7:56 PM


Simultaneous Communication by Telephony

and Telegraphy on a Telephone Circuit


A continuously transmitted telegraph signal should

not exceed a level of -13 dBm0. There should not

be more than 3 telephone circuits of this type per

group, nor more than the number of super-groups

in a wide-band system.

November 21, 2020 7:57 PM


Photo-telegraphy Transmissions Over Telephone Circuits

which are Entirely 4-Wire between Photo-telegraph Stations


The sent voltage for the photo-telegraph signal corresponding to

maximum amplitude should be so adjusted that the absolute

power level of the signal, at a zero-relative-level point, is 0 dBm0

for amplitude-modulation facsimile. In the former , the "black" level

is about 30 decibels lower than the "white" level.

November 21, 2020 7:57 PM


Power Levels for Data Transmission over Telephone Circuits :

      [  Private Wires on Carrier Systems ]


1) Maximum power output of subscriber's apparatus into line : 1 milliwatt.

2) Continuous-tone systems (for example, frequency modulation) :

  Maximum power level at zero-relative-level point : -10 dBm0,

  to be reduced to or below -20 dBm0 when data transmission

  is discontinued for any appreciable time.

3) Non-continuous-tone systems (for example, amplitude modulation) :

  Maximum power level at zero-relative-level point : -6 dBm0 , provided

  that busy-hour mean power in both directions of transmission added

  does not exceed 64 microwatts (-15 dBm0 mean level in each direction

  simultaneously). Also, the level of tones above 2400 hertz should

  confirm to recommendations for signaling tones in G.224 (H.51)

November 21, 2020 7:58 PM