InfoMagic Standards 1994 January/InfoMagic Standards - January 1994.iso/ccitt/1988/troff/3_1_05.tro

Text File | 1991-12-12 | 115KB | 3,628 lines

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.rs
\v | 5i'
.LP
\fBMONTAGE : FIN DE LA RECOMMANDATION G.125 EN T\* | TE DE CETTE PAGE\fR
.IP
\v'27P'
\fB1.3\fR     \fBGeneral characteristics of the 4\(hywire chain formed by\fR
\fBthe international circuits and national extension circuits\fR
.sp 1P
.RT
.PP

This subsection gives the overall characteristics recommended for the 4\(hywire
chain defined in Recommendation\ G.101,\ \(sc\ 2.
.sp 1P
.RT
.sp 2P
.LP
\fBRecommendation\ G.131\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBSTABILITY\ AND\ ECHO\fR
.EF '%    Fascicle\ III.1\ \(em\ Rec.\ G.131''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.131    %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1964; amended at Mar del Plata, 1968,\fR
\fIand Geneva, 1972, 1976,\fR
.sp 9p
.RT
.ce 0
.sp 1P
.ce 1000
\fIand 1980; Malaga\(hyTorremolinos, 1984 and Melbourne, 1988)\fR
.ce 0
.sp 1P
.LP
\fB1\fR     \fBStability of telephone transmission\fR
.sp 1P
.RT
.PP
The nominal transmission loss of international circuits having
been fixed, the principal remaining factors which affect the stability of
telephone transmission on switched connections are:
.RT
.LP
    \(em
    the variation of transmission loss with time and among
circuits (Recommendation\ G.151, \(sc\ 3);
.LP
    \(em
    the attenuation distortion of the circuits
(Recommendation\ G.151, \(sc\ 1);
.LP
    \(em
    the distribution of stability balance return losses
(Recommendation\ G.122, \(sc\(sc\ 2 and\ 3).
.bp
.PP
The stability of international connections has been calculated and the
results are displayed graphically in Figure\ 1/G.131, which shows the
proportion of connections (out of all the possible connections) likely to
exhibit a stability of less than or equal to 0\ dB or 3\ dB as a function
of the number of all analogue circuits comprising the 4\(hywire chain and
the mean values of stability balance return loss that may be assumed. Of
course the proportion of connections actually established which exhibit
a stability lower than or
equal to the values considered will be very much smaller.
.PP
\fINote\fR \ \(em\ If digital circuits are included in the 4\(hywire chain, the
stability is likely to be better than shown in Figure\ 1/G.131, as these
circuits will exhibit a lower transmission loss variability than is assumed
in that figure.
.PP
When interpreting the significance of the curves showing the
proportion of calls likely to have a stability of 3\ dB or less it should be
borne in mind that the more complicated connections will undoubtedly
incorporate a circuit equipped with an echo suppressor or canceller, in
which case the stability during conversation is very much higher.
.RT
.LP
.rs
.sp 34P
.ad r
\fBFigure\ 1/G.131, p.  \fR
.sp 1P
.RT
.ad b
.RT
.PP
The simplifying assumptions underlying the calculations are:
.LP
    a)
    National circuits are added to the international chain in
compliance with Recommendation\ G.122.
.LP
    b)
     The standard deviation of transmission loss among analogue international
circuits routed on groups equipped with automatic
regulation is 1\ dB. This accords with the assumptions used in
Recommendation\ G.122. The results of the 10th\ series
of tests by Study Group\ IV indicate that this target is
being approached in that 1.1\ dB was the standard deviation
of the recorded data and the proportion of unregulated
international groups in the international network is
significantly decreasing.
.bp
.LP
    c)
     The variations of transmission loss in the two directions of transmission
are perfectly correlated.
.LP
    d)
    The departure of the mean value of the transmission loss
from the nominal value is zero. As yet there is little information
concerning international circuits maintained between
4\(hywire points.
.LP
    e)
    No allowance has been made for the variations and
distortions introduced by the national and international exchanges.
.LP
    f
)
    The variation of transmission loss of circuits at
frequencies other than the test frequency is the same as that
at the test frequency.
.LP
    g)
     No account has been taken of attenuation distortion. This is felt to
be justifiable because low values of balance return
loss occur at the edges of the transmitted band and are thus
associated with higher values of transmission loss.
.LP
    h)
    All distributions are Gaussian.
.PP
Bearing in mind these assumptions, the conclusion is that the
Recommendations made by the CCITT are self\(hyconsistent and that if these
Recommendations are observed and the maintenance standard set for variation
of loss among circuits is achieved, there should be no instability problems
in the transmission plan. It is also evident that those national networks
which can
exhibit no better stability balance return loss than 3\ dB mean, 1.5\ dB
standard deviation are unlikely to seriously jeopardize the stability of
international connections as far as oscillation is concerned. However,
the near\(hysinging
distortion and echo effects that may result give no grounds for complacency
in this matter.
.PP
Details of the calculations are set out in\ [1].
.RT
.sp 2P
.LP
\fB2\fR     \fBLimitation of echoes\fR
.sp 1P
.RT
.PP
The main circuits of a modern telephone network providing
international communications are high\(hyvelocity carrier circuits on symmetric,
coaxial or optical fibre pairs or radio\(hyrelay systems. Echo control
devices
such as echo suppressors and echo cancellers are not normally used except on
connections involving very long international circuits. There is often no
general need for
echo control devices
in national networks but they may be required for the inland service in
large countries. Echo control devices may also be needed on loaded\(hycable
circuits (low\(hyvelocity circuits) used for
international calls.
.PP
Echoes may be controlled in one of two ways: either the overall
loss of the 4\(hywire chain of circuits may be adjusted so that echo currents
are sufficiently attenuated (which tacitly assumes a particular value for
the echo return loss) or an echo control device can be fitted.
.RT
.sp 1P
.LP
2.1
    \fITransmission loss adjustment\fR
.sp 9p
.RT
.PP
The curves of Figure\ 2/G.131 indicate the minimum value of the
overall loudness rating (OLR)
.FS
While Figure\ 2/G.131 is based on
nominal values of LR of trunk junction and trunk circuits, it refers to
minimum SLR and RLR values of subscriber systems.
.FE
in the echo path that must be
introduced if no echo suppressor is to be fitted. The OLR is shown as a
function of the mean one\(hyway propagation time. Supplement\ No.\ 2, at
the end of this fascicle, explains how these curves have been derived and
Annex\ A to this Recommendation gives an example of their application.
.PP
The solid curves are applicable to a chain of analogue circuits
which are connected together 4\(hywire. However, they may also be used for
circuits connected together 2\(hywire if precautions have been taken to ensure
good echo return losses at these points (i.e.\ averaged in accordance with
Recommendation\ G.122) for example, a mean value of 27\ dB with a standard
deviation of 3\ dB.
.PP
\fINote\fR \ \(em\ This value is only sufficient to assure average echo
losses (\fIa\fR \(hy\fIb\fR ) of (15\ +\ \fIn\fR )\ dB, as currently called
for in
Recommendation\ G.122\ \(sc\ 4.1.
.PP
The dashed curve is applicable to fully digital connections with
analogue subscriber lines (such as shown in Figure\ 2/G.111), and, under
certain assumptions (see Supplement\ No.\ 2), to fully digital connections
with digital subscriber lines (such as shown in\ \fIb)\fR of Figure\ 1/G.104.
In the latter case the echo path includes the acoustical path between earpiece
and mouthpiece of the handset.
.bp
.PP
When an international circuit is used only for comparatively short and
straightforward international connections the nominal transmission loss
between virtual analogue switching points may be increased in proportion
to the length of the circuit according to the following rule, if the use
of echo
control devices can thereby be avoided:
.RT
.LP
    \(em
    up\ to\ 500\ km\ route\ distance:
    0.5\ dB;
.LP
    \(em
    between\ 500\ km\ and\ 1000\ km\ route\ distance:
    1.0\ dB;
.LP
    \(em
    for\ every\ additional\ 500\ km\ or\ part\ thereof:
    0.5\ dB.
.PP
However, such a circuit may not form part of multicircuit
connections unless the nominal transmission loss is restored to
0.5\ dB.
.LP
.rs
.sp 35P
.ad r
\fBFigure\ 2/G.131, p.  \fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
2.2
    \fIEcho control devices\fR
.sp 9p
.RT
.PP
The preferred type of echo suppressor is a
terminal,
differential, half\(hyecho suppressor
operated from the far end. There are
several types of
half\(hyecho suppressor
in use in the international
network, one suitable only for use in connections with mean one\(hyway
propagation times not exceeding 50\ ms, referred to as a short\(hydelay
echo suppressor, and
the others suitable for use in connections with any
.bp
.PP
mean one\(hyway
propagation
time, especially times well over 50\ ms, referred to as a long\(hydelay echo
suppressor like those used on circuits routed on communication\(hysatellite
systems. The characteristics of the short\(hydelay echo suppressors are given
in\ [2]. The characteristics of echo suppressors which can be used on
connections with either short or long propagation times are given in\ [3]
and in Recommendation\ G.164 (echo suppressors with new functions). Another
type of
echo control can be obtained by echo cancellers. The characteristics are
given in Recommendation\ G.165.
.PP
From subjective test information received, it is concluded
that:
.RT
.LP
    1)
    Echo cancellers in accordance with Recommendation G.165
provide superior speech transmission performance (at the
0.05 confidence level) to that provided by:
.LP
    a)
    echo suppressors according to Recommendation G.161
(\fIOrange\ Book\fR );
.LP
    b)
    echo suppressors according to Recommendation G.164
with fixed break\(hyin differential sensitivity, FBDS;
.LP
    \fINote\fR \ \(em\ Two Administrations have the view that echo
cancellers according to Recommendation\ G.165 and echo
suppressors according to Recommendation\ G.164 with
adaptative break\(hyin differential sensitivity (ABDS)
provide about the same performance when the echo path loss
is considerably above the lower end of its range;
calculations based on Recommendation\ G.122, \(sc\ 2 and
assuming a minimum echo loss of 6\ dB, indicate that the
majority of echo path losses will be greater than the
minimum value.
.LP
    2)
    echo suppressors in accordance with Recommendation G.164
with ABDS provide superior speech transmission performance to
that provided by echo suppressors with FBDS.
.LP
    3)
    echo control devices of different types (i.e. echo
suppressors or cancellers in accordance with the
series\ G\ Recommendations) placed at opposite ends of a
connection will operate compatibly. In this case the subjective
quality perceived at one end is almost uniquely dependent on the
performance of the echo control device installed at the opposite
end.
.PP
\fINote\ 1\fR \ \(em\ Regional satellite circuits routed in parallel with
terrestrial circuits, without perceivable echo, will benefit from the use of
echo control devices of the best quality. Otherwise any degradation of the
normal quality by routing over the satellite circuit may be found objectionable
by the subscriber.
.PP
\fINote\ 2\fR \ \(em\ Bilateral agreement between Administrations may facilitate
the introduction in the network of echo control devices of better
quality.
.RT
.sp 1P
.LP
2.3
    \fIRules governing the limitation of echoes\fR
.sp 9p
.RT
.PP
The rules given below are subdivided into ideal rules and practical rules.
It is recognized that no practical solution to the problem could comply
with rules so exclusive and inflexible as the ideal rules. Practical rules
are suggested in the hope that they will ease the switching and economic
problems. They should not be invoked unless the ideal rules cannot reasonably
be complied with.
.RT
.sp 2P
.LP
2.3.1
    \fIRules for connections without echo control\fR
\fIdevices\fR
.FS
The rules in this Recommendation have been updated (to \fR
include echo cancellers)
and regrouped, compared with previous versions of Recommendation\ G.131. The
letters indicating the rules are the same as in previous versions of
Recommendation\ G.131 in order to provide a degree of continuity.
.FE
.sp 1P
.RT
.sp 1P
.LP
2.3.1.1
    \fIIdeal rule\ \(em\ Rule A\fR
.sp 9p
.RT
.PP
For a connection between any pair of local exchanges in different countries,
the probability of incurring the opinion \*Qunsatisfactory\*U due to
talker echo shall be less than\ 1%, when minimum practical nominal send and
receive loudness ratings are assumed for the talker's telephone and line.
.PP
\fINote\fR \ \(em\ Calls between a given pair of local exchanges may
encounter different numbers of 4\(hywire circuits, according to the routing
discipline and time of day. Figure\ 2/G.131 permits compliance with this rule
to be assessed for the separate parts of the total traffic which encounter\
1, 2, 3 . |  |  9 4\(hywire circuits, under certain conventional assumptions.
(See
Supplement\ No.\ 2 at the end of this fascicle.)
.bp
.RT
.sp 1P
.LP
2.3.1.2
    \fIPractical rule\ \(em\ Rule E\fR
.sp 9p
.RT
.PP
Recommendation\ Q.115\ [4] is a study of the application of Rules\ A
and\ E to the United Kingdom\(hyEuropean network relations.
.FE
For connections involving the longest national 4\(hywire extensions of
the two countries, a probability of incurring an \*Qunsatisfactory\*U opinion
due to echo not of 1% (Rule\ A) but of 10% can, by agreement between the
Administrations concerned, be tolerated. This Rule\ E
is valid only in
those cases where it would otherwise be necessary, according to Rule\ A
, to use an echo control device solely for these connections, and where
there is no need for echo control devices on connections between the regions
in the
immediate neighbourhood of the two international centres concerned.
.RT
.LP
2.3.2
    \fIRules for connections with echo control devices\fR
.sp 1P
.RT
.sp 2P
.LP
2.3.2.1
    \fIIdeal rules\fR
.sp 1P
.RT
.sp 1P
.LP
2.3.2.1.1
    \fIRule B\fR \v'3p'
.sp 9p
.RT
.LP
    1)
    Not more than the equivalent of one full echo suppressor
(i.e.\ two half\(hyecho suppressors) should be included in any
connection needing an echo suppressor. When there is more than
one full echo suppressor the conversation is liable to be
clipped; lockout can also occur.
.LP
    2)
    Circuits equipped with echo cancellers
(Recommendation\ G.165) can be connected together in tandem
without echo performance degradation.
.LP
    3)
    A circuit equipped with echo suppressors
(Recommendation\ G.164) can be connected with another circuit
equipped with echo cancellers (Recommendation\ G.165) without
additional performance degradation.
.LP
    \fINote\fR \ \(em\ The overall performance will not be better than
that provided by the poorer performing device.
.sp 1P
.LP
2.3.2.1.2
    \fIRule\ D\fR
.sp 9p
.RT
.PP
The half\(hyecho suppressors should be associated with the
terminating sets of the 4\(hywire chain of the complete connection.
This:
.RT
.LP
    \(em
    reduces the chance of speech being multilated by the echo
suppressors because the hangover times can be very short;
.LP
    \(em
     reduces the change of ineffective echo canceller operation as end delays
are short and minimum required echo losses can be
assured.
.sp 2P
.LP
2.3.2.2
    \fIPractical rules\fR
.sp 1P
.RT
.sp 1P
.LP
2.3.2.2.1
    \fIRule F\fR
.sp 9p
.RT
.PP
If, as is appreciated, Rule D above cannot be complied with, the
echo control device may be fitted at the international exchange or at an
appropriate national transit centre. However, each echo control device
should be located sufficiently near to the respective subscribers for the
end delays not to exceed the maximum value recommended in Recommendation\
G.161,
(\fIOrange\ Book\fR ) and Recommendations\ G.164 and\ G.165 of this fascicle.
For
countries of average size, this will normally mean that the originating and
terminating control devices will be in the countries of origin and destination
of the call.
.RT
.sp 1P
.LP
2.3.2.2.2
    \fIRule\ G\fR
.sp 9p
.RT
.PP
In isolated cases a full short\(hydelay echo suppressor may be fitted at
the outgoing end of a transit circuit (instead of two half\(hyecho suppressors
at the terminal centres) provided that neither of the two hangover times
exceeds 70\ ms. This relaxation may reduce the number of echo suppressors
required and may also simplify the signalling and switching arrangements.
It is emphasized that full echo suppressors must not be used indiscriminately;
the
preferred arrangement is two half\(hyecho suppressors as near the terminating
sets as possible. A full echo suppressor should be as near to the \*Qtime\(hycentre\*U
of the connection as possible, because this will require lower hangover
times.
.PP
Whether a full long\(hydelay echo suppressor or canceller can be used in
this circumstance is under study.
.bp
.RT
.sp 1P
.LP
2.3.2.2.3
    \fIRule\ K\fR
.sp 9p
.RT
.PP
On a connection that requires an echo suppressor, up to
the equivalent of two full echo suppressors (e.g.\ three half\(hyecho suppressors
or two half\(hyecho suppressors and a full one) may be permitted. Every
effort
should be made to avoid appealing to this relaxation because the equivalent
of two or more full echo suppressors, with long hangover times, on a connection
can cause severe clipping of the conversation and considerably increases the
risk of lockout. This rule does not apply to echo cancellers
(see Rule\ B).
.RT
.sp 1P
.LP
2.3.2.2.4
    \fIRule\ L\fR
.sp 9p
.RT
.PP
In general it will not be desirable to switch
out (or disable) the intermediate echo suppressors when a circuit equipped
with long\(hydelay echo control devices is connected to one with short\(hydelay
echo suppressors. However, it would be desirable to switch out (or disable)
the
intermediate echo suppressors if the mean one\(hyway propagation time of that
portion of the connection which would now fall between the terminal half\(hyecho
suppressors is not greater than 50\ ms, since the different types are likely
to be compatible. An intermediate echo canceller need not be switched
out.
.RT
.sp 2P
.LP
2.3.3
    \fIGeneral rules\fR
.sp 1P
.RT
.sp 1P
.LP
2.3.3.1
    \fIIdeal rule\ \(em\ Rule C\fR
.sp 9p
.RT
.PP
Connections that do not require echo control devices should not be fitted
with them, because they increase the fault rate and are an additional
maintenance burden.
.RT
.sp 2P
.LP
2.3.3.2
    \fIPractical rules\fR
.sp 1P
.RT
.sp 1P
.LP
2.3.3.2.1
    \fIRule\ H\fR
.sp 9p
.RT
.PP
In exceptional circumstances, such as breakdown, an
emergency route may be provided. The circuits of this route need not be
fitted with echo control devices if they are usable without them for a
short period. However, if the emergency routing is to last more than a
few hours, echo
control devices must be fitted according to Rules\ A to\ E above.
.RT
.sp 1P
.LP
2.3.3.2.2
    \fIRule\ J\fR
.sp 9p
.RT
.PP
It is accepted that a connection that does not require an echo
control devices may in fact be unnecessarily equipped with one or two half\(hyecho
suppressors, or a full echo suppressor or echo cancellers. (The presence
of an echo suppressor in good adjustment on a circuit with modest delay
times can
hardly be detected and in the case of echo cancellers it may improve the
overall performance of the connection.)
.PP
Where a terminating international exchange is accessible from an
originating international exchange by more than one route, and
.RT
.LP
    1)
     at least one route requires echo suppressors, and at least one route
does not; and
.LP
    2)
    the originating exchange is unable to determine which route  is to be used;
.LP
echo control devices should be connected in all cases.
.sp 1P
.LP
2.3.3.2.3
    \fIRule M\fR
.sp 9p
.RT
.PP
It has been found in actual practice that echo can be made
tolerable by providing loss in the circuit if the one\(hyway propagation time
(delay) of the echo is less than about 25\ ms. For delays longer than this,
too much circuit loss is needed to attenuate echo, and echo control devices
are
required.
.PP
\fINote\fR \ \(em\ The equivalent of this rule is stated in
Recommendation\ G.161, \(sc\ B.b. (\fIOrange\ Book\fR ). This rule has
not been expressed in earlier versions of Recommendation\ G.131.
.bp
.RT
.sp 1P
.LP
2.4
    \fIInsertion of echo control devices in a connection\fR
.sp 9p
.RT
.PP
Ways of inserting echo control devices in a connection which have been
considered are the following:
.RT
.LP
    1)
    provide a pool of echo control devices common to several
groups of circuits, and arrange for an echo control device to be
associated with any circuit that requires one (see
Recommendation\ Q.115\ [4]);
.LP
    2)
    arrange for the circuits to be permanently equipped with
echo control devices but switch them out (or disable them) when
they are not required (see\ [5]);
.LP
    3)
    divide the circuits of an international route into two
groups, one with and one without echo control devices and route
the connection over a circuit selected from the appropriate
group according to whether the connection merits an echo control
device. However, it is recognized that circuits may not be used
efficiently when they are divided into separate groups. This
must be borne in mind;
.LP
    4)
    conceive schemes in which the originating country and the
terminal country are divided into zones at increasing mean
radial distances from the international centre and determine the
nominal lengths of the national extensions by examining routing
digits and circuits\(hyof\(hyorigin.
.PP
Whichever method is used, due regard must be paid to the last
sentence of \(sc\ 2.1\ above. Methods of achieving the required reduction
of circuit losses are under study by the CCITT. The nature and volume of
the traffic
carried by a particular connection will also influence the economics of the
methods and hence the choice among them.
.PP
The CCITT is currently studying what recommendations are necessary to ensure
that the insertion of echo control devices in international connections
complies, overall, with the practical rules given above.
.PP
It should be appreciated that different continents need not use the
same method although the methods must be compatible to permit intercontinental
connections. There appears to be no great difficulty in arranging this.
.RT
.sp 1P
.LP
2.5
    \fISpeech processing devices\fR
.sp 9p
.RT
.PP
Some speech processing devices, such as
speech interpolation
devices
, have an inherent echo\(hysuppressor function. However, such devices may
only suppress echo during the
single talk mode
and not during
double talking
conditions (see Recommendation\ G.164, \(sc\ 1.7) unless they are equipped
to perform full echo\(hysuppressor functions. When devices without
full echo control are connected in tandem with echo cancellers, performance
degradation due to echo may occur during double talking conditions as the
intermediate echo canceller will not be effective during double talk.
\v'6p'
.RT
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation\ G.131)
.sp 9p
.RT
.ce 0
.ce 1000
\fBApplication of Recommendation G.131, \(sc\ 2\fR
.sp 1P
.RT
.ce 0
.PP
Recommendation G.131, \(sc 2.3.1.1, Rule A, requires, for each pair of
countries, an assessment of echo conditions for each possible pair of local
exchanges to ascertain whether the plot of corrected reference equivalent
of
echo path against mean one\(hyway propagation time for that pair of exchanges,
lies above or below the appropriate 1% line in Figure\ 2/G.131.
.sp 1P
.RT
.PP
The variables in the problem are indicated in Table\ A\(hy1/G.131 and illustrated
for all analogue connections in Figure\ A\(hy1/G.131 and for all
digital connections in Figure\ A\(hy2/G.131.
.PP
For a given pair of exchanges, all eight items are known or can be
estimated. A plot of overall loudness rating [1)\ +\ 2)\ +\ 3)\ +\ 4) of
Table\ A\(hy1/G.131] as a function of mean one\(hyway propagation time
[5)\ +\ 6)\ +\ 7) of Table\ A\(hy1/G.131] on Figure\ 2/G.131 may be assessed
in relation to the 1%
curve, for a given number of analogue circuits in the 4\(hywire chain for fully
analogue connections and mixed analogue/digital connections or, for fully
digital connections using the appropriate curve.
.bp
.RT
.ce
\fBH.T. [T1.131]\fR
.ce
TABLE\ A\(hy1/G.131
.ce
\fBQuantities needed for echo assessment\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
lw(228p) .
 {
\fIOverall loudness rating of the echo path\fR
, made up of the sum
of:
 }
.T&
lw(228p) .
 {
1)
the minimum of the sum of the values of the sending and
receiving loudness ratings of the local system of
country\ A (talker end));
 }
.T&
lw(228p) .
 {
2)
the nominal loudness rating from, and to, the virtual
analogue switching points (\fIa\fR
A\ and\ \fIb\fR
A) of the chain of national
circuits in country\ A, connecting the local exchange to the
international exchange;
 }
.T&
lw(228p) .
 {
3)
the nominal loudness rating in each direction of transmission of
the international chain;
 }
.T&
lw(228p) .
 {
4)
the echo loss (\fIa\fR
B\(hy\fIb\fR
B) of the
national system of country\ B (listener
end).
 }
.T&
lw(228p) .
 {
\fIMean one\(hyway propagation time\fR
,
made up of half the sum of the propagation
times of:
 }
.T&
lw(228p) .
 {
5)
the paths from the telephone set in
country A, to and from the virtual analogue switching
points\ \fIa\fR
A and\ \fIb\fR
A;
 }
.T&
lw(228p) .
 {
6)
the two directions of
transmission of the international chain;
 }
.T&
lw(228p) .
 {
7)
the path \fIa\fR
B\(hy\fIb\fR
B of country B.
 }
.T&
lw(228p) .
 {
In addition, there will be needed for fully analogue or
mixed analogue/digital
connections:
 }
.T&
lw(228p) .
 {
8)
the number of analogue circuits in the
4\(hywire chain (see Figure 3/G.101).
 }
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau A\(hy1/G.131 [T1.131], p. 3\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 28P
.ad r
\fBFigure A\(hy1/G.131, p.  \fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 35P
.ad r
\fBFigure A\(hy2/G.131, p.  \fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
A.1
    \fIFull analogue connections\fR (Figure A\(hy1/G.131)
.sp 9p
.RT
.PP
For the purpose of this Recommendation, it may be assumed that the principal
reflection at the listener's end occurs at the 4\(hywire/2\(hywire
terminating set, which may be assumed to be located at the primary exchange
associated with the listener's local exchange. The components of\ 4) of
Table\ A\(hy1/G.131 are then the losses\ \fIa\fR\dB\u\(hy\fIt\fR and\ \fIt\fR
\(hy\fIb\fR\dB\u, plus the echo balance return loss at the 2\(hywire port
of the terminating set. This return loss will be the mean overall, of the
off\(hyhook subscriber's lines, which may be
presented to the 2\(hywire port of the terminating set by the listener's local
exchange. (Figure\ 2/G.131 assumes that the standard deviation of the return
loss is 3\ dB.) If the mean value is not known, it may be assumed that 4) of
Table\ A\(hy1/G.131 is in accordance with Recommendation\ G.122, \(sc\
4, viz., a mean value of (15\ +\ \fIS\fR )\ dB where \fIS\fR is the sum
of the nominal losses in the two
directions of transmission of the circuits in the listener's national
4\(hywire\ chain (\fIS\fR is assumed to be 1\ dB in this case).
.PP
For a given pair of local exchanges, successive connections may
encounter different numbers of 4\(hywire circuits, and the total traffic may be
regarded as a number of packets of various proportions encountering from
one to nine 4\(hywire circuits. Each \*Qpacket\*U may be tested with the
aid of
Figure\ 2/G.131 and the results combined in order to assess whether Rule\ A is
complied with for the totality of traffic.
.bp
.PP
Figure\ A\(hy1/G.131 shows, as an example, an application of
Recommendation\ G.131, \(sc\ 2, where a listener's \fIa\fR \(hy\fIt\fR
\(hy\fIb\fR path is assumed to be in accordance with Recommendation\ G.122.
For simplicity, it is assumed that
100% of the traffic encounters the given conditions. Values for the example
are as follows:
.RT
.sp 2P
.LP
    \fITalker's country\ A\fR
.sp 1P
.RT
.sp 1P
.LP
Distance from local exchange A\d1\uto
international exchange
        1600\ km
.sp 9p
.RT
.LP
Assuming a velocity of propagation for the transmission systems of 250\
km/ms, 3\ FDM channel modulators and demodulators of 1.5\ ms each for
talker's country\ A and the international chain of circuits\ A to B, and
a 12\ ms constant for listener's country\ B (see Recommendation\ G.114).
.FE
Mean one\(hyway propagation time from local exchange
A\d1\uto international exchange
        \ 11\ ms
.LP
Simultaneous\(hyminimum sending and receiving loudness rating
(sum) of the local system
        \ \ 5\ dB
.LP
It is assumed that the loaded trunk\(hyjunction introduces an
additional 1\ dB (in each direction) when changing from nominal transmission
loss to loudness rating.
.FE
Loudness rating from local exchange to international exchange
(\fIb\fR\dA\u)
        \ \ 7\ dB
.LP
Loudness rating from international exchange to local exchange
(\fIa\fR\dA\u)
        \ \ 6\ dB
.LP
Number of 4\(hywire circuits
        \ \ 2
.sp 2P
.LP
    \fIInternational chain A to B\fR
.sp 1P
.RT
.sp 1P
.LP
Number of circuits
        \ \ 3
.FS
An unusually
large number, chosen only to illustrate the principle of addition of
loss.
.FE
.sp 9p
.RT
.LP
Distance
        3200\ km
.LP
Mean one\(hyway propagation time
        \ 17\ ms
.LP
Sum of loudness ratings in both directions 2\ \(mu\ 3\ \(mu\ 0.5 dB
        \ \ 3\ dB
.sp 2P
.LP
    \fIListener's country B\fR
.sp 1P
.RT
.ad r
Mean echo loss (\fIa\fR\dB\u\(hy\fIb\fR\dB\u)\ =\ (15\ +\ 1)\ dB
        \ 16\ dB
    (Rec.\ G.122)
.sp 9p
.RT
.ad b
.RT
.LP
Distance from international exchange to primary exchange
associated with local exchange B\d1\u(i.e.\ point of
principal reflection)
        1120\ km
.LP
Mean one\(hyway propagation time corresponding to above
distance
        \ 16\ ms
.LP
Number of 4\(hywire circuits
        \ \ 1
.sp 1P
.LP
Total number of 4\(hywire circuits = 2 + 3 + 1 = 6
.sp 9p
.RT
.LP
Total mean one\(hyway propagation time = 11 + 17 + 16 = 44\ ms
        (A\(hy1)
.LP
Total loudness rating of the echo path = 5 + 7 + 6 + 3 + 16 = 37 dB
        (A\(hy2)
.PP
If (A\(hy1) and (A\(hy2) are plotted on Figure\ 2/G.131, the point lies
below the 1% line for six 4\(hywire circuits, indicating a probability
of more
than 1% of incurring an \*Qunsatisfactory\*U opinion. The conclusion also
applies to other possible numbers of 4\(hywire circuits.
.sp 1P
.LP
A.2\fR     \fIFully digital connections\fR  | Figure A\(hy2/G.131)
.sp 9p
.RT
.PP
It may be assumed that the principal reflection at the listener's end occurs
at the 4\(hywire/2\(hywire terminating set, which is located at the
listener's local exchange. The components of\ 4) of Table\ A\(hy1/G.131
are then the losses\ \fIa\fR\dB\u\(hy\fIt\fR and \fIt\fR \(hy\fIb\fR\dB\uplus
the echo balance return loss at the
2\(hywire port of the terminating set. This return loss will be the mean,
overall, of the off\(hyhook subscriber's lines, which may be presented
to the 2\(hywire port of the terminating set by the listener's local exchange.
(Figure\ 2/G.131
assumes that the standard deviation of the return loss is 3\ dB.) If the mean
value is not known, it may be assumed that it is in accordance with
Recommendation\ G.122, \(sc\ 4.3, viz., a mean value of 11\ dB.
.bp
.PP
In order to apply Figure A\(hy2/G.131 the value of \fIn\fR is not required
in this case (as the digital circuits in the 4\(hywire chain do not contribute
to the overall circuit loss variability). However, the number of digital
exchanges has an effect on the propagation time, for instance, in accordance
with
Table\ 1/G.114, that each digital transit exchange adds 0.45\ ms to the mean
one\(hyway propagation time of the connection.
.PP
Figure A\(hy2/G.131 shows an example where the sum of the R and T pads
is either 6 or 7\ dB. Values for the example are as follows:
.RT
.sp 2P
.LP
    \fITalker's country A\fR
.sp 1P
.RT
.sp 1P
.LP
Distance from local exchange A\d1\uto international exchange
        1600\ km
.sp 9p
.RT
.LP
Assuming a velocity of propagation for
the transmission systems of 250\ km/ms, 4\ exchange delays of 0.45\ ms each and
0.3\ ms delay in the coder or decoder. (In practice a local digital exchange
will contribute a little more than 0.45\ ms, see Recommendation\ G.114.)
.FE
Mean one\(hyway propagation time from
local exchange\ A\d1\uto international exchange
        \ \ 8.5\ ms
.LP
Simultaneous\(hyminimum sending and receiving loudness rating
(sum) of the local system
        \ \ 5\ dB
.LP
Sum of loudness ratings in both directions of transmission
(\fIt\fR\d1\u\(hy\fIb\fR\dA\u)\ +\ (\fIa\fR\dA\u\(hy\fIt\fR\d1\u)
        \ \ 6\ dB
.sp 2P
.LP
    \fIInternational chain A to B\fR
.sp 1P
.RT
.sp 1P
.LP
Distance
        3200\ km
.sp 9p
.RT
.LP
Mean one\(hyway propagation
time
        \ 13.7\ ms
.FS
Assuming a velocity of propagation   for the
transmission systems of 250\ km/ms and 2\ exchange delays of 0.45\ ms each.
.FE
.LP
Loudness rating of international chain
        \ \ 0\ dB
.sp 2P
.LP
    \fIListener's country B\fR
.sp 1P
.RT
.sp 1P
.LP
Distance from local exchange B\d1\uto international exchange
        1600\ km
.sp 9p
.RT
.LP
Mean one\(hyway propagation time
        \ \ 8.5\ ms
.LP
Mean echo loss (\fIa\fR\dB\u\(hy\fIb\fR\dB\u)\ =\ (11\ +\ 7)\ dB
        \ 18\ dB
.LP
Total mean one\(hyway propagation time\ =\ 8.5\ +\ 13.7\ +\ 8.5
=\ 30.7\ ms
        (A\(hy3)
.LP
Total loudness rating of the echo path\ =\ 5\ +\ 6\ +\ 0\ +\ 18\ =\ 29\ dB
        (A\(hy4)
.PP
If (A\(hy3) and (A\(hy4) are plotted on Figure\ 2/G.131, the point lies
below the 1%\ line (and also the 10%\ line) for fully digital connections,
indicating a propability of more than 1% incurring an \*Qunsatisfactory\*U
opinion.
.sp 1P
.LP
    \fIConclusion\fR \v'3p'
.sp 9p
.RT
.LP
    a)
    An echo control device should be used on the connection;
or
.LP
    b)
    the loss in the echo path should be increased (but the
limitations of Recommendation\ G.121 must be observed).
.PP
\fINote\fR \ \(em\ It should be noted, when contemplating to increase the
loss in the echo path, that digital pads placed in digital circuits need
to be switched out for digital data signals (but not for voiceband data
signals) as they destroy the bit transparency for such signals.
.sp 1P
.LP
A.3
    \fIMixed analogue/digital connections\fR
.sp 9p
.RT
.PP
The examples given in Figures\ A\(hy1/G.131 and A\(hy2/G.131 allow the
construction of mixed analogue/digital connection models by combining the
appropriate elements of the two\ figures. The quantities stated in
Table
A\(hy1/G.131 can be calculated with these models. (Quantity\ 8) of this
table (number of circuits) should now be taken as the number of analogue
circuits in the 4\(hywire chain (thus not including the digital circuits). The
appropriate solid curve in Figure\ 2/G.131 will approximate the required echo
tolerance curve with good accuracy.
.bp
.PP
\fINote\fR \ \(em\ In mixed analogue/digital networks the propagation time
can become larger than in purely analogue or digital networks. The latter
occurs in particular when digital exchanges are connected with analogue
transmission
systems through PCM/FDM equipments in tandem or transmultiplexers. Many
different configurations may arise.
.RT
.sp 2P
.LP
    \fBReferences\fR
.sp 1P
.RT
.LP
[1]
    \fICalculation of the stability of international connection\fR
\fIestablished in accordance with the transmission and switching plan\fR ,
Green\ Book, Vol.\ III, Supplement No.\ 1, ITU, Geneva,\ 1973.
.LP
[2]
    CCITT Recommendation \fIDefinitions relating to echo suppressors\fR
\fIand characteristics of a far\(hyend operated, differential, half\(hyecho\fR
\fIsuppressor\fR , Blue\ Book, Vol.\ III, Rec.\ G.161, Section\ B, ITU,
Geneva,\ 1964.
.LP
[3]
    CCITT Recommendation \fIEcho\(hysuppressors suitable for circuits\fR
\fIhaving either short or long propagation times\fR , Orange\ Book, Vol.\ III,
Rec.\ G.161, Sections\ B and\ C, ITU, Geneva,\ 1977.
.LP
[4]
    CCITT Recommendation \fIControl of echo suppressors\fR , Vol.\ VI,
Rec.\ Q.115.
.LP
[5]
    CCITT \(em \fIInsertion and disablement of echo suppressors\fR , Blue
Book, Volume\ VI.1, Question\ 2/XI, Annex\ 3, ITU, Geneva,\ 1966.
\v'6p'
.sp 2P
.LP
\fBRecommendation\ G.132\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBATTENUATION\ DISTORTION\fR
.EF '%    Fascicle\ III.1\ \(em\ Rec.\ G.132''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.132    %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1964; Mar del Plata, 1968; Geneva, 1972 and Melbourne, 1988)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.PP
The network performance objectives for the variation with
frequency of transmission loss
in terminal condition of a worldwide 4\(hywire chain of 12\ circuits (international
plus national extensions), each one routed over a single group link, are
shown in Figure\ 1/G.132, which assumes that no use is made of high\(hyfrequency
radio
circuits or 3\(hykHz channel equipment.
.sp 1P
.RT
.PP
\fINote\ 1\fR \ \(em\ The design objectives contained in the Recommendation
cited in\ [1], for carrier terminal equipments are such that for a chain
of
6\ circuits (international and national extensions) in tandem, each circuit
being equipped with one pair of channel translating equipments, the attenuation
distortion would in most cases be less than 9\ dB between 300 and 3400\
Hz. For the case of 12\ circuits in tandem it can be expected that in most
cases the
attenuation distortion will not exceed 9\ dB between about 400 and 3000\
Hz. As far as the international chain is concerned, see
Recommendation\ G.141, \(sc\ 1.
.PP
\fINote\ 2\fR \ \(em\ It is only in a small proportion of international
connections that the 4\(hywire chain will in fact comprise 12\ circuits.
.PP
\fINote\ 3\fR \ \(em\ Limits given in Figure\ 1/G.132 should be met also
for mixed connections using the analogue\(hydigital equipments. Probably,
the number of
analogue\(hydigital equipment (pair codecs) for the mixed connections with\ 12
circuits does not exceed\ 6 (see Recommendation\ G.103, Annex\ B).
.PP
It should be recognized that a connection containing six coder\(hydecoder
pairs where each pair just meets the attenuation distortion requirements
found in Recommendation\ G.712 will not meet the attenuation distortion
requirement
found in Recommendation\ G.132 for 3400\ Hz.
.PP
However, it is likely that real coder\(hydecoder pairs will have
attenuation distortion performance better than in Recommendation\ G.712,
so for practical purposes the likelihood of not complying with Recommendation\
G.132 is very small.
.PP
\fINote\ 4\fR \ \(em\ Studies are being carried out by Study Group\ IV
and Study Group\ XII about how well this objective is being met in practice,
about the
expectation with which it should be met in future (taking account of Note\
2 and Note\ 3 and about any possible consequential need for notifications
to
Recommendations referring to equipments.
.bp
.RT
.LP
.rs
.sp 17P
.ad r
\fBFigure 1/G.132, p.  \fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
    \fBReference\fR
.sp 1P
.RT
.LP
[1]
    CCITT Recommendation \fI12\(hychannel terminal equipments\fR , Vol.\ III,
Rec.\ G.232, \(sc\ 1.
.sp 2P
.LP
\fBRecommendation\ G.133\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBGROUP\(hyDELAY\ DISTORTION\fR
.EF '%    Fascicle\ III.1\ \(em\ Rec.\ G.133''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.133    %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1964; amended at Geneva, 1980)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.PP
The network performance objectives for the permissible differences for
a worldwide chain of 12\ circuits
each on a single 12\(hychannel group link, between the minimum group delay
(throughout the transmitted frequency band) and the group delay at the lower
and upper limits of this frequency band are indicated in the Table\ 1/G.133.
.sp 1P
.RT
.PP
Group\(hydelay distortion is of importance over a band of frequencies where
the attenuation is of importance, i.e. at which the attenuation is less
than 10\ dB relative to the value at 800\ Hz. This will normally be the
case for frequencies higher than about 260\(hy320\ Hz and lower than about
3150\(hy3400\ Hz
respectively for the lower and upper limit of the frequency band as indicated
in Table\ 1/G.133.
.ce
\fBH.T. [T1.133]\fR
.ce
TABLE\ 1/G.133
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(120p) | cw(54p) | cw(54p) .
\\fB.\fR     {
Lower limit of
frequency band
(ms)
 }     {
Upper limit of
frequency band
(ms)
 }
_
.T&
cw(120p) | cw(54p) | cw(54p) .
International chain    30\fB.5\fR    15\fB.5\fR
.T&
cw(120p) | cw(54p) | cw(54p) .
 {
Each of the national 4\(hywire extensions
 }    15\fB.5\fR    \ 7.5
.T&
cw(120p) | cw(54p) | cw(54p) .
On the whole 4\(hywire chain    60\fB.5\fR    30\fB.5\fR
.TE
.LP
\fINote\fR
\ \(em\ Limits given in Table 1/G.133 should be met
both for analogue circuits and mixed
circuits with analogue and digital sections.
.nr PS 9
.RT
.ad r
\fBTable 1/G.133 [T1.133], p.  \fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 2P
.LP
\fBRecommendation\ G.134\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBLINEAR\ CROSSTALK\fR
.FS
Recommended
methods for the measurement of crosstalk are described in Annex\ A.
.FE
.EF '%    Fascicle\ III.1\ \(em\ Rec.\ G.134''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.134    %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1964; amended at Mar del Plata, 1968)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.LP
\fB1\fR     \fBLinear crosstalk between different 4\(hywire chains of\fR
\fBcircuits (analogue and mixed)\fR
.sp 1P
.RT
.PP
As a network performance objective, the signal\(hyto\(hycrosstalk ratio
which may exist between two 4\(hywire
chains of circuits comprising international and national circuits is restricted
by Recommendation\ G.151, \(sc\ 4.1, as regards circuits, and by
Recommendation\ Q.45\ [1], as regards international centres.
.RT
.sp 2P
.LP
\fB2\fR \fBLinear crosstalk between go and return channels of the 4\(hywire\fR
\fBchain of circuits (analogue and mixed)\fR
.sp 1P
.RT
.PP
As a network performance objective, the signal\(hyto\(hycrosstalk ratio
between the two directions of transmission of a 4\(hywire chain of circuits
is
restricted by Recommendation\ G.151, \(sc\ 4.2, as regards circuits and by
Recommendation\ Q.45\ [1] as regards international centres.
.RT
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation G.134)
.sp 9p
.RT
.ce 0
.ce 1000
\fBMethods for measuring crosstalk in exchanges,\fR
.sp 1P
.RT
.ce 0
.ce 1000
\fBon international telephone circuits\fR
.ce 0
.ce 1000
\fBand on a chain of international telephone circuits\fR
.ce 0
.PP
A.1
The method used for measuring crosstalk will depend on the
type of crosstalk. In general one or the other of the following two situations
will be encountered:
.sp 1P
.RT
.LP
    a)
    crosstalk in an exchange arising mainly from a single
source or from several nearby sources;
.LP
    b)
    crosstalk measured at the end of a circuit or chain of
circuits and which is the result of multiple sources of crosstalk
occurring at points along the circuit or chain of circuits.
The total crosstalk will depend on the relative phases of the
individual contributions and may accordingly vary greatly with
frequency. On long circuits or chains of circuits, difficulties
may arise when making crosstalk measurements at a single
frequency owing to small variations in the frequency of the
master oscillators supplying translating equipment at various
points along the circuit or chain of circuits.
.PP
A.2
Available methods for measuring crosstalk are as
follows
.FS
It is a question here of the measurement of the frequency (or
frequencies) to be used; the measure of the crosstalk for a given frequency
is described in\ [2]
.FE
:
.sp 9p
.RT
.LP
    a)
    single\(hyfrequency measurements (e.g. at 800\ Hz or 1000\ Hz);
.LP
    b)
    measurements made at several frequencies (e.g. at 500,
1000 and 2000\ Hz), the results being averaged on a current or voltage
basis;
.LP
    c)
     measurements made using a uniform spectrum random noise or closely spaced
harmonic series signal shaped in accordance
with a speech power density curve. Such measurements should
be made in accordance with the Recommendation cited in\ [3];
.LP
    d)
     voice/ear tests, in which speech is used as the disturbing source and
the crosstalk is measured by listening and comparing
its level with a reference source whose level can be adjusted by
some form of calibrated attenuating network.
.bp
.PP
A.3
Pending further study, the following methods are provisionally recommended
for \*Q
type tests
\*U and \*Q
acceptance tests
\*U involving   crosstalk measurement.
.sp 9p
.RT
.sp 1P
.LP
A.3.1
    \fICrosstalk in exchanges\fR
.sp 9p
.RT
.PP
Crosstalk should be measured at 1100\ Hz which, in the experience of some
Administrations, is equivalent to a measurement made with a conventional
telephone signal generator (Recommendation\ G.227\ [4]) and a
psophometer.
.RT
.sp 1P
.LP
A.3.2
    \fICrosstalk on an international telephone circuit\fR \fIor\fR
\fIchain of international telephone circuits\fR
.sp 9p
.RT
.PP
Crosstalk should be measured using a uniform spectrum random noise or closely
spaced harmonic series signal shaped in accordance with the speech power
density curve of Recommendation\ G.227\ [4]. The measurements should be
made in accordance with the Recommendation cited in\ [3].
.PP
\fINote\ 1\fR \ \(em\ In cases of difficulty with A.2.a) and A.2.b), voice/ear
tests are recommended.
.PP
\fINote\ 2\fR \ \(em\ In the case of telephone circuits used for voice\(hyfrequency
telegraphy the near\(hyend signal\(hyto\(hy
crosstalk\ ratio between the two
directions
of transmission should be measured at each of the telegraph channel carrier
frequencies, i.e. at each odd multiple of 60\ Hz from 420\ Hz to 3180\ Hz
inclusive. However, difficulty can arise in practice because of the effect
mentioned in A.1.b) above.
.RT
.sp 2P
.LP
    \fBReferences\fR
.sp 1P
.RT
.LP
[1]
    CCITT Recommendation \fITransmission characteristics of an\fR
\fIinternational exchange\fR , Vol.\ VI, Rec.\ Q.45.
.LP
[2]
    \fIMeasurement of crosstalk\fR , Green Book, Vol.\ IV.2, Supplement
No.\ 2.4, ITU, Geneva,\ 1973.
.LP
[3]
    CCITT Recommendation \fI12\(hychannel terminal equipments\fR , Vol.\ III,
Rec.\ G.232, \(sc\ 9.2.
.LP
[4]
    CCITT Recommendation \fIConventional telephone signal\fR , Vol.\ III,
Rec.\ G.227.
\v'6p'
.sp 2P
.LP
\fBRecommendation\ G.135\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBERROR\ ON\ THE\ RECONSTITUTED\ FREQUENCY\fR
.EF '%    Fascicle\ III.1\ \(em\ Rec.\ G.135''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.135    %'
.ce 0
.sp 1P
.ce 1000
\fI(Mar del Plata, 1968)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.PP
As the channels of any international telephone circuit should be suitable
for voice\(hyfrequency telegraphy, the network performance objective for
the accuracy of the virtual carrier
frequencies should be such that the difference between an audiofrequency
applied to one end of the circuit and the frequency received at the other
end should not exceed 2\ Hz, even when there are intermediate modulating
and
demodulating processes.
.sp 1P
.RT
.PP
To attain this objective, the CCITT recommends that the channel
and group carrier frequencies of the various stages should have the accuracies
specified in the corresponding clauses of Recommendation\ G.225\ [1].
.PP
Experience shows that, if a proper check is kept on the operation of oscillators
designed to these specifications, the difference between the
frequency applied at the origin of a telephone channel and the reconstituted
frequency at the other end hardly ever exceeds 2\ Hz if the channel has
the same composition as the 2500\(hykm hypothetical reference circuit for
the system
concerned.
.PP
Calculations indicate that, if these recommendations are followed, in the
4\(hywire chain forming part of the hypothetical reference connection defined
in Figure\ 1/G.103
.FS
In fact, the chain considered for these calculations
comprised 16 (instead of\ 12) modulator\(hydemodulator pairs to allow for the
possibility that submarine cables with equipments in conformity with
Recommendation\ G.235\ [2] might form part of the chain. No allowance was
made, however, for the effects of Doppler frequency\(hyshift due to inclusion
of a
non\(hystationary satellite; values for this shift are given in
CCIR\ Report\ 214\ [3].
.FE
there is about 1% probability that
the frequency difference between the beginning and the end of the connection
will exceed 3\ Hz and less than 0.1% probability that it will
exceed 4\ Hz.
.bp
.PP
The CCITT notes that in mixed circuits having several digital sections
the requirements concerning frequency error are met more easily since digital
systems do not change the frequency of an audio frequency channel.
.RT
.sp 2P
.LP
    \fBReferences\fR
.sp 1P
.RT
.LP
[1]
     CCITT Recommendation \fIRecommendations relating to the accuracy\fR \fIof
carrier frequencies\fR , Vol.\ III, Rec.\ G.225.
.LP
[2]
     CCITT Recommendation \fI16\(hychannel terminal equipments\fR , Vol.\
III, Rec.\ G.235.
.LP
[3]
    CCIR Report \fIThe effects of doppler frequency\(hyshifts and\fR
\fIswitching discontinuities in the fixed satellite service\fR , Vol.\ IV,
Report\ 214, ITU, Geneva,\ 1986.
\v'6p'
.IP
\fB1.4\fR     \fBGeneral characteristics of the 4\(hywire chain of international
circuits; international transit\fR
.sp 1P
.RT
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    \fBRecommendation\ G.141\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBATTENUATION\ DISTORTION\fR
.EF '%    Fascicle\ III.1\ \(em\ Rec.\ G.141''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.141    %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1964; amended at Mar del Plata, 1968\fR \fIand Geneva, 1972
and 1980)\fR
.sp 9p
.RT
.ce 0
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\fB1\fR     \fBAttenuation distortion\fR
.sp 1P
.RT
.sp 1P
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1.1
    \fIAll\(hyanalogue conditions\fR
.sp 9p
.RT
.PP
The design objectives recommended for carrier terminal equipment by the
Recommendation cited in [1] are such that for a chain of six circuits,
each equipped with a single pair of channel translating equipments in accordance
with that Recommendation, the network performance objective for the attenuation
distortion given by Figure\ 1/G.132 will in most cases be met. The distortion
contributed by the seven international centres is thereby included.
.PP
\fINote\fR \ \(em\ To assess the attenuation distortion of the international
chain, the limits indicated for international circuits in Recommendation\
G.151, \(sc\ 1 must not be added to the limits for international centres
mentioned in
Recommendation\ Q.45\ [2]. In fact, on the one hand, some exchange equipment
would be counted twice if this addition were made; on the other, the
specification limits of Recommendation\ Q.45\ [2] apply to the worst possible
connection through an international exchange, while the maintenance limits
of Recommendation\ G.151, \(sc\ 1 apply to the poorest international circuit.
The
specifications of the various equipments are such that the mean performance
will be appreciably better than could be estimated by the above\(hymentioned
addition.
.RT
.sp 1P
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1.2
    \fIMixed analogue/digital conditions\fR
.sp 9p
.RT
.PP
In the mixed analogue/digital period, it is expected that the
attenuation/frequency characteristics of the analogue carrier terminal
equipment that is to be used in international telephone connections will
continue to be governed by existing Recommendations that are relevant to
this type of circuit.
.PP
Where unintegrated PCM digital processes are to be included in
international telephone connections, it is recommended that the
attenuation/frequency characteristic of the bandpass filters associated with
such processes should comply with the more stringent version of
Figure\ 1/G.712\ [3]. The latter Recommendation applies specifically to
cases where integrated PCM digital processes are associated with
trunk junctions (toll connecting trunks), trunk circuits (intertoll trunks),
and international circuits.
.PP
With regard to the incorporation of unintegrated PCM digital processes
in local telephone networks, the required attenuation/frequency characteristics
of the bandpass filters involved are still under study.
.bp
.RT
.sp 2P
.LP
    \fBReferences\fR
.sp 1P
.RT
.LP
[1]
    CCITT Recommendation \fI12\(hychannel terminal equipments\fR , Vol. III,
Rec.\ G.232, \(sc\ 1.
.LP
[2]
     CCITT Recommendation \fITransmission characteristics of an international\fR
\fIexchange\fR , Vol.\ VI, Rec.\ Q.45.
.LP
[3]
    CCITT Recommendation \fIPerformance characteristics of PCM channels\fR
\fIbetween 4\(hywire interfaces at voice frequencies\fR , Vol.\ III, Rec.\
G.712,
Figure\ 1/G.712.
\v'6p'
.sp 2P
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\fBRecommendation\ G.142\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBTRANSMISSION\ CHARACTERISTICS\ OF\ EXCHANGES\fR
.EF '%    Fascicle\ III.1\ \(em\ Rec.\ G.142''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.142    %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1980; amended at Melbourne, 1988)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.PP
This Recommendation consists of two parts. The first part, \(sc\ 1,
is concerned with the voice\(hyfrequency transmission characteristics of
international analogue exchanges. The information involved is encompassed
within Recommendation\ Q.45\ [1]. The second part, \(sc\ 2, is concerned
with the
voice\(hyfrequency transmission considerations that should be taken into
account in the design of digital exchanges and their incorporation into
the network.
The digital exchanges referred to include local exchanges and transit exchanges
(national and international). The transmission considerations relate primarily
to the properties which digital exchanges should possess to enable them
to
.sp 1P
.RT
.LP
operate under different and changing network conditions with respect to the
content of analogue, mixed analogue/digital and all\(hydigital plant.
.PP
Detailed transmission characteristics for digital exchanges are
contained in Recommendations\ Q.551, Q.552, Q.553 and Q.554 (Fascicle\ VI.5).
.sp 2P
.LP
\fB1\fR     \fBInternational analogue exchange\fR
.sp 1P
.RT
.PP
The commissioning objectives for the transmission requirements to be respected
by an international analogue exchange are included in
Recommendation\ Q.45 or Q.45 | fIbis\fR .
.RT
.sp 2P
.LP
\fB2\fR     \fBDigital exchanges\fR
.sp 1P
.RT
.sp 1P
.LP
2.1
    \fIDigital processes\ \(em\ Effect on transmission\fR
.sp 9p
.RT
.PP
Digital (TDM) exchanges, to varying degrees, are required to
include such digital processes as analogue\(hyto\(hydigital coders,
digital\(hyto\(hyanalogue decoders and digital recoding processes, examples
of which are companding law converters and digital pads. The extent to
which such
digital processes might be included in a digital exchange is determined
by the network environment in which the exchange is to operate (i.e.,\
all\(hyanalogue,
mixed analogue/digital or all\(hydigital).
.PP
Digital processes such as those referred to above, attract
transmission penalties. These penalties can be expressed in terms of \*Qunits
of transmission impairment\*U.
.PP
A limit is placed on the permissible accumulation of units of
transmission impairment in an international telephone connection. Details
of the planning rule resulting from this limit and the penalties introduced
by individual digital processes are given in Recommendations\ G.101, \(sc\
4
and\ G.113, \(sc\ 3.
.PP
In accordance with Recommendation G.113, \(sc\ 3 it is provisionally
recommended that no more than 14\ units of transmission impairment be permitted
to accumulate in an international connection. Of these 14\ units, a maximum
of 5\ units could be introduced by each national extension and a maximum
of 4\ units by the international portion. Since one 8\(hybit PCM codec
pair (coder and
decoder) introduces 1\ unit of transmission impairment, it is clear that
unintegrated PCM digital processes involving analogue/digital conversions,
.PP
(e.g.\ codecs) or digital processes involving the recoding of information
(e.g.\ digital pads) should not be allowed to proliferate in an uncontrolled
fashion. Figure\ 1/G.142 shows some of the transmission paths that might be
established through a digital exchange and the \*Qunits of transmission
impairment\*U attributable to the digital processes in these paths.
.bp
.RT
.LP
.rs
.sp 47P
.ad r
\fBFigure 1/G.142, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
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2.2
    \fITransmission loss through a digital exchange\fR
.sp 9p
.RT
.PP
The 4\(hywire digital switching function at a digital exchange should introduce
a nominal transmission loss of 0\ dB. Thus, in Figure\ 1/G.142 (Case\ 1)
if a 0\ dBm0 sinusoidal test signal is introduced at the analogue terminals
of an ideal coder connected to the input of a digital switch, a Digital
Reference Sequence (DRS) should be transmitted unaltered through the switch
and produce a 0\ dBm0 sinusoidal signal at the analogue terminals of a
decoder connected to the output of the digital switch.
.PP
Except for the transmission loss considered above (and perhaps
the possible loss due to exchange wiring) all transmission losses which
are to be introduced by a digital exchange, either in a digital or analogue
form,
are to be governed by the applicable transmission plan (see \(sc\ 2.4
below).
.RT
.sp 1P
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2.3
    \fIRelative levels\fR
.sp 9p
.RT
.PP
On digital paths within an all\(hydigital network, relative levels
have no real meaning or use. However, as long as a substantial portion
of the worldwide telephone network is of an analogue nature, it is necessary
and
useful to assign relative levels to digital exchanges.
.PP
The relative levels assigned to a digital exchange are applicable at the
virtual analogue switching points of the exchange. The virtual analogue
switching points are theoretical points as explained in
Recommendation\ G.101, \(sc\ 5.1. The concept of applying relative levels
at the
virtual analogue switching points of a digital exchange is dealt with in
Recommendations\ G.101, \(sc\ 4.2 and\ G.101, \(sc\ 5.2.
.PP
In accordance with Recommendation G.101, \(sc\ 5.2 the send relative
level at an international digital exchange should be \(em3.5\ dBr. In the
case of digital exchanges in national extensions, the send relative levels
should be
governed by the applicable national transmission plan.
.PP
With regard to the receive relative level at a digital exchange, this level
is related to the transmission loss of the circuits terminating at the
exchange. In the case of an international digital exchange, it is desirable
to have the receive relative level at \(em3.5\ dBr to avoid having to introduce
digital pads. But see the general Note in Recommendation\ G.101, \(sc\ 4.2 for
exceptions. In the case of national extensions, the receive relative levels,
as in the case of the send relative levels, are to be determined on the
basis of the applicable national transmission plan.
.RT
.sp 1P
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2.4
    \fIEcho and stability control\fR
.sp 9p
.RT
.PP
The overall echo and stability losses presented by a national
extension are a function of the relevant transmission losses and, in the
case of the use of 2\(hywire conversion circuits, the balance return loss
introduced
by the 2\(hywire/4\(hywire conversion circuit. Both contributions need to
be considered in the design of digital local exchanges where there is generally
scope for improving the echo and stability losses. Such improvements are
likely to be needed as connections in digital networks will tend to have
lower losses and longer delays than analogue connections with a consequent
worsening in echo performance.
.RT
.sp 1P
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2.4.1
    \fITransmission loss contribution\fR
.sp 9p
.RT
.PP
The requirements for controlling stability and echo on
international connections under all\(hydigital or mixed analogue/digital
network conditions are dealt with in Recommendation\ G.122. In accordance
with the
latter Recommendation, the national extensions are to be mainly responsible
for effecting this control. Arrangements for doing so are dealt with in
Recommendation\ G.121, \(sc\ 6.
.PP
Recommendation G.121, \(sc\ 6 provides the framework within which
individual national transmission plans are to provide for the necessary
features to effect the required control. In the case of a digital 4\(hywire
national extension (i.e.,\ all\(hydigital down to the local exchange but with
2\(hywire analogue subscriber lines), the control can be effected entirely
at the local exchange. Where the national extension is to be of a mixed
analogue/digital nature, the control under some national transmission plans
might be distributed among the different parts of the national extension but
the main burden would in general still lie with the local exchange.
Figure\ 1/G.142 contains examples of some of the different arrangements that
might be encountered at a digital exchange.
.bp
.PP
The arrangement in Case 1 of Figure 1/G.142 deals with the
termination of a digital circuit at what might be a national or international
digital exchange. In this particular case, the circuit is to be operated
without introducing additional loss at the exchange.
.PP
The arrangement in Case 2 of Figure 1/G.142 also deals with
the termination of a digital circuit at a national or international digital
exchange. However, in this case, the relevant transmission plan requires
that loss should be associated with the circuit at the exchange through
the medium of digital pads. See \(sc\ 2.6 below regarding the use of digital
pads.
.PP
The arrangement in Case 3 of Figure 1/G.142 deals with the
termination of a 2\(hywire subscriber's line at a digital local exchange.
The pads designated\ R and\ T are pad symbols intended to represent loss
or level
adjustment made in the analogue portion. Recommendation\ G.121, \(sc\ 6
is concerned with the appropriate choice of values for R and\ T.
.PP
The arrangement in Case 4 of Figure 1/G.142 is similar to that of
Case\ 3 except that the losses\ R and\ T are shown as being provided in the
digital portion. See \(sc\ 2.6 below regarding the use of digital pads.
.PP
The arrangement in Cases 5, 6 and 7 of Figure 1/G.142 deals with
the termination of analogue circuits at a national or international digital
exchange. In Case\ 5, an analogue pad\ (L) is used to develop the required
loss of the circuit in accordance with the relevant transmission plan.
Case\ 6 is similar to Case\ 5 except that a digital pad\ (L) is used to
develop the
required circuit loss. Case\ 7 is also similar to Case\ 5 except that the
.PP
analogue pad\ (L) as well as the A/D\ coder and D/A\ decoder are provided
as part of the transmission equipment associated with the circuit rather
than by
equipment that is built\(hyin as part of the switching system. Although
not shown in Figure\ 1/G.142, the A/D\ coders, the D/A\ decoders, the 2\(hywire/4\(hywire
terminating units and the pads involved in Cases\ 2, 3 and\ 4 can also be
provided as part of the transmission equipment on the transmission side
of the exchange rather than by equipment that is built\(hyin as part of
the switching
system.
.RT
.sp 1P
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2.4.2
    \fIBalance return loss contribution\fR
.sp 9p
.RT
.PP
The contribution of balance return loss to the overall echo and
stability losses is illustrated in Cases\ 3 and\ 4 of Figure\ 1/G.142 which
show the situation of 2\(hywire local lines terminating on a digital local
exchange.
The achieved balance return loss is determined by the match between the
impedance presented by the 2\(hywire local line and customer terminating
apparatus and the balancing impedance chosen for the digital exchange line
card.
.PP
In many designs of digital local exchange there is no 2\(hywire switch
and the 2\(hywire line is permanently connected to the line card. This
arrangement has significant advantages for balance return loss as there
is likely to be a significant reduction in the range of impedances presented
to any single line card. It is then possible to choose a line card balancing
impedance more
closely matched to the local line impedances and obtain an improvement in
balance return loss compared with the conventional compromise impedances.
.PP
The optimum balancing impedance will not be the same for all
Administrations as it needs to take into account the local cable types used
together with the range of customer apparatus impedances. It is possible
that the use of different exchange balancing impedances for different local
line
classes will give an improvement in performance at the expense of some
increase in network Administration. In general it has been found that the
use of
balancing networks which resemble the impedance presented by local cable
give the optimum performance. Examples of balancing impedances adopted
by a number of Administrations are given in Recommendation\ Q.552.
.PP
Further improvement in balance return loss is possible where the
impedance of the customer apparatus can be influenced by the Administration.
Telephone instruments with an input impedance close to the impedance of the
local cable can result in an improvement in the balance return loss at the
digital local exchange in the order of 10\ dB on short local lines.
.RT
.sp 1P
.LP
2.5
    \fILocal transmission\fR
.sp 9p
.RT
.PP
On local calls between subscribers served by the same
digital   local exchange
, the switching of 2\(hywire subscriber lines such as those
shown in Figure\ 1/G.142, Case\ 3, results in an equipment arrangement which
takes on the appearance of a voice\(hyfrequency repeater\ \(em see Figure\
2/G.142. As is well known, such an arrangement must include sufficient
loss around the loop to provide for an adequate margin of stability. To
provide for this loss, some 2\(hywire to 2\(hywire attenuation may be acceptable
in some cases. The attenuation might be supported by the national transmission
plan, as it provides adequate loudness rating distribution for local calls.
However, in cases where the
2\(hywire to 2\(hywire attenuation is to be comparable to that generally
prevailing at an analogue exchange, i.e.,\ approximately 0\ dB, adequate
balance return
losses must be provided at the 2\(hywire/4\(hywire junctions. This could entail
increasing the existing values of balance return loss at these points.
Methods for doing this are under study by Study Group\ XII.
.bp
.PP
Increasing the balance return losses as referred to above should
also be beneficial to the control of echo and stability in national connections
beyond the local exchange as well as on international connections.
.RT
.LP
.rs
.sp 26P
.ad r
\fBFigure 2/G.142, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
2.6
    \fISidetone and input impedance\fR
.sp 9p
.RT
.PP
Digital local exchanges can have a significant influence on
the sidetone performance of telephone instruments, particularly those
instruments on relatively short local lines. The reason for this can be
seen in Figure\ 2/G.142 where the impedance presented by the exchange to
the local line is a function of the input impedance of the line card and
the characteristics of the singing and echo path within the exchange.
.PP
For optimum sidetone performance on short local lines the input
impedance of the exchange line card should be close to the anti\(hysidetone
impedance of the telephone instrument. In the case where the telephone
instrument is designed to give good sidetone performance on long local lines
this anti\(hysidetone impedance is likely to be close to the characteristic
impedance of the 2\(hywire local cable. This would lead to the digital local
exchange also presenting an impedance close to that of the 2\(hywire local
cable.
.PP
On longer local lines the exchange impedance will have less effect on the
sidetone performance as the impedance presented to the telephone is masked
by the local cable impedance.
.PP
The final choice of exchange impedance needs to take into account a
number of factors:
.RT
.LP
    \(em
    telephone set impedance and sensitivity characteristics;
.LP
    \(em
    local line network characteristics;
.LP
    \(em
    digital exchange current feeding arrangements,
.LP
the objective being that the customer should not see a worsening in sidetone
performance when connected to a digital exchange. The impedance chosen by a
number of Administrations are given in Recommendation\ Q.552 and it is clear
that there is a considerable difference between the impedances which reflects
the differences between the national networks.
.bp
.sp 1P
.LP
2.7
    \fIDigital pads\fR
.sp 9p
.RT
.PP
The use of a digital pad to produce the required transmission loss in a
digital path attracts a transmission penalty. This penalty has to come
out of the allowance of \*Qunits of transmission impairment\*U allotted
to the national and international portions of international connections\
\(em see
Recommendation\ G.113, \(sc\ 3. Additionally, since digital pads involve
the use of digital recoding processes, the use of such pads in paths where
bit integrity must be preserved is unattractive. This can be an important
consideration
where multipurpose networks are contemplated. Consequently, if digital pads
must be introduced, arrangements should be made to switch them out or to
bypass them.
.RT
.sp 1P
.LP
2.8
    \fITransmission delay\fR
.sp 9p
.RT
.PP
Transmission delays through digital exchanges could be significant. For
example, such delays could have the effect of decreasing the length of
connections on which echo control devices (e.g.,\ echo suppressors or echo
cancellers) should be applied. Transmission delays at digital local exchanges
(or at digital PBXs) could in some cases also affect the impedance match
between subscriber lines and the exchange (or PBX) in a way that could
adversely affect subscriber sidetone. Transmission delays through digital
exchanges should, therefore be minimized. See Recommendation\ G.114, \(sc\
2 for
details of the delay introduced by various items of digital equipment and
systems.
.PP
For transmission delays that might be encountered at digital
exchanges; see Recommendation\ Q.551.
.RT
.sp 2P
.LP
    \fBReference\fR
.sp 1P
.RT
.LP
[1]
     CCITT Recommendation \fITransmission characteristics of an international\fR
\fIexchange\fR , Vol.\ VI, Rec.\ Q.45.
\v'1P'
.LP
.sp 2P
.LP
\fBRecommendation\ G.143\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBCIRCUIT\ NOISE\ AND\ THE\ USE\ OF\ COMPANDORS\fR
.EF '%    Fascicle\ III.1\ \(em\ Rec.\ G.143''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.143    %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1964; amended at Mar del Plata, 1968;\fR
.sp 9p
.RT
.ce 0
.sp 1P
.ce 1000
\fIGeneva, 1972 and 1980\fR \fIand Malaga\(hyTorremolinos, 1984)\fR
.ce 0
.sp 1P
.LP
\fB1\fR     \fBNoise objectives for telephony\fR
.sp 1P
.RT
.sp 1P
.LP
1.1
    \fIPrinciple\fR
.sp 9p
.RT
.PP
Taking into account the network performance objectives for noise
allowed in national networks
(Recommendation\ G.123), it is desirable that the circuit performance objective
for the mean psophometric
power in any hour of the total noise generated by a chain of six international
circuits, some of which may exceed 2500\ km in length, on a connection
used for international telephone calls, should not exceed 50 | 00\ picowatts
referred to a zero relative level point of the first circuit in the chain
(level
\(em43\ dBm0p).
.PP
Of course, a lower value of the total noise may be expected when the international
chain consists of only a small number of international circuits, not exceeding
2500\ km in length and conforming to Recommendation\ G.152 (in
particular, the circuit performance objective for the noise of such circuits
is that the mean psophometric power in any hour does not exceed 10 | 00\
pW at a
zero level point on the circuit, level \(em50\ dBm0p).
.PP
However, as connections longer than 25 | 00\ km will be set up, the
CCITT recommends, as an objective, that on sections longer than 2500\ km used
for international traffic, line equipment be supplied with a circuit
performance objective for noise not greatly exceeding \fIL\fR \ picowatts on a
circuit \fIL\fR \ km long (see\ [1]). There is obvious advantage in working
to the
same standard on short sections when this can reasonably be done.
.bp
.PP
\fINote\ 1\fR \ \(em\ Noise objectives for maintenance purposes are the
subject of Recommendation\ M.580\ [2]. Table\ 4/M.580 of that Recommendation
is
reproduced here:
.RT
.ce
\fBH.T. [T1.143]\fR
.ce
TABLE\ 4/M.580
.ce
\fBMaintenance noise objectives for public telephone circuits\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
lw(84p) | cw(18p) | cw(24p) | cw(18p) | cw(24p) | cw(18p) | cw(24p) | cw(18p) .
Distance (km)    < | 20    321  to  640    641  to  1600    1601  to  2500    2501  to  5000    5001  to  10 | 00    10 | 01 to  20 | 00
_
.T&
lw(84p) | cw(18p) | cw(24p) | cw(18p) | cw(24p) | cw(18p) | cw(24p) | cw(18p) .
Noise (dBm0p)    \(em55    \(em53    \(em51    \(em49    \(em46    \(em43    \(em40
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 4/M.580 [T1.143], p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
.sp 2
\fINote\ 2\fR \ \(em\ Strictly speaking, the noise objective for
communication\(hysatellite systems (see Recommendation\ G.153, \(sc\ 3)
cannot be
expressed in the form of a given number of picowatts per km. See also the
Note of Recommendation\ M.580\ [2].
.sp 1P
.LP
1.2
    \fINoise produced by equipment\fR
.sp 9p
.RT
.PP
The equipment design objective for noise produced by the modulating equipment
in the international chain of circuits in the longest hypothetical
reference connection (see Figure\ 1/G.103) can be estimated on the assumption
that such equipment comprises:
.RT
.LP
    \(em
    6\ channel\(hymodulation pairs, or 8 to 10 if 3\(hykHz\(hyspaced
channel equipment is used on transoceanic routes;
.LP
    \(em
    12 to 14\ group\(hymodulation pairs;
.LP
    \(em
    18 to 24\ supergroup\(hymodulation pairs;
.LP
for all of which a total circuit performance for the combined psophometric
power of 5000 to 7000\ pW0p (at a point
of zero relative level on the first circuit of the international chain of
4\(hywire circuits) is a generous assumption.
.PP
The equipment design objective of \(em67\ dBm0p for the hourly\(hymean
psophometric power
level at each international switching point quoted in Recommendation\ Q.45\
[3] is equivalent to about 2000\ pW0p at a point of zero relative level
on the first circuit in the 4\(hywire chain.
.PP
It may thus be seen that the equipment design objective for the noise produced
by the equipment does
not constitute a large part of the network performance objective for the
total noise generated by the international chain.
.RT
.sp 1P
.LP
1.3
    \fIDivision of the overall circuit performance objective for noise\fR
.sp 9p
.RT
.PP
The land sections in the international chain, set up on cable
carrier systems or on radio\(hyrelay links, should in principle afford
circuits of the quality defined above. In practice, by agreement between
Administrations, the circuit performance objective for noise could be shared
between the
submarine and overland systems in such a way that the submarine cable systems
contribute at a somewhat lower rate, e.g.\ 1\ pW/km, and the overland systems
contribute at a somewhat higher rate, e.g.\ a maximum of 2\ pW/km. This result
may be achieved either by setting up special systems, or by a proper choice
of channels in systems designed to the 3\ pW/km objective.
.PP
\fINote\fR \ \(em\ In some countries, overland systems forming part of a
circuit substantially longer than 2500\ km (e.g. 5000\ km or more) have been
constructed with the same circuit performance objective for noise as the
submarine cable system, i.e.\ 1\ pW/km.
.bp
.RT
.sp 1P
.LP
1.4
    \fICircuits operated with speech concentrators\fR
.FS
For example, TASI (Time Assignment Speech Interpolation) of CELTIC (Concentrateur
exploitant
les temps d'inoccupation des circuits); see Recommendation\ G.163.
.FE
.sp 9p
.RT
.PP
It would be desirable for all the circuits making up a group for
use with a concentrator system to have approximately the same noise power
level under operating conditions.
.RT
.sp 2P
.LP
The instantaneous compandors that are associated with certain
transmission systems are considered to be an
integral part of these systems.
.FE
\fB2\fR     \fBUse of \fR \fBsyllabic compandors\fR \u,\d\
.FS
For
characteristics of syllabic compandors for telephony on high capacity long
distance systems, see Recommendation\ G.166.
.FE
\u,\d\
.FS
See Annex\ A for
further considerations relating to the use of syllabic compandors.
.FE
.sp 1P
.RT
.PP
For many years, international (and national) circuits
will continue to be provided on existing transmission systems which have
been designed to other standards, e.g.\ 4\ pW/km, as given in Recommendation | .152.
Furthermore, the circuit noise produced by transmission systems can increase
above the values originally achieved because of ageing effects, and changes
of system loading. There is therefore a need for a simple practical criterion
that can be applied for planning purposes to an international circuit to
.PP
determine if, as far as noise power is concerned, it is suitable for
establishing multicircuit worldwide telephone connections or whether it
can be made suitable by fitting compandors
.
.PP
It is recommended that, for the present, the systematic use of
compandors conforming to Recommendation\ G.162 in the long\(hydistance
national and international network be restricted.
.PP
Compandors conforming to Recommendation G.166 may be used in the
network provided planning is done to minimize the number of compandored
circuits in tandem. It is desirable to have at most one compandored circuit
in a connection. Preliminary results obtained by one Administration indicate
that for voice operation no more than three compandored circuits in tandem
should be allowed. Some high speed modems (9.6\ kbit/s) may experience
difficulty on a
connection with even one compandored circuit. To ensure compliance not more
than one compandored circuit should be used in the international segment.
Additional information is required before a firm planning rule can be
established including possible application in national extensions on circuits
with moderate noise levels.
.PP
It must be pointed out that the action of a compandor doubles the
effect of any variations in the transmission loss occurring in that part
of the circuit which lies between the compressor and the expander and for
this reason compandors, if needed, should be fitted at the ends of circuit
sections
provided by inherently stable line transmission systems such as submarine
cable systems.
.PP
The following planning rule is recommended by the CCITT as a guide for
deciding whether an international circuit requires a compandor:
.PP
If the hourly\(hymean psophometric circuit noise power level of an
international circuit substantially longer than 2500\ km (e.g.\ 5000\ km
or more) is less than \(em44\ dBm0p (at a point of zero relative level
on the circuit) no
compandor is necessary.
.PP
If the circuit noise power level is \(em44\ dBm0p (40 | 00 pW0p) or
greater, a compandor should be fitted.
.PP
It is, of course, to be understood that circuits of
length 2500\ km or less will always meet the appropriate general noise
objectives (Recommendation\ G.222\ [4]) without the need for compandors.
.PP
\fINote\ 1\fR \ \(em\ This rule has been devised to make possible the planning
of the international telephone network, using presently available circuits.
It should in no way be interpreted as relaxation of the design objectives
recommended in \(sc\ 1 of this Recommendation, nor should it be applied for
maintenance purposes
(see\ Note\ 1 of \(sc\ 1.1 above).
.PP
\fINote\ 2\fR \ \(em\ The compandors used should conform to the limits
proposed in Recommendation\ G.162 or in Recommendation\ G.166.
.PP
\fINote\ 3\fR \ \(em\ In accordance with the Recommendation cited in [5],
circuits with a noise power level of \(em37\ dBm0p or worse are removed from
service.
.bp
.RT
.sp 2P
.LP
\fB3\fR     \fBNoise limits for telegraphy\fR
.sp 1P
.RT
.PP
Noise limits for telegraphy are given in
Recommendation\ H.22\ [6].
.RT
.sp 2P
.LP
\fB4\fR     \fBNoise limits for data transmission\fR
.sp 1P
.RT
.PP
The following objectives are acceptable for data transmission at
data signalling rates not exceeding 1200\ bit/s. It is expected that the
values actually experienced on many circuits and connections will be
better than the following limits.
.RT
.sp 1P
.LP
4.1
    \fILeased circuits for data transmission\fR
.sp 9p
.RT
.PP
A reasonable limit for uniform spectrum random noise for a data
transmission \fIleased\fR circuit, assuming that plant liable to impulsive
noise
interference is avoided, and as high a modulation rate as possible is to be
used without significant error rate, would appear to be \(em40\ dBm0p.
.RT
.sp 1P
.LP
4.2
    \fISwitched connections\fR
.sp 9p
.RT
.PP
For switched connections a limit of, say, \(em36\ dBm0p
without compandors may be taken for interconti
nental circuits on which
compandors may be used.
.RT
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation G.143)
.sp 9p
.RT
.ce 0
.ce 1000
\fBAdditional considerations relating to\fR \fBthe use of syllabic compandors\fR
.sp 1P
.RT
.ce 0
.ce 1000
(The following information was available from Study Group XII)
.sp 9p
.RT
.ce 0
.PP
This annex addresses compandor advantage in \(sc\ A.1, followed by a recommendation
of the permissible advantage limits for planning purposes
(\(sc\ A.2). A requirement of circuit stability between compressor and
expander is given in \(sc\ A.3, and \(sc\(sc\ A.4 and\ A.5 deals with aspects
of system loading and
companded circuits in tandem.
.sp 1P
.RT
.LP
.sp 1P
.LP
A.1
    \fICompandor advantage\fR
.sp 9p
.RT
.PP
To define \fBcompandor advantage\fR , assume:
.RT
.LP
    a)
    an international circuit not equipped with compandors and
contributing \fIN\fR \ dBm0 of noise to the overall end\(hyto\(hyend
connection (including typical national extensions) and meeting
the noise objectives of Recommendation\ G.152 or
Recommendation\ G.153, and
.LP
    b)
    the same international circuit equipped with compandors and
connected to typical national extensions, yielding the noise
performance subjectively equivalent to or better than that of
the circuit described in\ a), while contributing \fIN\fR `\ dBm0
of noise in between compressor and expander.
.PP
Then the compandor advantage for the international circuit of\ b) is defined
as (\fIN\fR `\ \(em\ \fIN\fR )\ dB.
.LP
.sp 1P
.LP
A.2
    \fICompandor advantage limit\fR
.sp 9p
.RT
.PP
For planning purposes, the compandor advantage defined in \(sc A.1
should not exceed\ 10\ dB.
.PP
\fINote\fR \ \(em\ It should be emphasized that this value applies to the
international portion of the connection only. Other portions of the connection
could permit a higher value when selected with due regard to the effect
it has on the total noise of the end\(hyto\(hyend connection during the
presence of the
signal.
.RT
.sp 1P
.LP
A.3
    \fICircuit stability\fR
.sp 9p
.RT
.PP
The international circuit between compressor and expander should
have an insertion loss which, when considered over a long period of time,
has a standard deviation not exceeding\ 0.75\ dB.
.bp
.RT
.sp 1P
.LP
A.4
    \fICircuit loading\fR
.sp 9p
.RT
.PP
It is generally advisable to select the unaffected level of the
compandor equal to \(em10\ dBm0. However, if Administrations mutually desire to
operate at a different value of unaffected level, it should be selected such
.PP
that it results in a system loading which minimizes total distortion due to
noise, intermodulation, or other load\(hydependent characterisics and should
always be dictated by the allowable compandor advantage limit.
.RT
.sp 1P
.LP
A.5
    \fICompandored circuits in tandem\fR
.sp 9p
.RT
.PP
The following paragraphs apply to circuits fitted with compandors according
to Recommendation\ G.162.
.PP
Results of experiments with compandored circuit links in tandem
show that two compandored links in tandem can produce a noticeable degradation
only if the second link exceeds, by a considerable margin, the recommended
compandor advantage limit of 10\ dB. The experiment was admittedly designed
to uncover gross effects by limiting the subjective judgement to only seven
persons per test condition.
.PP
The conclusion drawn was that two links in tandem, each of which is
limited to 10\ dB compandor advantage, will not pose a restriction to users.
This however, does not constitute sufficient guidance for application for
the number of compandored links permissible in an end\(hyto\(hyend international
connection.
.RT
.sp 2P
.LP
    \fBReferences\fR
.sp 1P
.RT
.LP
[1]
    CCITT \fIRed Book\fR , Vol. V | fIbis\fR , Annexes B and C, ITU, Geneva, 1965.
.LP
[2]
    CCITT Recommendation \fISetting\(hyup and lining\(hyup an international\fR
\fIcircuit for public telephony\fR , Vol.\ IV, Rec.\ M.580.
.LP
[3]
     CCITT Recommendation \fITransmission characteristics of an international\fR
\fIexchange\fR , Vol.\ VI, Rec.\ Q.45.
.LP
[4]
     CCITT Recommendation \fINoise objectives for design of\fR \fIcarrier\(hytransmission
systems of 2500\ km\fR , Vol.\ III, Rec.\ G.222.
.LP
[5]
    CCITT Recommendation \fISetting\(hyup and lining\(hyup an international\fR
\fIcircuit for public telephony\fR , Vol.\ IV, Rec.\ M.580, \(sc\ 6.
.LP
[6]
    CCITT Recommendation \fITransmission requirements of international\fR
\fIvoice\(hyfrequency telegraph links (at\ 50, 100 and 200\ bauds)\fR ,
Vol.\ III,
Rec.\ H.22.
\v'6p'
.LP
.IP
\fB1.5\fR     \fBGeneral characteristics of international telephone
circuits and national extension circuits\fR
.sp 1P
.RT
.sp 2P
.LP
    \fBRecommendation\ G.151\fR
.RT
.sp 2P
.ce 1000
\fBGENERAL\ PERFORMANCE\ OBJECTIVES\ APPLICABLE\ TO\ ALL\ MODERN\fR
.EF '%    Fascicle\ III.1\ \(em\ Rec.\ G.151''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.151    %'
.ce 0
.sp 1P
.ce 1000
\fBINTERNATIONAL\ CIRCUITS\ AND\ NATIONAL\ EXTENSION\ CIRCUITS\fR
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1964; amended at Mar del Plata, 1968\fR \fIand Geneva, 1972
and 1980)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.LP
\fB1\fR     \fBAttenuation distortion\fR
.sp 1P
.RT
.PP
The circuit performance objectives for attenuation distortion of
international circuits and national extension circuits should individually
be such that the network performance objectives of Recommendation\ G.132
are
complied with. Recommendation\ G.232\ [1] gives equipment design objectives.
.bp
.PP
It follows from the Recommendations mentioned above that, as a rule,   the
frequency band effectively transmitted
by a telephone circuit,
according to the definition adopted by the CCITT (i.e.\ the band in which the
attenuation distortion does not exceed 9\ dB compared with the value for
800\ Hz), will be a little wider than the 300\(hy3400\ Hz band, and for
a single
pair of channel terminal equipments of this type, the attenuation distortion
at 300\ Hz and 3400\ Hz should never exceed 3\ dB and in a large number
of equipments should not average more than 1.7\ dB (see Graphs\ A and\
B in Figure\ 1/G.232\ [2]). Even more complex circuits, and circuits using
terminal equipments with
3\(hykHz\(hychannel spacing in accordance with Recommendation\ G.235\ [3],
should
satisfy the limits in Figure\ 1/G.151; to ensure that these limits are
respected, equalizers are inserted, if necessary, when the circuits are
set up (Recommendation\ M.580\ [4]).
.RT
.LP
.rs
.sp 35P
.ad r
\fBFigure 1/G.151, p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
\fINote\ 1\fR \ \(em\ The CCITT examined the possibility of recommending a
specific frequency below 300\ Hz as the lower limit of the frequency band
effectively transmitted, taking the following considerations into
account:
.LP
    1)
    The results of subjective tests carried out by certain
Administrations show that it is possible to improve transmission
quality if the lower limit of the transmitted frequency band is
reduced from 300\ Hz to 200\ Hz. These tests show a definite
increase in the loudness of the received speech, and also in
the quality of the transmission as judged by opinion tests;
the improvement in articulation is, on the other hand, very
slight.
.bp
.LP
    2)
    However, such an extension would probably have the following disadvantages:
.LP
    a)
    it would slightly increase the cost of equipment;
.LP
    b)
    it would introduce some difficulties in balancing the
terminating sets at the ends of the 4\(hywire chain, if it
were desired to use 4\(hywire circuits without exceeding the
values of nominal transmission loss recommended in the new
transmission plan;
.LP
    c)
    it would increase the possible susceptibility to
interference, whether as subjective noise or as disturbances
interfering with carrier equipment (see the Recommendation cited in\ [5])
or affecting compandor gain;
.LP
    d)
    the additional energy transmitted in consequence of
extending the band could increase the loading of carrier
systems;
.LP
    e)
    the out\(hyof\(hyband signalling systems recognized by
the CCITT could not be used.
.PP
In view of the above, the CCITT has issued the aforementioned
Recommendations concerning signals transmitted at frequencies between 300
and 3400\ Hz.
.PP
\fINote\ 2\fR \ \(em\ In applying the Recommendations, Administrations may
mutually agree to transmit signals at frequencies below 300\ Hz over
international circuits. Every Administration may, of course, decide to
transmit signals at frequencies below 300\ Hz over its national extension
circuits,
provided it is still able to apply the CCITT transmission plan to international
communications.
.RT
.sp 2P
.LP
\fB2\fR     \fBGroup delay\fR
.sp 1P
.RT
.PP
The group\(hydelay performance objectives of international circuits
and national extension circuits should be such that the network performance
objectives of Recommendations\ G.114 and\ G.133 are met.
.RT
.LP
.sp 2P
.LP
\fB3\fR     \fBVariations of transmission loss with time\fR
.sp 1P
.RT
.PP
The CCITT recommends the following circuit performance objectives [objective\
a) has been used to assess the stability of international
connections\ \(em\ see Recommendation\ G.131, \(sc\ 1]:
.RT
.LP
    a)
     The standard deviation of the variation in transmission loss of a circuit
should not exceed 1\ dB. This objective can be obtained
already for circuits on a single group link equipped with
automatic regulation and should be obtained for each national
circuit, whether regulated or not. The standard deviation
should not exceed 1.5\ dB for other international circuits.
.LP
    b)
     The difference between the mean value and the nominal value of the transmission
loss for each circuit should not exceed
0.5\ dB.
.sp 2P
.LP
\fB4\fR     \fBLinear crosstalk\fR
.FS
The methods recommended for measuring
crosstalk are described in Annex\ A to Recommendation\ G.134.
.FE
.sp 1P
.RT
.sp 1P
.LP
4.1
    \fIBetween circuits\fR
.sp 9p
.RT
.LP
.PP
The circuit performance objective for the near\(hyend or far\(hyend
crosstalk ratio (intelligible crosstalk only)
measured at audio\(hyfrequency at trunk exchanges between two complete
circuits in terminal service position should not be less than 65\ dB.
.PP
\fINote\ 1\fR \ \(em\ When a minimum noise level of at least 4000 pW0p is
always present in a system (e.g.\ this may be the case in satellite systems,
for example) a reduced crosstalk ratio of 58\ dB between circuits is acceptable.
.PP
\fINote\ 2\fR \ \(em\ Coaxial pair cables complying with Recommendations
G.622\ [6] and G.623\ [7] already allow this condition to be fulfilled
if it is assumed that the frequency bands for which crosstalk is caused
by the cable and those for which crosstalk is due to the equipments are
not the same. On the
other hand FDM systems on symmetric pair cables do not always allow a limit
more stringent than 58\ dB to be met.
.PP
\fINote\ 3\fR \ \(em\ In cases where the length of a homogeneous section of a
real transmission system substantially exceeds the length of a homogeneous
section of the HRC, the 65\ dB limit may not be met in all cases for all the
channels in the system.
.bp
.RT
.sp 2P
.LP
4.2
    \fIBetween the go and return channels of a 4\(hywire circuit\fR
.sp 1P
.RT
.sp 1P
.LP
4.2.1
    \fIOrdinary telephone circuit\fR (see Note\ 1 below)
.sp 9p
.RT
.PP
Since all ordinary telephone circuits may also be used as VF
telegraph bearers, the circuit performance objective for the near\(hyend
crosstalk ratio between the two directions of transmission should be at
least
43\ dB.
.RT
.sp 1P
.LP
4.2.2
    \fICircuits used with a \fR \fIspeech concentrator\fR
.sp 9p
.RT
.PP
For circuits and circuit sections used to interconnect terminal
speech concentrator equipments, near\(hyend crosstalk between any two channels
will appear in the form of crosstalk between circuits and hence the circuit
performance objective for the total near\(hyend crosstalk ratio introduced
between speech concentrators should not be less than 58\ dB. (See Notes\
2 and\ 4
below.)
.RT
.sp 1P
.LP
4.2.3
    \fICircuits used with modern echo suppressors, for example\fR
\fIhigh\(hyaltitude satellite circuits\fR
.sp 9p
.RT
.PP
The circuit performance objective for the near\(hyend crosstalk ratio of
any circuit equipped with terminal
far\(hyend operated, half\(hyecho suppressors of modern design should not
be less
than 55\ dB. This is to avoid nullifying the effect of the suppression loss
introduced by modern echo suppressors. (See Notes\ 2, 3 and\ 4 below.)
.PP
\fINote\ 1\fR \ \(em\ Telephone circuits which are not equipped with (or
used in
conjunction with) modern echo suppressors designed for long propagation
times are referred to in \(sc\ 4.2.1 above. Circuits which can form part
of switched
connections with a long propagation time and which then lie between terminal
half\(hyecho suppressors of modern design should, wherever possible, conform to
the higher standards given in this \(sc\ 4.2.3.
.PP
\fINote\ 2\fR \ \(em\ The channel\(hytranslating equipment provides the
principal
go\(hyto\(hyreturn crosstalk path on circuits or circuit\(hysections routed
on carrier systems with modern translating and line transmission equipment
(but see Note\ 4 below). It should be noted that crosstalk paths between
the high\(hyfrequency
input and the high\(hyfrequency output and also between the voice\(hyfrequency
input and the voice\(hyfrequency output on channel\(hytranslating equipments
contribute to the go\(hyto\(hyreturn crosstalk ratios of circuits and circuit
sections. Both these paths must be taken into account when considering
circuits or circuit sections used between terminal speech concentrator
equipments or modern echo
suppressors. The following cases arise:
.RT
.sp 1P
.LP
    \fISpeech concentrators\fR
.sp 9p
.RT
.PP
Both the high\(hyfrequency path and the voice\(hyfrequency path
contribute to the crosstalk ratio.
.RT
.sp 1P
.LP
    \fIEcho suppressors\fR \v'3p'
.sp 9p
.RT
.LP
    1)
    A circuit comprising one circuit section between far\(hyend
operated, half\(hyecho suppressors: the high\(hyfrequency path
is dominant.
.LP
    2)
     A circuit comprising more than one circuit section between the suppressors:
at points where channel\(hytranslating equipments
are connected together at voice\(hyfrequency. The voice frequency
crosstalk\ path of one equipment is effectively in parallel
with the high\(hyfrequency crosstalk path
of\ the\ other, so that
both must be taken into account.
.LP
    3)
     More than one circuit between the suppressors: this occurs when intermediate
adjacent half\(hyecho suppressors are switched out
(or disabled) and the go\(hyto\(hyreturn crosstalk arises in a
fashion analogous to that described in 2) above, circuits
replacing circuit sections.
.PP
\fINote\ 3\fR \ \(em\ If channel equipments just conforming to the
Recommendation cited in\ [8] are used on a circuit comprising three
circuit sections, then assuming r.m.s. addition of crosstalk paths the
crosstalk ratio would be approximately 60\ dB.
.PP
\fINote\ 4\fR \ \(em\ If channel equipments used on a circuit comprising three
circuit sections just comply with the Recommendation cited in\ [9], then the
.PP
least go\(hyto\(hyreturn crosstalk ratio, assuming r.m.s. addition of the
various
paths, would be approximately 56\ dB which is 2\ dB less than is required for
speech concentrators in \(sc\ 4.2.2 above. However, the assumptions are most
pessimistic and there is not likely to be any difficulty in practice. The
limit for echo suppressor in \(sc\ 4.2.3 above is complied with.
.bp
.PP
\fINote\ 5\fR \ \(em\ Some types of symmetrical\(hypair line transmission
systems
introduce significantly low go\(hyto\(hyreturn crosstalk ratios on the derived
circuits and wherever possible such systems should not be used to provide
circuits or circuit sections for use with speech concentrators or modern
echo suppressors.
.PP
\fINote\ 6\fR \ \(em\ Some attention must be given to the unbalance of
the audio parts of FDM channel equipments if the crosstalk of 65\ dB is
not to be
diminished by crosstalk in station cabling due to unbalanced cable terminating
equipment.
.RT
.sp 2P
.LP
\fB5\fR     \fBNonlinear distortion\fR
.sp 1P
.RT
.PP
Experience has shown that telephone circuits set up on systems for which
the CCITT has issued recommendations (the elements of which systems,
taken separately, meet the relevant nonlinearity requirements) are equally
suitable, as far as nonlinearity is concerned, for telephone and
voice\(hyfrequency telegraph transmission.
.PP
\fINote\fR \ \(em\ In carrier telephone circuits, the nonlinear distortion
produced by the line amplifiers and by modulation stages other than the
.PP
channel\(hytranslating equipment can be ignored. Hence the above remarks are
applicable to circuits of any length.
.RT
.sp 2P
.LP
\fB6\fR     \fBError on the reconstituted frequency\fR
.sp 1P
.RT
.PP
See Recommendation\ G.135.
.RT
.sp 2P
.LP
\fB7\fR     \fBInterference at harmonics from the mains and other low\fR
\fBfrequencies\fR
.sp 1P
.RT
.PP
Signals carried by transmission systems are sometimes modulated by interfering
signals from mains frequency power supplies, induced voltages
caused by railway traction currents and from other sources. This unwanted
modulation can take the form of amplitude or phase modulation or a combination
of both. This interference may be characterized by the level of the strongest
unwanted side component when a sine wave signal is applied with a power
of 1\ mW at the point of zero relative level (0\ dBm0) on a telephone circuit.
The
circuit performance objective for the maximum admissible level of the unwanted
side components on a complete telephone circuit should then not exceed
\(em45\ dBm0 (i.e.\ the minimum side component attenuation should be 45\
dB). This circuit
performance objective should apply to all low frequency interfering signals
up to about 400\ Hz.
.PP
\fINote\ 1\fR \ \(em\ This level was found to be acceptable for circuits for
FM and AM VF\(hytelegraphy, facsimile transmission, speech, telephone signalling
and data transmission.
.PP
\fINote\ 2\fR \ \(em\ For limits applicable to sound\(hyprogramme circuits, see
the Recommendation cited in\ [10].
.PP
\fINote\ 3\fR \ \(em\ The main causes of interference due to power sources
are:
.RT
.LP
    a)
    residual ripples at the terminals of d.c. supply which
are directly transmitted to equipments through the power\(hyfed
circuits;
.LP
    b)
    the a.c. to the dependent power\(hyfed stations in some
systems, which interferes through the power\(hyseparating filter or through
the iron tapes of coaxial pairs;
.LP
    c)
     the induction voltages in the d.c. supply line to power\(hyfed dependent
stations in some systems;
.LP
    d)
     the amplitude and phase unwanted modulations of the various carriers
due to cause a) which are increased in the
frequency\(hymultiplying equipments.
.PP
\fINote\ 4\fR \ \(em\ The effect of the modulation process is that an input
signal of frequency \fIf\fR \ Hz will produce, for example, corresponding
output
signals at frequencies\ \fIf\fR , \fIf\fR \ \(+-\ 50, \fIf\fR \ \(+-\ 100,
\fIf\fR \ \(+-\ 150\ Hz,\ etc.
.sp 2P
.LP
\fB8\fR     \fBSingle tone interference in telephone circuits\fR
.sp 1P
.RT
.PP
The single tone interference level in a telephone circuit should
not be higher than \(em73\ dBm0p (provisional value, pending the conclusion of
studies by Study Group\ XII). Psophometric weighting should only be accounted
for when the frequency of the interference is well defined.
.bp
.RT
.LP
.sp 2P
.LP
    \fBReferences\fR
.sp 1P
.RT
.LP
[1]
    CCITT Recommendation \fI12\(hychannel terminal equipments\fR , Vol.\ III,
Rec.\ G.232.
.LP
[2]
    \fIIbid.\fR , Figure 1/G.232, Graphs\ A and\ B.
.LP
[3]
    CCITT Recommendation \fI16\(hychannel terminal equipments\fR , Vol.\ III,
Rec.\ G.235.
.LP
[4]
    CCITT Recommendation \fISetting\(hyup and lining\(hyup an international\fR
\fIcircuit for public telephony\fR , Vol.\ IV, Rec.\ M.580.
.LP
[5]
    CCITT Recommendation \fI12\(hychannel terminal equipments\fR , Vol.\ III,
Rec.\ G.232, \(sc\ 6.
.LP
[6]
    CCITT Recommendation \fICharacteristics of 1.2/4.4\(hymm coaxial cable\fR
\fIpairs\fR , Vol.\ III, Rec.\ G.622.
.LP
[7]
    CCITT Recommendation \fICharacteristics of 2.6/9.5\(hymm coaxial cable\fR
\fIpairs\fR , Vol.\ III, Rec.\ G.623.
.LP
[8]
    CCITT Recommendation \fI12\(hychannel terminal equipments\fR , Vol.\ III,
Rec.\ G.232, \(sc\ 9.1.
.LP
[9]
    \fIIbid.\fR , \(sc\ 9.3.
.LP
[10]
    CCITT Recommendation \fIPerformance characteristics of 15\(hykHz type\fR
\fIsound\(hyprogramme circuits\fR , Vol.\ III, Rec.\ J.21, \(sc\ 3.1.7.
\v'1P'
.LP
.sp 2P
.LP
\fBRecommendation\ G.152\fR
.RT
.sp 2P
.ce 1000
\fBCHARACTERISTICS\ APPROPRIATE\ TO\ \fR \fBLONG\(hyDISTANCE\ CIRCUITS\fR
.EF '%    Fascicle\ III.1\ \(em\ Rec.\ G.152''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.152    %'
.ce 0
.sp 1P
.ce 1000
\fBOF\ A\ LENGTH\ NOT\ EXCEEDING\ 2500\ km\fR
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1964; amended at Mar del Plata, 1968 and Geneva, 1972\fR
\fIand 1980)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.PP
This Recommendation applies to all modern international circuits not more
than 2500\ km in length. It also applies to national trunk circuits
in an average\(hysize country, and which may be used in the 4\(hywire chain
of an
international connection.
.sp 1P
.RT
.PP
It is understood that, should an extension circuit more than
2500\(hykm long be used in a large country, it will have to meet all the
recommendations applicable to an international circuit of the same length.
.LP
.sp 2P
.LP
\fB1\fR \fBCircuits on land or submarine cable systems or on line\(hyof\(hysight\fR
\fBradio\(hyrelay systems\fR
.sp 1P
.RT
.PP
The circuits in question are mostly set up in cable or radio\(hyrelay link
carrier systems, such that the noise objectives of
Recommendation\ G.222\ [1] are applicable to a circuit with the same make\(hyup
as the hypothetical reference circuit 2500\(hykm long.
.PP
A consequence of Recommendation\ G.222\ [1] is that, for a circuit
\fIL\fR \(hykm long (\fIL\fR \ \(=\ 2500\ km), the circuit performance
objective for the mean
psophometric noise power during any hour should be of the order of
4\ \fIL\fR \ picowatts, excluding very short circuits and those with a very
complicated composition, this latter case being dealt with in
Recommendation\ G.226\ [2].
.RT
.sp 2P
.LP
\fB2\fR     \fBCircuits on tropospheric\(hyscatter radio\(hyrelay systems\fR
.sp 1P
.RT
.PP
The CCIR has defined a hypothetical reference circuit and fixed
circuit performance objectives in its Recommendations\ 396\ [3] and 397\ [4]
respectively.
.RT
.LP
.sp 2P
.LP
\fB3\fR     \fBCircuits on open\(hywire carrier systems\fR
.sp 1P
.RT
.PP
The Recommendation cited in\ [5] contains relevant noise
objectives.
.PP
\fINote\fR \ \(em\ Recommendation\ M.580\ [6] deals with noise objectives for
maintenance purposes. See Note\ 1 of Recommendation\ G.143, \(sc\ 1.1.
.bp
.RT
.sp 2P
.LP
    \fBReferences\fR
.sp 1P
.RT
.LP
[1]
     CCITT Recommendation \fINoise objectives for design of\fR \fIcarrier\(hytransmission
systems of 2500\ km\fR , Vol.\ III, Rec.\ G.222.
.LP
[2]
    CCITT Recommendation \fINoise on a real link\fR , Vol.\ III, Rec.\ G.226.
.LP
[3]
    CCIR Recommendation \fIHypothetical reference circuit for\fR
\fItrans\(hyhorizon radio\(hyrelay systems for telephony using\fR \fIfrequency\(hydivision
multiplex\fR , Vol.\ IX, Rec.\ 396, ITU,
Geneva,\ 1986.
.LP
[4]
    CCIR Recommendation \fIAllowable noise power in the hypothetical\fR
\fIreference circuit of trans\(hyhorizon radio\(hyrelay systems for telephony\fR
\fIusing frequency\(hydivision multiplex\fR , Vol.\ IX, Rec.\ 397, ITU,
Geneva,\ 1986.
.LP
[5]
    CCITT Recommendation \fIGeneral characteristics of systems\fR
\fIproviding 12 carrier telephone circuits on an open\(hywire pair\fR ,
Vol.\ III, Rec.\ G.311, \(sc\ 8.
.LP
[6]
     CCITT Recommendation \fISetting\(hyup and lining\(hyup an international\fR
\fIcircuit for public telephony\fR , Vol.\ IV, Rec.\ M.580.
.sp 2P
.LP
\fBRecommendation\ G.153\fR
.RT
.sp 2P
.ce 1000
\fBCHARACTERISTICS\ APPROPRIATE\ TO\ INTERNATIONAL\ CIRCUITS\fR
.EF '%    Fascicle\ III.1\ \(em\ Rec.\ G.153''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.153    %'
.ce 0
.sp 1P
.ce 1000
\fBMORE\ THAN\ 2500\ KM\ IN\ LENGTH\fR
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1964; amended at Mar del Plata, 1968, and Geneva, 1972\fR
\fIand 1980)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.PP
These circuits should meet the general requirements set forth
in Recommendation\ G.151 and should, in addition, according to the kind of
system on which they are set up, meet the particular provisions of \(sc\(sc\
1, 2,
3\ and 4\ below.
.sp 1P
.RT
.LP
.PP
\fINote\ 1\fR \ \(em\ Some circuits which do not meet the noise objectives
specified in the present Recommendation can nevertheless be used for telephony
(if they are fitted with compandors), telegraphy or data transmission
(\(sc\(sc\ 2, 3\ and\ 4 of Recommendation\ G.143; Table\ 1/G.153 summarizes
these
Recommendations).
.PP
\fINote\ 2\fR \ \(em\ Recommendation\ M.580\ [1] deals with noise
objectives for maintenance purposes. See Note\ 1 of
Recommendation\ G.143, \(sc\ 1.1).
.RT
.sp 2P
.LP
\fB1\fR     \fBCircuits more than 2500 km in length on cable or radio\(hyrelay\fR
\fBsystems, with no long submarine cable section\fR
.sp 1P
.RT
.PP
In many cases circuits of this kind, between 2500\ km and about
25 | 00\ km long will, throughout most of their length, be carried in land\(hycable
systems or radio\(hyrelay systems already used to give international circuits
not more than 2500\ km long, and designed on the basis of the objectives
already recommended for such systems in Recommendation\ G.222\ [3].
.PP
Moreover, it is unlikely that the number of channel demodulations
will exceed that envisaged in the corresponding part of the longest
international connection referred to in Recommendation\ G.103. There will
also be cases where it will be possible to establish such circuits on systems
designed on the basis of national hypothetical reference circuits of the
type referred to in the Recommendation cited in\ [4]. This being so, the
CCITT issues the following recommendations:
.RT
.sp 1P
.LP
1.1
    \fIVariations in transmission loss with time\fR
.sp 9p
.RT
.PP
Automatic level adjustment should be used on each group link on
which the circuit is routed. In addition, all possible steps should be
taken to reduce changes of transmission loss with time.
.RT
.sp 1P
.LP
1.2
    \fIPerformance objectives for circuit noise\fR
.sp 9p
.RT
.PP
It is provisionally recommended that systems to provide such
international circuits not more than 25 | 00\ km long should be designed
on the basis of the noise objectives at present recommended for 2500\(hykm
hypothetical reference circuits.
.bp
.RT
.ce
\fBH.T. [T1.153]\fR
.ce
TABLE\ 1/G.153
.ce
\fBNoise objectives or limits\fR
.ce
 | ua\d\u)\d \fBfor very long circuits
.ce
providing various services\fR
.ce
 | ub\d\u)\d
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(24p) sw(24p) | cw(90p) sw(90p) , c | c | c | c.
Psophometric  power    Type of objective or limit
pW0p    dBm0p     {
For a connection, a chain of circuits,
or a leased circuit
 }     {
For a circuit which may form part
of a switched connection
 }
_
.T&
cw(24p) | cw(24p) | lw(90p) | lw(90p) .
\ 40 | 00    \(em44         {
Limit for a telephone circuit used without a
compandor (Recommendation G.143, \(sc 2)
 }
.T&
cw(24p) | cw(24p) | lw(90p) | lw(90p) .
\ 50 | 00    \(em43     {
Objective for a chain of 6 international circuits,
obtained in practice by a combination of circuits with
circuit performance objectives of 1,
2 or 4 pW/km (Recommen
dation\ G.143,\ \(sc\ 1)
 }
.T&
cw(24p) | cw(24p) | lw(90p) | lw(90p) .
\ 80 | 00    \(em41     {
Limit for FM VF telegraphy, in accordance with CCITT
standards (Recommen
dation\ H.22 | 2])
 }
.T&
cw(24p) | cw(24p) | lw(90p) | lw(90p) .
100 | 00    \(em40     {
Limit for data transmission over a leased circuit
(Recommendation\ G.143, \(sc 4.1)
 }
.T&
cw(24p) | cw(24p) | lw(90p) | lw(90p) .
250 | 00    \(em36         {
Acceptable for data transmission over
the switched
network (Recommen
dation\ G.143,\ \(sc\ 4.2). A circuit exceeding this
limit without a compandor cannot be used in a chain of
6 telephone circuits, even if it is equipped with a
compandor (Recommen
dation\ G.143,\ \(sc\ 2)
 }
.T&
rw(24p) | cw(24p) | lw(90p) | lw(90p) .
10\u6\d    \(em30     {
Tolerable for a certain system of synchronous telegraphy
(Recommendation H.22 | 2])
 }
.TE
.LP
\ua\d\u)\d
Only the mean psophometric power over one hour has been indicated,
referred to a point of zero relative level of the
international circuit, or of the first circuit of the chain.
.LP
\ub\d\u)\d
The noise limits are determined according to the minimum
performance requirements of each service. The noise
objectives are commissioning objectives for various
transmission systems.
.nr PS 9
.RT
.ad r
\fBTABLE 1/G.153 [T1.153], p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
Whenever possible lower noise objectives should be sought and it is
recognized that in some large countries systems forming part of a circuit
substantially longer than 2500\ km (e.g.\ 5000\ km) are constructed according
.PP
to the principles referred to in the Recommendation cited in\ [4]. Alternatively
lower noise figures can be obtained by a suitable choice of telephone channels
making up the circuits. Provisionally the short\(hyterm noise performance
objectives for circuits of this kind of length up to about 7500\ km are as
follows:
.PP
The one\(hyminute mean noise power shall not exceed 50 | 00\ pW (\(em43\
dBm0p) for more than 0.3% of any month and the unweighted noise power,
measured or
calculated with an integrating time of 5\ ms, shall not exceed 10\u6\d\ pW
(\(em30\ dBm0) for more than 0.03% of any month. It is to be understood
that these objectives are derived pro rata from the objectives for circuits
of 2500\ km
length (Recommendation\ G.222\ [3]); for lengths between
2500\ and 7500\ km
proportionate intermediate values should apply.
.PP
The CCITT is not yet able to recommend objectives for short\(hyterm noise
performance on circuits of the above type which exceed 7500\ km in length.
.bp
.RT
.sp 2P
.LP
\fB2\fR     \fBCircuits more than 2500 km with a long submarine cable section\fR
.sp 1P
.RT
.sp 1P
.LP
2.1
    \fIAttenuation distortion\fR
.sp 9p
.RT
.PP
A circuit of this kind may, for reasons of economy, comprise
terminal equipments with carriers spaced 3\ kHz apart, in accordance with
Recommendation\ G.235\ [5].
.PP
If terminal equipment be used with carrier spacing of 4\ kHz, it must at
least meet the requirements of Recommendation\ G.232\ [6]. Some countries
use improved terminal equipment in circuits permanently used for intercontinental
operation.
.RT
.sp 2P
.LP
2.2
    \fIPerformance objectives for circuit noise attributable to the\fR
\fIsubmarine cable section\fR
.sp 1P
.RT
.sp 1P
.LP
2.2.1
    \fIWithout compandor\fR
.sp 9p
.RT
.PP
The circuit performance objective for the mean noise per hour of a very
long submarine\(hycable system designed for use without compandors and
with no restrictions for telephony, voice\(hyfrequency telegraphy and data
transmission should not exceed 3\ pW/km on the worst channel. The circuit
performance
objective for the mean noise power for each direction of transmission,
extended over all the channels used for the longest circuits, should not
exceed
1\ pW/km.
.PP
\fINote\fR \ \(em\ However, it would be desirable that the circuits in
a group to be operated with a speech concentrator system
.FS
See footnote\ 2) in
Recommendation\ G.143, \(sc\ 2.
.FE
should all have more or less the same noise
level.
.RT
.LP
.sp 1P
.LP
2.2.2
    \fIWith compandor\fR
.sp 9p
.RT
.PP
At present, the CCITT does not propose to study systems which, by relying
on the \fIsystematic\fR use of compandors, have noise objectives which
are greatly different from those of \(sc\ 2.2.1 above.
.RT
.sp 1P
.LP
2.3
    \fIPerformance objectives for circuit noise attributable to other\fR
\fIsections\fR
.sp 9p
.RT
.PP
The other sections of the circuit should comply with the
recommendations given in \(sc\ 1 of this Recommendation.
.RT
.sp 2P
.LP
\fB3\fR     \fBCircuits on communication\(hysatellite systems\fR
.sp 1P
.RT
.PP
The CCIR and the CCITT are considering the extent to which
circuits set up on communication\(hysatellite systems may be integrated
into the worldwide network; some of the limitations on the use of such
circuits are
outlined in Recommendation\ Q.13\ [7].
.PP
The CCIR has made recommendations as far as circuit noise is
concerned and has defined a hypothetical reference circuit (CCIR
Recommendation\ 352\ [8]) and the allowable noise power in this reference
circuit (CCIR Recommendation\ 353\ [9]).
.RT
.sp 2P
.LP
\fB4\fR \fBCircuits more than 2500 km in length set up on open\(hywire
lines\fR
.sp 1P
.RT
.PP
Paragraph\ 4 is not published in this Book, but can be found under Part\
D of Recommendation\ G.153, \fIOrange\ Book\fR , ITU, Geneva,\ 1977.
.RT
.LP
.sp 2P
.LP
    \fBReferences\fR
.sp 1P
.RT
.LP
[1]
    CCITT Recommendation \fISetting\(hyup and lining\(hyup an international\fR
\fIcircuit for public telephony\fR , Vol.\ IV, Rec.\ M.580.
.LP
[2]
    CCITT Recommendation \fITransmission requirements of international\fR
\fIvoice\(hyfrequency telegraph links (at\ 50, 100 and 200\ bauds)\fR ,
Vol.\ III, Rec.\ H.22.
.LP
[3]
     CCITT Recommendation \fINoise objectives for design of\fR \fIcarrier\(hytransmission
systems of 2500\ km\fR , Vol.\ III, Rec.\ G.222.
.bp
.LP
[4]
    \fIIbid.\fR , \(sc\ 3.
.LP
[5]
    CCITT Recommendation \fI16\(hychannel terminal equipments\fR , Vol.\ III,
Rec.\ G.235.
.LP
[6]
    CCITT Recommendation \fI12\(hychannel terminal equipments\fR , Vol.\ III,
Rec.\ G.232.
.LP
[7]
    CCITT Recommendation \fIThe international routing plan\fR , Vol.\ VI,
Rec.\ Q.13.
.LP
[8]
    CCIR Recommendation \fIHypothetical reference circuits for\fR
\fItelephony and television in the fixed satellite service\fR , Vol.\ IV,
Rec.\ 352, ITU, Geneva, 1986.
.LP
[9]
    CCIR Recommendation \fIAllowable noise power in the hypothetical\fR
\fIreference circuit for frequency\(hydivision multiplex telephony in the\fR
\fIfixed satellite service\fR , Vol.\ IV, Rec.\ 353, ITU, Geneva, 1986.
.LP
.rs
.sp 44P
.sp 2P
.LP
\fBMONTAGE : RECOMMANDATION G.161 SUR LE RESTE DE CETTE PAGE\fR
.sp 1P
.RT
.LP
.bp