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.rs
.\" Troff code generated by TPS Convert from ITU Original Files
.\" Not Copyright ( c) 1991
.\"
.\" Assumes tbl, eqn, MS macros, and lots of luck.
.TA 1c 2c 3c 4c 5c 6c 7c 8c
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.ds CF
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.EN
.nr LL 40.5P
.nr ll 40.5P
.nr HM 3P
.nr FM 6P
.nr PO 4P
.nr PD 9p
.po 4P
.rs
\v | 5i'
.LP
\s9\fBMONTAGE:\ REC.\ R.113 EN T\* | TE DE CETTE PAGE\fR
.RT
.sp 2P
.LP
\v'12P'
\fBRecommendation\ G.114\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBMEAN\ ONE\(hyWAY\ PROPAGATION\ TIME\fR
.EF '% Fascicle\ III.1\ \(em\ Rec.\ G.114''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.114 %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1964; amended Mar del Plata, 1968, Geneva, 1980;\fR
.sp 9p
.RT
.ce 0
.sp 1P
.ce 1000
\fIMalaga\(hyTorremolinos, 1984 and Melbourne, 1988)\fR
.ce 0
.sp 1P
.PP
The times in this Recommendation are the means of the propagation times
in the two directions of transmission in a connection. When opposite
directions of transmission are provided by different media (e.g.\ a satellite
channel in one direction and a terrestrial channel in the other) the two
times contributing to the mean may differ considerably.
.sp 1P
.RT
.sp 2P
.LP
\fB1\fR \fBLimits for a connection\fR
.sp 1P
.RT
.PP
It is necessary in an international telephone connection to limit the propagation
time between two subscribers. As the propagation time is
increased, subscriber difficulties increase, and the rate of increase of
difficulty rises. Relevant evidence is given in references\ [1] to [10],
particularly with regard to\ b) below.
.PP
As a network performance objective, the CCITT therefore \fIrecommends\fR
| he following limitations on
mean one\(hyway propagation times
when echo sources exist and appropriate echo control devices, such as
echo
suppressors
and
echo cancellers
, are used:
.RT
.LP
a)
0 to 150 ms, acceptable.
.LP
\fINote\fR \ \(em\ Echo suppressors specified in Recommendation\ G.161
of the Blue\ Book [11] may be used for delays not exceeding 50\ ms (see
Recommendation\ G.131, \(sc\ 2.2).
.LP
b)
150 to 400 ms, acceptable, provided that increasing care
is exercised on connections when the mean one\(hyway propagation
time exceeds about 300\ ms, and provided that echo control
devices, such as echo suppressors and echo cancellers, designed
for long\(hydelay circuits are used;
.LP
c)
above 400 ms, unacceptable. Connections with these delays
should not be used except under the most exceptional
circumstances.
.PP
Until such time as additional, significant information permits
Administrations to make a firmer determination of acceptable delay limits,
they should take full account of the documents referred to under References
in
selecting, from alternatives, plans involving delays in range\ b) above.
.PP
\fINote\ 1\fR \ \(em\ The above values refer only to the propagation time
between two subscribers. However, for other purposes (e.g.\ in Recommendation\
G.131) the mean one\(hyway propagation time of an echo path is to be estimated.
The values
in \(sc\ 2 may be used in such estimations.
.bp
.PP
\fINote\ 2\fR \ \(em\ There is good evidence that echo cancellers fitted
at both ends of a long\(hydelay connection generally yield superior performance
over
current types of echo suppressors. (For further details, see \(sc\ 2.2 of
Recommendation\ G.131.)
.PP
\fINote\ 3\fR \ \(em\ It should be noted that although an echo suppressor
and an echo canceller on the same connection are compatible (they can satisfactorily
interwork), the full benefits of echo cancellers are only experienced when
both ends are so equipped. In particular, an Administration unilaterally
replacing
.PP
its echo suppressors with echo cancellers will cause little benefit to
its own subscriber on international connections if the echo suppressor
still remains at the other end.
.PP
\fINote\ 4\fR \ \(em\ Available experimental data (Annex A) has indicated that
connections with delays somewhat greater than 400\ ms may be acceptable
provided that echo cancellers conforming to the specifications of Rec.\
G.165, or other echo control devices with equivalent performance, are used.
However, the use of connections with delays greater than 400\ ms is not
recommended at present and is under study in Question\ 27/XII.
.PP
\fINote\ 5\fR \ \(em\ The use of equipment that introduces clipping, noise
contrast, low echo return loss enhancement or other impairments that may
degrade echo performance (such as may be the case with hands free telephones,
especially in a changing noise environment) may have to be controlled to
achieve acceptable transmission quality on connections with delays in the
range from\ 150 to\ 400\ ms. This subject is under study in Question\ 11/XII.
.RT
.sp 2P
.LP
\fB2\fR \fBValues for circuits\fR
.sp 1P
.RT
.PP
In the establishment of the general interconnection plan within the limits
in \(sc\ 1 the one\(hyway propagation time of both the national extension
circuits and the international circuits must be taken into account. The
propagation time of circuits and connections is the aggregate of several
components; e.g.\ group delay in cables and in filters encountered in FDM
modems of different types. Digital transmission and switching also contribute
delays. The conventional planning values given in \(sc\ 2.1 may be used
to estimate
the total propagation time of specified assemblies which may form circuits
or connections.
.RT
.LP
.sp 1P
.LP
2.1
\fIConventional planning\fR
\fIvalues of propagation time\fR
.sp 9p
.RT
.PP
Provisionally, the conventional planning values of propagation
time in Table\ 1/G.114 may be used.
.RT
.sp 1P
.LP
2.2
\fINational extension circuits\fR
.sp 9p
.RT
.PP
The main arteries of the national network should consist of
high\(hyvelocity propagation lines. In these conditions, the propagation time
between the international centre and the subscriber farthest away from it in
the national network will be as follows:
.RT
.LP
a)
in purely analogue networks, the propagation time will
probably not exceed:
\v'6p'
.sp 1P
.ce 1000
12\ +\ (0.004\ \(mu\ distance in kilometres) ms.
.ce 0
.sp 1P
.LP
.sp 1
Here the factor 0.004 is based on the assumption that national
trunk circuits will be routed over high\(hyvelocity plant (250\ km/ms).
The 12\ ms constant term makes allowance for terminal equipment and
for the probable presence in the national network of a certain
quantity of loaded cables (e.g.\ three pairs of channel translating
equipments plus about 160\ km of H\ 88/36 loaded cables). For an average
size country (see Figure\ 2/G.103) the one\(hyway propagation time will be
less than 18\ ms;
.LP
b)
in mixed analogue/digital networks, the propagation time can
generally be estimated by the equation given for purely analogue
networks. However under certain unfavourable conditions
increased delay may occur compared with the purely analogue
case. This occurs in particular when digital exchanges are
connected with analogue transmission systems through PCM/FDM
equipments in tandem, or transmultiplexers. With the growing
degree of digitization the propagation time will gradually
approach the condition of purely digital networks;
.bp
.LP
.ce
\fBH.T. [T1.114]\fR
.ce
TABLE\ 1/G.114
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(66p) | cw(48p) | cw(66p) .
Transmission medium {
Contribution to one\(hyway propagation time
} Remarks
_
.T&
lw(66p) | cw(48p) | lw(66p) .
{
Terrestrial coaxial cable or radio relay system;
FDM and digital transmission
} 4 \(*ms/km {
Allows for delay in repeaters and regenerators
}
_
.T&
lw(66p) | lw(48p) | lw(66p) , l | l | ^ .
{
Allows for delay in repeaters and regenerators
}
_
.T&
lw(66p) | cw(48p) | lw(66p) .
{
Satellite system
\(em\ 14 | 00 km altitude
\(em\ 36 | 00 km altitude
} . 110 ms 260 ms Between earth stations only
.TE
.LP
Half the sum of propagation times in both directions of
transmission
.LP
\ua\d\u)\d
These values allow for group\(hydelay distortion around frequencies
of peak speech energy and for delay of intermediate higher order
multiplex and through\(hyconnecting equipment.
.LP
\ub\d\u)\d
This value refers to FDM equipments designed to be used with a
compandor and special filters.
.LP
\uc\d\u)\d
For satellite digital communications where the transmultiplexer
is located at the earth station, this value may be increased to
3.3\ ms.
.LP
\ud\d\u)\d
These are mean values: depending on traffic loading, higher
values can be encountered, e.g.\ 0.75\ ms (1.950\ ms, 1.350\ ms
or 1.250\ ms) with 0.95\ probability of not exceeding. (For details,
see Recommendation\ Q.551.)
.LP
\ue\d\u)\d
Echo cancellers, when placed in service, will add a one\(hyway
propagation time of up to 1\ ms in the send path of each echo
canceller. This delay excludes the delay through any codec
in the echo canceller. No significant delay should be incurred
in the receive path of the echo canceller.
.nr PS 9
.RT
.ad r
\fBTABLEAU 1/G.114 [T1.114], p. 1\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
c)
in purely digital networks between exchanges (e.g.\ an IDN),
the propagation time as defined above will probably not
exceed:
\v'6p'
.sp 1P
.ce 1000
3 + (0.004 \(mu distance in kilometers) ms.
.ce 0
.sp 1P
.LP
.sp 1
The 3 ms constant term makes allowance for one PCM coder or
decoder and five digitally switched exchanges.
.LP
\fINote\fR \ \(em\ The value 0.004 is a mean value for coaxial cable systems
and radio\(hyrelay systems; for optical fibre systems 0.005 is to
be used;
.LP
d)
in purely digital networks between susbscribers (e.g.\ an
ISDN), the delay of c) above has to be increased by up to 3.6\ ms
if burst\(hymode (time compression multiplexing) transmission is
used on 2\(hyW local subscriber lines.
.sp 1P
.LP
2.3
\fIInternational circuits\fR
.sp 9p
.RT
.PP
International circuits
.FS
For short nearby links,
telecommunications cables operated at voice frequencies may also be used
in the conditions set out in the introduction to Sub\(hysection\ 5.4 of
Fascicle\ III.2.
.FE
will use high\(hyvelocity transmission systems,
e.g.\ terrestrial cable or radio\(hyrelay systems, submarine systems or
satellite systems. The planning values of \(sc\ 2.1 may be used.
.PP
The magnitude of the mean one\(hyway propagation time for circuits on
high altitude communication satellite systems makes it desirable to impose
some routing restrictions on their use. Details of these restrictions are
given in Recommendation\ Q.13\ [12]. (See also Annex\ A below.)
\v'1P'
.RT
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation G.114)
.sp 9p
.RT
.ce 0
.ce 1000
\fBLong propagation delay and echo related\fR
.sp 1P
.RT
.ce 0
.ce 1000
\fBconsiderations for telephone circuits\fR
.ce 0
.LP
A.1
\fIIntroduction\fR
.sp 1P
.RT
.PP
International connections (see Figure 1/G.103 or Figure\ 1/G.104)
comprising submarine cables, may involve a maximum one\(hyway transmission
delay of about 170\ ms. This Annex addresses the basic issues of national
and
international connections which inherently entail comparatively larger
one\(hyway transmission delays.
.PP
A one hop satellite connection even with an ISL (Inter\(hySatellite
Link) of moderate length introduces one\(hyway transmission delay within the
recommended limit of 400\ ms. However, a careful analysis of the additional
probable delay contributions by digital signal processing (e.g.\ TDMA, DSI,
DCME, 16\ kbit/s and 32\ kbit/s low bit rate encoding, bit\(hyregeneration,
packet\(hyswitching,\ etc.), among other sources, has led to the notion
that the
recommended limit of 400\ ms mean one\(hyway propagation delay may be unnecessarily
restrictive.
.PP
In light of recent technical improvements in echo\(hycontrol
techniques, it is feasible to consider an extension to this limit.
Administrations are encouraged to take note of the continuing nature, as
well as need, of further investigations in this area.
.RT
.LP
.bp
.PP
In order to analyse this problem further, consider that two
distinct
types of effects must be considered in connection with the mean one\(hyway
propagation time; namely, echo\(hyrelated speech quality impairments and pure
(transit) delay related conversational difficulty. Echo control devices,
i.e.\ echo suppressors and especially echo cancellers, can be suitably
employed for overcoming the former effect.
.PP
The 4\(hywire circuits provides a close approximation to echo\(hyfree
connections, assuming minimum acoustic coupling across the handset. In
the long run with expansion of the ISDN implementation, use of 4\(hywire
circuits is
expected in grow. However, 2\(hywire circuits and their accompanying hybrid
connection, as well as other componentes causing echo, are still likely
to be present in vaying degrees during the foreseeable future. Thus, the
use of
modern echo cancellers in satellite circuits is currently regarded as the
most effective method for overcoming the echo problem, provided that the
characteristics of the echo path to be modeled by the echo canceller are
linear and time invariant, or varying only slowly compared with the convergence
speed of the echo canceller.
.PP
A brief discussion of delay measurements, their effect on circuit
quality and the subscriber reaction are provided below.
.RT
.sp 2P
.LP
A.2
\fIEffect of long transmission delays on the subscriber\fR
.sp 1P
.RT
.sp 1P
.LP
A.2.1
\fIEarly measurements\fR
.sp 9p
.RT
.PP
Figure A\(hy1/G.114 shows the effect of long transmission delay on the
difficulty of conversation experienced by the subscriber. Curve\ 1 is the
result of investigations in\ 1964 and\ 1965 [5, 8 \fIet\ al\fR .] where
the performance of the first operational satellite Early Bird was tested
in circuits between France, the United Kingdom, the United States and the
Federal Republic of Germany. The circuits were equipped with early versions
of various echo suppressors, had a certain amount of noise power (about
20 | 00\ pW0p), and had different bandwidths on the TAT\(hy3 cable route
(230\(hy3200\ Hz) as opposed to the satellite
(170\(hy3400\ Hz). Curve\ 1 (F/P) shows the same interview results on the
basis of a fair\(hyor\(hypoor opinion rating by the subscribers.
.PP
From curve 1 it can be seen that, at about 400 ms of delay, more than 50%
of the subscribers have difficulties with the conversation. A 40% value
of difficulty corresponds to a delay of about 300\ ms. On the other hand,
the
percentage of fair\(hyor\(hypoor opinions of the subscribers is about 15%
lower than
.PP
the percentage of difficulties. This may result from the fact that some
of the inquired customers, in spite of the difficulties they had, found
the received speech quality good or excellent.
.PP
On the basis of these observations, 300 ms of delay was selected as
the threshold of difficulty and 400\ ms as the maximum allowable delay in
international connections for telephony in earlier versions of Rec.\ G.114.
.PP
In addition to these results, other ealier results exist. Williams and
Moye [30, 31] investigated the effect of unsuppressed echo on conversations
over simulated telephone links with different values of echo return loss and
with flat or shaped echo\(hypath frequency characteristics.
.PP
Curves 2, 5 and 6 show the results for connections with echo return
losses of 37\ dB (shaped), 37\ dB (flat) and 50\ dB (flat or shaped). Curve\ 4
shows laboratory test results [32] or simulated connections equipped with
echo suppressors and with an echo return loss of about 20\ dB. These test
results
were obtained using a linear time invariant echo path.
.PP
Figure\ A\(hy1/G.114 also includes some recent results obtained from
circuits with long delay but which were equipped with modern echo cancellers
with an echo return loss of about 18\ dB [29] (see \(sc\ A.2.3).
.PP
From curves 2 to 6 (which obtained better methods of echo control or high
echo return loss values) it can be seen that the influence of longer
propagation delay on the difficulties of conversations is much smaller than
indicated by curve\ 1, which used earlier versions of echo suppressors.
.PP
Other investigations summarized in [33] which were obtained from
circuits having only pure transmission delay (i.e.\ echo free 4\(hywire
circuits), have shown that mean one\(hyway propagation delays up to 600\
ms appear to have
no significant influence on the subjective judgements of telephone
subscribers.
.bp
.RT
.LP
.rs
.sp 24P
.ad r
\fBFigure, p. 2\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 27P
.ad r
\fBFigure A\(hy1/G.114 [T2.114), p. 3\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
A.2.2
\fILater measurements\fR
.sp 9p
.RT
.PP
Following technical advancement, design developments and
performance enhancements of echo cancellers [16\(hy19], experiments were
conducted by Helder and Lopiparo\ [20], DiBiaso [21], Post and Silverthorn\
[22], and
others to evaluate the subjective performance of echo suppressors and echo
cancellers on satellite and terrestrial facilities in the U.S., Canada and
other domestic satellite networks.
.PP
Helder and Lopiparo [20] reported results of testing of certain
terrestrial, half\(hyhop satellite
.FS
Half\(hyhop connection refers to the situation when the forward link is
via satellite but the return link is terrestrial (or vice\(hyversa).
.FE
, and one\(hyhope satellite circuits in the U.S. in 1976
and\ 1977. DiBiaso's report\ [21] is based on a study of tests and subjective
evaluation of echo control methods performed during\ 1975\(hy77 by the American
Telephone and Telegraph Company (AT&T) and others using the U.S. domestic
satellite system (COMSTAR), together with conventional analog echo suppressors
(ES), digital echo suppressors (DES) [23] and experimental echo cancellers
(EC) [24\(hy25], and examining the cases of terrestrial, half\(hyhop satellite,
one\(hyhope satellite and two\(hyhop satellite connections, respectively.
A detailed account of these test results is provided elsewhere\ [26]. A
summary of these
.PP
test results, represented in terms of the percent of calls rated unacceptable
for the various cases mentioned above, is reproduced here in Figure\ A\(hy2/G.114.
The graph demonstrates the improvement possible through the use of the
digital echo suppressor and echo canceller in the half\(hyhop and one\(hyhop
satellite
connections, respectively, to yield performances in these two cases practically
equivalent to the terrestrial circuits with echo suppressors. Basically,
similar conclusions were reached by using somewhat different criteria for
performance and quality; e.g.\ percent of calls terminated early or percent
of calls replaced, or percent of calls needing operator assistance.
.RT
.LP
.rs
.sp 24P
.ad r
\fBFigure A\(hy2/G.114, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
In 1978, Post and Silverthorn [22] performed an evalutation of
nine experimental conditions characterized by generically different methods
of echo control on the Trans\(hyCanada Telephone System\ (TCTS) satellite
and
certain terrestrial links. Figure\ A\(hy3/G.114 provides a partial summary
of their results in terms of percent of interviews that judged the terrestrial,
echo
canceller\(hyequipped satellite (S/EC) and echo suppressor\(hyequipped
satellite
(S/ES) circuits as excellent, good, fair, or poor as regards to quality.
Figure\ A\(hy4/G.114 provides a summary of analogous test results as derived
from similar domestic and international satellite and terrestrial networks\
[22].
These results serve to illustrate the near equivalence of the performance of
satellite circuits equipped with echo cancellers and long\(hyhaul terrestrial
circuits with echo suppressors. These results also demonstrate the poorer
performance of echo suppressors as compared to echo cancellers in the satellite
link. Consequently, echo suppressors are not considered optimal for satellite
links and only echo cancellers are recommended to be employed. For terrestrial
applications, the improvement resulting from the use of echo cancellers
is
expected to be only marginal; and system economy may still justify the
use of echo suppressors in the terrestrial links.
.PP
The above observations confirm the conclusion that the difficulties
experienced by telephone users of satellite networks is primarily due to
echo related impairments associated with the long propagation delay. This
impairment can be sufficiently reduced with the use of echo cancellers
to yield a
performance for one\(hyhop satellite connections practically equivalent
to that of terrestrial connections [27\(hy28].
.RT
.LP
.rs
.sp 20P
.ad r
\fBFigure A\(hy3/A\(hy4/G.114, p. 5 et 6\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
A.2.3
\fIRecent and future measurements\fR
.sp 9p
.RT
.PP
In 1987, Communications Satellite Corp. (COMSAT) of the U.S.A.
performed a series of tests to determine the effectiveness of echo cancellers
in terrestrial and satellite circuits, using echo cancellers conforming
to
Rec.\ G.165 and a callback interview procedure as per Rec.\ P.77, Annex\ A.
Details of the procedure were presented recently\ [29] and a summary of the
results is shown in Figure\ A\(hy1/G.114, curve\ 3 giving a plot of the percent
difficulty as a function of mean one way propagation time. A one way delay
value of 45\ ms over terrestrial circuits was taken as a reference, and the
effect of increasing the delay value to 300\ ms and 500\ ms over terrestrial
and satellite links was evaluated.
.bp
.PP
It was concluded on the basis of the COMSAT results that no
significant difference between 45\ ms and 300\ ms delays resulted for the
\*Qpercent difficulty\*U score. At a 500\ ms delay, the percent difficulty
score
approximately doubled (from\ 7.3% to\ 15.8%), but this value is still
considerably smaller than earlier results of over 60%\ [13].
.PP
The above results support the view that connections with delays
somewhat greater than 400\ ms may be accepted provided that echo cancellers
conforming to the specifications of Recommendation\ G.165 or other echo
control devices with equivalent performance are used. This may permit accommodation
of signal processing and Inter Satellite Links\ (ISL) with moderate angular
separations, without causing any significant or noticeable degradations.
.PP
Further tests, measurements and evaluation of subjective performance using
state\(hyof\(hythe\(hyart echo cancellers in modern satellite connections
should prove to be useful to determine what, if any, additional improvements
over
these results are likely or achievable.
.RT
.sp 1P
.LP
A.3
\fISummary and conclusions\fR
.sp 9p
.RT
.PP
The transmission impairments associated with long delay circuits
are best analysed by separating the echo\(hyinduced degradation and the
subjective difficulty due to pure delay. Clearly, as shown by the tests
cited above, echo suppressors (with fixed break\(hyin sensitivity) used
in satellite circuits are
far less efficient than echo cancellers. The effectiveness of echo cancellers
in removing the echo effect and the associated impairments is sufficient
to
yield high or acceptable performance in a long delay satellite circuit.
Further improvement in the performance of echo cancellers and the associated
satellite circuits are continuing. Thus, under these conditions the dominant
impairments are associated with the pure delay component.
.PP
A number of recent works and continuing interest in the area indicate the
possibility of developing and utilizing even more improved and efficient
echo cancellers. VLSI fabrication of echo cancellers is also a viable option
and this is expected to lead to a significantly lower cost for equipping
long delay telephone circuits. Thus, with the use of such suitable devices,
the
comparatively larger pure delay in international connections is not expected
to cause the degree of degradation in the channel quality or efficiency
as was
experienced in earlier tests without echo control or with echo suppressors
with fixed break\(hyin sensitivity. Appropriate use of echo cancellers
has been shown
.PP
to indeed provide international or national satellite connections yielding
quality and performance practically equivalent to the terrestrial connections
for telephony. These results only refer to electric echo and additional
studies are necessary to determine the effect of acoustic echo (see Note
5 of
Question\ 27/XII).
.RT
.sp 2P
.LP
\fBReferences\fR
.sp 1P
.RT
.LP
[1]
CCITT \fIRed Book\fR , Vol. V | fIbis\fR , Annex E (United States), ITU,
Geneva,
1965.
.LP
[2]
\fIIbid:\fR , Annex F (United Kingdom).
.LP
[3]
\fIIbid:\fR , Annex 4 to Question 6/XII (Italy).
.LP
[4]
CCITT \fIRed Book\fR , Vol. V, Supplements No. 1 to No.\ 6, ITU,
Geneva,\ 1985.
.LP
[5]
BARSTOW (J. | .): Results of User Reaction Tests on Communication via
Early Bird Satellite, \fIProgress in Astronautic Aeronautics\fR , 19,
Academic Press, New\ York and London,\ 1966.
.LP
[6]
HELDER (G. | .): Customer Evaluation of Telephone Circuits with Delay,
\fIBell System Technical Journal\fR , 45, September\ 1966, pp.\ 1157\(hy1191.
.LP
[7]
RICHARDS (D. | .): Transmission Performance of Telephone Connections
Having Long Propagation Times, \fIHet P.T.T.\(hyBedriff\fR , XV, No.\ 1/2,
May\ 1967, pp.\ 12\(hy24.
.LP
[8]
KARLIN (J. | .): Measuring the Acceptability of Long\(hyDelay
Transmission Circuits used During the Early Bird Transatlantic Tests in
1965, \fIHet P.T.T.\(hyBedriff\fR , May\ 1967, pp.\ 25\(hy31.
.LP
[9]
De JONG (C.): Observations on Telephone Calls Between the Netherlands
and the U.S.A., \fIHet P.T.T.\(hyBedriff\fR , May\ 1967, pp.\ 32\(hy36.
.LP
[10]
HUTTER (J.): Customer Response to Telephone Circuits Routed via a
Synchronous\(hyOrbit Satellite, \fIP.O.E.E.J.\fR , Vol.\ 60, October\ 1967,
p.\ 181.
.LP
[11]
CCITT Recommendation, \fIDefinitions relating to echo suppressors and\fR
\fIcharacteristics of a far\(hyend operated, differential, half\(hyecho
suppressor\fR ,
Blue\ Book, Vol.\ III, Rec.\ G.161, ITU, Geneva,\ 1965.
.LP
[12]
CCITT Recommendation, \fIThe international routing plan\fR , Vol.\ VI,
Rec.\ Q.13.
.LP
[13]
CCITT Recommendation, \fIMean One Way Propagation Time\fR , \fIRed Book\fR ,
Vol.\ III, Rec.\ G.114, ITU, Malaga\(hyTorremolinos, 1984.
.bp
.LP
[14]
CCIR Report, \fIThe effects of transmission delay in the fixed satellite\fR
\fIservice\fR . Vol.\ IV, pp.\ 29\(hy37, Report\ 383\(hy4, ITU, Geneva,
1982.
.LP
[15]
DECKER (H.): Die fur lange Fernsprechleitungen Zulassige
Ubertragungszeit, \fIEuropaischer Fernsprechdienst\fR , 19832, Heft No.\
8, 1931,
pp.\ 133\(hy135.
.LP
[16]
SONDHI (M. | .): An Adaptive Echo Canceller, \fIBell Systems Technical\fR
\fIJournal\fR , Vol.\ 46, March\ 1967, pp.\ 497\(hy511.
.LP
[17]
CAMPANELLA (S. | .), SUYDERHOUD (H. | .) and ONUFRY (M.): Analysis
of an Adaptive Impulse Response Echo Canceller, \fICOMSAT Technical Review\fR ,
Vol.\ 2, No.\ 1, Spring\ 1972, pp.\ 1\(hy36.
.LP
[18]
SUYDERHOUD (H. | .), CAMPANELLA (S.) and ONUFRY (M.): Results and
Analysis of Worldwide Echo Canceller Field Trial, \fICOMSAT Technical Review\fR
,
Vol.\ 5, No.\ 2, Fall\ 1975, pp.\ 253\(hy273.
.LP
[19]
HORNA (O. | ): Echo Canceller with Adaptive Transversal Filter
Utilizing Pseudo\(hylogarithmic Coding, \fICOMSAT Technical Review\fR ,
Vol.\ 7, No.\ 2, Fall\ 1977, pp.\ 393\(hy428.
.LP
[20]
HELDER (G. | .) and LOPIPARO (P. | .): Improving Transmission on Domestic
Satellite Circuits, \fIBell Laboratories Record\fR , Vol.\ 55, No.\ 8,
October\ 1977, pp.\ 202\(hy207.
.LP
[21]
DIBIASO (L. | .): Satellite User Reaction Tests: A Subjective Evaluation
of Echo Control Methods, \fINational Telecommunications Conference Record\fR
,
Vol.\ 3, 1979, pp.\ 48.6.1\(hy48.6.6.
.LP
[22]
POST (J. | .) and SILVERTHORN (R. | .): Results of a Subjective
Comparison of Echo Control Devices in Terrestrial and Satellite Trunks,
\fINational Telecommunications Conference Record\fR , Vol.\ 3, 1979,
pp.\ 48.4.1\(hy48.4.5.
.LP
[23]
CCITT \(em Contribution COM XV\(hyNo.\ 86, (Annex\ II to Question\ 10/XV),
January\ 1978.
.LP
[24]
DUTTWEILER (D. | .): A twelve\(hychannel digital echo canceller,
\fIIEEE Transactions on Communications\fR , Vol.\ COM\(hy26, No.\ 5, May\ 1978.
.LP
[25]
CCITT Rec. G.165 for echo cancellers.
.LP
[26]
CCITT \(em Contribution COM XII\(hyNo.\ 165 (also COM XV\(hyNo.\ 112),
June\ 1979.
.LP
[27]
CCITT \(em Contribution COM XVI\(hyNo.\ 65, Study Period\ 1973\(hy1976.
.LP
[28]
CCITT \(em Contribution COM XII\(hyNo.\ 154, April\ 1979.
.LP
[29]
CCITT \(em Contribution COM XII\(hyNo.\ 177 \(em WP XII/3, June\ 1987.
.LP
[30]
WILLIAMS (G.): Subjective Evaluation of Unsuppressed Echo in Simulated
Long Delay Telephone Communications. \fIProc. 5th Internat. Sympos. Human\fR
\fIFactors in Telecommun.\fR , London, 1970, paper\ 2.2.
.LP
[31]
WILLIAMS (G.) and MOYE (L. | .): Subjective evaluation of unsuppressed
echo in simulated long\(hydelay telephone communications. \fIProc. IEE\fR
118 (1971), No.\ 3/4, pp.\ 401\(hy408.
.LP
[32]
HUTTER (J.): The effect of echo suppressors and echo return loss on the
performance of circuits having a long propagation time. \fIPost Office
Research\fR \fIDepartment Report\fR No.\ 153, 1970.
.LP
[33]
CCITT \(em Contribution COM XII\(hyNo. 199, 1984\(hy1988 Study Period.
.sp 2P
.LP
\fBBibliography\fR
.sp 1P
.RT
.LP
SETZER (R.): Echo Control for RCA Americom Satellite Channels, \fIRCA Engineer\fR
, Vol.\ 25, No.\ 1, June\(hyJuly\ 1979, pp.\ 72\(hy76.
.LP
YAMAMOTO (S.) \fIet\ al.\fR | Adaptive Echo Canceller with Linear Predictor,
\fITrans. Inst. Electron. Commun. Eng. Japan\fR , Vol.\ E62, No.\ 12, December\
1979, pp.\ 851\(hy857.
.LP
WEHRMANN (R.), VAN DER LIST (J.) and MEISSNER (P.): Noise\(hyInsensitive
Compromise Gradient Method for the Adjustment of Adaptive Echo Cancellor,
\fIIEEE\ Trans. Communication\fR , Vol.\ COM\(hy28, No.\ 5, May\ 1980,
pp.\ 753\(hy759.
.LP
CAVANAUGH (J. | .), HATCH (R. | .) and NEIGH (J. | .): Model for the Subjective
Effects of Listener Echo on Telephone Connections, \fIBell Systems Technical\fR
\fIJournal\fR , Vol.\ 59, No.\ 6, July\(hyAugust\ 1980, pp.\ 1009\(hy1060.
.LP
SONDHI (M. | .) and BERKLEY (D. | .): Silencing Echoes on the Telephone
Network, \fIProc. IEEE\fR , Vol.\ 68, No.\ 8, August\ 1980, pp.\ 948\(hy963.
.LP
DUTTWEILER (D. | .): Bell's Echo\(hyKiller Chip, \fIIEEE Spectrum\fR ,
Vol.\ 17, No.\ 10, October\ 1980, pp.\ 34\(hy37.
.LP
MEISSNER (P.), WEHRMANN (R.) and VAN DER LIST (J.): Comparative Analysis of
Kalman and Gradient Methods for Adaptive Echo Cancellation, \fIAEU Arch\fR
\fIElectron Uebertrag Electron Commun.\fR , Vol.\ 34, No.\ 12, December\ 1980,
pp.\ 485\(hy492.
.bp
.LP
HORNA (O.A.): Extended Range Echo Cancellers, \fIProceedings of IEEE\fR
\fISOUTHEASTCON Regional Conf.\fR 81, Huntsville, 5\(hy8 April\ 1981, pp.\
846\(hy853.
.LP
FURUYA (N.) \fIet al.\fR | High Performance Custom VLSI Echo Canceller,
\fIIEEE\fR
\fIInternational Conference on Communications\fR , Chicago, 23\(hy26 June\
1985,
pp.\ 46.1.1\(hy46.1.7.
.LP
ITO (Y.), MARUYAMA (Y.) and FURUYA (N.): An Acoustic Echo Canceller for
Teleconferencing, \fIibid\fR , pp.\ 1498\(hy1502.
.LP
CIOFFI (J. | .) and KAILATH (T.): An Efficient, RLS, Data Driven Echo Canceller
for Fast Initialization of Full\(hyDuplex Data Transmission, \fIibid\fR
,
pp.\ 1503\(hy1507.
.sp 2P
.LP
\fBRecommendation\ G.117\fR
.RT
.sp 2P
.ce 1000
\fBTRANSMISSION\ ASPECTS\ OF\ UNBALANCE\ ABOUT\ EARTH\fR
.EF '% Fascicle\ III.1\ \(em\ Rec.\ G.117''
.OF '''Fascicle\ III.1\ \(em\ Rec.\ G.117 %'
.ce 0
.sp 1P
.ce 1000
\fB(DEFINITIONS\ AND\ METHODS)\fR
.ce 0
.sp 1P
.ce 1000
\fI(Geneva,\ 1980; amended at Malaga\(hyTorremolinos, 1984 and Melbourne,\fR |
\fI1988)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.LP
\fB1\fR \fBObjective\fR
.sp 1P
.RT
.PP
This Recommendation gives a comprehensive set of prescriptive
measurements of various balance parameters for one\(hyport and two\(hyport
networks. These are intended for use either in the field or in the factory
with
relatively simple test apparatus (e.g.\ standard transmission oscillators,
level measuring sets), and a special test bridge. Measuring arrangements
for
assessing the degree of unbalance are covered in Recommendation\ O.121\ [1],
which are consistent with this Recommendation.
.PP
The definitions and methods are so devised that the results obtained from
separately\(hymeasured (or specified) items of equipment
(e.g.\ feeding\(hybridges, cable pairs, audio inputs to channel translating
equipment,\ etc.) can be meaningfully combined though not necessarily by
simple decibel addition. This allows the performance of a tandem connection
of such
items to be predicted or at least, bounds determined for that performance.
Performance in this sense means those features affected by unbalanced
conditions, e.g.\ level of impulsive noise, sensitivity to longitudinal
exposure, crosstalk ratios,\ etc.
.RT
.sp 2P
.LP
\fB2\fR \fBPrinciples of the scheme of nomenclature\fR
.sp 1P
.RT
.PP
Many different terms have been used throughout the literature
concerning unbalance about earth, some conflicting, or in other respects
inadequate. The descriptive titles of the quantities given in this
Recommendation are based on the following principles which have been
adopted:
.RT
.LP
a)
Mode \fIconversion\fR , e.g. a poor (unbalanced) termination will
develop an unwanted transverse signal when excited by a
longitudinal signal. The measure of this effect is here termed
\fIlongitudinal conversion ratio\fR , and when expressed in
transmission units
\fIlongitudinal conversion loss\fR , or
LCL
.
.LP
b)
When a two\(hyport is involved where for example an excitation
at one port produces a signal at the other port, then the
designation will include the word \fItransfer\fR , for example
\fIlongitudinal conversion transfer ratio\fR and the corresponding
\fIloss\fR , LCTL.
.LP
c)
The impedance of the longitudinal path presented by a test
object is a key parameter. The term \fIlongitudinal impedance\fR \fIratio\fR
and the corresponding decibel expression, \fIlongitudinal\fR
\fIimpedance loss\fR , are used to characterize the particular
measurement defined.
.LP
d)
Active devices which are sources of signals (e.g. an
oscillator, the output port of an amplifier) are additionally
characterized by the amount of unwanted longitudinal signal that
is present in the output. The key word \fIoutput\fR is now included,
to give \fIlongitudinal output voltage\fR , and the corresponding
\fIlongitudinal output level\fR . When such unwanted signals are
expressed as a proportion of the wanted (transverse) signal the
key phrase is \fIoutput signal balance ratio\fR , the decibel
expression of which is \fIoutput signal balance\fR .
.bp
.LP
e)
Devices which continuously respond to signals
(e.g.\ level\(hymeasuring sets, the input port of an amplifier) and
which can in principle respond to unwanted longitudinal signals
by reason of internal mechanisms (i.e.\ even if their input
impedances were perfectly balanced) are characterized by
measures containing the words \fIinput interference\fR . These
measures are \fIinput longitudinal interference ratio\fR and the
.LP
corresponding decibel expression \fIinput longitudinal\fR \fIinterference
loss\fR . The long\(hyestablished and well\(hydefined
\fIcommon\(hymode rejection ratio\fR is maintained. The term
\fIsensitivity coefficient\fR is avoided, since this is widely used
in the Directives\ [2] and the work of Study Group\ V with a
rather specialized meaning.
.LP
f
)
When a two\(hyport network is involved, the input and
output signals may not be the same, for example, they may have
different levels, frequencies (FDM modems) or structure
(PCM multiplex equipments). These aspects should be taken
into account when formulating proposals for the item under
test.
.LP
g)
In the case of receiving devices in which the operation is
not a linear continuous function of the level of the input
signal (e.g.\ a group\(hydelay measuring set or a data modem) the
key principle is the \fIthreshold\fR level of the interference; this
is the level at or above which an unacceptable amount of
.LP
degradation of performance or misoperation occurs. Thus
\fIlongitudinal interference threshold voltage\fR and the
corresponding \fIlevels\fR are obtained.
.sp 2P
.LP
\fB3\fR \fBSummary of the descriptive terms used\fR
.sp 1P
.RT
.sp 1P
.LP
3.1
\fIOne\(hyport networks\fR \v'3p'
.sp 9p
.RT
.LP
a)
transverse reflexion factor (transverse return loss:
TRL),
.LP
b)
transverse conversion ratio (loss: TCL),
.LP
c)
longitudinal conversion ratio (loss: LCL),
.LP
d)
longitudinal impedance ratio (loss: LIL),
.LP
e)
transverse output voltage (level: TOL),
.LP
f
)
longitudinal output voltage (level: LOL).
.PP
(Voltages e) and f
) are unwanted signals uncorrelated to the wanted signals.)
.sp 2P
.LP
3.2
\fITwo\(hyport networks\fR
.sp 1P
.RT
.sp 1P
.LP
3.2.1
\fISeparate measurement\fR
.sp 9p
.RT
.LP
.PP
For each port taken separately the one\(hyport measures:
.RT
.LP
a)
transverse reflexion factors (transverse return losses:
TRL),
.LP
b)
transverse conversion ratio (loss: TCL),
.LP
c)
longitudinal conversion ratios (losses: LCL),
.LP
d)
longitudinal impedance ratios (losses: LIL),
.LP
e)
transverse output voltage (levels: TOL),
.LP
f
)
longitudinal output voltage (levels: LOL).
.sp 1P
.LP
3.2.2
\fIMeasurement combined\fR
.sp 9p
.RT
.PP
In addition the following transfer parameters are for each of the two directions
of transmission:
.RT
.LP
a)
transverse transfer ratios (losses: TTL),
.LP
b)
transverse conversion transfer ratios (losses: TCTL),
.LP
c)
longitudinal transfer ratios (losses: LTL),
.LP
d)
longitudinal conversion transfer ratios (losses:
LCTL).
.bp
.sp 1P
.LP
3.3
\fISignal generating devices\fR \v'3p'
.sp 9p
.RT
.LP
a)
Output signal balance ratio (losses: OSB).
.PP
This is in addition to the six one\(hyport measures listed in \(sc\ 3.1.
.sp 1P
.LP
3.4
\fISignal receiving devices\fR \v'3p'
.sp 9p
.RT
.LP
a)
Input longitudinal interference ratio (loss: ILIL).
.LP
b)
Longitudinal interference threshold voltage (level).
.PP
These are in addition to the six one\(hyport measures listed in
\(sc\ 3.1. If the wanted signal is longitudinal (e.g.\ as in a signalling
system)
and the interfering voltage transverse, replace the word \fIlongitudinal\fR
with\fR \fItransverse\fR in the descriptive terms.
.sp 2P
.LP
\fB4\fR \fBDefinitions and measuring techniques based on idealized measuring\fR
\fBarrangements\fR
.sp 1P
.RT
.PP
The illustrated definitions in this section assume ideal test
bridges (with lossless infinite\(hyinductance centre\(hytapped coils),
zero impedance voltage generators and infinite\(hyimpedance voltmeters.
.PP
An important aspect of this set of mutually consistent measurements is
that the test bridge provides simultaneously defined reference terminations
.PP
of \fIZ\fR \ ohms for the transverse paths, and \fIZ\fR /4\ ohms for the
longitudinal
paths.
From this starting point, the performance of cascaded items, each measured
in the prescribed fashion, can be calculated. This takes account of the
fact that the cascaded items do not, in general, exhibit the reference
impedances
provided by the test conditions.
.PP
It simplifies the mathematical treatment if the reference impedance is
nonreactive and this also accords with the important objective of being
able
to use readily\(hyavailable transmission test\(hyapparatus to obtain field and
factory measurement results.
.PP
The ideal test bridge configuration used in the following pages is
shown in Figure\ 1/G.117.
.PP
The transverse and longitudinal sources \fIE\fR\d\fIT\fR\uand \fIE\fR\d\fIL\fR\uare
activated as required by the particular measurement being made. In
Figure\ 6/G.117, neither source is active, and the bridge then provides only
passive terminations of\ \fIZ\fR and\ \fIZ\fR /4.
.PP
\fINote\fR \ \(em\ It would have been in keeping with traditional transmission
theory for the parameters to be defined in terms of half the open\(hycircuit
e.m.f. However, to harmonize with Recommendation\ O.121, this Recommendation
defines some parameters in terms of \fIV\fR\d\fIT\fR\\d1\u. If the input
impedance of the device under test is nominally
equal to the driving device, then the two methods are equivalent.
.RT
.LP
.rs
.sp 11P
.ad r
\fBFigure 1/G.117, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 2P
.LP
4.1
\fIOne\(hyport networks\fR
.sp 1P
.RT
.sp 1P
.LP
4.1.1
\fITransverse reflexion factor\fR \fI(return loss)\fR
| see Figure 2/G.117)
.sp 9p
.RT
.LP
.rs
.sp 34P
.ad r
\fBFigure 2/G.117, p.
.sp 1P
.RT
.ad b
.RT
.LP
.sp 12
.bp
.sp 1P
.LP
4.1.2
\fITransverse conversion ratio (loss)\fR | see Figure 3/G.
117)
.sp 9p
.RT
.LP
.rs
.sp 20P
.ad r
\fBFigure 3/G.117, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
4.1.3
\fILongitudinal conversion ratio (loss)\fR | see
Figure\ 4/G.117)
.sp 9p
.RT
.LP
.rs
.sp 26P
.ad r
\fBFigure 4/G.117, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
4.1.4
\fILongitudinal impedance ratio (loss)\fR | see Figure
5/G.117)
.sp 9p
.RT
.LP
.rs
.sp 23P
.ad r
\fBFigure 5/G.117 p.
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
4.1.5
\fITransverse and longitudinal output voltages (levels)\fR
| see Figure\ 6/G.117)
.sp 9p
.RT
.LP
.rs
.sp 23P
.ad r
\fBFigure 6/G.117 p.
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
4.2
\fITwo\(hyport networks\fR
.sp 9p
.RT
.PP
These follow similar principles to those defined for one\(hyport
networks but now signals can be transferred from one port to the other.
The two ports are distinguished by the subscripts\ 1/1` for one end and\
2/2` for the
other. There are two types of measurements:
.RT
.LP
\(em
those in which the excitation and response are at the same
side of the network; these are as already defined for a one\(hyport
but will carry a single subscript\ 1/1 or\ 2/2` as appropriate;
.LP
\(em
those in which the excitation and response are at opposite
sides of the network. The designation will contain the word
transfer and the symbol two subscripts, the order of which
indicates the direction of transmission.
.sp 1P
.LP
4.2.1
\fITransverse reflexion factors (return losses)\fR | see
Figure\ 7/G.117)
.sp 9p
.RT
.LP
.rs
.sp 23P
.ad r
\fBFigure 7/G.117, p. \fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 15
.bp
.sp 1P
.LP
4.2.2
\fITransverse transfer ratios (losses)\fR \fIand\fR
\fIconversion transfer ratios (losses)\fR (see Figure\ 8/G.117)
.sp 9p
.RT
.LP
.rs
.sp 30P
.ad r
\fBFigure 8/G.117, p.
.sp 1P
.RT
.ad b
.RT
.LP
.sp 15
.bp
.sp 1P
.LP
4.2.3
\fILongitudinal transfer ratios (losses)\fR \fIand\fR
\fIconversion transfer ratios (losses)\fR (see Figure\ 9/G.117)
.sp 9p
.RT
.LP
.rs
.sp 34P
.ad r
\fBFigure 9/G.117, p.
.sp 1P
.RT
.ad b
.RT
.LP
.sp 10
.bp
.sp 1P
.LP
4.3
\fISignal generating devices\fR
.sp 9p
.RT
.LP
.PP
In addition to the six one\(hyport measures already defined, an
additional measure is required to control the amount of unwanted signal
correlated with the wanted signal delivered by the device to the circuit
it is connected to. This special measure is the output signal balance ratio
(loss).
.RT
.sp 1P
.LP
4.3.1
\fIOutput signal balance ratio (loss)\fR | see Figure\ 10/G.117)
.sp 9p
.RT
.LP
.rs
.sp 30P
.ad r
\fBFigure 10/G.117, p.
.sp 1P
.RT
.ad b
.RT
.LP
.sp 9
.bp
.sp 1P
.LP
4.4
\fISignal receiving devices\fR
.sp 9p
.RT
.PP
In addition to the six one\(hyport measures already defined,
additional measures are required for signal receiving devices to control
their sensitivity to unwanted signals. Two cases are important. Firstly,
there are
receiving devices in which the response is a linear, continuous function
of the wanted signal level, e.g.\ the indication of a level\(hymeasuring
set. In this case unwanted signals give rise to \fIinaccuracy\fR .
.PP
In the other kind of receiver such as data modems, group\(hydelay
distortion measuring sets, signalling receivers, unwanted signals cause
errors or \fImisoperation\fR . Two additional measures are defined.
.RT
.sp 1P
.LP
4.4.1
\fIInput longitudinal interference ratio (loss)\fR | see
Figure\ 11/G.117)
.sp 9p
.RT
.LP
.rs
.sp 28P
.ad r
\fBFigure 11/G.117, p.
.sp 1P
.RT
.ad b
.RT
.LP
.sp 10
.bp
.sp 1P
.LP
4.4.2
\fILongitudinal interference threshold voltage (level)\fR
| see
Figure\ 12/G.117)
.sp 9p
.RT
.LP
.rs
.sp 28P
.ad r
\fBFigure 12/G.117, p.
.sp 1P
.RT
.ad b
.RT
.LP
.sp 19
.bp
.sp 2P
.LP
\fB5\fR \fBOther measurement definitions\fR
.sp 1P
.RT
.sp 1P
.LP
5.1
\fICommon\(hymode rejection ratio\fR
.sp 9p
.RT
.PP
This is another quantity that is appropriate to signal receivers
and is measured in accordance with the principle shown in Figure\ 13/G.117,
the input terminals being short\(hycircuited and then energized together.
.RT
.LP
.rs
.sp 22P
.ad r
\fBFigure 13/G.117, p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
It is clear that this measure is similar to the input longitudinal interference
ratio but since there is no transverse signal (by reason of the
short circuit) no longitudinal/transverse conversion mechanism within the
test\(hyobject is excited. In general, there is no simple relationship
between the two measures, as can be seen from the generalized measuring
instrument
illustrated in Figure\ 14/G.117, in which the input impedance is unbalanced
and the gain ratios of the two halves of the differential amplifier are
also
slightly different. Provided the value for\ \(*e" is as in Figure\ 14/G.117
and\ ?63\ <<\ 1, the various balance parameters are as indicated. This
assumes the
common mode rejection ratio is not twice the input longitudinal interference
ratio, i.e.\ there is not a 6\(hydB difference between their decibel values.
.LP
.rs
.sp 10P
.LP
.bp
.LP
.rs
.sp 21P
.ad r
\fBFIGURE 14/G.117, p. 20\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
\fBReferences\fR
.sp 1P
.RT
.LP
[1]
CCITT Recommendation \fIMeasuring arrangements to assess the degree\fR
\fIof unbalance about earth\fR , Vol.\ IV, Rec.\ O.121.
.LP
[2]
CCITT \fIDirectives concerning the protection of telecommunication\fR
\fIlines against harmful effects from electricity lines\fR , Chapter\ XVI,
ITU, Geneva,\ 1978.
.LP
[3]
CCITT Recommendation \fILogarithmic quantities and units\fR ,
Vol.\ XIII, Rec.\ 574, ITU, Geneva,\ 1986.
.LP
[4]
CCITT Recommendation \fISpecification for a psophometer for use on\fR
\fItelephone\(hytype circuits\fR , Vol.\ IV, Rec.\ O.41.
.LP
.rs
.sp 17P
.LP
.bp