.rs
.\" Troff code generated by TPS Convert from ITU Original Files
.\"                 Not Copyright ( c) 1991 
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.\" Assumes tbl, eqn, MS macros, and lots of luck.
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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
\fBMONTAGE: FIN DE LA RECOMMANDATION Q.552 EN\(hyT\* | TE DE CETTE PAGE\fR 
.sp 2P
.LP
\v'31P'
\fBRecommendation\ Q.553\fR 
.RT
.sp 2P
.ce 1000
\fBTRANSMISSION\ CHARACTERISTICS\ AT\ 4\(hyWIRE\ ANALOGUE\ INTERFACES\fR 
.EF '%	Fascicle\ VI.5\ \(em\ Rec.\ Q.553''
.OF '''Fascicle\ VI.5\ \(em\ Rec.\ Q.553	%'
.ce 0
.sp 1P
.ce 1000
\fBOF\ A\ DIGITAL\ EXCHANGE\fR 
.ce 0
.sp 1P
.LP
\fB1\fR 	\fBGeneral\fR 
.sp 1P
.RT
.PP
This Recommendation provides characteristics for:
.RT
.LP
	\(em
	4\(hywire analogue interfaces (Type C\d1\\d1\u, C\d1\\d2\uand
C\d1\\d3\u),
.LP
	\(em
	input and output connections with 4\(hywire analogue
interfaces, and
.LP
	\(em
	half connections with 4\(hywire analogue interfaces,
.LP
in digital transit and combined local and transit exchanges in accordance 
with the definitions given in Recommendation\ Q.551, particularly in Figures\ 
1/Q.551 and\ 2/Q.551. 
.PP
The characteristics of the input and output connections of a given interface 
are not necessarily the same. The characteristics of half connections are 
not necessarily identical for different types of interfaces. 
.PP
This Recommendation is intended for switched connections that may be part 
of an international long\(hydistance connection via 4\(hywire line circuits 
interconnected by 4\(hywire exchanges. Since 4\(hywire analogue interfaces 
of digital exchanges may connect with circuits which are used for both 
international and national traffic, the same values recommended for international 
connections may also be used for connections entirely within the national 
network. 
.bp
.RT
.LP
\fB2\fR 	\fBCharacteristics of interfaces\fR 
.sp 1P
.RT
.sp 2P
.LP
2.1
	\fICharacteristics common to all 4\(hywire analogue interfaces\fR 
.sp 1P
.RT
.sp 1P
.LP
2.1.1
	\fIExchange impedance\fR 
.sp 9p
.RT
.sp 1P
.LP
2.1.1.1
	\fINominal value\fR 
.sp 9p
.RT
.PP
The nominal impedance at the 4\(hywire input and output interfaces
should be 600\ ohms, balanced.
.RT
.sp 1P
.LP
2.1.1.2
	\fIReturn loss\fR 
.sp 9p
.RT
.PP
The return loss, measured against the nominal impedance, should not be 
less than 20\ dB over the frequency range 300\ Hz to 3400\ Hz. 
.PP
\fINote\fR \ \(em\ For output measurement, the exchange test point T\di\umust 
be driven by a PCM signal corresponding to the decoder output value 
number\ 0 for the \(*m\(hylaw or decoder output value number\ 1 for the 
A\(hylaw. (See 
Recommendation\ Q.551, \(sc\ 1.2.3.1.)
.RT
.sp 1P
.LP
2.1.2
	\fIImpedance unbalance about earth\fR 
.sp 9p
.RT
.PP
The value for the Longitudinal Conversion Loss (LCL) defined in
Recommendation\ G.117, \(sc\ 4.1.3, with the circuit under test in the normal
talking state, should exceed the minimum values of Figure\ 1/Q.553, in
accordance with Recommendations\ Q.45 | fIbis\fR and\ K.10.
.RT
.LP
.rs
.sp 17P
.ad r
\fBFigure\ 1/Q.553, p.\fR 
.sp 1P
.RT
.ad b
.RT
.PP
\fINote\ 1\fR \ \(em\ An Administration may adopt other values and in some
cases a wider bandwidth, depending upon actual conditions in its telephone
network.
.PP
\fINote\ 2\fR \ \(em\ A limit may also be required for the Transverse Conversion 
Loss (TCL) as defined in Recommendation\ G.117, \(sc\ 4.1.2, if the exchange 
termination is not reciprocal with respect to the transverse and longitudinal 
connections. A suitable limit would be 40\ dB to ensure an adequate near\(hyend 
crosstalk attenuation between interfaces.
.PP
\fITest method\fR 
.PP
LCL should be measured in accordance with the principles given in Recommendation\ 
O.9, \(sc\(sc\ 2.1 and 3. Figure\ 2/Q.553 shows the basic measuring 
arrangement.
.PP
Measurements of the longitudinal and transverse voltages should be
performed by means of a frequency\(hyselective level meter.
.bp
.RT
.LP
.rs
.sp 20P
.ad r
\fBFigure\ 2/Q.553, p.\fR 
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
2.1.3
	\fIRelative levels\fR 
.sp 9p
.RT
.PP
In assigning the relative levels to the interfaces, the limiting of \*Qdifference 
in transmission loss between the two directions of transmission\*U in Recommendation\ 
G.121, Annex\ A has been taken into account. For the national 
extension this is the value \*Qloss (t\(hyb)\(hyloss(a\(hyt)\*U. (See the 
text in the cited Recommendation for guidance.) This difference is limited 
to \(+- | \ dB. However, to allow for additional asymmetry of loss in the 
rest of the national network, 
only part of this difference can be used by the digital exchange.
.RT
.sp 1P
.LP
2.1.3.1
	\fINominal levels\fR 
.sp 9p
.RT
.PP
The nominal relative levels at the 4\(hywire analogue input and output 
interfaces of the digital exchange depend on the type of equipment which 
is 
connected to the exchange. (See Figure\ 1/Q.551.)
.PP
In practice it may be necessary to compensate for the loss between the 
output interfaces of the digital exchange and the input ports of the connected 
equipment to fulfill transmission plan conditions. The definition of adjustable 
steps for and the location of this compensation (digital exchange or connected 
equipment) is within national competence. 
.PP
Nominal values of relative levels are given in \(sc\(sc\ 2.2.1, 2.3.1 and
2.4.1 for the different types of half connections.
.RT
.sp 1P
.LP
2.1.3.2
	\fITolerances of relative levels\fR 
.sp 9p
.RT
.PP
The difference between the actual relative level and the nominal
relative level should lie within the following ranges:
.RT
.LP
	\(em
	input relative level:
	\(em0.3 to +0.7 dB;
.LP
	\(em
	output relative level:
	\(em0.7 to +0.3 dB.
.PP
These differences may arise, for example, from design tolerances, cabling 
(between analogue equipment ports and the DF) and adjustment 
increments.
.PP
\fINote\fR \ \(em\ Adjustment of the relative level should be made in accordance 
with Recommendation\ G.712, \(sc\ 15. 
.RT
.sp 1P
.LP
2.2
	\fICharacteristics of interface\fR 
\fIC\fR\d\fI1\fR\\d\fI1\fR\u
.sp 9p
.RT
.PP
According to Figure 1/Q.551, the interface C\d1\\d1\uof a digital exchange 
is intended to interwork with the channel translating equipment of an FDM 
system. 
.bp
.RT
.sp 1P
.LP
2.2.1
	\fIValues of nominal levels\fR 
.sp 9p
.RT
.PP
The nominal values of relative levels at the channel translating
equipment are specified in Table\ 2/G.232 for the two recommended cases. With
the pads in the channel translating equipment set to zero, these values
are:
.RT
.ce
\fBH.T. [T1.553]\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
lw(24p) | cw(42p) | cw(42p) .
	Case 1	Case 2
_
.T&
cw(24p) | cw(42p) | cw(42p) .
R	+4.0 dBr	+7.0 dBr
.T&
cw(24p) | cw(42p) | cw(42p) .
S	\(em14.0 dBr	\(em16.0 dBr
_
.TE
.nr PS 9
.RT
.ad r
\fBTable [T1.553], p.\fR 
.sp 1P
.RT
.ad b
.RT
.PP
The nominal values of relative levels at the digital exchange must be adjusted 
to compensate for the total loss between the interface of the 
digital exchange and the channel translating equipment. Therefore:
\v'6p'
.sp 1P
.ce 1000
\fIL\fR\di\u= \fIR\fR \(em \fIA\fR\d\fIR\fR\u
.ce 0
.sp 1P
.ce 1000
\fIL\fR\do\u= \fIS\fR + \fIA\fR\d\fIS\fR\u
.ce 0
.sp 1P
.LP
.sp 1
.LP
where
.LP
	\fIA\fR\d\fIR\fR\u\ =\ total loss in the receive path
.LP
	\fIA\fR\d\fIS\fR\u\ =\ total loss in the send path
.sp 1P
.LP
2.3
	\fICharacteristics of\fR 
\fIinterface C\fI\d\fI1\fR\\d\fI2\fR\u
.sp 9p
.RT
.PP
According to Figure 1/Q.551, the interface C\d1\\d2\uof a digital exchange 
is intended to interwork with the incoming and outgoing relay set of an 
analogue 4\(hywire exchange. (See Figure 1/Q.45 | fIbis\fR .) 
.RT
.sp 1P
.LP
2.3.1
	\fIValues of nominal levels\fR 
.sp 9p
.RT
.PP
The nominal values of relative levels at the relay set of an
analogue exchange are consistent with Table\ 2/G.232 for the two recommended
cases. These values are:
.RT
.ce
\fBH.T. [T2.553]\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
lw(24p) | cw(42p) | cw(42p) .
	Case 1	Case 2
_
.T&
cw(24p) | cw(42p) | cw(42p) .
R	\(em14.0 dBr	\(em16.0 dBr
.T&
cw(24p) | cw(42p) | cw(42p) .
S	+4.0 dBr	+7.0 dBr
_
.TE
.nr PS 9
.RT
.ad r
\fBTable [T2.553], p.\fR 
.sp 1P
.RT
.ad b
.RT
.PP
The nominal values of relative levels at the digital exchange must be adjusted 
to compensate for the total loss between the interface of the 
digital exchange and the relay sets of the analogue exchange. Therefore:
\v'6p'
.sp 1P
.ce 1000
\fIL\fR\di\u= \fIR\fR \(em \fIA\fR\d\fIR\fR\u
.ce 0
.sp 1P
.ce 1000
\fIL\fR\do\u= \fIS\fR + \fIA\fR\d\fIS\fR\u
.ce 0
.sp 1P
.LP
.sp 1
where
.LP
	\fIA\fR\d\fIR\fR\u\ =\ total loss in the receive path
.LP
	\fIA\fR\d\fIS\fR\u\ =\ total loss in the send path
.bp
.sp 1P
.LP
2.4
	\fICharacteristics of\fR 
\fIinterface C\fI\d\fI1\fR\\d\fI3\fR\u
.sp 9p
.RT
.PP
According to Figure 1/Q.551 the interface C\d1\\d3\uof a digital
exchange is intended to connect to a 4\(hywire analogue switching stage. (See
Figure\ 1/G.142, case\ 5.)
.RT
.sp 1P
.LP
2.4.1
	\fIValues of nominal levels\fR 
.sp 9p
.RT
.PP
The nominal values of relative levels are determined by the
relative levels of the analogue 4\(hywire switching stages in the national
transmission plans. For example, if these relative levels are identical with
the virtual analogue switching point of \(em3.5\ dBr in both directions of
transmission, the nominal input and output levels of a C\d1\\d3\uinterface
are:
\v'6p'
.RT
.sp 1P
.ce 1000
\fIL\fR\di\u= \fIL\fR\do\u= \(em3.5\ dBr
.ce 0
.sp 1P
.PP
.sp 1
Different levels at the switching stages and transmission loss
between interface C\d1\\d3\uand the switching stages can require adjusting
these levels.
.LP
\fB3\fR 	\fBCharacteristics of half connections\fR 
.sp 1P
.RT
.sp 2P
.LP
3.1
	\fICharacteristics common to all 4\(hywire analogue interfaces\fR 
.sp 1P
.RT
.sp 1P
.LP
3.1.1
	\fITransmission loss\fR 
.sp 9p
.RT
.sp 1P
.LP
3.1.1.1
	\fINominal value\fR 
.sp 9p
.RT
.PP
The nominal transmission loss, according to Recommendation\ Q.551
\(sc\ 1.2.4.1, is defined for input and output connections of a half connection
with 4\(hywire analogue interface in \(sc\(sc\ 3.2.1, 3.3.1 and 3.4.1.
.RT
.sp 1P
.LP
3.1.1.2
	\fITolerances of transmission loss\fR 
.sp 9p
.RT
.PP
The difference between the actual transmission loss and the nominal transmission 
loss of an input or output connection of the same half connection according 
to \(sc\ 2.1.3.2 should lie within the following values: 
.PP
\(em0.3 to +0.7 dB.
.PP
These differences may arise for example, from design tolerances,
cabling (between analogue equipment ports and the DF) or adjustment
increments.
.RT
.sp 1P
.LP
3.1.1.3
	\fIShort\(hyterm variation of loss with time\fR 
.sp 9p
.RT
.PP
When a sine\(hywave test signal at the reference frequency of 1020\ Hz 
and at a level of \(em10\ dBm0 (if preferred, the value 0\ dBm0 may be 
used) is 
applied to a 4\(hywire analogue interface of any input connection, or a 
digitally simulated sine\(hywave signal of the same characteristic is applied 
to the 
exchange test point T\di\uof any output connection, the level at the
corresponding exchange test point T\do\uand the 4\(hywire analogue interface
respectively, should not vary by more than\ \(+- | .2\ dB during any 10\(hyminute 
interval of typical operation under the steady state condition permitted
variations in the power supply voltage and temperature.
.RT
.sp 1P
.LP
3.1.1.4
	\fIVariation of gain with input level\fR 
.sp 9p
.RT
.PP
With a sine\(hywave test signal at the reference frequency of 1020\ Hz 
and at a level between \(em55 dBm0 and +3\ dBm0 applied to the 4\(hywire 
analogue 
interface of any input connection, or with a digitally simulated sine\(hywave
signal of the same characteristic applied to the exchange test point T\di\uof 
any output connection, the gain variation of that connection, relative 
to the gain at the input level of \(em10\ dBm0, should lie within the limits 
given in 
Figure\ 3/Q.553.
.PP
The measurement should be made with a frequency selective meter to
reduce the effect of the exchange noise. This requires a sinusoidal test
signal.
.bp
.RT
.LP
.rs
.sp 20P
.ad r
\fBFigure\ 3/Q.553, p.\fR 
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
3.1.1.5
	\fILoss distortion with frequency\fR 
.sp 9p
.RT
.PP
According to Recommendation\ Q.551, \(sc\ 1.2.5, the loss distortion
with frequency of any input or output connection should lie within the 
limits shown in the mask of Figures\ 4/Q.553,\ a) and\ b), respectively. 
The preferred 
input level is \(em10\ dBm0.
.RT
.sp 1P
.LP
3.1.2
	\fIGroup delay\fR 
.sp 9p
.RT
.PP
\*QGroup delay\*U is defined in the Blue Book, Fascicle\ I.3.
.RT
.sp 1P
.LP
3.1.2.1
	\fIAbsolute group delay\fR 
.sp 9p
.RT
.PP
See Recommendation Q.551, \(sc 3.3.1.
.RT
.sp 1P
.LP
3.1.2.2
	\fIGroup delay distortion with frequency\fR 
.sp 9p
.RT
.PP
Taking the minimum group delay, in the frequency range between
500\ Hz and 2500\ Hz, of the input or output connection as the reference, the
group delay distortion of that connection should lie within the limits 
shown in the template of Figure\ 5/Q.553. Group delay distortion is measured 
in 
accordance with Recommendation\ O.81.
.RT
.sp 2P
.LP
3.1.3
	\fINoise\fR 
.sp 1P
.RT
.sp 1P
.LP
3.1.3.1
	\fIWeighted noise\fR 
.sp 9p
.RT
.PP
Two components of noise must be considered: noise arising from the coding 
process and noise from the exchange power supply and other analogue 
sources transmitted through signalling circuits. The first component is 
limited by Recommendation\ G.714, \(sc\(sc\ 9 and\ 10 to \(em66\ dBm0p 
for an input connection; and to \(em75\ dBm0p for an output connection. 
The other component is limited by 
Recommendation\ G.123, \(sc\ 3 to \(em(67+3)\ dBm0p =\ \(em70 dBm0p for 
one 4\(hywire 
analogue interface.
.bp
.RT
.LP
.rs
.sp 47P
.ad r
\fBFigure 4/Q.553, p. 6\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 18P
.ad r
\fBFigure 5/Q.553, p. 7\fR 
.sp 1P
.RT
.ad b
.RT
.PP
This leads to the following maximum values for the overall weighted
noise at the output interfaces of a half connection of a digital
exchange:
.RT
.LP
	\(em
	Input connection:
	\(em64.5 dBm0p
	for equipment with
signalling on the speech wires;
.LP
	\(em66.0 dBm0p
	for equipment with
signalling on separate wires.
.LP
	\(em
	Output connection:
	\(em68.8 dBm0p
	for equipment with
signalling on the speech wires;
.LP
	\(em75.0 dBm0p
	for equipment with
signalling on separate wires.
.sp 1P
.LP
3.1.3.2
	\fIUnweighted noise\fR 
.sp 9p
.RT
.PP
This noise will be more dependent on the noise on the power supply and 
on the rejection ratio. 
.PP
\fINote\fR \ \(em\ The need for and value of this parameter are both under
study. Recommendations\ Q.45\fIbis\fR , \(sc\ 2.5.2 and G.123, \(sc\ 3 
must also be 
considered.
.RT
.sp 1P
.LP
3.1.3.3
	\fIImpulsive noise\fR 
.sp 9p
.RT
.PP
Limits should be placed on impulsive noise arising from sources
within the exchange; these limits are under study. Pending the results 
of this study, Recommendation Q.45 | fIbis\fR , \(sc\ 2.5.3 may give some 
guidance on the subject of controlling impulsive noise with low frequency 
content. 
.PP
\fINote\ 1\fR \ \(em\ The sources of impulsive noise are often associated with
signalling functions (or in some cases the power supply) and may produce 
either transverse or longitudinal voltage at 4\(hywire interfaces. 
.PP
\fINote\ 2\fR \ \(em\ The disturbances to be considered are those to speech or
modem data at audio frequencies, and also those causing bit errors on parallel 
digital lines carried in the same cable. This latter case, involving impulsive 
noise with high frequency content, is not presently covered by the measurement 
procedure of Recommendation\ Q.45 | fIbis\fR . 
.RT
.sp 1P
.LP
3.1.3.4
	\fISingle frequency noise\fR 
.sp 9p
.RT
.PP
The level of any single frequency (in particular the sampling
frequency and its multiples), measured selectively at the interface of an
output connection should not exceed \(em50\ dBm0.
.PP
\fINote\fR \ \(em\ See Recommendation Q.551, \(sc 1.2.3.1.
.bp
.RT
.sp 1P
.LP
3.1.4
	\fICrosstalk\fR 
.sp 9p
.RT
.PP
For crosstalk measurements auxiliary signals are injected as
indicated in Figures 6 to 9/Q.553. These signals are:
.RT
.LP
	\(em
	the quiet code (see Recommendation\ Q.551, \(sc\ 1.2.3.1);
.LP
	\(em
	a low level activating signal. Suitable activating signals
are, for example, a band limited noise signal (see
Recommendation\ O.131), at a level in the range \(em50\ to \(em60\ dBm0
or
a sine\(hywave signal at a level in the range from \(em33\ to \(em40\ dBm0.
Care must be taken in the choice of frequency and the filtering
characteristics of the measuring apparatus in order that the
activating signal does not significantly affect the accuracy of
the crosstalk measurement.
.sp 2P
.LP
3.1.4.1
	\fICrosstalk measured with analogue test signal\fR 
.sp 1P
.RT
.sp 1P
.LP
3.1.4.1.1
	\fIFar\(hyend and near\(hyend crosstalk\fR 
.sp 9p
.RT
.PP
A sine\(hywave test signal at the reference frequency of 1020\ Hz and at 
a level of 0\ dBm0, applied to an analogue 4\(hywire input interface, should 
not produce a level at either output of any other half connection exceeding 
\(em73\ dBm0 for a near\(hyend crosstalk (NEXT) path and \(em70\ dBm0 for 
a far\(hyend 
crosstalk (FEXT) path. These paths are shown in Figure\ 6/Q.553.
.RT
.LP
.rs
.sp 15P
.ad r
\fBFigure\ 6/Q.553, p.\fR 
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
3.1.4.1.2
	\fIGo\(hyto\(hyreturn crosstalk\fR 
.sp 9p
.RT
.PP
A sine\(hywave test signal at any frequency in the range 300\(hy3400\ Hz 
and at a level of 0\ dBm0, applied to the 4\(hywire interface of an input 
connection, should not produce a level exceeding \(em66\ dBm0 at the analogue
output of the same half connection. See Figure\ 7/Q.553.
.RT
.LP
.rs
.sp 14P
.ad r
\fBFigure\ 7/Q.553, p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 2P
.LP
3.1.4.2
	\fICrosstalk measured with digital test signal\fR 
.sp 1P
.RT
.sp 1P
.LP
3.1.4.2.1
	\fIFar\(hyend and near\(hyend crosstalk\fR 
.sp 9p
.RT
.PP
A digitally simulated sine\(hywave test signal at the reference
frequency of 1020\ Hz and at a level of 0\ dBm0, applied to an exchange test
point T\di\u, should not produce a level exceeding \(em70\ dBm0 for near\(hyend
crosstalk (NEXT) or \(em73\ dBm0 for far\(hyend crosstalk (FEXT), at either 
output of any other half connection. (See Figure\ 8/Q.553.) 
.RT
.LP
.rs
.sp 17P
.ad r
\fBFigure\ 8/Q.553, p.\fR 
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
3.1.4.2.2
	\fIGo\(hyto\(hyreturn crosstalk\fR 
.sp 9p
.RT
.PP
A digitally simulated sine\(hywave test signal, at any frequency in
the range 300\(hy3400\ Hz and at a level of 0\ dBm0, applied to an exchange 
test 
point T\di\uof an output connection, should not produce a crosstalk level
exceeding \(em66\ dBm0 at the exchange test point T\do\uof the corresponding
input connection. See Figure\ 9/Q.553.
.RT
.LP
.rs
.sp 15P
.ad r
\fBFigure\ 9/Q.553, p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
3.1.5
	\fITotal distortion including quantizing distortion\fR 
.sp 9p
.RT
.PP
With a sine\(hywave test signal at the reference frequency of 1020\ Hz 
(see Recommendation\ O.132) applied to the 4\(hywire interface of an input 
connection, or with a digitally simulated sine\(hywave signal of the same
characteristic applied to the exchange test point T\di\uof an output
connection, the signal\(hyto\(hytotal distortion ratio, measured at the 
respective 
outputs of the half connection with a proper noise weighting (see
Table\ 4/G.223) should lie above the limits shown in Figure\ 10/Q.553 for
signalling on separate wires and in Figure\ 11/Q.553 for signalling on the
speech wires.
.PP
\fINote\fR \ \(em\ The sinusoidal test signal is chosen to obtain results
independent of the spectral content of the exchange noise.
.RT
.LP
.rs
.sp 14P
.ad r
\fBFigure\ 10/Q.553, p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 16P
.ad r
\fBFigure 11/Q.553, p.\fR 
.sp 1P
.RT
.ad b
.RT
.PP
The values of Figure 11/Q.553 include the limits for the coding
process given in Figure\ 5/G.714 and the allowance for the noise contributed 
via signalling circuits from the exchange power supply and other analogue 
sources which is limited to \(em(67+3) dBm0p\ =\ \(em70\ dBm0p for one 
4\(hywire analogue interface by Recommendation\ G.123, \(sc\ 3. 
.sp 1P
.LP
3.1.6
	\fIDiscrimination against out\(hyof\(hyband signals applied to the\fR 
\fIinput interface\fR 
.sp 9p
.RT
.PP
(Applicable only to input connections.)
.bp
.RT
.sp 1P
.LP
3.1.6.1
	\fIInput signals above 4.6 kHz\fR 
.sp 9p
.RT
.PP
With any sine\(hywave signal in the range from 4.6\ kHz to 72\ kHz
applied to the 4\(hywire interface of a half connection at a level of \(em25\ 
dBm0, 
the level of any image frequency produced in the time slot corresponding 
to the input connection should be at least 25\ dB below the level of the 
test signal. This value may need to be more stringent to meet the overall 
requirement. 
.RT
.sp 1P
.LP
3.1.6.2
	\fIOverall requirement\fR 
.sp 9p
.RT
.PP
Under the most adverse conditions encountered in a national network the 
half connection should not contribute more than 100\ pW0p of additional 
noise in the band 10\ Hz\(hy4\ kHz at the output of the input connection, as a
result of the presence of out\(hyof\(hyband signals at the input port of 
the input 
connection.
.RT
.sp 1P
.LP
3.1.7
	\fISpurious out\(hyof\(hyband signals received at the output interface\fR 
.sp 9p
.RT
.PP
(Applicable only to an output connection.)
.RT
.sp 1P
.LP
3.1.7.1
	\fILevel of individual components\fR 
.sp 9p
.RT
.PP
With a digitally simulated sine\(hywave test signal in the frequency range 
300\(hy3400\ Hz and at a level of 0\ dBm0 applied to the exchange test 
point T\di\uof a half connection, the level of spurious out\(hyof\(hyband 
image signals 
measured selectively at a 4\(hywire interface of the output connection 
should be lower than \(em25\ dBm0. This value may need to be more stringent 
to meet the 
overall requirement.
.RT
.sp 1P
.LP
3.1.7.2
	\fIOverall requirement\fR 
.sp 9p
.RT
.PP
Spurious out\(hyof\(hyband signals should not give rise to unacceptable 
interference in the equipment connected to the digital exchange. In particular, 
the intelligible and unintelligible crosstalk in a connected FDM channel 
should not exceed a level of \(em65\ dBm0 as a consequence of the spurious 
out\(hyof\(hyband 
signals at the half connection.
.RT
.sp 2P
.LP
3.2
	\fICharacteristics for\fR 
\fIinterface C\fI\d\fI1\fR\\d\fI1\fR\u
.sp 1P
.RT
.sp 1P
.LP
3.2.1
	\fINominal value of transmission loss\fR 
.sp 9p
.RT
.PP
According to the relative levels defined in \(sc\ 2.2.1, the nominal
transmission losses of a half connection with a C\d1\\d1\uinterface
are:
.RT
.LP
	\(em
	Input connection:
	\fIR\fR \(em \fIA\fR\d\fIR\fR\u
.LP
	\(em
	Output connection:
	\(em\fIS\fR \(em \fIA\fR\d\fIS\fR\u
.PP
See \(sc 2.2.1 for definitions for \fIR\fR , \fIS\fR , \fIA\fR\d\fIR\fR\uand
\fIA\fR\d\fIS\fR\u.
.sp 2P
.LP
3.3
	\fICharacteristics for\fR 
\fIinterface C\fI\d\fI1\fR\\d\fI2\fR\u
.sp 1P
.RT
.sp 1P
.LP
3.3.1
	\fINominal value of transmission loss\fR 
.sp 9p
.RT
.PP
According to the relative levels defined in \(sc\ 2.3.1 the nominal
transmission losses of a half connection with a C\d1\\d2\uinterface
are:
.RT
.LP
	\(em
	Input connection:
	\fIR\fR \(em \fIA\fR\d\fIR\fR\u
.LP
	\(em
	Output connection:
	\(em\fIS\fR \(em \fIA\fR\d\fIS\fR\u
.PP
See \(sc 2.2.1 for definitions for \fIR\fR , \fIS\fR , \fIA\fR\d\fIR\fR\uand
\fIA\fR\d\fIS\fR\u.
.sp 2P
.LP
3.4
	\fICharacteristics for\fR 
\fIinterface C\fI\d\fI1\fR\\d\fI3\fR\u
.sp 1P
.RT
.sp 1P
.LP
3.4.1
	\fINominal value of transmission loss\fR 
.sp 9p
.RT
.PP
According to the relative levels defined in \(sc\ 2.4.1 the nominal
transmission losses of a half connection with a C\d1\\d3\uinterface
are:
.RT
.LP
	\(em
	Input connection:
	\(em3.5 dB,
.LP
	\(em
	Output connection:
	\ 3.5 dB.
.PP
Different levels at the switching stages and transmission loss
between interface C\d1\\d3\uand the switching stages can require adjusting
these losses.
.bp
.sp 2P
.LP
\fBRecommendation\ Q.554\fR 
.RT
.sp 2P
.ce 1000
\fBTRANSMISSION\ CHARACTERISTICS\ AT\ DIGITAL\ INTERFACES\fR 
.EF '%	Fascicle\ VI.5\ \(em\ Rec.\ Q.554''
.OF '''Fascicle\ VI.5\ \(em\ Rec.\ Q.554	%'
.ce 0
.sp 1P
.ce 1000
\fBOF\ A\ DIGITAL\ EXCHANGE\fR 
.ce 0
.sp 1P
.LP
\fB1\fR 	\fBGeneral\fR 
.sp 1P
.RT
.PP
The field of application of this Recommendation is found in
Recommendation Q.500.
.PP
The signals taken into consideration are passed through the following interfaces 
as described in Recommendations\ Q.511 and Q.512 and 
Figure\ 1/Q.551.
.RT
.LP
	\(em
	Interface A is for primary rate digital signals at
2048\ kbit/s or 1544\ kbit/s.
.LP
	\(em
	Interface B is for secondary rate digital signals at
8448\ kbit/s or 6312\ kbit/s.
.LP
	\(em
	Interface types V are for digital subscriber line
access.
.PP
Interface types V may appear remote from the exchange through the use of 
digital transmission facilities. When this occurs, there should be no 
impact on transmission parameters other than delay.
.PP
Detailed transmission characteristics of the digital interface
ports are given in \(sc\ 2 of this Recommendation.
.PP
\(sc 3 covers the requirements for transmission characteristics
of the half\(hyconnections between the digital interfaces and the exchange test
points. The half\(hyconnection comprises an input connection (the one\(hyway
64\ kbit/s path from the interface to the test point) and an output connection 
(the one\(hyway 64\ kbit/s path from the test point to the interface) as 
defined in Recommendation\ Q.551. Requirements are given for the input 
connection and the output connection characteristics and the two are not 
necessarily identical. 
.PP
The overall characteristics of a connection through the digital
exchange involving two interfaces can be obtained by suitably combining the
values for the characteristics of the two half\(hyconnections. For example, the
overall characteristics of the connection between a Z interface and the A
interface are obtained by combining the Z\ interface half\(hyconnection
characteristics given in \(sc\ 3.3 of Recommendation\ Q.552 with the A\ 
interface 
half\(hyconnection requirements given in \(sc\ 3.1 of this Recommendation.
.PP
Where bit integrity is maintained on the digital half\(hyconnection and 
the error performance requirements are met, the digital half\(hyconnection 
will 
add no impairment to the voice\(hyband performance of a complete connection
through the switch (with the exception of delay). For this reason the digital 
half\(hyconnection requirements do not include the conventional voice band 
parameters.
.PP
(The cases where bit integrity is not maintained are for further
study.)
.PP
The values given are to be considered as either \*Qdesign\*U or
\*Qperformance objectives\*U according to the explanation of the terms given in
Recommendation\ G.102 (Transmission performance objectives and recommendations) 
and the particular context. These objectives should be met under the most 
adverse specified timing and synchronization conditions as given in
Recommendation\ Q.541, \(sc\ 3.
.RT
.sp 2P
.LP
\fB2\fR 	\fBCharacteristics of interfaces\fR 
.sp 1P
.RT
.PP
This section covers requirements for interfaces A, B, V.
.PP
These interfaces must meet the requirements for timing and
synchronization given in Recommendation\ Q.541, \(sc\ 3.
.RT
.sp 1P
.LP
2.1
	\fIInterface characteristics common to digital interfaces\fR 
.sp 9p
.RT
.PP
The general characteristics of the interfaces A, B, V\d2\u, V\d3\u, V\d4\uare 
given in Recommendations\ G.703, G.704, G.705, G.706, Q.511 
and\ Q.512.
.RT
.sp 1P
.LP
2.2
	\fIInterface characteristics at\fR 
\fIinterface A\fR 
.sp 9p
.RT
.PP
The physical and electrical characteristics of interface A are
given in \(sc\(sc\ 2\ and 6 of Recommendation\ G.703.
.bp
.RT
.sp 1P
.LP
2.2.1
	\fIJitter and wander tolerance at the exchange input\fR 
.sp 9p
.RT
.PP
Jitter and wander tolerance is the ability of the exchange to
accept phase deviations on incoming signals without introducing slips or
errors.
.PP
The jitter/wander tolerance at input A should comply:
.RT
.LP
	\(em
	with Recommendation G.824, \(sc\ 3.1.1, for the A interface at
1544\ kbit/s;
.LP
	\(em
	with Recommendation G.823, \(sc\ 3.1.1, for the A interface at
2048\ kbit/s.
.PP
This specification may not be applicable to inputs used solely for synchronization 
purposes (i.e.\ for deriving the internal timing of the 
exchange).
.sp 1P
.LP
2.3
	\fIInterface characteristics at\fR 
\fIinterface B\fR 
.sp 9p
.RT
.PP
The physical and electrical characteristics of interface B are
given in \(sc\(sc\ 3\ and 7 of Recommendation G.703.
.RT
.sp 1P
.LP
2.3.1
	\fIJitter and wander tolerance at the exchange input\fR 
.sp 9p
.RT
.PP
Jitter and wander tolerance is the ability of the exchange to
accept phase deviations on incoming signals without introducing slips or
errors.
.PP
The jitter/wander tolerance at input B should comply:
.RT
.LP
	\(em
	with Recommendation G.824, \(sc\ 4.2.2, for the B interface at
6312\ kbit/s;
.LP
	\(em
	with Recommendation G.823, \(sc 3.1.1, for the B interface
at 8448\ kbit/s.
.PP
This specification may not be applicable to inputs used solely for synchronization 
purposes (i.e. for deriving the internal timing of the 
exchange).
.sp 1P
.LP
2.4
	\fIInterface characteristics at\fR 
\fIinterface V\fI\d\fI1\fR\u
.sp 9p
.RT
.PP
The functional characteristics of the basic access digital section between 
the V\d1\uand T reference\(hypoint are defined in Recommendations\ Q.512 
and\ I.AA. The characteristics and parameters of a digital transmission 
system which may form part of the digital section for the ISDN basic rate 
access are given in Recommendation\ I.AB. 
.RT
.sp 1P
.LP
2.5
	\fIInterface characteristics at other\fR 
\fIV\(hytype interfaces\fR 
.sp 9p
.RT
.PP
The other V\(hytype interfaces will have transmission characteristics of 
the A and B interfaces as given in \(sc\(sc\ 2.2 and\ 2.3 above. 
.RT
.sp 2P
.LP
\fB3\fR 	\fBCharacteristics of 64 kbit/s half connections\fR 
.sp 1P
.RT
.PP
This section covers the essential digital characteristics of
64\ kbit/s half connections. Where these requirements are met, the digital 
half connection will add no impairment to the voice band performance of 
a complete connection through the exchange (with the exception of delay). 
The voice band performance of digital half connections may therefore be 
interpreted by 
assuming that ideal send and receive sides (see Recommendation\ G.714) are
connected to the digital inputs and outputs respectively. The digital half
connection requirements also ensure that any connection through the exchange
using a pair of digital half connections will provide acceptable performance
for non\(hyvoice 64\ kbit/s digital services.
.RT
.sp 2P
.LP
3.1
	\fIHalf connection characteristics common to all digital\fR 
\fIinterfaces\fR 
.sp 1P
.RT
.sp 1P
.LP
3.1.1
	\fIError performance\fR 
.sp 9p
.RT
.PP
The design objective long\(hyterm mean Bit Error Ratio (BER) for a single 
pass of a 64\ kbit/s connection through an exchange between the digital 
transmissionB/Fswitching interfaces should be 1 in\ 10\u9\d or better. 
This 
corresponds to 99.5% error\(hyfree minutes assuming that the occurrence 
of errors has a Poisson distribution. 
.bp
.RT
.sp 1P
.LP
3.1.2
	\fIBit integrity\fR 
.sp 9p
.RT
.PP
Bit integrity will be maintained if called for to support 64
kbit/s non\(hytelephony services.
.PP
\fINote\fR \ \(em\ It is understood that to meet this requirement, digital
processing devices such as \(*m/A\ law converters, echo suppressors and digital
pads must be disabled for non\(hytelephony calls requiring bit integrity. The
means of disabling these devices has yet to be determined. (See
Recommendation\ Q.551, \(sc\ 1.2.6.1.)
.RT
.sp 1P
.LP
3.1.3
	\fIBit sequence independence\fR 
.sp 9p
.RT
.PP
No limitation should be imposed by the exchange on the number of
consecutive binary ones or zeros or any other binary pattern within the
64\ kbit/s paths through the exchange.
.RT
.sp 1P
.LP
3.1.4
	\fIAbsolute group dela\fR y
.sp 9p
.RT
.PP
The requirements for absolute group delay are given in \(sc\ 3.3.1 of Recommendation 
Q.551. 
.RT
.LP
.rs
.sp 36P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.sp 1P
.ce 1000
\v'12P'
\s12PART\ II
\v'4P'
.RT
.ce 0
.sp 1P
.ce 1000
\fBSUPPLEMENTS\ TO\ THE\ Q.500\ SERIES\ OF\ RECOMMENDATIONS\fR 
.ce 0
.sp 1P
.LP
.rs
.sp 33P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.LP
Montage page 174 = page blanche
.sp 1P
.RT
.LP
.bp
.sp 2P
.LP
\fBSupplement No.\ 1\fR 
.RT
.sp 2P
.ce 1000
\fBDEFINITION\ OF\ RELATIVE\ LEVELS,\ TRANSMISSION\ LOSS\fR 
.EF '%	Fascicle\ VI.5\ \(em\ Suppl.\ No.\ 1''
.OF '''Fascicle\ VI.5\ \(em\ Suppl.\ No.\ 1	%'
.ce 0
.ce 1000
\fBAND\ ATTENUATION/FREQUENCY\ DISTORTION\ FOR\ DIGITAL\ EXCHANGES\fR 
.ce 0
.sp 1P
.ce 1000
\fBWITH\ COMPLEX\ IMPEDANCES\ AT\ Z\ INTERFACES\fR 
.ce 0
.sp 1P
.LP
\fB1\fR 	\fBIntroduction\fR 
.sp 1P
.RT
.PP
During the studies of Study Group\ XI on transmission
characteristics of exchanges it has been recognized that digital local
exchanges may require complex impedances at the subscriber line interface 
(see Recommendation\ Q.552). These complex impedances result in difficulties 
with 
defining relative levels, transmission loss and attenuation/frequency
distortion.
.PP
This Supplement gives the basis for coherent definitions which are in accordance 
with the principles outlined by Study Group\ XII in the G.100 series of 
Recommendations, Fascicle\ III.1. 
.RT
.sp 2P
.LP
\fB2\fR 	\fBRelative levels\fR 
.sp 1P
.RT
.PP
There is a clear statement by Study Group\ XII that relative levels (\fIL\fR 
) \(em even at ports of complex impendance\ \(em relate to power (in general, 
apparent power) at a reference frequency of 1000\ Hz. Accordingly, at a 
point of zero relative level (i.e.\ transmission reference point, 
cf.\ Recommendation\ G.101, item \(sc\ 5.3.1) and at an impedance\ \fIZ\fR 
, the reference power of 1\ mW 
.FS
Watt is the unit of apparent power as well as of real
power.
.FE
(at 1000\ Hz) corresponds to a voltage:
\v'6p'
.RT
.ce 1000
\fIU\fR\do\u\ =\ 
@ sqrt { \~mW\~\(mu\ | \fIZ\fR | } @ 
.ce 0
.ad r
(1)
.ad b
.RT
.PP
.sp 1
It follows that generally at a point of relative level \fIL\fR the
voltage will be
\v'6p'
.ce 1000
\fIU\fR \ =\ 10\fI\fI
\u\fIL\fR\d\u/\d\u2\d\u0\d\ \(mu\ 
@ sqrt { \~mW\~\(mu\ | \fIZ\fR | } @ 
.ce 0
.ad r
(2)
.ad b
.RT
.LP
.sp 1
and that consequently the level \fIL\fR can be expressed as
\v'6p'
.ce 1000
\fIL\fR = 20 log
@ { fIU\fR } over { sqrt { 1~mW~\(mu | \fIZ\fR | } } @ 
.ce 0
.ad r
(3)
.ad b
.RT
.PP
.sp 1
This is the basis for a coherent definition of transmission loss, and subsequently 
of attenuation/frequency distortion, as derived below. 
.PP
\fINote\fR \ \(em\ In the future, measurements should be made at 1020 Hz.
.RT
.sp 2P
.LP
\fB3\fR 	\fBnominal transmission loss\fR 
.sp 1P
.RT
.PP
In the field of telecommunications, it is a well\(hyestablished
practice to define the nominal transmission loss (\fINL\fR ) between two 
points as the difference between the relative levels associated with these 
points. If, 
for instance, for a \*Qconnection through a digital exchange\*U the relative 
level at the input is \fIL\fR\di\u, and at the output, \fIL\fR\do\u, then 
the 
nominal loss is
\v'6p'
.RT
.ce 1000
\fINL\fR = \fIL\fR\di\u\(em \fIL\fR\do\u
.ce 0
.ad r
(4)
.ad b
.RT
.LP
.rs
.sp 9P
.ad r
\fBFigures 1 and 2, p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
Taking into account that according to the definition of the power reference 
circuit (Figure 1), \fIE\fR  | is frequency\(hyindependent, one obtains 
from equations (3) and (4) the nominal loss. 
\v'6p'
.ce 1000
\fINL\fR = 20 log
@ left | { fIE\fR } over { fIU\fR (1000~Hz) } right | @  + 10 log
@ left | { fIZ\fR~\d~02~\u (1000~Hz) } over { fIZ\fR~\d~01~\u (1000~Hz) } right | @ 
.ce 0
.ad r
(5)
.ad b
.RT
.PP
.sp 1
It may be noted that equation (5) represents the \*Qcomposite loss\*U (ITU 
definition 05.20) at 1000\ Hz. The composite loss is the only measure of 
attenuation that allows adding of the losses of \*Qhalf\(hychannels\*U 
(i.e.\ A\(hyD and D\(hyA) regardless of the specific impendances at the 
input and output ports. 
.sp 2P
.LP
\fB4\fR 	\fBattenuation/frequency distortion\fR 
.sp 1P
.RT
.PP
\*QAttenuation distortion\*U or \*Qloss distortion\*U is the result of
imperfect amplitude/frequency response and is generally specified in addition 
to the relative levels of a transmission section, from which the nominal 
transmission loss is derived. The definition of the attenuation/frequency
distortion (\fILD\fR ) is well established: it is the difference between 
the actual response of voltage versus frequency \fIU\fR ( 
\fIf\fR )
and the ideal (planned) response of voltage versus frequency
\fIU\fR *
(
\fIf\fR ), referred to the corresponding
difference at 1000\ Hz:
\v'6p'
.RT
.ad r
\fILD\fR \ =\ 
@ left [ 20~log left | { fIE\fR } over { fIU\fR ( \fIf\fR ) } right | \(em~20~log left | { fIE\fR } over { fIU\fR~* ( \fIf\fR ) } right |  right ] @ 
\(em 
@ left [ 20~log left | { fIE\fR } over { fIU\fR (1000~Hz) } right | \(em~20~log left | { fIE\fR } over { fIU\fR~* (1000~Hz) } right |  right ] @ 
(6)
\v'8p'
.ad b
.RT
.PP
.sp 1
Equation (6) can be rewritten as follows:
\v'6p'
.ce 1000
\fILD\fR = 20 log
@ left | { fIU\fR (1000~Hz) } over { fIU\fR ( \fIf\fR ) } right | @ 
\(em 20 log
@ left | { fIU\fR~* (1000~Hz) } over { fIU\fR~* ( \fIf\fR ) } right | @ 
.ce 0
.ad r
(7)
.ad b
.RT
.LP
.sp 1
.PP
For practical reasons the ideal response of voltage versus
frequency, \fIU\fR *
(
\fIf\fR ), is flat. Taking this into
account, equation\ (7) reduces further to
\v'6p'
.ce 1000
\fILD\fR = 20 log
@ left | { fIU\fR (1000~Hz) } over { fIU\fR ( \fIf\fR ) } right | @ 
.ce 0
.ad r
(8)
.ad b
.RT
.PP
.sp 1
It should be noted that equation (8) is valid regardless of
whether \fIZ\fR\d0\\d1\uis equal to \fIZ\fR\d0\\d2\uor not. However,
impedance matching at input (\fIZ\fR\d0\\d1\u`\ \( =\ \fIZ\fR\d0\\d1\u) 
and output (\fIZ\fR\d0\\d2\u`\ \( =\ \fIZ\fR\d0\\d2\u) is assumed. 
A measurement in accordance with equation (8) is entirely in
conformity with existing measuring techniques.
.sp 2P
.LP
\fB5\fR 	\fBConclusions\fR 
.sp 1P
.RT
.PP
Nominal transmission loss and attenuation/frequency distortion are essential 
loss parameters. Their definitions in Sections\ 3 and\ 4 are based on the 
definition of relative (power) levels at 1000\ Hz in accordance with Study 
Group\ XII which has stated the following advantages: 
.RT
.LP
	1)
	an illustrative indication of passband performance
(especially with regard to band\(hyedge distortion and extraneous
ripples);
.LP
	2)
	a loss definition in accordance with the relative level
definition;
.LP
	3)
	the loss values are relevant to singing margin evaluation;
.LP
	4)
	the loudness insertion loss will be (almost) equal to the
exchange loss; 
.LP
	5)
	additivity with a fair degree of accuracy;
.LP
	6)
	the definition is also suitable for half exchange loss
currently envisaged by Study Group\ XI.
.bp
.sp 2P
.LP
\fBSupplement No.\ 2\fR 
.RT
.sp 2P
.ce 1000
\fBIMPEDANCE\ STRATEGY\ FOR\ TELEPHONE\ INSTRUMENTS\fR 
.EF '%	Fascicle\ VI.5\ \(em\ Suppl.\ No.\ 2''
.OF '''Fascicle\ VI.5\ \(em\ Suppl.\ No.\ 2	%'
.ce 0
.sp 1P
.ce 1000
\fBAND\ DIGITAL\ LOCAL\ EXCHANGES\fR \fB\ IN\ THE\ BRITISH\ TELECOM\ NETWORK\fR 
.ce 0
.sp 1P
.LP
\fB1\fR 	\fBIntroduction\fR 
.sp 1P
.RT
.PP
When planning the introduction of digital local exchanges it is
essential to take into account the subjective performance offered to customers. 
This will, of course, include provision of overall loudness ratings within 
an acceptable range of values. Noise, distortion and other impairments 
also need to be adequately controlled. However, it is also important to 
consider those 
parameters largely influenced by the impedances associated with telephone
instruments, local lines and exchanges. In particular acceptance values of
sidetone and echo/stability losses need to be obtained. These parameters are
influenced by the choice of:
.RT
.LP
	i)
	Input and balance impedances of telephone instruments,
.LP
	ii)
	Input and balance impedances of the digital exchange
hybrid,
.LP
	iii)
	Impedances of the 2\(hywire local lines.
.PP
This contribution outlines the impedance strategy adopted for
telephone instruments and digital local exchanges in the British Telecom
network. It is shown that there are major advantages in adopting complex
impedances both for the exchange hybrid and for new telephone instruments. 
The contribution includes calculations of sidetone, echo and stability 
balance 
return losses based on a sample of 1800\ local lines in the British Telecom
network.
.sp 2P
.LP
\fB2\fR 	\fBImpedance strategy for a digital local exchange\fR 
.sp 1P
.RT
.PP
2.1
In order to adequately control echo and stability losses in the digital 
network the nominal hybrid balance impedance ZB for lines of up to 
10\ dB attenuation is based on a 3\ element network. This network consists 
of a resistor in series with a parallel resistor/capacitor combination,\ 
i.e.: 
.sp 9p
.RT
.LP
.rs
.sp 8P
.ad r
\fBFigure 1, p.\fR 
.sp 1P
.RT
.ad b
.RT
.PP
With appropriate component values it has been found that this
network can give significantly improved echo and stability balance return
losses compared with a resistive network.
.PP
2.2
The nominal exchange input impedance ZI is also based on a 3
element network of the same form as the balance impedance\ ZB. This network,
with suitable component values, is required to give an acceptable sidetone
performance on the lower loss lines. It has been found that a 600\ \(*W 
resistive input impedance gives unacceptable sidetone performance on these 
lower loss 
lines.
.sp 9p
.RT
.sp 2P
.LP
\fB3\fR 	\fBImpedance strategy for telephone instruments\fR 
.sp 1P
.RT
.PP
It should be noted that the digital local exchange is designed to operate 
with a low feeding current (\( = 40\ mA). The telephone instrument will 
therefore be operating as though it were connected to a long line on a
conventional analogue exchange. In particular, any regulation function 
will be disabled. 
.bp
.PP
The input impedance of present instruments is, under low current
feeding conditions, substantially resistive. It has been found that there 
is a significant improvement in echo/stability balance return losses at 
the exchange hybrid if the telephone input impedance is also made complex. 
The preferred 
impedance is close to the design value for the exchange balance
impedance\ ZB.
.RT
.sp 2P
.LP
\fB4\fR 	\fBBackground to calculated results\fR 
.sp 1P
.RT
.PP
This section includes the results of calculating STMR values, echo and 
stability balance return losses for a range of local connections. 
.PP
Four groups of exchange lines have been used where the groups have
mean attenuations of 1\ dB, 3\ dB, 6\ dB and 9\ dB. Each group consists 
of at least 100\ samples of local lines in the British Telecom network 
with attenuations 
within 1\ dB of the mean value for the group.
.PP
Two telephone instruments have been used with identical
characteristics except for input impedance. One instrument retains a
conventional, substantially resistive impedance; the other instrument uses a
complex capacitive input impedance. The sidetone balance impedance is, 
in both cases, designed to match long lengths of 0.5\ mm Cu\ cable. 
.PP
Two cases for the exchange hybrid impedances are considered. The
strategy outlined in Section\ 2 is used i.e.,\ complex input and balance
impedance, and for comparison purposes, a conventional \*Qtransmission 
equipment\*U hybrid is assumed with nominal 600\ \(*W input and balance 
impedances. 
.PP
Using a computer program, values of echo and stability balance return losses, 
and sidetone masking rating are calculated for the four exchange line groups 
with the two telephone instruments and two exchange line hybrids. 
.RT
.sp 2P
.LP
\fB5\fR 	\fBResults\fR 
.sp 1P
.RT
.sp 1P
.LP
5.1
	\fISidetone values\fR 
.sp 9p
.RT
.PP
For this case the comparison is made between a 600\ \(*W exchange input 
impedance and a complex input impedance. (It should be noted that the STMR 
values have been calculated as in Recommendation\ P.79 of the Blue Book).
.PP
\fINote\fR \ \(em\ The exchange input impedance has the following approximate
values:
.PP
R\d1\u\ =\ 300\ \(*W, R\d2\u\ =\ 1000\ \(*W, C\ =\ 220\ nF (see Figure\ 1).
.PP
The results are summarized in Table\ 1 below:
.RT
.ce
\fBH.T. [T1.2]\fR 
.ce
TABLE\ 1
.ce
\fBCalculated values of STMR\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(60p) | cw(24p) sw(24p) sw(24p) sw(24p) , ^  | c s s s 
^  | c | c | c | c.
Exchange termination	Mean value of STMR (dB)
	 {
Attenuation of local line group (dB)
 }	1	3	6	9
_
.T&
lw(60p) | cw(24p) | cw(24p) | cw(24p) | cw(24p) .
600 \(*W	\ 2.6	\ 5.2	\ 8.1	12.4
_
.T&
lw(60p) | cw(24p) | cw(24p) | cw(24p) | cw(24p) .
Complex termination	13.9	14.8	12.7	13.0
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 1 [T1. p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
It is clear from Table\ 1 that a 600\ \(*W termination gives far from satisfactory 
results with shorter local lines which will include at least 50% of local 
lines in the British Telecom network. Use of a complex input impedance 
improves these STMR values by approximately 10\ dB and the values are closer 
to the recommended values given in Recommendation\ G.121. 
.PP
These results show that a complex input impedance is essential for the 
case of sensitive telephone instruments directly connected to digital exchange 
hybrids. The performance with a resistive impedance is in fact worse than 
the performance on a conventional analogue exchange because of the low 
feeding 
current and impedance masking effect of the digital exchange.
.RT
.sp 1P
.LP
5.2
	\fIEcho and stability balance return losses\fR 
.sp 9p
.RT
.PP
As far as impedance is concerned the most important factor is the choice 
of the balance impedance for the exchange line hybrid as this determines 
the network echo and stability performance. Initially a comparison is made 
between a 600\ \(*W impedance and a complex impedance assuming existing 
telephone instruments. Having chosen a balance impedance it is then shown 
that a further improvement can be made by considering the telephone input 
impedance. 
.RT
.sp 1P
.LP
5.2.1
	\fIExchange balance impedance\fR 
.sp 9p
.RT
.PP
Table\ 2 below shows the summarized results for mean values of echo balance 
return loss (calculated according to Recommendation\ G.122, 
Volume\ III.1, of the Blue Book), and stability balance return loss.
.PP
\fINote\fR \ \(em\ The complex balance impedance has approximate values
R\d1\u= 370 \(*W, R\d2\u= 620 \(*W, C = 310 nF (see\ Figure\ 1).
.RT
.ce
\fBH.T. [T2.2]\fR 
.ce
TABLE\ 2
.ce
\fBCalculated values of mean echo (stability)\fR 
.ce
\fBbalance return losses\fR 
.ce
\fBassuming\fR 
.ce
\fBexisting telephone input impedance\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(72p) | cw(36p) sw(36p) sw(36p) sw(36p) , ^  | c s s s 
^  | c | c | c | c.
Exchange balance impedance	 {
Mean value of echo (stability) balance
return loss dB
 }
	 {
Attenuation of local line group dB
 }	1	3	6	9
_
.T&
lw(72p) | cw(36p) | cw(36p) | cw(36p) | cw(36p) .
600 \(*W	22.5 (13.9)	12.9 (7.5)	\ 9.4 (6.2)\ 	\ 8.3 (6.0)\ 
_
.T&
lw(72p) | cw(36p) | cw(36p) | cw(36p) | cw(36p) .
Complex impedance	10.2 (8.0)\ 	13.8 (9.1)\ 	15.2 (11.2)	17.1 (12.9)
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 2 [T2.2], p.\fR 
.sp 1P
.RT
.ad b
.RT
.PP
.sp 3
In addition to calculating mean values for the distributions it is important 
to consider the edges of the distributions. This is especially true for 
echo and stability performance where it is the worst case values that are 
likely to cause network difficulties. 
.PP
Table\ 3 shows the minimum values of calculated echo and stability
balance return losses for the samples of lines considered. The values for
stability balance return loss are those given in brackets.
.bp
.RT
.LP
.ce
\fBH.T. [T3.2]\fR 
.ce
TABLE\ 3
.ce
\fBCalculated values of minimum echo (stability)\fR 
.ce
\fBbalance return losses\fR 
.ce
\fBassuming existing\fR 
.ce
\fBtelephone input impedance\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(72p) | cw(36p) sw(36p) sw(36p) sw(36p) , ^  | c s s s 
^  | c | c | c | c.
Exchange balance impedance	 {
\fIMinimum\fR
value of echo (stability)
balance
return loss dB
 }
	 {
Attenuation of local line group dB
 }	1	3	6	9
_
.T&
lw(72p) | cw(36p) | cw(36p) | cw(36p) | cw(36p) .
600 \(*W	20 (13)	11 (5)	\ 8 (4)	\ 6 (3)
_
.T&
lw(72p) | cw(36p) | cw(36p) | cw(36p) | cw(36p) .
Complex impedance	\ 9 (7)\ 	11 (7)	12 (9)	11 (7)
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 3 [T3.2], p.\fR 
.sp 1P
.RT
.ad b
.RT
.PP
With the exception of the 1\ dB sample of lines it can be seen from Table\ 
2 that the complex impedance results in mean values for the distributions 
which are higher than the corresponding values using a 600\ \(*W impedance. 
The 
improvement is particularly marked for the higher loss exchange lines. 
When the minimum values of the distributions are also taken into account 
(Table\ 3) there is a clear advantage in using the complex balance impedance. 
A similar 
advantage would also be obtained with non\(hyspeech devices such as data modems
which have an impedance similar to that of the telephone instrument (assuming 
a low feeding current). 
.sp 1P
.LP
5.2.2
	\fITelephone input impedance\fR 
.sp 9p
.RT
.PP
Having chosen a suitable complex balance impedance for the exchange hybrid, 
the options for changing the telephone input impedance can be 
considered. Tables\ 4 and 5 present calculated results for the distributions 
of echo and stability balance return losses at the exchange hybrid, comparing 
the effect of complex and resistive telephone input impedances. 
.PP
\fINote\fR \ \(em\ The input impedance has nominal values
R\d1\u\ =\ 370\ \(*W, R\d2\u\ =\ 620\ \(*W, C\ =\ 310\ nF. (See Figure 1.)
.RT
.ce
\fBH.T. [T4.2]\fR 
.ce
TABLE\ 4
.ce
\fBCalculated value of mean echo (stability)\fR 
.ce
\fBbalance return losses\fR 
.ce
\fBassuming complex\fR 
.ce
\fBexchange balance impedance\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(72p) | cw(36p) sw(36p) sw(36p) sw(36p) , ^  | c s s s 
^  | c | c | c | c.
Telephone input impedance	 {
\fIMean\fR
value of echo (stability)
balance return loss dB
 }
	 {
Attenuation of local line group dB
 }	1	3	6	9
_
.T&
lw(72p) | cw(36p) | cw(36p) | cw(36p) | cw(36p) .
Resistive	10.2 (8.0)\ 	13.8 (9.1)\ 	15.2 (11.2)	17.1 (12.9)
_
.T&
lw(72p) | cw(36p) | cw(36p) | cw(36p) | cw(36p) .
Complex	29.0 (23.6)	21.0 (13.9)	16.9 (12.8)	17.0 (11.8)
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 4 [T4.2], p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [T5.2]\fR 
.ce
TABLE\ 5
.ce
\fBCalculated value of minimum echo (stability) balance return losses\fR 
.ce

.ce
\fBassuming complex exchange balance impedance\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(72p) | cw(36p) sw(36p) sw(36p) sw(36p) , ^  | c s s s 
^  | c | c | c | c.
Telephone input impedance	 {
\fIMinimum\fR
value of echo (stability)
balance return loss dB
 }
	 {
Attenuation of local line group dB
 }	1	3	6	9
_
.T&
lw(72p) | cw(36p) | cw(36p) | cw(36p) | cw(36p) .
Resistive	\ 9 (7)\ 	11 (7)\ 	12 (9)\ 	11 (7)
_
.T&
lw(72p) | cw(36p) | cw(36p) | cw(36p) | cw(36p) .
Complex	24 (18)	15 (11)	13 (10)	10 (7)
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 5 [T5.2], p.\fR 
.sp 1P
.RT
.ad b
.RT
.PP
The results in Tables 4 and 5 show a significant improvement in
echo and stability balance return losses for the lower loss local lines. 
There is little difference for the higher loss lines as the balance return 
loss is 
primarily determined by the cable characteristics. It can be concluded that
there is a clear advantage in designing future telephone instruments with a
complex input impedance.
.sp 2P
.LP
\fB6\fR 	\fBNew telephone instruments in the existing analogue network\fR 
.sp 1P
.RT
.PP
In \(sc\ 5.2.2 the advantages of a complex telephone input
impedance have been illustrated when used with digital exchanges. However,
there are also advantages if these instruments are used on conventional
analogue exchanges.
.PP
The sidetone balance impedance of instruments is generally optimised around 
the capacitive impedance of unloaded cable. If the telephone input 
impedance is also capacitive then the sidetone performance of instruments on
own exchange calls can be improved. The improvement will be most marked when
both instruments are on short lines hence the sidetone is largely determined 
by the input impedance of the other instrument. This situation is widely 
encountered on analogue PABXs where the majority of extensions are of low
loss.
.RT
.sp 2P
.LP
\fB7\fR 	\fBApplication to other voiceband terminal equipment\fR 
.sp 1P
.RT
.PP
The discussions in this paper have concentrated on telephone
instruments. However the conclusions concerning telephone input impedance 
can equally be applied to other voiceband equipment, e.g.,\ data modems. 
Work in 
Study Group\ XII has shown that higher speed modem services require signal to
listener echo ratios approaching 25\ dB for successful operation. If the data
modem adopts a complex input impedance then the improvements in stability
balance return losses (and hence signal to listener echo ratio) discussed in
\(sc\ 5.2.2 can be obtained.
.RT
.sp 2P
.LP
\fB8\fR 	\fBSummary and conclusions\fR 
.sp 1P
.RT
.PP
This paper has considered aspects of an impedance strategy for the local 
network with the introduction of digital local exchanges and new 
telephone instruments.
.PP
Calculations based on a large sample of local lines in the British
Telecom network have shown that:
.RT
.LP
	i)
	The input impedance of the digital exchange must take into
account the sidetone performance of the telephone instruments.
To provide acceptable sidetone performance it has been found
necessary to provide a complex input impedance which more
closely matches the sidetone balance impedance of the telephone
instrument.
.bp
.LP
	ii)
	Adopting a complex exchange balance impedance gives a
significant improvement in echo and stability balance return
losses. This improvement is considered necessary to provide
adequate echo performance in the digital network without
requiring extensive use of echo control devices.
.LP
	iii)
	A further improvement in echo and stability losses is
obtained by using a complex input impedance for new telephone
instruments. This impedance also improves the sidetone
performance of connections on analogue exchanges.
.LP
	iv)
	The conclusions are also relevant to other voiceband
apparatus. Signal to listener echo ratios on voiceband data
connections can be improved if the modems use a complex input
impedance.
.LP
.rs
.sp 42P
.ad r
Blanc
.ad b
.RT
.LP
.bp