home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Internet Standards
/
CD2.mdf
/
ccitt
/
1992
/
q
/
q552.asc
< prev
next >
Wrap
Text File
|
1991-12-31
|
58KB
|
995 lines
All drawings appearing in this Recommendation have been done in Autocad.
Recommendation Q.552
TRANSMISSION CHARACTERISTICS AT 2-WIRE ANALOGUE INTERFACES OF
DIGITAL EXCHANGE
1 General
This Recommendation provides characteristics for:
- 2-wire analogue interfaces (Type C2 and Z),
- input and output connections with 2-wire analogue interfaces, and
- half-connections with 2-wire analogue interfaces,
in accordance with definitions given in Recommendation Q.551 particularly in
Figure 1/Q.551.
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.
This Recommendation is valid for equipment that may terminate an
international long-distance connection via 4-wire circuits interconnected by
4-wire exchanges. It also includes, in a separate category, characteristics for
interfaces which cannot terminate an international connection and are therefore
entirely national in application.
2 Characteristics of interfaces
Note - For measuring 2-wire analogue interface conditions it is necessary
to apply a quiet code, i.e. a PCM signal corresponding to decoder output value 0
(m-law) or output value 1 (A-law), with the sign bit in a fixed state, to the
exchange test point Ti, when no test signal is stipulated.
2.1 Characteristics of interface C2
The recommended values of interfaces C2 are valid for digital exchanges
including PABXs with transit functions and routing capabilities for originating
and terminating traffic. Depending on the type of traffic to be handled, two
different sets of relative levels are required. This suggests subdivision into
C21 and C22 interface specifications. The interface C21 provides the termination
of outgoing and incoming international long distance connections and possible
national connections, with the exchange acting as transit switch. The interface
C22 provides for the connection of a 2-wire trunk line. A typical example is the
interconnection of a Z interface with a C22 interface in a local exchange for
routing through the 2-wire analogue trunk network. A C22 interface cannot be part
of the international 4-wire chain (see Figure 2/Q.551).
2.1.1 Exchange impedance
2.1.1.1 Nominal value
Nominal values of exchange impedance should be defined depending on
national conditions. The definition shall include a test network for the exchange
impedance. Administrations may want to adopt different test networks
corresponding to the cable types used (e.g. unloaded and loaded).
2.1.1.2 Return loss
The return loss of the impedance presented by a C2 interface against the
test network for the exchange impedance should comply with the limits given in
Figure 1/Q.552.
Figure 1/Q.552 - CCITT 72230
2.1.2 Impedance unbalance about earth
The longitudinal conversion loss (LCL), defined in Recommendation G.117, S
4.1.3, should exceed the minimum values of Figure 2/Q.552 with the equipment
under test in the normal talking state, in accordance with Recommendation K.10.
Note 1 - An Administration may adopt other values and in some cases a
wider bandwidth, depending on actual conditions in its telephone network.
Note 2 - A limit may also be required for the transverse conversion loss
(TCL), as defined in Recommendation G.117, S 4.1.2, if the exchange termination
is not reciprocal with respect to the transverse and longitudinal paths. A
suitable limit would be 40 dB to ensure an adequate near-end crosstalk
attenuation between interfaces.
Figure 2/Q.552 - CCITT 65091
Fascicle VI.5 - Rec. Q.552 PAGE1
Test method
Longitudinal conversion loss should be measured in accordance with the
principles given in Recommendation O.121, SS 2.1 and 3. Figure 3/Q.552 shows an
example of the basic measuring arrangement for digital exchanges.
Measurements of the longitudinal and transverse voltages should preferably
be done with a frequency-selective level meter.
Figure 3/Q.552 - CCITT 65101
2.1.3 Longitudinal interference threshold level
Under study.
2.1.4 Relative levels
2.1.4.1 Nominal levels
2.1.4.1.1 Interface C21
C21 interfaces should meet the recommended values for Z interfaces in S
2.2.4.1 if no loss compensation comparable to S 2.2.4.3 is provided.
2.1.4.1.2 Interface C22
To adjust the transmission loss of a digital transmission section to the
values of national transmission planning for local or national traffic, depending
on the relative levels given in SS 2.1.4.1.1 and 2.2.4.1, the following ranges
encompass the requirements for C22 interfaces of a large number of
administrations:
- input level: Li = +3.0 to -7.0 dBr in 0.5 dB steps;
- output level: Lo = +1.0 to -8.0 dBr in 0.5 dB steps.
According to Annex E of Recommendation G.121 (column 2 of Table
E-1/G.121), the range of transmission loss from 1.0 to 8.0 dB for the digital
transmission section encompasses the requirements of a large number of
administrations.
In order to compensate loss on long toll or junction lines, an
administration may, to satisfy local conditions, choose values of relative levels
derived from the basic values as follows:
L`i = Li + x dB
L`o = Lo - x dB
where x should take a negative value. The value of x is in national competence.
Such compensation of loss require careful selection and application of balance
networks.
It has been recognized that it is not necessary for a particular design of
equipment to be capable of operating over the entire level range.
2.1.4.2 Tolerances of relative levels
The difference between the actual relative level and the nominal relative
level should lie within the following values:
- input relative level: -0.3 to +0.7 dB;
- output relative level: -0.7 to +0.3 dB.
These differences may arise, for example, from design tolerances, cabling
between analogue ports and the (DF), and adjustment increments.
Note - Level adjustment procedures are given in Recommendation G.715, S
2.1.
2.2 Characteristics of interface Z
The recommended values of interface Z are valid for digital local
exchanges, PABXs and digital remote units. For PABXs, see Recommendation Q.551, S
2.1.1
2.2.1 Exchange impedance
2.2.1.1 Nominal value
The principal criterion governing the choice of the nominal value of the
exchange impedance is to ensure an adequate sidetone performance for telephone
sets, particularly those operated on short lines. If this criterion is met, the
impedance will also be suitable for subscriber lines fitted with voice band
modems.
As a general rule a complex exchange impedance with a capacitive reactance
is necessary to achieve satisfactory values of stability, echo and sidetone. For
additional information, see Supplement No. 2, Fascicle VI.5 of the CCITT Blue
Book and Recommendations G.111 and G.121.
The use of the preferred configuration below will minimize the diversity
of types of exchange impedances. At present no unique component values can be
recommended. However, to provide guidance for administrations, examples of
nominal values chosen by some administrations are given in Table 1/Q.552.
PAGE22 Fascicle VI.5 - Rec. Q.552
TABLE 1/Q.552
CCITT 53830
Test networks for exchange impedances being considered
Rs (ohms) Rp (ohms) Cp (farads)
NTT 600 infinity 1 m
Austria, FRG 220 820 115 n
USA 900 infinity 2.16 m
BT 300 1000 220 n
New Zealand 370 620 310 n
Note 1 - The test network and the component values
represent a configuration that exhibits the required
exchange impedance. It need not necessarily
correspond to any actual network provided in the
exchange interface.
Note 2 - The range of component values reflects the
fact that there are substantial differences in the
sensitivity and sidetone performance of the various
telephone instruments throughout the world. In
general, the combination of short lines and sensitive
telephone sets might be rather common in the future
due to increased use of remote concentration. In
order to control sidetone performance,
Administrations need to take into account telephone
set parameters. Not only should the parameters of
existing telephone sets be considered but also the
parameters that may be desirable in the future to
allow improvement in sidetone performance to be
achieved.
Note 3 - It may be necessary to group the subscriber
lines of a particular exchange into classes, each
requiring a different exchange impedance of the Z
interface.
2.2.1.2 Return loss
Tolerances are needed for values of exchange impedance. For this purpose
the return loss of the impedance presented by a 2-wire port against the test
network for the exchange impedance should comply with limits which depend on the
particular conditions of the subscriber network considered. These are given in
the template of Figure 1/Q.552.
Some administrations may want to specify higher values. Examples of limit
values for the return loss, currently accepted by some administrations, are given
in Table 2/Q.552 for guidance.
TABLE 2/Q.552
Examples of limit values of return loss against the exchange impedance
FRG 14 dB at 300 Hz, rising (log f scale) to 18 dB at 500 Hz
remaining at 18 dB to 2000 Hz and then falling (log f
scale) to 14 dB at 3400 Hz.
NTT 22 dB: 300-3400 Hz.
Fascicle VI.5 - Rec. Q.552 PAGE1
BT 18 dB: 200-800 Hz; 20 dB: 800-2000 Hz; 24 dB: 2000-4000
Hz.
USA 20 dB: 200-500 Hz; 26 dB: 500-3400 Hz.
Austria 14.5 dB at 300 Hz, rising (log f scale) to 18 dB at 500 Hz
remaining at 18 dB to 2500 Hz and then falling (log f
scale) to 14.5 dB at 3400 Hz.
Note - The 12 dB spread in values stems from the difference in telephone set
sensitivities.
2.2.2 Impedance unbalance about earth
The longitudinal conversion loss (LCL) of the Z interface should meet the
values given in S 2.1.2 and Figure 2/Q.552, measured in accordance with the test
method given in Figure 3/Q.552.
2.2.3 Longitudinal interference threshold levels
The signalling and transmission performance of the Z interface can be
degraded when the subscriber line is exposed to an electromagnetic field of
sufficiently high intensity. The value of induced interference energy causing
performance degradation may be below a level which would cause permanent damage
or operate protective devices. Longitudinal interference may come from power or
traction lines or radio frequency sources.
Radio frequency interference tests at the Z interface should be in
accordance with Recommendations of the K-Series (intended by Study Group V).
Longitudinal interference tests relative to power and traction line
sources should be performed according to Figure 4/Q.552.
Interference up to the interference threshold level should not affect
signalling and transmission more than the limits stated below. Measurements
should be performed using quiet code at the exchange test point Ti.
There are two groups of parameters to be observed while performing the
tests:
i) signalling related parameters;
ii) transmission related parameters, i.e. noise parameters.
For group i) the performance of the signalling parameters mentioned in
Recommendation Q.543 should be tested in a go - no go procedure under normal
operating conditions.
For group ii) two test steps should be performed under normal operating
conditions, the first step without and the second one with the longitudinal test
generator connected to the coupling network. The additional noise in the second
test step should not contribute more than:
LEN = Y1 pWp using sinusoidal longitudinal test signal with X1
volts rms;
LEN = Y2 pWp using longitudinal EMF test signal with defined harmonic
content (e.g., triangular waveform with X2 volts zero to
peak).
The values Y1 and Y2 of the noise power must be specified depending on the
interface the noise measuring set is connected to, i.e. the analogue interface at
the termination T representing subscriber apparatus or the digital interface at
the exchange test point To. The noise measuring set should be provided with a
notch filter to exclude the activating signal at the nominal reference frequency.
The associated noise level limit results from the use of the equations
given in SS 3.3.2.1 and 3.3.3 of this Recommendation.
Note 1 - The values of X1 and X2 need further study. (Some administrations
reported an X1 value of 15 volts and an X2 value of 25 volts.)
Note 2 - The value of the induced noise power LEN needs further study.
(Attention is drawn to S 3.1.6.2 of this Recommendation and to S 1 of
Recommendation G.123.)
Test method
Figure 4/Q.552 - CCITT 89630
The longitudinal interference test generator should provide the
longitudinal interference EMF with the fundamental frequency of the interference
source (as appropriate to national conditions, i.e. 16 2/3 Hz, 50 Hz or 60 Hz)
with a sinusoidal waveshape, and additionally with a waveshape having a certain
amount of harmonic content, e.g. a triangular waveshape.
The coupling network CN1) should represent a typical subscriber line
1) The exact definition of the harmonic content and the coupling network is for
further study.
PAGE22 Fascicle VI.5 - Rec. Q.552
(length, type of cable) exposed to power or traction line interference. The
impedance of the coupling path within the network should be primarily capacitive.
(One RPOA reported an impedance of -j 1.17 kohm at 60 Hz for each capacitor
indicated in Figure 4/Q.552.)
The termination T representing subscriber apparatus should provide for an
appropriate loop current and the requested internal impedance of the reference
frequency signal generator.
Note 1 - Annex A gives an example of a CN applicable to the measuring
arrangement of Figure 4/Q.552, the application of which needs further study.
Note 2 - The measuring arrangement in Figure 4/Q.552 covers the general
use of subscriber equipment, as recommended in Recommendation K.4, without low
impedance to earth, especially without signalling using earth return. National
deviations from this general case need to be considered for each special type of
subscriber circuit.
2.2.4 Relative levels
Operation of the Z interface in the ranges of relative levels given below
is recommended when the interface terminates an entirely 4-wire international
long-distance connection. Pairs of input and output levels can be chosen for
internal, local, or national long-distance traffic in a wider range if these
connections can be discriminated from international ones for correct level
switching. If digital pads are used, the additional distortion must be considered
(see Recommendation G.113, Table 1/G.113).
In assigning the relative levels for international long-distance
connections to the interface it should be noted that:
- The limiting of "difference in transmission loss between the two
directions of transmission" in Recommendation G.121, S 6.4 must be
taken into account. For the national extension this is the value "loss
(t-b)-loss(a-t)". (See the text in the cited Recommendation for
guidance.) This difference is limited to ▒ 4 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.
- If within the ranges of Li and Lo given under SS 2.2.4.1.1 and
2.2.4.1.2, the values are chosen such that Li - Lo 6 dB and if
adequate balance networks are used (e.g., S 3.1.8 and Figure 11/Q.552),
the requirements of Recommendation G.121, S 6 (Incorporation of PCM
digital processes in national extensions) as well as for Recommendation
G.122 (Stability and echo loss) will be satisfied.
2.2.4.1 Nominal levels
2.2.4.1.1 Input relative level
According to Annex C to Recommendation G.121 (columns 1, 2 and 3 of Table
C-1/G.121), the following range of input relative level for all types of
connections (internal, local, national and international) encompasses the
requirements of a large number of administrations.
Li = 0 to +2.0 dBr
Note 1 - Recommendation G.101, S 5.3.2.3 indicates that if the minimum
nominal send loudness rating (SLR) of the local system under the same conditions
is not less than -1.5 dB, then the peak power of the speech will be suitably
controlled. It follows that, for instance, the value Li = 0 dBr (lower limit of
the range for Li) is suited to a send loudness rating -1.5 dB.
Note 2 - The values given above are in conformity with current national
practices and with the existing text of Recommendation G.101. However, the latter
is itself partly based on a very old investigation (which Study Group XII has
been asked to review) of the relationship between loudness ratings and speech
levels. This may, in the near future, lead to amending the basis of objectives,
so that it may be useful to allow wider design margins.
2.2.4.1.2 Output relative level
According to Annex C to Recommendation G.121 (column 3 of Table
C-1/G.121), the following range of output relative level for international
long-distance connections encompasses the requirements of a large number of
administrations.
Lo = -5.0 to -8.0 dBr
The chosen value may be used for connections entirely within a national
network as well.
If the connection type can always be detected, the nominal output relative
levels for local or national connections can take other values in accordance with
Fascicle VI.5 - Rec. Q.552 PAGE1
national transmission planning. According to Annex C to Recommendation G.121
(columns 1 and 2 of Table C-1/G.121) the following range encompasses the
requirements of a large number of administrations:
Lo = 0 to -8.0 dBr
It has been recognized that it is not necessary for a particular design of
equipment to be capable of operating over the entire range.
2.2.4.2 Tolerances of relative levels
The difference between the actual relative level and the nominal relative
level should lie within the following limits:
- input relative level: -0.3 to +0.7 dB,
- output relative level: -0.7 to +0.3 dB.
These differences may arise, for example, from design tolerances, cabling
(between analogue ports and the DF) and adjustment increments. Short-term
variation of loss with time as discussed in S 3.1.1.3 is not included.
Note - Procedures for adjusting relative level are given in Recommendation
G.715, S 2.1.
2.2.4.3 Consideration of short and long subscriber lines
In order to compensate for the loss of short or long subscriber lines, an
administration may choose values of the relative levels derived from the basic
values as follows:
L`i = Li + x dB
L`o = Lo - x dB
The value of x is within national competence (e.g., x = 3 dB for short
subscriber lines).
If values of L`i and L`o are chosen as indicated, the loss difference with
respect to the conditions given in S 2.2.4.1 will be left unchanged.
The use of values of x < 0 requires careful selection of balance networks;
values of x < -3 dB are not recommended.
3 Characteristics of half-connections
For interfaces C2 this Recommendation is valid for digital local and
transit exchanges and for C21 interfaces of PABXs connected to the digital local
exchange by a digital transmission system.
For interface Z this Recommendation is valid for digital local and
combined local/transit exchanges, for PABXs and for digital remote units, each
connected to the digital local exchange by a digital transmission system. For
further information concerning PABXs, see Recommendation Q.551, S 2.1.1.
Note - In measuring an input connection it is necessary to apply a quiet
code, i.e. a PCM signal corresponding to decoder output value 0 (m-law) or output
value 1 (A-law) with the sign bit in a fixed state to the exchange test point Ti.
(See Recommendation Q.551, S 1.2.3.1.)
3.1 Characteristics common to all 2-wire analogue interfaces
3.1.1 Transmission loss
3.1.1.1 Nominal value
The nominal transmission loss according to Recommendation Q.551, S 1.2.4.1
is defined in SS 3.2.1 and 3.3.1 for input and output connections of
half-connections with a 2-wire analogue interface.
3.1.1.2 Tolerances of transmission loss
The difference between the actual transmission loss and the nominal
transmission loss of an input or output connection, according to SS 2.1.4.2 and
2.2.4.2 should lie within the following range:
-0.3 to +0.7 dB
These differences may arise, for example, from design tolerances, cabling
(between analogue equipment ports and the DF) and adjustment increments.
Short-term variation of loss with time as discussed in S 3.1.1.3 is not included.
3.1.1.3 Short-term variation of loss with time
When a sine-wave test signal at the reference frequency of 1020 Hz and at
a level of -10 dBm0 is applied to the 2-wire analogue interface of any input
connection, or a digitally simulated sine-wave signal of the same characteristic
is applied to the exchange test point Ti of any output connection, the level at
the corresponding exchange test point To and the 2-wire analogue interface
respectively should not vary by more than ▒ 0.2 dB during any 10-minute interval
of typical operation under the steady state condition permitted variations in the
power supply voltage and temperature.
3.1.1.4 Variation of gain with input level
With a sine-wave test signal at the reference frequency 1020 Hz and at a
PAGE22 Fascicle VI.5 - Rec. Q.552
level between -55 dBm0 and +3 dBm0 applied to the 2-wire analogue interface of
any input connection, or with a digitally simulated sine-wave signal of the same
characteristic applied to the exchange test point Ti of any output connection,
the gain variation of that connection, relative to the gain at an input level of
-10 dBm0, should lie within the limits given in Figure 5/Q.552.
The measurement should be made with a frequency-selective level meter to
reduce the effect of the exchange noise. This requires a sinusoidal test signal.
Figure 5/Q.552 - CCITT 67340
3.1.1.5 Loss distortion with frequency
The loss distortion with frequency of any input or output connection
according to Recommendation Q.551, S 1.2.5 should lie within the limits shown in
the mask of Figure 6/Q.552 a) or 6/Q.552 b) respectively using an input level of
-10 dBm0.
Note - The limits of this clause shall not apply to Z half-connections
which include equalization for the distortion in the subscriber line.
Figure 6/Q.552 - T1102880-86 AND T1102890-86
3.1.2 Group delay
"Group delay" is defined in the Yellow Book, Fascicle X.1.
3.1.2.1 Absolute group delay
See Recommendation Q.551, S 3.3.1.
3.1.2.2 Group delay distortion with frequency
Taking as the reference the minimum group delay, in the frequency range
between 500 Hz and 2500 Hz, of the input or output connection, the group delay
distortion of that connection should lie within the limits shown in the template
of Figure 7/Q.552. Group delay distortion is measured in accordance with
Recommendation O.81.
Figure 7/Q.552 - CCITT 72250
These requirements should be met at an input level of -10 dBm0.
3.1.3 Single frequency noise
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 -50 dBm0.
Note - See Recommendation Q.551, S 1.2.3.1.
3.1.4 Crosstalk
For crosstalk measurements, auxiliary signals are injected as indicated in
Figures 8/Q.552 and 9/Q.552. These signals are:
- the quiet code (see Recommendation Q.551, S 1.2.3.1);
- 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 -50 to -60 dBm0 or a sine-wave signal at a level in
the range from -33 to -40 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.
3.1.4.1 Input crosstalk
A sine-wave test signal at the reference frequency of 1020 Hz and at a
level of 0 dBm0, applied to an analogue 2-wire interface, should not produce a
level in any other half-connection exceeding -73 dBm0 for near-end crosstalk
(NEXT) and -70 dBm0 for far-end crosstalk (FEXT) (see Figure 8/Q.552).
Figure 8/Q.552 - T1102900-86
3.1.4.2 Output crosstalk
A digitally simulated sine-wave test signal at the reference frequency of
1020 Hz applied at a level of 0 dBm0 to an exchange test point Ti, should not
produce a level in any other half connection exceeding -70 dBm0 for near-end
crosstalk (NEXT) and -73 dBm0 for far-end crosstalk (FEXT) (see Figure 9/Q.552).
Figure 9/Q.552 - T1102910-86
3.1.5 Total distortion including quantizing distortion
With a sine-wave test signal at the reference frequency of 1020 Hz (see
Recommendation O.132) applied to the 2-wire interface of an input connection, or
with a digitally simulated sine-wave signal of the same characteristic applied to
Fascicle VI.5 - Rec. Q.552 PAGE1
the exchange test point Ti of an output connection, the
signal-to-total-distortion ratio, measured at the corresponding outputs of the
half connection with a proper noise weighting (see Table 4/G.223) should lie
above the limits given in SS 3.2.3, Figures 13/Q.552 and 14/Q.552 for interface
C2 and S 3.3.3, Figure 15/Q.552 for interface Z.
Note - The sinusoidal test signal is chosen to obtain results independent
of the spectral content of the exchange noise.
3.1.6 Discrimination against out-of-band signals applied to the input interface
(Only applicable to input connections.)
3.1.6.1 Input signals above 4.6 kHz
With sine-wave signal in the range from 4.6 kHz to 72 kHz applied to the
2-wire interface of an input connection at a level of -25 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.
3.1.6.2 Overall requirement
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 to 4 kHz at the output of the input connection, as a result of the
presence of out-of-band signals at the 2-wire interface of the input connection.
3.1.7 Spurious out-of-band signals received at the output interface
(Only applicable to an output connection.)
3.1.7.1 Level of individual components
With a digitally simulated sine-wave signal in the frequency range
300-3400 Hz and at a level of 0 dBm0 applied to the exchange test point Ti of a
half connection, the level of spurious out-of-band image signals measured
selectively at the 2-wire interface of the output connection should be lower than
-25 dBm0. This value may need to be more stringent to meet the overall
requirement.
3.1.7.2 Overall requirement
Spurious out-of-band signals should not give rise to unacceptable
interference in equipment connected to the digital exchange. In particular, the
intelligible and unintelligible crosstalk in a connected FDM channel should not
exceed a level of -65 dBm0 as a consequence of spurious out-of-band signals at
the half-connections.
3.1.8 Echo and stability
Terminal Balance Return Loss (TBRL) as defined in S 3.1.8.1 is introduced
in order to characterize the exchange performance required to comply with the
network performance objective of Recommendation G.122 with respect to echo. The
TBRL of an equipment port is measured in the talking state as in an established
connection through a digital exchange.
The parameter "Stability Loss", as defined in Recommendation G.122,
applies to the worst terminating conditions encountered at a 2-wire interface in
normal operation.
3.1.8.1 Terminal Balance Return Loss (TBRL)
The term TBRL is used to characterize an impedance balancing property of
the 2-wire analogue equipment port.
The expression for TBRL is:
TBRL = 20 log eq \x\le\ri(\f( Zo + Zb, 2 Zo)) .eq \x\le\ri(\f( Zt +
Zo, Zt - Zb))
where
Zo exchange impedance of a 2-wire equipment port
Zb impedance of the balance network presented at a 2-wire equipment
port
Zt impedance of the balance test network
Some administrations have found that it is advantageous to choose Zo = Zb
in order to optimize TBRL. In this case the expression reduced to
TBRL = 20 log eq \x\le\ri(\f( Zt + Zb, Zt - Zb))
and the balance test network will be identical to the test network for the
exchange impedance.
The balance test network should be representative of the impedance
conditions to be expected from a population of terminated lines connected to
2-wire interfaces, as determined by the national transmission planning.
The TBRL is related to the loss aio between the exchange test point Ti and
To of a half connection as follows:
PAGE22 Fascicle VI.5 - Rec. Q.552
TBRL = aio - (ao+ ai)
where ao and ai are the losses between the exchange test point Ti and the 2-wire
port and between the 2-wire equipment port and the exchange test point To,
respectively.
TBRL can thus be determined by measurement of aio provided the sum (ao +
ai) is known. This can be derived in several ways:
a) ao and ai are assigned their nominal values NLo and NLi as defined in
SS 3.2.1 and 3.3.1. Then:
TBRL = = aio -(NLo + NLi)
b) ao is measured with the load matched to the exchange impedance as
actual transmission loss ALo and ALi (see S 3.1.1.2). Then:
TBRL = aio -(ALo + ALi)
c) the loss aio is measured with the 2-wire equipment port open- and
short-circuited, giving losses a`io y and a``io respectively.
TBRL = aio -eq \f( a`io + a``io, 2)
Method b) provides the most accurate results.
Figure 10/Q.552 - CCITT 59692
Using the arrangement of Figure 10/Q.552 and sinusoidal test signals, the
measured TBRL should exceed the limits shown in Figure 11/Q.552.
Figure 11/Q.552 - CCITT 56221
Figure 12/Q.552 gives examples of balance test networks adopted by some
administrations for unloaded subscriber lines. These examples may provide
guidance for other administrations in order to minimize the diversity of types of
test networks.
Note - Some administrations may need to adopt several balance test
networks to cover the various types of unloaded and loaded cables.
Figure 12/Q.552 - CCITT 56231
3.1.8.2 Stability loss
The stability loss should be measured between the exchange test points Ti
and To of a half-connection (Figure 10/Q.552) by terminating the 2-wire interface
with stability test networks representing the "worst terminating condition
encountered in normal operation". Some administrations may find that open- and
short-circuit terminations are sufficiently representative of worst-case
conditions. Other administrations may need to specify, for example, an inductive
termination to represent the worst-case condition.
With worst-case terminating conditions on the 2-wire interface of a
half-connection, the stability loss Ti to To measured as aio should be:
Stability Loss = aio │ x;
where x is under study for sinusoidal signals at all frequencies between 200 Hz
and 3600 Hz. This frequency band is determined by the filters used in the
interface designs.
The need for requirements outside this frequency band is also under study.
Where the digital exchange is connected to the international chain using
only 4-wire switching and transmission, the half connection of the digital
exchange may provide the total stability loss of the national extension. The
value of stability loss (SL) that is required for a 2-wire interface is a matter
of national control provided that the requirements of Recommendation G.122 are
met. A SL value of 6 dB at all frequencies between 200 Hz and 3600 Hz will ensure
that the G.122 requirements are met. However, SL values of between 6 dB and 0 dB
will formally comply with the present requirements of G.122 (Red Book 1984) but
further study is required to provide guidance in this area. One administration
has found that a value of 3 dB is satisfactory in its environment.
Note - It is suggested that the half-connection of a digital PABX, or of a
digital remote unit, when connected to the digital local exchange by a digital
transmission system, should also meet the requirements of S 3.1.8.
Fascicle VI.5 - Rec. Q.552 PAGE1
3.2 Characteristics of interface C2
3.2.1 Nominal value of transmission loss
According to the relative levels defined in S 2.1.4.1, the nominal
transmission losses of input or output connections NLi and NLo of a half
connection with C2 interfaces are in the following ranges:
C21 interfaces
NLi = 0 to 2.0 dB for all types of connections
NLo = 0 to 8.0 dB for international connections
0 to 8.0 dB for local or national connections
C22 interfaces
NLi = 3.0 to -7.0 dB ü
ì for all types of connections
NLo = 8.0 to -1.0 dB Φ
It has been recognized that it is not necessary for a particular design of
equipment to be capable of operating over the entire range of nominal
transmission losses.
If a loss compensation is applied the nominal loss NLi and NLo should be
corrected by the value of x dB chosen in connection with SS 2.1.4.1.2 or 2.2.4.3.
3.2.2 Noise
3.2.2.1 Weighted noise
For the calculation of noise, worst case conditions at the C2 interface
are assumed. The band limiting effect of the encoder on the noise was not taken
into account. For a more exact calculation further study is necessary.
3.2.2.1.1 Output connection
Two components of noise must be considered. One of these arises from the
quiet decoder, the other from analogue sources, such as signalling equipment. The
first component is limited by Recommendation G.714, S 10 as receiving equipment
noise to -75 dBm0p; the other component by Recommendation G.123, S 3 to -(67+3)
dBm0p = -70 dBm0p for one 2-wire analogue interface. This results in the maximum
value for the overall weighted noise in the talking state at the C2 interface of
a digital exchange of:
-68.8 dBm0p for equipment with signalling on the speech wires,
-75.0 dBm0p for equipment with signalling on separate wires.
3.2.2.1.2 Input connection
Two components of noise must be considered. One of these arises from the
encoding process, the other from analogue sources, e.g. signalling equipment. The
first component is limited by Recommendation G.714, S 9 as idle channel noise to
-66 dBm0p; the other component by Recommendation G.123, S 3 to -(67+3) dBm0p =
-70 dBm0p for one 2-wire analogue interface. This results in the maximum value
for the overall weighted noise in the talking state at the exchange test point To
of a digital exchange of:
-64.5 dBm0p for equipment with signalling on the speech wires,
-66.0 dBm0p for equipment with signalling on separate wires.
3.2.2.2 Unweighted noise
This noise will be more dependent on the noise on the power supply and on
the rejection ratio.
Note - The need for and value of this parameter are both under study.
Recommendations Q.45 bis, S 2.5.2 and G.123, S 3 must also be considered.
3.2.2.3 Impulsive noise
It will be necessary to place limits on impulsive noise arising from
sources within the exchange; these limits are under study. Pending the results of
this study, Recommendation Q.45 bis, S 2.5.3 may give some guidance on the
subject of controlling impulsive noise with low frequency content.
Note 1 - 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 C2 interfaces.
Note 2 - 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 bis.
3.2.3 Values of total distortion
The total distortion including quantizing distortion of a half-connection
with a C2 interface is measured in accordance with S 3.1.5.
The signal-to-total-distortion ratio for a half-connection at interface C2
PAGE22 Fascicle VI.5 - Rec. Q.552
should lie above the limits shown in Figure 13/Q.552 for equipment with
signalling on separate wires, and in Figure 14/Q.552 for equipment with
signalling on the speech wires both measured in the talking state.
Figure 13/Q.552 - CCITT 46061
Figure 14/Q.552 - T1102920-86
The values of Figure 14/Q.552 include the limits for the encoding process
given in Figure 4/G.714 and the allowance for the noise contributed via
signalling circuits from the exchange power supply and other analogue sources
(e.g., analogue coupling), which is limited to -(67+3) dBm0p = -70 dBm0p for one
C2 analogue interface by Recommendation G.123, S 3.
3.3 Characteristics of interface Z
3.3.1 Nominal value of transmission loss
According to the relative levels defined in S 2.2.4.1, the nominal
transmission losses of input or output connections NLi and NLo of a
half-connection with Z interfaces are in the following ranges:
NLi = 0 to 2.0 dB for all types of connections
NLo = 5.0 to 8.0 dB for international connections
0 to 8.0 dB for internal, local or national connections.
If a compensation for the loss of short or long subscriber lines is
applied, the nominal loss NLi and NLo should be corrected by the value of x dB
chosen in connection with S 2.2.4.3.
3.3.2 Noise
3.3.2.1 Weighted noise
For the calculation of noise, worst-case conditions at the Z interface are
assumed. The band limiting effect of the encoder on the noise has not been taken
into account. For a more exact calculation further study is necessary.
3.3.2.1.1 Output connection
Two components of noise must be considered. One of these, e.g. noise
arising from the decoding process, is dependent upon the output relative level.
The other, e.g. power supply noise from the feeding bridge, is independent of the
output relative level. The first component is limited by Recommendation G.714, S
10 as receiving equipment noise to -75 dBm0p; the other component is assumed by
Recommendation G.123, Annex A to be 200 pWp (-67 dBmp). This can be caused by the
main DC power supply and auxiliary DC-DC converters.
Information about the subject of noise on the DC power supply is given in
Supplement No. 13 to the G-Series Recommendations (Orange Book, Volume III-3).
The total psophometric power allowed at a Z interface with a relative
output level of Lo dB may be approximated by the formula:
PTNo = PAN + 10eq \b\bc\( (\f( 90 + LINo + Lo, 10)) pWp
The total noise level is given by:
LTNo = 10 logeq \b\bc\( (\f( PTNo, 1 pW)) - 90 dBmp
where
PTNo : total weighted noise power for the output connection of the
local digital exchange;
PAN : weighted noise power caused by analogue functions according to
Recommendation G.123, Annex A for local exchanges, i.e. 200 pWp;
LINo : receiving equipment noise (weighted) for PCM translating
equipment according to Recommendation G.714, S 10, i.e., -75
dBm0p;
Lo : output relative level of a half-channel of a local digital
exchange according to S 2.2.4.1.2, e.g., 0 to -8.0 dBr;
LTNo : total weighted noise level for the output connection of the
local digital exchange.
For the range of output relative levels according to S 2.2.4.1.2 the
resulting total psophometric powers and the total noise levels for the output
connection are:
Lo = -5.0 -6.0 -7.0 -8.0 dBr
0
PTNo = 231 210 208
Fascicle VI.5 - Rec. Q.552 PAGE1
206 205 pWp
LTNo = -66.4 -66.8 -66.8 -66.9 -66.9 dBmp
3.3.2.1.2 Input connection
Two components of noise must be considered. One of these, e.g. noise
arising from the encoding process, is dependent upon the output relative level.
The other, e.g. power supply noise from the feeding bridge, must be corrected by
the input relative level for calculation at the exchange test point To. The first
component is limited by Recommendation G.714, S 9 as idle channel noise to -66
dBm0p; the other component is assumed by Recommendation G.123, Annex A to be 200
pWp (-67 dBmp) which results in -67 dBmp - Li at the exchange test point To.
The total psophometric power allowed at the exchange test point To with a
relative input level of Li dB may be approximated by the formula:
PTNi = PAN . 10eq \f( -Li, 10) + 10eq \b\bc\( (\f( 90 + LINi, 10)) pWp
PAGE22 Fascicle VI.5 - Rec. Q.552
and the total noise level by
LTNi = 10 logeq \b\bc\( (\f( PTNi, 1 pW)) - 90 dBm0p
where
PTNi : total weighted noise power for the input connection of the local
digital exchange;
PAN : weighted noise power caused by analogue functions according to
Recommendation G.123, Annex A for local exchanges, i.e. 200 pWp;
LINi : idle channel noise (weighted) for the input connection of a digital
local exchange according to Recommendation G.714, S 9 i.e., -66
dBm0p;
Li : input relative level of a half-channel of a local digital
exchange according to S 2.2.4.1.1, e.g. 0 and +1 dBr;
LTNi : total weighted noise level for the input connection of the local
exchange.
For the relative levels according to S 2.2.4.1.1, the resulting
psophometric power and the total noise levels for the input connection are:
Li = 0 +1.0 +2.0 dBr
= 451 410 377 pW0p
PTNi
= -63.5 -63.9 -64.2 dBm0p
LTNi
Note - The calculation above is intended to account for the worst case. No
band limiting effect of the encoder on the noise was taken into account.
3.3.2.2 Unweighted noise
This noise will be more dependent on the noise on the power supply and on
the rejection ratio.
Note - The need for and value of this parameter are both under study.
Recommendation G.123, S 3 must also be considered.
3.3.2.3 Impulsive noise
It will be necessary to place limits on impulsive noise arising from
sources within the exchange; these limits are under study.
Note 1 - The sources of impulsive noise are often associated with
signalling functions (or in some cases the power supply and the ringing voltage)
and may produce either transverse or longitudinal voltages at Z interfaces.
Note 2 - The disturbances to be considered are those to speech or modem
data at audio frequencies, and also those causing bit errors on parallel digital
subscriber 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 bis.
3.3.3 Values of total distortion
The total distortion including quantizing distortion on half connections
with Z interfaces is measured in accordance with S 3.1.5.
Fascicle VI.5 - Rec. Q.552 PAGE1
The signal-to-total distortion ratio required for a half connection may be
approximated by the formula:
eq \f(S, NT) = Ls + Lr - 10 log eq \b\bc\[ ( 10\b\bc\( (\f( Ls + Lr - S/N, 10))
+ 10\b\bc\( (\f( LN, 10)))
where
eq \f(S,NT) resulting signal-to-total distortion ratio for input or
output connections in digital local exchanges;
Ls : signal level of the measuring signal in dBm0;
Lr : for input connections, input relative level Li in dBr for output
connections, output relative level Lo in dBr;
S/N : signal-to-total distortion ratio for PCM translating equipment in
Recommendation G.714;
LN : weighted noise caused by analogue functions according to
Recommendation G.123, Annex A for local exchanges, i.e. -67 dBmp at
the Z interface.
One resulting template for the signal-to-total distortion ratio of input
and output connections in a local exchange is shown in Figure 15/Q.552 a) and b)
as an example.
The values of Figure 15/Q.552 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 -67 dBmp for a Z interface (with feeding) by Recommendation
G.123, Annex A. As an example, the mean relative levels according to S 2.2.4.1
are assumed to be Li = 0 dBr and Lo = -7 dBr.
Note - For an input connection the calculation above is assumed to be the
worst case. No band limiting effect of the encoder on the noise was taken into
account.
Figure 15/Q.552 - T1102940-86
PAGE22 Fascicle VI.5 - Rec. Q.552