Internet Standards/CD2.mdf/ccitt/1992/g/g715.asc

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6.    Recommendation G.715



SEPARATE PERFORMANCE CHARACTERISTICS

FOR THE ENCODING AND DECODING SIDE                OF PCM
CHANNELS APPLICABLE TO 2-WIRE INTERFACES



1.    General



    The CCITT,



considering



    (a)  that Recommendation G.712 defines the performance of point-to-
point PCM systems between 4-wire voice-frequency ports;



    (b)  that with the introduction of digital switching into telecommunica-
tion networks, many PCM systems will not be operated on a point-to-
point basis. In these instances a particular PCM send side will be associ-
ated no longer with a particular distant PCM receive side. Furthermore,
the combination is likely to vary on a call by call basis;



    (c)  that for digital signals crossing an international border, the send and
receive sides of PCM systems are likely to be of different origin;



    (d)  that it is necessary to achieve compatibility between send and receive
side interconnections as can arise in the situations outlined above,



recommends



    That for those PCM systems for which there is a need for separate speci-
fication, the requirements given below should be met for the separate
send and receive sides when measured at the 2-wire voice-frequency
ports. These specifications should ensure that, if not stated, otherwise,
any combination of PCM multiplexes corresponding to the specifications
meets also

Recommendation G.713.



    The parameters and values specified in this Recommendation apply to
the use of PCM equipment connected to analogue trunks or to analogue
exchanges. When PCM equipment is connected directly to analogue sub-
scriber lines, different values for some of the parameters may be
required.

Recommendation Q.552 contains those values. They may also be applied
if the PCM equipment is directly connected to an analogue local
exchange that is virtually transparent with regard to the impedances con-
nected to its ports and the subscriber lines are short (e.g., less than 500
meters).



    In deriving the limits, an allowance has been included for the effect of
possible signalling functions and/or line current feeding on the transmis-
sion performance. The limits should be met when any signalling function
is in the normal speaking condition, but excluding any dynamic signal-
ling conditions, e.g., metering.



    The limits do not, in general, have any allowance for the effects of line
current noise. The permissible amount of line current noise and the need
for allowances are under study.



Note - In the following paragraphs, the concepts of a "standard digital
generator" and "a standard digital analyzer" should be assumed and these
are defined as follows:

    A standard digital generator is a hypothetical device which is absolutely
ideal, i.e., a perfect digital to analogue converter followed by an ideal low
pass filter (assumed to have no attenuation/frequency distortion and no
envelope delay distortion), and which may be simulated by a digital pro-
cessor.



    A standard digital analyzer is a hypothetical device which is absolutely
ideal, i.e., a perfect digital to analogue coverter followed by an ideal low
pass filter (assumed to have no attenuation/frequency distortion and no
envelope delay distortion), and which may be simulated by a digital pro-
cessor.



    Recommendation O.133 contains information about test equipment
based on these concepts. Account should be taken of the measurement
accuracy provided by test equipment designed in accordance with that
Recommendation.

    The following specifications are based on ideal measuring equipment.
Therefore, they do not include any margin for measurement errors.



    To avoid level errors produced as a result of the use of test frequencies
which are sub-multiples of the PCM sampling rate, the use of integer
sub-multiples of 8 kHz should be avoided.



    Where a nominal reference frequency of 1 020 Hz is indicated (measure-
ment of attenuation/frequency distortion and adjustment of relative lev-
els), the actual frequency should be 1 020 Hz +2 Hz, -7 Hz in accordance
with Recommendation 0.6.



    For an interim period administrations may for practical reasons, need to
use a reference frequency of normally 800 kHz.



2.    Adjustment of actual relative levels



2.1    The gain of the encoding side should be adjusted by connecting its
output to a standard digital analyzer and applying a sine-wave signal at a
nominal frequency of 1 020 Hz at a level of 0 dBm0 to the voice-fre-
quency input.  The adjustment should result in an output level of 0 dBm0
+ 0.4 dB and should be made under typical conditions of power supply
voltage, humidity and temperature.



    The load capacity of the encoding side may be checked by applying a
sine-wave signal at a frequency of 1 020 Hz at its voice-frequency input.
The level of this signal should be initially well below Tmax and should
then be slowly increased. The input level should be measured at which
the first occurence is observed of the character signal corresponding to
the extreme quantizing interval for both positive and negative values.
Tmax is taken as being 0.3 dB greater than the measured input level.



    This method allows Tmax to be checked for both positive and negative
amplitudes and the values thus obtained should be within 0.4 dB of the
theoretical load capacity (i.e., +3.14 dBm0 for the A-law or +3.17 dBm0
for the ╡-law).



2.2    The decoding side should be adjusted to conform with º 4 of Recom-
mendation G.711 within a tolerance of + 0.4 dB.



Note* - The use of another digital periodic sequence representing a nom-
inal reference frequency of 1 020 Hz at a nominal level of -10 dBm0 (or 0
dBm0) isacceptable, provided that the theoretical level accuracy is better
than

+ 0.03 dB.



3.    Short-term and long-term variation of loss with time



3.1    When a sine-wave signal at a nominal frequency of 1 020 Hz and at a
level of -10 dBm0 (preferred value; 0 dBm0 may be used) is applied to
any voice-frequency input, the level measured at the corresponding time
slot output of a standard digital analyzer should not vary by more than +
0.1 dB during any

10-minute interval of typical operation nor by more than + 0.3 dB during
any one year under the permitted variations in the power supply voltage
and temperature.

3.2    When a digitally simulated sine-wave signal at a frequency of 1 020
Hz and at a level of -10 dBm0 (preferred value; however the 0 dBm0
sequence of Recommendation G.711, Tables 5 and 6 may be used) is
applied to any channel time slot at the decoder input, the level measured
at the corresponding

voice-frequency output should not vary by more than + 0.1 dB during
any

10-minute interval of typical operation, nor by more than + 0.3 dB during
any one year under the permitted variations in the power supply voltage
and temperature.



4.    Impedance of voice-frequency ports



4.1    Nominal impedance



    No single value of impedance is recommended.



    The most widely used value of nominal impedance at 2-wire audio input
and outputs ports is 600 ohms resistive (balanced). Some administrations
adopt values of 600 ohms + 2.16 ╡F or 900 ohms + 2.16 ╡F, and one
administration uses 900 ohms resistive, the latter representing a compro-
mise value suitable for loaded and unloaded cables.



Note - Some examples of complex impedances used in connection with
subscriber lines can be found in Recommendation Q.552, º 2.2.1.



4.2    Return loss



    The return loss, measured against the nominal impedance, should meet
the limits given below:





+ûûûûûûûûûûûûûûûûûûûûûûûûûû+

                         _Frequency        Return   _

                         _Range            Loss     _

                         +ûûûûûûûûûûûûûûûûûûûûûûûûûû+

                         _300 to 600 Hz    > 12 dB  _

                         _600 to 3 400 Hz  > 15 dB  _

                         +ûûûûûûûûûûûûûûûûûûûûûûûûûû+









ûûûûûûûûûûûûûûû

*  This note should be deleted, if a corresponding note is being inserted in
Recommendation G.711 by Study Group XVIII.



Note - Reflections due to impedance and balance impedance mismatches
at 2w-4w interfaces may cause severe sidetone and echo problems in the
network. Administrations need to adopt a suitable impedance strategy,
including tolerances, to ensure an adequate transmission quality. (For fur-
ther information, see Recommendation G.121 º5, and Supplement 10 of
Vol. VI.)



5.    Longitudinal balance



    The longitudinal balance parameters referred to below are defined in
Recommendation 0.9 which also gives some information about the
requirements of test circuits (Note 1). The value of Z in the driving test
circuit should be

750 ohms + 20%.



    a)    The longitudinal conversion loss (see Recommendation 0.9, º 2.1)
as measured at the input port of the encoding side should not be less
than the limits shown in Figure 1/G.715.



    b)    The longitudinal conversion loss (see Recommendation 0.9, º 2.1)
as measured at the output port of the decoding side should not be less
than the limits shown in Figure 1/G.715.



Note 1 - Attention is drawn to Recommendation 0.9, º 3, which shows
the equivalence between a number of different test driving circuits and
also includes information concerning the inherent balance requirements
of the test bridge.



Note 2 - Attention is drawn to the fact that these values represent mini-
mum requirements. The magnitude of potential longitudinal signal volt-
ages depends, for example, on system use, the system environment, the
location of hybrid transformers and attenuators, and may therefore vary
for different administrations. Some administrations have found it neces-
sary to specify higher values for longitudinal conversion loss to ensure
that transverse voltages caused by possible longitudinal signal voltages
are sufficiently small.



Note 3 - The possible need to introduce limits for frequencies below 300
Hz, in particular at 50 or 60 Hz, is under study. Overall rejection of longi-
tudinal interference can be achieved by a combination of good longitudi-
nal balancing and high filtering (see º 11.2).

Note 4 -    The measurements should be made selectively.



6.    Relative levels at voice frequency ports



    On account of differences in network transmission plans and equipment
utilization, administrations have differing requirements for the range of
relative levels to be provided. It would appear that the following ranges
would encompass the requirements of a large number of administrations:



    -     input level (encoding side) 0 to -5 dBr in 0.5 dB steps;



    -     output level (decoding side) -2 to -7.5 dBr in 0.5 dB steps.



    It has been recognized that it is not necessarily appropriate for a particu-
lar design of equipment to be capable of operating over the entire range.



Note - The requirements in this paragraph are different from the require-
ments in Recommendation Q.552, º 2.1.4.



7.    Attenuation/frequency distortion of the encoding or the decoding side



    The variations with frequency of the attenuation of any channel should be
within the limits shown in the mask of Figure 2/G.715.



    The nominal reference frequency is 1 020 Hz.



    The preferred input power level is -10 dBm0, in accordance with Recom-
mendation 0.6. As an alternative a level of 0 dBm0 may be used if com-
plex nominal impedances are used, the measuring method to be applied is
described in Recommendation Q.551, º 1.2.5 and in Annex A to Recom-
mendation G.121.



8.    Group delay



Note - The following are design objectives only. It does not seem neces-
sary to define special test equipment to make these measurements
between the voice-frequency input and the digital output and between the
digital input and the voice-frequency output.



8.1    Absolute group delay



8.1.1    Absolute group delay of the encoding side at the frequency of mini-
mum group delay should not exceed 450 microseconds.



8.1.2    The absolute group delay of the decoding side at the frequency of
minimum group delay should not exceed 300 microseconds.



8.2    Group delay distortion with frequency of the encoding or decoding
side



    The group delay distortion should lie within the limits shown in the mask
of Figure 3/G.715



    The minimum value of group delay for each side is taken as the reference
for the group delay distortion.



8.3    Input level



    The requirements of º 8.1 and º 8.2 above should be met at an input
power level of -10 dBm0 (preferred value) 0 dBm0 may be used in accor-
dance with Recommendation 0.6.



9.    Weighted noise measured at the encoding side



    With the input ports of the channel terminated in the nominal impedance,
the idle channel noise should not exceed -66 dBm0p.



10.    Weighted noise measured at the decoding side



    Noise contributed by the decoding equipment alone should be less than

-75 dBm0p when its input is driven by a PCM signal (quiet code) corre-
sponding  to the decoder output value number 0 for the ╡-law or decoder
output value number 1 for the A-law.



11.    Discrimination against out-of-band input signals (only applicable to
encoding side)



11.1    Input signal above 4.6 kHz



    With any sine-wave signal in the range from 4.6 kHz to X kHz applied to
the input port of the channel at a suitable level, the level of any image fre-
quency produced in the time slot corresponding to the channel should, as
a minimum requirement, be at least 25 dB below the level of the test sig-
nal.



Note - The value X is under study, but it should be at least 150 kHz.



    It has been found that a suitable test level is -25 dBm0.



11.2    Signal below 300 Hz



    No particular value is recommended.



Note 1 - While some administrations have no particular requirement in
this respect some other administrations have found it necessary to pro-
vide at least 20 to 26 dB rejection at the encoding side at frequencies
across the band

15 - 60 Hz.



Note 2 - Overall rejection of longitudinal interference can be achieved by
a combination of good longitudinal balancing (see º 5) and high pass fil-
tering.



12.    Spurious out-of-band signals at channel output (only applicable to
decoding side)



    With a digitally simulated sine-wave signal in the frequency range

300 - 3 400 Hz and at a level of 0 dBm0 applied to a channel time slot at
the decoder input, the level of spurious out-of-band image signals mea-
sured selectively at the output port should as a minimum requirement be
lower than  -25dBm0.



    Attention is drawn to the importance of the attenuation characteristic in
the range 3 400 to 4 600 Hz. Although other attenuation characteristics
can satisfy the requirement of º 12.1 above, the filter template of Figure
4/G.713 gives adequate protection against out-of-band signals.



13.    Single frequency noise from the encoding or decoding side



    The level of any single frequency (in particular for the decoding side at
the sampling frequency and its multiples) measured selectively, should
not exceed -50 dBm0.



14.    Total distortion, including quantizing distortion



    Two alternative methods are recommended. It should be noted that the
two test methods are not exactly equivalent. The noise test method
(Method 1) gives fairly smooth curves. The sine-wave method (Method
2) can be more sensitive in identifying possible localized codec imperfec-
tions. Thus the two methods respond to practical codec impairments in
slightly different ways.



Note 1 - Some administrations have taken the position that the require-
ments of both test methods should be met. Other administrations are of
the opinion that meeting the requirements of either test method is suffi-
cient to meet network performance requirements. In practice, administra-
tions may choose to use only one method in production testing and
operational situations.



Note 2 - There is a slight possibility that an adverse combination of
encoding and decoding sides might not meet the overall requirements of

Recommendation G.713. To minimize this possibility some administra-
tions suggested that encoding and decoding sides of the same design
should always meet the overall requirements of Recommendation G.713.



Note 3 - The limits for Methods 1 and 2 do not include any allowance for
additional noise which might be present when signalling takes place on
the two wires. The derivation of limits for this case, taking account of the
philosophy adopted in Recommendation Q.551, is under study.



14.1    Method 1 (Encoding side)



    With a noise signal corresponding to Recommendation 0.131 applied to
the input port of a channel, the ratio of signal-to-total distortion power
should lie above the limits shown in Figure 4a/G.715.



14.2    Method 1 (Decoding side)



    With a digitally simulated noise signal corresponding to

Recommendation 0.131 applied to the time slot of any telephone channel,
the ratio of signal-to-total distortion power should lie above the limits
shown in Figure 4b/G.715. The value in the mask includes the distortion
power of an ideal encoder.



14.3    Method 2 (Encoding side)



    With a sine-wave signal at a nominal frequency of 1 020 Hz (preferred
value) or 820 Hz (see Recommendation 0.132) applied to the input port
of a channel, the ratio of signal-to-total distortion power measured with
the proper noise weighting (see Table 4/G.223) should lie above the lim-
its shown in

Figure 5/G.715.



14.4    Method 2 (Decoding side)



    With a digitally simulated sine-wave signal at a nominal frequency of
1020Hz (preferred value) or 820 Hz (see Recommendation 0.132)
applied to the time slot of any channel, the ratio of signal-to-total distor-
tion power measured with the proper noise weighting (see Table 4/G.223)
should lie above the limits shown in Figure 5/G.715.



15.    Variation of gain with input level



    Two alternative methods are recommended (see comments in º 14).



Note - There is a slight possibility that an adverse combination of encod-
ing and decoding sides might not meet the overall requirements of Rec-
ommendation G.713. To minimize this possibility encoding and decoding
sides of the same design should always meet the overall requirements of
Recommendation G.713.



15.1    Method 1 (Encoding side)



    With a band limited noise signal as defined in Recommendation 0.131,
applied to the input port of any channel at a level between -55 dBm0 and

-10 dBm0, the gain variation of that channel, relative to the gain at an
input level of -10 dBm0, should lie within the limits of Figure 6a/G.715.
The measurement should be limited to the frequency band 350 - 550 Hz
in accordance with the filter characteristics defined in Recommendation
0.131, º 3.2.1.



    Furthermore, with a sine-wave signal in the frequency range

700 - 1 100 Hz applied to the input port of any channel at a level between

-10 dBm0 and +3 dBm0, the gain variation of that channel, relative to the
gain at an input level of -10 dBm0 should lie within the limits of Figure
6b/G.715. The measurement should be made selectively.



15.2    Method 1 (Decoding side)



    With a digitally simulated band limited noise signal, corresponding to
Recommendation 0.131, applied to the time slot of any telephone channel
at a level between -55 and -10 dBm0, the gain variation of that channel,
relative to the gain at an input level of -10 dBm0, should lie within the
limits of

Figure 6a/G.715 below. The measurements should be limited to the fre-
quency band 350 - 550 Hz in accordance with the filter characteristics
defined in Recommendation 0.131, º 3.2.1.



    Furthermore, with a digitally simulated sine-wave signal in the frequency
range 700 - 1 100 Hz applied to the time slot of any telephone channel at
a level between -10 dBm0 and +3 dBm0, the gain variation of that chan-
nel, relative to the gain at an input level of -10 dBm0, should lie within
the limits of Figure 6b/G.715. The measurement should be made selec-
tively.



15.3    Method 2 (Encoding side)



    With a sine-wave signal in the frequency range 700 to 1 100 Hz applied
to the input port of any channel at a level between -55 dBm0 and +3
dBm0, the gain variation of that channel, relative to the gain at an input
level of  -10dBm0, should lie within the limits given in Figure 7/
G.715. The measurement should be made selectively.



15.4    Method 2 (Decoding side)



    With a digitally simulated sine-wave signal in the frequency range

700 - 1 100 Hz applied to the time slot of any telephone channel at a level
between -55 dBm0 and +3 dBm0, the gain variation of that channel, rela-
tive to the gain at an input level of -10 dBm0, should lie within the limits
given in

Figure 7/G.715. The measurement should be made selectively.



16.    Crosstalk measurements with sine-wave signals



16.1    General



    For the crosstalk measurements auxillary signals are injected as indicated
in Figures 8 and 9/G.715. These signals are:



        -    the quiet code, i.e., a PCM signal corresponding to decoder output
value number 0 (╡-law) or output value number 1 (A-law) (with the
sign bit in a fixed state);



         -    a low level activating signal. Suitable activating signals are for
example, a band limited noise signal (see

Recommendation 0.131), 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 accu-
racy of the crosstalk measurement.



16.2    Far-end and near-end crosstalk measured with analogue test signal



    The crosstalk between individual channels of a multiplex should be such
that with a sine-wave signal in the frequency range 700 to 1 100 Hz and
at a level of 0 dBm0 applied to a voice-frequency input port, the crosstalk
level produced in any other channel should not exceed -73 dBm0 for
NEXT and -70 dBm0 for FEXT (see Figure 8/G.715).



16.3    Far-end and near-end crosstalk measured with digital test signal



    The crosstalk between individual channels of a multiplex should be such-
that with a digitally simulated sine-wave signal in the frequency range
700 to 1 100 Hz and at a level of 0 dBm0 applied to the digital input, the
crosstalk level received in any other channel should not exceed -70 dBm0
for NEXT and

-73 dBm0 for FEXT (see Figure 9/G.715).



17.    Echo and stability



17.1    Terminal balance return loss (TBRL)



    This quantity characterizes the equipment performance required to com-
ply with the network performance objective of Recommendation G.122
in respect of echo. The TBRL is defined as the Balance Return Loss (see
definition in Recommendation Q.552, º 3.1.8.1) measured against a bal-
ance test network. It is related to the "Half-Loop Loss" HLL, i.e., the loss
between the digital test input point, Ti and the digital test output point, To
(see Figure 10/G.715) as follows:



    HLL = Ti to To loss = Pi + Po + TBRL (dB)



where Pi and Po are the measured values of loss in the equivalent circuit
of Figure 10/G.715 which represent all the loss between the digital test
point and the 2-wire point, or conversely at the measurement frequency.



    The TBRL should be measured in the arrangement of Figure 10/G.715
with a sinusoidal test signal at frequencies across the telephone band cov-
ering the bandwidth 300 to 3 400 Hz.



    Values for the nominal balance impedance and for the maximum devia-
tion of this impedance from the nominal value, differ from one adminis-
tration to another. The range of impedances presented at the 2-wire port
during normal operation also varies considerably. Administrations will
need to establish their own requirements for TBRL taking account of
national or international transmission plans. As a minimum requirement,
the TBRL limits shown in

Figure 11/G.715 should be met when the 2-wire port is terminated with a
balance test network which is representative of the impedance conditions
expected in the speaking condition from a population of 2-wire trunks
connected to the PCM muldex. The limits are provisional.



17.2    Stability loss (SL)



    The stability loss is defined as the minimum value of the half-loop loss
measured in the arrangement of Figure 10/G.715. The stability loss
should be measured between Ti and To by terminating the 2-wire port
with stability test networks representing the worst case terminating con-
dition encountered in normal operation. Some administrations may find
that open circuit and short circuit terminations are sufficiently representa-
tive of worst case conditions. Other administrations may need to specify,
for example, an inductive termination to represent that worst case condi-
tion.



    The stability loss at any frequency can be expressed as follows:



                        SL > Pi + Po - X dB



where Pi and Po are measured values of loss, at the measurement fre-
quency, under normal terminating conditions at the 2-wire port. X is a
factor dependent on the interaction between the 2-wire input impedance,
the 2-wire balance impedance and the impedance actually applied at the
2-wire port. X can be computed or measured by the methods described in
Recommendation Q.552.



    The 2-wire input and balance impedances at a 2w-4w interface usually
have to be optimized by administrations with regard to echo and
sidetone. The worst case terminations depend on the actual network con-
ditions. Thus, the value of X is fully determined by network conditions
and on the impedance strategy. Values between 0 and 3 dB have been
observed in practice.



    Administrations should choose the nominal values of Pi and Po taking
account of the value of X for their particular operating conditions and of
national and international transmission plans for overall network stability
(see Recommendation G.122)



18.    Interference from signalling

    The characterization of such interference by separate measurements
requires four different types of measurements, as for crosstalk (see

Figure 12/G.715). In each case the maximum level of interference in one
channel should not exceed -63 dBm0p when signalling (10 Hz signal
with a 50/50 duty ratio) is active simultaneously on all channels.



Note - The value of X is under study.