All drawings appearing in this Recommendation have been done in Autocad.
Supplement No. 2
IMPEDANCE STRATEGY FOR TELEPHONE INSTRUMENTS
AND DIGITAL LOCAL EXCHANGES IN THE BRITISH TELECOM NETWORK
1 Introduction
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:
i) Input and balance impedances of telephone instruments,
ii) Input and balance impedances of the digital exchange hybrid,
iii) Impedances of the 2-wire local lines.
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.
2 Impedance strategy for a digital local exchange
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.:
Figure 1 - CCITT 88130
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.
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.
3 Impedance strategy for telephone instruments
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.
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.
4 Background to calculated results
This section includes the results of calculating STMR values, echo and
stability balance return losses for a range of local connections.
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.
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.
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 "transmission equipment" hybrid is assumed
with nominal 600 W input and balance impedances.
Using a computer program, values of echo and stability balance return
Fascicle VI.5 - Suppl. No. 2 PAGE1
losses, and sidetone masking rating are calculated for the four exchange line
groups with the two telephone instruments and two exchange line hybrids.
5 Results
5.1 Sidetone values
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).
Note - The exchange input impedance has the following approximate values:
R1 = 300 W, R2 = 1000 W, C = 220 nF (see Figure 1).
The results are summarized in Table 1 below:
TABLE 1
Calculated values of STMR
Mean value of STMR (dB)
Exchange termination Attenuation of local line group
(dB)
1 3 6 9
600 W 2.6 5.2 8.1 12.4
Complex termination 13.9 14.8 12.7 13.0
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.
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.
5.2 Echo and stability balance return losses
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.
5.2.1 Exchange balance impedance
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.
Note - The complex balance impedance has approximate values R1 = 370 W, R2
= 620 W, C = 310 nF (see Figure 1).
TABLE 2
Calculated values of mean echo (stability) balance return losses assuming existing
telephone input impedance
Attenuation of local line group dB
Exchange balance Mean value of echo (stability) balance return 1loss
impedance dB
1
PAGE4 Fascicle VI.5 - Suppl. No. 2
3 6 9
600 W 22.5 12.9 (7.5) 9.4 (6.2) 8.3 (6.0)
(13.9)
Complex impedance 10.2 (8.0) 13.8 (9.1) 15.2 17.1
(11.2) (12.9)
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.
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.
TABLE 3
Calculated values of minimum echo (stability) balance return losses assuming existing
telephone input impedance
Minimum value of echo (stability) balance return loss
dB
Exchange balance Attenuation of local line group dB
impedance
1 3 6 9
600 W 20 (13) 11 (5) 8 (4) 6 (3)
Complex impedance 9 (7) 11 (7) 12 (9)
Fascicle VI.5 - Suppl. No. 2 PAGE1
11 (7)
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-speech devices such as data modems which have an impedance
similar to that of the telephone instrument (assuming a low feeding current).
5.2.2 Telephone input impedance
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.
Note - The input impedance has nominal values R1 = 370 W, R2 = 620 W, C =
310 nF. (See Figure 1.)
TABLE 4
Calculated value of mean echo (stability) balance return losses assuming complex exchange
balance impedance
Mean value of echo (stability) balance return loss
dB
Telephone input impedance Attenuation of local line group dB
1 3 6 9
Resistive 10.2 (8.0) 13.8 (9.1) 15.2 17.1
(11.2) (12.9)
Complex 29.0 21.0 16.9 17.0
(23.6) (13.9) (12.8) (11.8)
TABLE 5
Calculated value of minimum echo (stability) balance return losses assuming complex
exchange balance impedance
Mean value of echo (stability) balance return loss
dB
Telephone input impedance Attenuation of local line group dB
1 3 6 9
PAGE4 Fascicle VI.5 - Suppl. No. 2
Resistive 9 (7) 11 (7) 12 (9) 11 (7)
Complex 24 (18) 15 (11) 13 (10) 10 (7)
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.
6 New telephone instruments in the existing analogue network
In S 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.
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.
7 Application to other voiceband terminal equipment
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 S 5.2.2 can be obtained.
8 Summary and conclusions
This paper has considered aspects of an impedance strategy for the local
network with the introduction of digital local exchanges and new telephone
instruments.
Calculations based on a large sample of local lines in the British Telecom
network have shown that:
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.
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.
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.
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.
Fascicle VI.5 - Suppl. No. 2 PAGE1