1 - History of Ericsson - History of Ericsson

introduction of a transmission check-before conversation starts-betvveen
an outgoing and incoming control switching point located as close to the
hybrids as possible. The control of international calls may be split in
national and international sections.
D With level regulation in the checking points the loss variations due to loss
deviation combinations can be compensated for and only a loss for echo
protection should remain.
• The echo protection loss in the 4-wire path can be reduced by normalizing
the characteristics of the 2-wire circuits and telephone instruments to give
a better matching with the 4-wire circuits.
• A transmission check on a worldwide basis, using only a relatively small
number of actual LD calls as test calls, would give supervision and maintenance
people in different areas and administrations a valuable help in
tracing difficult transmission faults, as 4-wire connexions with too poor
properties can, when desired, be automatically locked for investigation by
the people concerned.
• As the control problems are worldwide, there will undoubtedly be a number
of intricate problems to study and solve, which will take time. It would
be advisable, however, to prepare new 4-wire switching points, so that
control equipment can be introduced later on at a reasonable cost.
• The attempts to calculate and minimize the inconvenience to users from
irregularities such as technical faults, congestion and too poor intelligibility
are interesting, as planning, supervision and maintenance people will get a
better picture of how to extend and maintain a plant economically having
regard to the telephone users' demands.
• We have to teach the non-technical users to apply a certain conversation
technique on very long distance calls with long propagation times and
with interlinked echo suppressors, in order to prevent confusion and
"halloa-calls with the resulting prolongation of conversation time.
APPENDIX 1
Isoopinion Contours and Mean Opinion Score
Deviations
In the calculation of the isoopinion contours MARKMAN 4 has used equation
y = 3.35 + /^(X+2.5) 2 (5)
where Y is the mean opinion score
X is the junction loss between terminal exchanges
8 is a negative noise factor depending on circuit noise level as given
in Markman's table 4.
In the CCITT investigations a loss of 6 db for subscribers' sets and lines
was added to the junction loss. Hence we can put X = SyRE - 6 db and transform
equation (5) into
3 35 — Y
SyRE= I/- -5— + 3.5db
V - p
We get the isoopinion contours by keeping Y constant and inserting the
^-values corresponding to different noise levels as given in Markman's table 4.