OPTICOM OPERA - V 3.5 User Manual page 145

C H A P T E R 6 : T E L E P H O N Y
T E S T I N G
1.0, x ≤ 1.7
{
Y =
–0.157268 x3 + 1.386609 x2 – 2.504699 x + 2.023345, x > 1.7
This mapping was submitted to the ITU-T SG12 with the intention of extending
P.862 by an annex or appendix. SG12 however clearly rejected this proposal,
and we, OPTICOM are fully in line with this rejection for the following reasons:
•
The PESQ MOS has the best overall performance. If a user requires the
mapping of the PESQ score to another listening test, he has to perform his
own mapping in any case. PESQ-LQ will be as wrong as any other
parameter in this case.
•
Having a second MOS-like parameter is confusing.
•
Applying a second third order polynome to the already third order mapped
PESQ MOS doubles the mathematical degree of freedom. This will
increase the correlation on the data which were used for the parameter
fitting ("training"), but it also increases the risk of complete failure on other
data.
Similar is the situation with a MOS mapped to the full P.800 scale. Although in
OPERA we use a linear mapping in this case only, we do not recommend using
this value.
Both parameters are supplied in OPERA, due to the sole reason, that customers
wanted to see them. In our opinion it is scientifically wrong to use them, and
we do not recommend it. However we do recommend application of a linear
mapping between the PESQ MOS and subjective results if a direct comparison
to a specific listening test is required. This will compensate for differences in the
MOS scales used by the listeners and by PESQ. The actual scale varies slightly
between listening tests, which means in test 1 File a may be graded as a 3.2,
while in test 2 it may score 3.4, while PESQ will always give the same result.
The ETSI e-model as defined in ITU-T G.107 [ITUT107] is a planning tool that
assigns a certain equipment impairment factor Ie to each piece of equipment in
the transmission chain. These Ie values are then summed up and combined
with several other parameters to give the final R factor or R rating. This R Rating
is an estimate of the quality that can be expected if the network is realised the
way it is planned. Although the e-model is an excellent planning tool, it can
never replace measurements on the final network, since it has to make some
very wide ranging assumptions. R ranges from 0 for terrible quality up to 100
for "users are very satisfied". Values below 50 are generally interpreted as
"nearly all users are dissatisfied". Of course there is a well defined relation
between R and the MOS score. To allow for the comparison between the
estimates from the network planning phase and the QoS of the live network,
OPERA provides the R factor as well. It is directly derived from the overall
MOS, as it is calculated by PESQ [MÖLL02]. Please note, that the R value
presented here is derived directly from the PESQ MOS. It takes neither delay,
nor echo or attenuation into account and is in fact more corresponding to the
G.107 I
value than to the R factor (which is a conversational measure, rather
e
than a listening quality index).
B A N D V O I C E Q U A L I T Y
139
G.107 Rating, R
Factor