Medias Mediatrix 1204, Справочное руководство

Страница: 310 / 390

Chapter 29 - Troubleshooting Calling Issues
284 Mediatrix 1204
S
OLUTION
: The problem is country-specific. See “Input Sound Level Limitations” on page 212 for details
on how to solve it.
D
ESCRIPTION
: I hear an echo when making a call.
P
OSSIBLE
C
AUSES
: The Mediatrix 1204 gateway includes an ITU-T G.168 echo canceller that attempts
to diminish echoes on telephone calls that pass through the Mediatrix 1204. Typically, the Mediatrix
1204 is only one component in a network of equipment that is used to carry a telephone call between
two parties. There may be several possible causes to the issue.
• For each SCN port on the Mediatrix 1204, there are two directions of voice traffic. In
one direction, voice signals from a remote VoIP gateway arrive at the Mediatrix 1204 as
IP packets and are converted to analog signals which are transmitted by the Mediatrix
1204 to the SCN device attached to its SCN port. In the other direction, the Mediatrix
1204 receives analog signals from the device attached to its SCN port and converts the
signals to IP packets and forwards the packets to the remote VoIP gateway. Anytime
that analog equipment is used to carry voice signals, some fraction of the signal
transmitted to the analog equipment will be reflected back to the Mediatrix 1204’s
receiver as echo. The Mediatrix 1204’s ITU-T G.168 echo canceller will estimate what
portion of the signal received from the SCN port is energy echoed by the SCN network
and what portion is voice signal generated by the party at the SCN port. The Mediatrix
1204’s echo canceller then attenuates the echo portion of the received signal while
passing the party’s speech unmodified. Note that the Mediatrix 1204’s echo canceller
only operates on echoes that are generated towards its receiver. Recall that the
Mediatrix 1204 is only one component of a network of equipment used to carry a
telephone conversation and one possible cause of echo, for example, is that the
equipment between the Mediatrix 1204 and either party’s telephone is generating echo.
• Another possible cause of echo is that the ITU-T G.168 echo cancellers have operating
limits. One such limit is the delay between the transmission of a signal to an analog
interface and the reception of that signal’s echo. The typical limit of the Mediatrix
1204’s echo canceller is 16 ms. If the SCN equipment generates echo at delays greater
than 16 ms, the Mediatrix 1204 will have difficulty generating an accurate estimate of
the echo signal and so the echo will pass through the Mediatrix 1204 largely
unattenuated and audible.
• Lastly, even when operating between operating limits, an ITU-T G.168 echo canceller
echo estimate is not completely accurate and there will be some audible residual echo.
However, an ITU-T G.168 echo canceller includes a so-called Non-Linear Processor
(NLP) which masks the audible residual echo. The NLP includes a means of estimating
whether or not the SCN party is active. If that party is estimated to be inactive, then the
NLP will replace the received signal with background noise, thereby removing any echo
(including residual echo) completely. If the SCN party is estimated to be active, then the
NLP is not used during those instances and the ITU-T G.168 echo estimator is used to
attenuate the echo but the residual echo (if any) will be audible. It is typical, then for
echo to be more audible when both parties are speaking at the same time since the
NLP will be inactive in such instances and users will hear residual echo.
D
ESCRIPTION
: I am attempting to establish a modem-modem connection on the SCN side through a Mediatrix
1204 analog gateway, but the signal is not recognized.
P
OSSIBLE
C
AUSE
: This problem occurs because of the many analog-to-digital-to-analog conversions
performed on the modem signal.
S
OLUTION
: Mediatrix can not guarantee successful modem transmissions using the Mediatrix 1204.