Q-Flex Satellite Modem Installation and Operating Handbook
6-91
Figure 6-49 Test Screen
The
Loopback
setting allows a number of loopbacks to be selected to aid fault diagnosis
and resolution of system level problems.
•
IF (local)
is used to loopback transmit to receive internally within the modem.
•
Interface (local and remote)
is used to loopback receive to transmit and is
normally set on one of the pair of modems (usually the remote modem).
•
The other modes are more specialized and cause a loopback at various points
within the Tx/Rx processing chains.
Note that the loopback of IP has been disabled. This would cause the modem to
retransmit on its terrestrial port packets that it has received and this can confuse
intelligent switches, which conclude that the original sending device has moved and is
now on a different port, which can cause traffic storms.
With respect to the
RF Test Modes
, selecting Tx CW causes a pure carrier to be
transmitted rather than the normal modulated carrier. This is useful for evaluating phase
noise and spurious but should not be used over a live satellite link without the operator’s
permission. Selecting
Modulator alternate 1,0
causes an alternating 1,0,1,0 …… pattern
to be fed to the modulator at the symbol rate. This causes two discrete frequencies to be
generated, spaced at the symbol rate about the (suppressed) carrier. This is useful for
evaluating the carrier suppression of the modulator but should not be used over a live
satellite link.
6.2.12 BER Test
The internal PRBS BER Tester shown in
Figure 6-50
allows pseudo-random bit patterns
to be injected into the main traffic or overhead channel and the BER results to be
monitored. Use of the ESC and AUX channels allows continuous real traffic BER
performance monitoring whilse the modem carries traffic.
A facility exists to inject errors into the generated PRBS stream and see these reflected
within the error count.