WARNING
WHEN ACTIVATING LOOP2 UNDER CONDITIONS THAT SHDSL IS USED
TO TRANSMIT ETHERNET DATA, IT IS NECESSARY THAT THE DEVICE
IS DISCONNECTED FROM THE ETHERNET NETWORK!
3.2.4.1 Analogue Loop back
During the analogue loop back test, the SHDSL transceiver receives the transmitted signal from
its own transmitter. The analogue loop back function (the STARTAL command is used to activate
the analogue loop back) is used to test the equipment itself.
The analogue loop back causes a non-urgent alarm of the local unit and an urgent alarm of the
remote unit.
WARNING
TO PERFORM THE ANALOG LOOPBACK, THE CABLE SHOULD BE
DISCONNECTED FROM THE UNIT!
3.2.4.2 Performance Monitoring
The transmission performance of a link can be monitored in two different ways. The signal quality
is typically used during installation and maintenance procedures, whereas the G.826 error
performance parameters are used for long term evaluation of operating links and during
acceptance testing.
The Noise Margin (NM) provides qualitative performance information of a specific link. The NM
command is used to activate this test. This parameter is calculated according to ITU-T G.991.2
and is an efficient tool for determining the qualitative performance of an SHDSL link.
During acceptance testing, it is recommended to set the line rate or choose cable pairs (at a fixed
line rate) so that the NM value is no less that 6 dB.
An NM of 0dB in the presence of a Gaussian noise would yield an expected Bit-Error-Ratio
of 10
-7
.
3.2.4.3 G.826 Performance Monitoring
The error performance monitoring of a SHDSL link is performed according to ITU-T Rec. G.704.
The evaluation of the G.826 error performance parameters is based on CRC (Cyclic Redundancy
Check) error detection. CRC generation and detection are performed separately for the E1
interfaces and SHDSL interfaces.
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