ETHERLINK II - Repeater
Manual
Page 38 of 106
3.6.4.2 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 G.991.2 and is
an efficient tool for determining the qualitative performance of an xDSL 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 be 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
.
3.6.4.3 G.826 performance monitoring
The error performance monitoring of a digital DSL 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 xDSL
interfaces.
Figure 3.17 G.826 performance evaluation.
On the E1 side, four CRC4 check bits are generated per sub-multiframe (SMF) and compared
with the corresponding bits of the next SMF. If they do not match, the CRC4 error counter is
incremented.
On the xDSL side, six CRC6 check bits are generated per xDSL frame.
CRC6 errors are used by the software to count the block errors of the xDSL channel and to
evaluate its error performance according to ITU-T Rec. G.826.
G.826
Counters
xDSL
CRC6
CRC4
CRC4
CRC6
Slave
Master
C
R
C
4
C
R
C
4
C
R
C
6
/F
E
B
E
Tx
E1
Rx
Tx
E1
Rx
C
R
C
4
Tx
E1
Rx
C
R
C
4
C
R
C
6
/F
E
B
E
Tx
E1
Rx
G.826
Counters
Recei
ver
Gener
ator
Recei
ver
Gener
ator
Recei
ver
Gener
ator
Recei
ver
Gener
ator
Gener
ator
Gener
ator
Recei
ver
Recei
ver
