TELCOLINK DT - AccessDSL NTU
User Manual
Version: 2.5
Page 19 of 66
4 PERFORMANCE MONITORING
The transmission performance of an xDSL link can be monitored in two different ways. The
xDSL signal quality is typically used during installation and maintenance procedures, whereas
the G.826 error performance parameters are intended to be used for long-term evaluation of
operating xDSL links. Refer also to the “SQ” and “G826” monitor commands described in the
“Black Box Network Services Monitor” section.
4.1
G.826 Performance Monitoring
The G.826 error performance parameters provide
quantitative
performance information of a
specific loop. They are intended to be used for long-term evaluation of operating xDSL links.
The evaluation of the G.826 error performance parameters is based on CRC (Cyclic
Redundancy Check) error detection:
Slave
CRC4
CRC6
TX
RX
E1
Test
Gen.
CRC4/E-bit
G.826
Evaluation
Gen.
Test
CRC6/FEBE
xDSL
Channel
Master
CRC6
CRC4
Test
Gen.
CRC4/E-bit
G.826
Evaluation
Gen.
Test
CRC6/FEBE
TX
RX
E1
Figure 4-1: G.826 Performance Evaluation
CRC generation and detection on the LTU/NTU are handled separately for the E1 side and the
xDSL side, while on the ADD-DROP Repeater CRC generation and detection are handled
separately for the E1 side and both xDSL sides.
On the E1 side, four CRC4 check bits are generated per sub-multiframe (SMF) and compared
with the corresponding CRC4 bits in the following SMF. If they do not match, the CRC4 error
counter is incremented. The opposite station is informed of detected CRC4 errors by setting E-
bits in the transmitted frames. At the same time, the E-Bits from the opposite station are
counted and can be used for performance monitoring.
Similarly, on the xDSL side, six CRC6 check bits are generated per xDSL frame for each
channel and direction. For signaling detected block-errors in the return direction, the FEBE-bits
are used. The HDSL G.826 performance of the opposite unit is calculated according to these
FEBE-bits.
CRC6 errors are used by software to count the block-errors of the respective xDSL channel and
to evaluate its error performance according to ITU-T Rec. G.826.
For the E1 interface, calculations according to G.826 are only possible in the framed mode with
CRC4 option enabled. In framed mode with CRC4 option disabled only FAS-errors are
detected.
The estimation of a
bit-error rate
is not within the scope of the G.826 calculations.
