Troubleshooting guide

MX2820 System Manual
5-102 61186003L1-1D
Line Loopback
Line loopback performs a loop of the STS-1 back to the network. This loopback occurs just
prior to the HDB3/B8ZS decoder of the LIU; therefore, any coding violations received by the
STS-1 will be inserted back into the network without modification. See Figure 5-73 for an
illustration of this text.
NOTE
If a Line Loopback is active when the MX2820 STS-1 is operating in
the Free-Run timing mode, the timing source for the STS-1 is effec
-
tively removed from the circuit. Therefore, it is up to the test equip-
ment or network to provide STS-1 timing into the circuit for the
duration of the test.
Figure 5-73. Line Loopback Test
Digital Loopback
A Digital Loopback loops the entire STS-1 back to the local loop side. The end effect of this test
is a loopback of all VT/Ports after passing through the STS-1 mapper and partially through
the LIU. The incoming STS-1 data is ignored and the outgoing STS-1 stream is substituted in
its place just prior to entering the STS-1 encoder block of the LIU. This data will be processed
through the “Transmit Clock Duty Cycle Adjust” PLL and the “HDB3/B3ZS Encoder” block. At
this point, the data will be looped back to the “HDB3/B3ZS Encoder” block. This test is illus-
trated in Figure 5-74.
STS-1
MAPPER
T1/E1 LIU
(1 of 28)
T1/E1 LIU
(28 of 28)
STS-1
LIU
CODEC
(1 of 28)
CODEC
(28 of 28)