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MX2800 M13 Multiplexer User Manual Loopbacks, Section 6
61200290L1-1G 6-7
Line Loopback
Line Loopback performs a loop of the DS3 back to the network. This loopback occurs just prior to the DS3
framer and B3ZS decoder, but it makes full use of the DS3 LIU in both receive and transmit directions;
therefore, any coding violations received by the DS3 will be inserted back into the network without
modification. See Figure 6-9 on page 6-7 for an illustration of this test.
NOTE
If a Line loopback is active when the MX2800 is operating in the Local
timing mode, the timing source for the DS3 is effectively removed from
the circuit. Therefore, it is up to the test equipment or network to provide
DS3 timing into the circuit for the duration of the test.
Figure 6-9. Line Loopback Test
Digital Loopback
A Digital Loopback loops the entire DS3 back to the local loop side. The end effect of this test is a
loopback of all T1/E1s after being fully multiplexed and de-multiplexed to and from a DS3. The incoming
DS3 data is ignored, and the outgoing DS3 stream is substituted in its place just prior to exiting the DS3
framer. This test is illustrated in Figure 6-10.
Figure 6-10. Digital Loopback
DSX-3
LIU
M23
MUX/DMUX
M12/G.747
MUX/DMUX
(1 of 7)
F
R
A
M
E
R
D
S
2
F
R
A
M
E
R
D
S
3
....
....
T1/E1 LIU
(1 of 28)
CODEC
(28 of 28)
CODEC
(1 of 28)
T1/E1 LIU
(28 of 28)
DSX-3
LIU
M23
MUX/DMUX
M12/G.747
MUX/DMUX
(1 of 7)
F
R
A
M
E
R
D
S
2
F
R
A
M
E
R
D
S
3
....
T1/E1 LIU
(1 of 28)
CODEC
(28 of 28)
CODEC
(1 of 28)
T1/E1 LIU
(28 of 28)
DSX-3
LIU
M23
MUX/DMUX
M12/G.747
MUX/DMUX
(1 of 7)
F
R
A
M
E
R
D
S
2
F
R
A
M
E
R
D
S
3
....
....
T1/E1 LIU
(1 of 28)
CODEC
(28 of 28)
CODEC
(1 of 28)
T1/E1 LIU
(28 of 28)
DSX-3
LIU
M23
MUX/DMUX
M12/G.747
MUX/DMUX
(1 of 7)
F
R
A
M
E
R
D
S
2
F
R
A
M
E
R
D
S
3
....
T1/E1 LIU
(1 of 28)
CODEC
(28 of 28)
CODEC
(1 of 28)
T1/E1 LIU
(28 of 28)