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TROCU DP Bidirectional Loopback Support
The TROCU DP will execute a bidirectional loopback
when performing an OCU loopback at the TROCU
DP. If the Total Reach DDS-R detects a bidirectional
loopback during power-up synchronization, the Total
Reach DDS-R allows data to pass on the 4-wire
interface by entering into the pass-thru mode (Figure
8). This allows a standard portable DDS test set,
connected to the 4-wire customer interface of the
Total Reach DDS-R, to verify the integrity of the
2-wire loop by transmitting a test pattern and
examining the returning data for synchronization and
errors. The Total Reach DDS-R LBK indicator
always flashes during a TROCU DP bidirectional
loopback.
If the TROCU DP bidirectional loopback is invoked
after the Total Reach DDS-R achieves
synchronization, the Total Reach DDS-R will not pass
or receive data from the CPE or DDS test set. This is
consistent with current DDS testing methods and is
referred to as the TROCU DP bidirectional loopback
normal mode (Figure 9). For testing purposes, the
installer may choose to initiate the Total Reach
DDS-R to pass-thru mode. Once the bidirectional
loopback is executed in normal mode, unseat and
reseat the Total Reach DDS-R and allow the unit to
train-up. Once trained, the Total Reach DDS-R will
revert to pass-thru mode for further testing.
TROCU DP
Bidirectional
loopback
Figure 8. TROCU DP Bidirectional Loopback
Pass-Thru Mode
TROCU DP
Bidirectional
loopback
Figure 9. TROCU DP Bidirectional Loopback
Normal Mode
6
TR DDS-R
Test Unit
Data
Pass-Thru
TR DDS-R
X
DSU/CSU
X
Open Loop
Section 64291043SAALL-5, Issue 5
Repeater Loopback (56 kbps)
A repeater loopback code transmitted from a remote or
local test system causes the TROCU DP to notify the
Total Reach DDS-R of a CSU loopback request. Upon
detection of a CSU loopback request, the Total Reach
DDS-R causes a loopback toward the network by
connecting the customer-side transmit and receive
together. Circuit integrity can then be verified by
transmitting bit error rate (BER) data from the test
system and examining the returning data for
synchronization and errors.
While in loopback, the Total Reach DDS-R can detect
the All-1s Repeater Release Code sequence, and upon
recognition of this sequence, drops its loopback and
reverses sealing current to the DSU/CSU. The All-1s
Repeater Release Code sequence is the non-latching
CSU loopback code alternating with an All-1s data
pattern for approximately 2 seconds. See Table 5 for
alternating loopback sequences.
Some test systems and test sets support both a CSU
(channel loopback) and a 56 kbps repeater loopback.
If repeater loopback is available, it will only loop the
Total Reach DDS-R. When CSU or channel loopback
is selected, some test systems automatically send the
All-1s Repeater Release Code to release loopbacks
from any regenerating devices on the loop, including
the Total Reach DDS-R. If the test system or test set
does not support a 56 kbps repeater loopback, a CSU
or channel loopback should be selected and the All-1s
Repeater Release Code injected for approximately 2
seconds to release a Total Reach DDS-R loopback
prior to looping the DSU/CSU.
Table 5. Alternating Loopback Sequences
F
u
n
t c
o i
n
A
i t c
e v
o l
p o
a b
k c
M
a
n i
a t
n i
o l
p o
a b
k c
a
d n
t
s e
f t
r o
b
t i
r e
o r
s r
C
e l
r a
o l
p o
a b
k c
X
=
D
o
t ' n
a c
e r
b
t i
R
e
e c
v i
d e
B
t y
s e
F
o
r u
c
n o
e s
u c
i t
e v
b
t y
s e
f o
p s
c e
i f i
d e
o l
p o
a b
k c
o c
e d
X
0
0 1
0 1
0 1
-
O
C
U
X
0
0 1
0 1
0 0
-
C
S
U
X
0
0 1
1 1
0 0
-
D
S
U
D
a
a t
y b
e t
a
e t l
n r
i t a
g n
i w
h t
o l
p o
a b
k c
o c
e d
e
a x
m
p
: e l
X
D
D
D
D
D
D
1
X /
0
0 1
0 1
0 1
F
o
r u
c
n o
e s
u c
i t
e v
a d
a t
b
t y
s e
i w
h t
u o
a t
p s
c e
i f i
d e
o l
p o
a b
k c
o c
e d
64291043SAALL-5E
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