A-3
Section 61245207L2-5, Issue 3
61245207L2-5C
LOOPBACK ACTIVATION/DEACTIVATION
This section describes loopback activation and
deactivation methods for the ADTRAN Fractional T1
system. Loopback activation and deactivation is
controlled with the push-button located on the front
panel, through craft interface, or latching loopback
codes.
Both the FNID and the FT1 DP have a push-button
located on the front panel labeled REM LBK or RLB
which controls the activation of remote loopback.
Pressing REM LBK or RLB on the FNID activates
FT1 DP CPE loopback. Pressing REM LBK or RLB
on the FT1 DP activates FNID network loopback or
repeater loopback when a repeater is present. Remote
loopback is deactivated by pressing the REM LBK or
RLB or waiting for the 20-minute loopback timeout, if
enabled.
The FNID and FT1 DP (when available) have a craft
interface located on the front panel of the device. The
craft interface allows access to the FT1 device through
an RS-232-type interface. All loopbacks for the FNID
and FT1 DP can be controlled from the craft interface.
The craft interface on the FNID can activate and
deactivate the FNID network loopback and the FNID
CPE loopback. The FNID craft interface can also
remotely activate and deactivate the FT1 DP network
loopback, FT1 DP CPE loopback, and the repeater
network loopback. The craft interface on the FT1 DP
can activate and deactivate the FT1 DP network
loopback and the FT1 DP CPE loopback. The FT1
DP craft interface can also remotely activate and
deactivate the FNID network loopback and the FNID
CPE loopback and the repeater network loopback.
FT1 DP, FT1 Repeater, and FNID network loopbacks
can be activated by the DDS latching loopback
sequence. The latching loopback sequence is detected
in the primary DS0 of the FT1 system. When the
loopback is activated on the device, the whole FT1
bandwidth is looped. The FT1 DP, FT1 Repeater, and
FNID each respond to a different loopback select code
allowing for sectionalization of the network during
testing.
Table A-1
is a list of FT1 devices and their
loopback select codes.
Latching loopbacks for the FT1 system are activated
by transmitting the following latching loopback
sequence in the primary DS0 of the FT1 system:
A. Minimum of 35 transition in progress (TIP) bytes
(N0111010).
B. Minimum of 35 loopback select code (LSC) bytes as
defined in Table 1.
C. Minimum of 100 loopback enable (LBE) bytes
(N1010110).
D. Minimum of 35 all 1s bytes (S1111111), plus a
minimum of 100 LBE bytes. (N-1) iterations, where
N is the number of channel units of the same type
(i.e., same LSC) that lie between the test center and
the loopback to be operated. This step is only used
when there are identical channel units in tandem.
E. Minimum of 32 far end voice (FEV) bytes
(N1011010).
A 25-second watchdog timer is activated between the
35 TIP bytes and 35 LSC bytes, between the 35 LSC
bytes and 100 LBE bytes, and between the 100 LBE
bytes and 32 FEV bytes. The timer requires the
correct receipt of the latter sequences less than 25
seconds after receipt of the prior sequence. This
prevents inadvertent setting of the latching loopbacks.
Latching loopbacks for the FT1 system are
deactivated by transmitting the following latching
loopback sequence in the primary DS0 of the FT1
system:
Minimum of 35 TIP bytes (N0111010).
LOOPUP STATE
In the loopup state, the active FT1 loopback will
provide a continuous loop for the FT1 bandwidth.
The data flow is continuously monitored for the loop
deactivation commands to deactivate the loopbacks.
Also, a 20-minute loopback timeout is checked. If the
loopback timeout is enabled, any loopback that is in
the loopup state for 20 minutes will be deactivated.
Table A-1. FT1 Loopback Select Codes
FT1 Device
Loopback Select
Loopback Select
Code Name
Code Byte
FT1 DP .............................. OCU loopback .................. N1010101
FT1 Repeater #1 ................ NEI Position 1 loopback ... N1000001
FT1 Repeater #2 ................ NEI Position 2 loopback ... N1000001
FNID without repeater ...... NEI Position 1 loopback ... N1000001
FNID with one repeater .... NEI Position 2 loopback ... N1000001
FNID with two repeaters .. NEI Position 3 loopback ... N1000001
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