Section 150-611-100
Revision 02
Page 9
9. LOOPBACKS
9.01
HiGain-2 has a family of loopback options.
The most important of these is the Smart-
Jack loopback which enables the HRU-612 to
respond to the standard 2/3 in 5 Smart-Jack in-band
loopback codes. This option can be enabled or
disabled from either the front panel settings or the
dumb terminal system settings menu.
9.02
In addition to the Smart-Jack loopback,
HiGain-2 can be configured for one of five
special in-band loopback command sequences.
These are selected from the SPLB user option
shown in Table 2 and Figure 8. The loopback
locations are shown in Figure 5.
9.03
GNLB is the HiGain-2 Generic loopback
code. The GNLB allows 4 in 7 or 6 in 7 in-
band codes to loop-up the HLU (NLOC or CREM
respectively) and 3 in 7 or 4 in 7 in-band codes to
loop-up the HRU (NREM or CLOC respectively).
NLOC and NREM are issued from the HLU DS1
interface. CLOC and CREM are issued from the
HRU DS1 interface. All looped states are
terminated (looped-down) with the 3 in 5 loop-down
code. All commands must be present for 5 seconds
before HiGain-2 responds. Table 3 lists the test
procedures that apply when using the GNLB mode.
9.04
The A1LB loopback selection (Table 4)
complies with that proposed for VHDSL
systems in the T1E1.4/92 recommendation with the
following additions:
1) Query loopback
2) IOR powerdown
3) Three loopback time-out choices
4) Initiation from either end
5) Repeating bit error signatures
6) Alternate Query loopback
These additions make A1LB identical to A2LB
described below. It is given a separate identity to
allow future T1E1 enhancements to be added
without affecting A2LB.
9.05
A2LB through A5LB are four special
addressable repeater loopback functions
which are supported by the List 6 version of
HiGain-2. These loopbacks provide HiGain-2 with
sophisticated maintenance and trouble shooting
tools. Tables 5 through 8 list the details of these
Special Loopback (SPLB) functions. A2LB and
A5LB are patterned after the Teltrend addressable
T1 repeater loopbacks. A3LB and A4LB are
patterned after the Wescom addressable T1
repeater loopbacks. All four SPLBs have been
enhanced to handle the specific requirements of the
following HiGain-2 customers:
•
A2LB (Teltrend) = Southwestern Bell
•
A3LB (Wescom) = New England Tel.
•
A4LB (Wescom Mod 1) = New York Tel.
•
A5LB (Teltrend Mod 1) = Southern New
England Tel. (SNET)
9.06
A5LB differs from A2LB in that A5LB does
not block the arming (3 in 5) code from
exiting the HLU into the network. A2LB can be
configured to either block this arming code after two
seconds, and replace it with the AIS code, or to
unblock it by executing the FAR-END ACTIVATE
code. Since A5LB never blocks the arming code
from exiting the HLU, it does not need this FAR-
END ACTIVATE code. A3LB differs from A4LB in
that A3LB supports the additional 1 in 6 smart jack
loopback command. Refer to the PairGain HiGain-2
Intelligent Repeater Application Note # 910 Part #
325-910-100 for more SPLB details.
9.07
When T1 loopback tests are performed on
the HiGain-2 system with metallic loopback
connections at either end, the DS1 code that exists
at the metallic loopback interface may be different
from the DS1 code being received at the opposite
end when the DS1 user code is set to AUTO. This
is caused by the fact that in the AUTO DS1 code
mode, the HLU and HRU set their own code
independent of each other. Each end sets its
transmit code to match its receive code. Thus if one
end is receiving AMI and the other B8ZS, their
codes are different. For example, if the HRU has a
metallic loopback and the HLU’s receive pattern’s
code is changed from AMI to B8ZS and then the all
0 pattern is sent into the HLU, the HLU changes to
its B8ZS mode while the HRU remains in its AMI
mode and thus loops all 0’s. This causes the HRU
to indicate a LOS condition which then causes the
HLU to output the AIS pattern
9.08
HiGain-2 may take longer than normal to
respond to inband loopback commands
when its framing mode is set to UNFR and the
inband commands are sent in either an SF or ESF
mode. The frame bits override the command bits
and cause errors in the command sequence. These
errors cause HiGain-2 to reject some sequences.
This can extend the detection interval.
